Facoltà di Ingegneria - COnnecting REpositories · Figure 2.3-21 VIGVs schedule for T1000,...

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A.A. 2010-2011

Università di Pisa

Facoltà di Ingegneria

A study on the integration of the

IP Power Offtake system within the Trent 1000

turbofan engine

Author: Leonardo Lupelli

Supervisors: Fabrizio Paganucci

Torsten Geis

This information is given in good faith based upon the latest information available to Rolls-Royce plc, no warranty or

representation is given concerning such information, which must not be taken as establishing any contractual or

other commitment binding upon Rolls-Royce plc or any of its subsidiary or associated companies

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TABLE OF CONTENTS

FIGURES .................................................................................................................................... 6

ENGINE STATIONS ................................................................................................................. 10

SUMMARY ............................................................................................................................... 11

PART ONE – UNDERSTANDING OF THE IP POWER OFFTAKE SYSTEM ........................... 12

1.1 INTRODUCTION ...................................................................................................................... 13

1.2 LOAD REQUIREMENTS ............................................................................................................ 16

1.2.1 Interaction with the aircraft ........................................................................................ 16

1.2.2 Normal conditions ...................................................................................................... 16

1.2.3 Max conditions / peak levels ...................................................................................... 16

1.2.4 Transient loads .......................................................................................................... 17

1.2.5 Maximum P/O level for an IP/HP/LP system .............................................................. 18

1.2.6 Requirements: ........................................................................................................... 19

1.3 DESCRIPTION OF KEY WHOLE ENGINE SYSTEMS AND ATTRIBUTES AFFECTED BY THE

DEFINITION OF THE IP P/O SYSTEM .................................................................................................. 20

1.3.1 Accessories speed range .......................................................................................... 20

1.3.2 NMix variation ........................................................................................................... 22

1.3.3 Air/Oil Systems .......................................................................................................... 23

1.3.4 Thrust Response ....................................................................................................... 24

1.3.5 Thrust Asymmetry ..................................................................................................... 24

1.3.6 Icing .......................................................................................................................... 25

1.3.7 Bleed valve PR .......................................................................................................... 25

1.3.8 VSV overclosure........................................................................................................ 25

1.4 MECHANICAL ASPECTS ........................................................................................................... 25

1.4.1 Shaft critical speed .................................................................................................... 25

1.4.2 SAGB/IGB heat management and Windage .............................................................. 26

1.4.3 Effects of higher torque ............................................................................................. 27

1.4.4 Resonance during surge events ................................................................................ 27

1.4.5 Sustained Torsional Oscillations (STO) ..................................................................... 28

1.5 MAIN DRIVES TO MOVE FROM AN HP P/O SYSTEM TO AN IP P/O SYSTEM ................................... 28

1.5.1 Introduction ............................................................................................................... 28

1.5.2 Power extraction........................................................................................................ 29

2 PART TWO - A COMPARISON BETWEEN THE IP AND THE HP P/O SYSTEM ............. 31

2.1 ENGINE WEIGHT ..................................................................................................................... 32

2.1.1 Differences between IP & HP P/O systems ............................................................... 32

2.1.2 Extracting power from the core .................................................................................. 32

2.1.3 Starting the engine .................................................................................................... 33

2.1.4 Engine stability .......................................................................................................... 33

2.1.5 Engine weight estimation ........................................................................................... 33

2.2 FUEL BURN ............................................................................................................................ 34

2.2.1 How the IP P/O system affects the fuel burn ............................................................. 34

2.2.2 Flight conditions ........................................................................................................ 34

2.2.3 Impact on engine performance .................................................................................. 35

2.3 STARTING .............................................................................................................................. 36

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2.3.1 Introduction to the coupling device and Mark 1 SAGB ............................................... 36

2.3.2 Problems with coupling devices ................................................................................. 36

2.3.3 Differences between IP and HP starting system ........................................................ 37

2.3.4 Windmill - Relight capability ....................................................................................... 41

2.3.5 Hot start .................................................................................................................... 42

2.3.6 Requirements for the P/O system: ............................................................................. 43

2.3.7 Requirements for stakeholders: ................................................................................. 43

2.4 IDLE SETTING IN THE CONTEXT OF IP P/O SYSTEM.................................................................... 45

2.4.1 Introduction ............................................................................................................... 45

2.4.2 The boundaries for the idle setting ............................................................................ 45

2.5 EFFECTS OF THE IP P/O SYSTEM ON THERMODYNAMIC ASPECTS .............................................. 47

2.5.1 Introduction ............................................................................................................... 47

2.5.2 Effects on Core Matching .......................................................................................... 48

2.6 HOW THE DIFFERENT P/O SYSTEMS AFFECT THE SURGE MARGIN .............................................. 52

2.6.1 Introduction ............................................................................................................... 52

2.6.2 Definition of the available Surge Margin .................................................................... 53

2.6.3 HPC Stability ............................................................................................................. 54

2.6.4 P/O effects on HPC worst case assessment ............................................................. 57

2.6.5 IPC stability ............................................................................................................... 57

2.6.6 P/O effects on IPC worst case assesment ................................................................. 59

2.6.7 LP power offtake ....................................................................................................... 59

2.6.8 Core size ................................................................................................................... 59

2.6.9 Operability / HBV control ........................................................................................... 60

2.6.10 Operability / VSV control ........................................................................................... 60

2.6.11 Cruise conditions ....................................................................................................... 63

2.6.12 Effect of P/O on core size for increased BPR ............................................................ 64

2.6.13 Requirements for P/O system.................................................................................... 66

2.6.14 Requirements for stakeholders .................................................................................. 66

2.8 SECONDARY AIR SYSTEM ....................................................................................................... 67

2.8.1 Controls of Handling Bleed Valves with IP P/O system ............................................. 67

2.8.2 Sealing, IPT issue ..................................................................................................... 69

2.8.3 Bearing chamber issue .............................................................................................. 71

2.8.4 ESS anti-icing system ............................................................................................... 71

2.8.5 Reliability – FMECA issues ....................................................................................... 72

3 PART THREE – ANALISYS OF THE IP P/O SYSTEM ..................................................... 73

3.1 SYSTEM ENGINEERING ........................................................................................................... 74

3.1.1 System engineering and system thinking .................................................................. 74

3.1.2 Emergent properties .................................................................................................. 75

3.2 DEFINE REQUIREMENTS .......................................................................................................... 78

3.2.1 Summary ................................................................................................................... 78

3.2.2 Requirements of the IP P/O system........................................................................... 79

3.2.3 Context Diagram and definition of boundaries ........................................................... 79

3.2.4 Stakeholder analysis ................................................................................................. 85

3.2.5 Systemic Textural Analysis ........................................................................................ 86

3.2.6 Viewpoint Analysis .................................................................................................... 87

3.2.7 Functional Modelling ................................................................................................. 88

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3.2.8 Sensitivity Analysis .................................................................................................... 90

3.2.9 Quality Functional Deployment .................................................................................. 90

4 PART FOUR - REVIEW OF THE TRENT 1000 IP P/O SYSTEM ..................................... 100

4.1 STAKEHOLDERS DISCUSSION ................................................................................................ 101

4.1.1 Introduction ............................................................................................................. 101

4.1.2 Compressors ........................................................................................................... 101

4.1.3 Turbines .................................................................................................................. 102

4.1.4 Air and Oil systems ................................................................................................. 102

4.1.5 Transmission, Structures and Drives ....................................................................... 104

4.1.6 FMECA ................................................................................................................... 106

4.2 REQUIREMENTS – FUNCTIONS CORRELATION ......................................................................... 108

4.3 ERMS REPORT .................................................................................................................... 108

4.3.1 ERMS Reliability categories .................................................................................... 108

4.3.2 ERMS Status Categories ......................................................................................... 109

4.3.3 Trent 1000 ERMS log .............................................................................................. 109

4.4 LESSONS LEARNT LOG ......................................................................................................... 111

5 APPENDIX 1 – IDLE CONTROL PARAMETERS............................................................ 111

6 APPENDIX 2 – SYS-ML MODEL OF THE IP P/O SYSTEM ............................................ 113

6.1 SYSML LANGUAGE ............................................................................................................... 113

6.2 REQUIREMENTS ................................................................................................................... 113

6.3 BLOCK DEFINITION DIAGRAM (BDD) ...................................................................................... 114

6.4 INTERNAL BLOCK DIAGRAM (IBD).......................................................................................... 114

6.5 ACTIVITY DIAGRAM ............................................................................................................... 114

6.6 USE CASE DIAGRAMS ........................................................................................................... 114

7 APPENDIX 3 - FLUID COUPLING DEVICE .................................................................... 115

7.1.1 Accommodation of Failure cases ............................................................................. 119

7.1.2 Additional requirements for coupling devices ........................................................... 120

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FIGURES

Figure 1.1-1 Pressure and temperature stations for Trent 1000 .............................................. 10

Figure 1.1-2 IP Power offtake system layout .......................................................................... 13

Figure 1.1-3 Trent 1000 AGB and accessories layout ............................................................ 14

Figure 1.1-4 Trent 1000 Engine cut-off ................................................................................... 15

Figure 1.2-5 Step load characteristics used to model transient loads ..................................... 17

Figure 1.3-6 Trent 1000 idle thrust estimation Vs. day temperature with different IP shaft SR 21

Figure 1.3-7 N3 and N2 variation with IP P/O for constant NMix ............................................. 23

Figure 1.5-14 Power offtake levels of different engines .......................................................... 29

Figure 1.5-15 P/O levels Vs. P/O architectures ...................................................................... 30

Figure 2.2-18 Effects of weight increase on fuel burn ............................................................. 35

Figure 2.3-20 Layout of the IP P/O with low speed mechanical clutch .................................... 37

Figure 2.3-21 VIGVs schedule for T1000, Cranking Vs High speed ....................................... 39

Figure 2.3-22 Starting procedure with IP P/O and dual starter (with coupling device). ............ 40

Figure 2.3-23 Starting procedure with IP shaft crank .............................................................. 41

Figure 2.3-25 Starting time requirement, temperature envelope ............................................. 44

Figure 2.5-26 Typical HP compressor map with constant speed and constant efficiency

iso-lines ................................................................................................................................ 47

Figure 2.5-27 Pressure conditions for turbines ....................................................................... 49

Figure 2.5-28 P/O effects on turbines' power .......................................................................... 52

Figure 2.6-29 Residual surge margin and threats to WL and surge line .................................. 54

Figure 2.6-30 Effects on the HPC working line: IP P/O and HP P/O ....................................... 55

Figure 2.6-31 Loop shaping of the HPC with IP P/O system ................................................... 56

Figure 2.6-34 Changes on IPC working line with IP or HP P/O system, HBV effect is shown for

Hp P/O ................................................................................................................................... 58

Figure 2.6-36 Effects of the switched air system on the HPC and IPC stability margin ........... 59

Figure 2.6-24 VSV open loop schedule .................................................................................. 61

Figure 2.6-25 IPC speed triangle control with VSV ................................................................. 62

Figure 2.6-39 Flowchart for HBR requirements ...................................................................... 65

Figure 2.8-42 Effect of bleed air on compressor stages .......................................................... 69

Figure 2.8-43 Effect of min P26/P44 on resulting thrust at low idle ......................................... 71

Figure 2.8-44 Cooling air flow as function of P26/P44 ............................................................ 71

Figure 2.8-45 Trent 1000 ESS anti iceing system ................................................................... 72

Figure 3.1-47 Solution definition procedure, logic scheme ...................................................... 76

Figure 3.1-48 Requirements flowdown diagram ..................................................................... 77

Figure 3.1-49 Solution definition using System Engineering, preliminary design tools ............ 77

Figure 3.1-50 The "W diagram" .............................................................................................. 78

Figure 3.2-51 Power Offtake System physical boundaries and interfaces (cyan balloons) ...... 80

Figure 3.2-52 IP power offtake context diagram ..................................................................... 81

Figure 3.2-53 Engine context diagram for systems related to the IP P/O system .................... 82

Figure 3.2-54 IP P/O system Function Flow Diagram ............................................................. 89

Figure 3.2-55 QFD1 structure ................................................................................................. 92

Figure 3.2-56 IP P/O system Functional Reqirements Tree (Red boxes are top-level functions)

............................................................................................................................................... 97

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Figure 3.2-57 IP P/O system QFD1 results ............................................................................. 98

Figure 3.2-58 QFD1 results: System attributes rating ............................................................. 99

Figure 3.2-59 QFD1 results: Functional requirements rating .................................................. 99

Figure 6.6-60 Fluid coupling device ...................................................................................... 115

Figure 6.6-61 SAGB with breather, high speed fluid coupling device .................................... 117

Figure 6.6-62 SAGB, low speed fluid coupling device .......................................................... 119

© 2012 Rolls-Royce plc

©2012 Rolls-Royce plc

The information in this document is the property of Rolls-Royce plc and may not be copied, or communicated to a third party, or

used, for any purpose other than that for which it is supplied without the express written consent of Rolls-Royce plc.

Acronyms

A/C Aircraft

ACU Acceleration Control Unit

AFDX Avionics Full Duplex network

AGB Accessory Gearbox

Amb Ambient

AOHE Air Oil Heat Exchanger

APU Auxiliary Power Unit

B787 Boeing 787

BPR By-Pass Ratio

BV Bleed Valve

CACV Cooling Air Control Valve

CFB Centrifugal Breather

DP Differential Pressure

DPT Differential Pressure Transducer

EAI Engine Anti Ice

ECS Environmental Control System

EDP Engine Driven Pump (hydr.pump)

EDP Engine Development Program

EEC Engine Electronic Controller

EGT Exhaust gas Temperature

EIS Entry Into Service

EMI Electromagnetic Interference

EMU Engine Monitoring Unit

EPDC Electrical Power Distr. Centre

ESS Engine Section Stators

FAA Federal Aviation Administration

FADEC Full Authority Digital Engine Control

FBH Front Bearing Housing

fh flight hour

FOGV Fan Outlet Guide Vanes

FOHE Fuel Oil Heat Exchanger

HBV Handling Bleed Valve

GBX Gearbox

Gpm Gallons Per Minute

HBPR High BPR

HMU Hydro-Mechanical Unit

HP High Pressure

HPC High Pressure Compressor

HPT High Pressure Turbine

hp horse power

IFSD In Flight Shut Down

IGB Intermediate Gearbox

IGV Inlet Guide vane

IP Intermediate Pressure

IPC Intermediate Pressure Compr.

IPT Intermediate Pressure Turbine

IOM Input-Output Module

Kg Kilogram

KOZ Keep Out Zone

KT Knot

KVA Kilo Volt Ampere

Lbs Pounds

L/G Landing Gear

Lpm Liters per minute

LP Low Pressure

LPC Low Pressure Compressor

LPT Low Pressure Turbine

LPTCCV LPT Case Cooling Valve

LVDT Linear Variable Differ. Transducer

MCT Maximum Continuous Thrust

Mn Mach number

MoU Master of Understanding

MTBF Mean Time Between Failures

MTBR Mean Time Between Removals

MTO Maximum Take-Off

N1 (or NL) LP spool rotation speed

N2 (or NI) IP spool rotation speed

N3 (or NH) HP spool rotation speed

NM NMix rotation speed

NMDot Rate of change of NMix

OEI One Engine Inoperative

OGV Outlet Guide Vanes

P0 Ambient pressure

P160 Fan Exit Pressure

P24 Engine Intake Pressure

P26 IPC Exit Pressure


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P30 HPC Delivery Pressure

P50 LPT exit pressure

Pamb Ambient pressure

PCE Pre cooler

PMA Permanent Magnet Alternator

P/O Power Off-take

Pph Pound Per Hour

Psi Pound per square inch

PWR Power

RAT Ram Air Turbine

RCI Resolve Customer Issue

RDC Remote Data Concentrator

RDS Radial Drive Shaft

R-R Rolls-Royce plc

RTO Rejected Take-Off

S/C Short Circuit

SAGB Step Aside Gear Box

SASV Secondary Air System Salves

SFC Specific Fuel Consumption

SID System Interface Document

SLS Sea Level Static

T/O Take Off

TBH Tail Bearing Housing

TCAF Turbine Cooling Air Front

TCAR Turbine Cooling Air Rear

TCC Turbine Case Cooling

TCCV Turbine Case Cooling Valve

TET Turbine Entry Temperature

TGT Turbine Gas Temperature

TOD Top Of Descent

VFG Variable Frequency Generator

VFSG Variable Frequency Starter Generator

WL Working Line


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ENGINE STATIONS

Figure 1.1-1 Pressure and temperature stations for Trent 1000

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Summary

The progressive increase of power extracted from the engine for the electrical aircraft has raised

the issue of the competitive position of 2-shaft vs. 3-shaft engines and, within the 3-shaft architecture,

the effects of using a different shaft to drive the accessory gearbox.

The aim of this report is to define the key parameters and features of an IP P/O system, how they

are influenced, what are the systems and stakeholders involved and, qualitatively, how they are

influenced. This is done in order to explain and better understand the decisions made during the

design and development of the T1000, why certain solutions have been selected and others rejected,

which areas of P/O design require particular attention and what are the lessons learnt.

The first part of this document is aimed at better understanding the theory and the rationale behind

the IP P/O system and its influences on the engine. In order to do this different topics are discussed,

starting from Load requirements, Idle setting, Engine weight, Fuel burn, Starting performance,

Thermodynamic and Mechanical aspects, Secondary air system features and Noise.

The second part is aimed at reviewing the design procedure using the Systems Thinking

approach, i.e. considering the P/O system not just as the sum of its components but as a system

with different properties, functions and effects. The review of the design process starts from the

Definition of Requirements, proceeds with the Stakeholders Analysis (only the most affected

are considered in detail) and ends with a review of the system on the actual engine (Trent 1000

Pack B): relation between requirements, functions and implementation, review of ERMS and

Lessons Learnt databases and FMECA report.


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PART ONE – UNDERSTANDING OF THE IP POWER OFFTAKE SYSTEM


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1.1 Introduction

The progressive increase of power extracted from the engine for the aircraft has raised the issue of

the competitive position of 2-shaft vs. 3-shaft engines since, as will be widely discussed later, the

former engines are less affected by this change. For the 3-shaft engines this resulted in the

requirement of using a different shaft to drive the accessory gearbox.

With the term “power offtake system” we refer to the whole of the parts, mechanisms and functional

systems that allow taking the power from the core of the engine and making it available for the

accessory gearbox (AGB). The main assemblies that constitute the P/O system are the Internal Gear

Box (IGB), the Step Aside Gear Box (SAGB) and the External Gear Box (EGB). The External Gear

Box (EGB) is the assembly of Transfer Gear Box (TGB) and Accessories Gear Box (AGB). Although

the turbine and the core shaft are not considered as part of the P/O system, but they are required to

extract the thermal power from the gas flow, convert it into mechanical power and transfer it to the

IGB.

The SAGB is driven from the Radial Drive Shaft (RDS) that emerges from the IGB: it reduces the

rotational speed and creates an offset of the shaft. Doing this allows the design of a shorter and

lighter fan case. Additional length would be required if using the shaft from the IGB wich, without the

SAGB, would cause bigger problems with bounds for rotational speed, load, natural frequencies and

dynamic instability of the shaft. Without the SAGB and with a longer fan case it would also resulted in

a requirement to move the thrust reversal system backwards. This would also have resulted in a

AGB

EGB

TGB

SAGB

IGB

Figure 1.1-2 IP Power offtake system layout


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longer and heavier nacelle and a potential increase in drag. Moreover, reducing the shaft speed

provide a solution to some important problems. Connecting the IGB directly to the TGB would have

required a very long shaft rotating at high speed. This would have been problematic from the

prospective of its dynamic behaviour.

It will be evident from the next chapters why the design of the RDS and the SAGB has been crucial

for the development of the IP P/O system.

On the Trent 1000, for the first time in the history of the Trent Engines Family, there is an

intermediate-pressure power offtake system (IP P/O) instead of a high-pressure power offtake

system. This means that the power delivered to the AGB is taken from the shaft that connects the IP

compressor and the IP turbine instead that from the high-pressure shaft. This solution deeply affects

many systems on the engine in a different way compared with the traditional HP P/O:

Internal air system,

Bleeding valve system,

IGV/VSV‟s control system,

Oil system,

Electric system and generators,

EEC logic,

Figure 1.1-3 Trent 1000 AGB and accessories layout

Moreover it redefines operability limits and steady state performance, changes compressors matching

and influences at different levels a number of engine characteristics.

The main benefits gained from the IP P/O system are:

1. Can easily handle increased offtake requirements well beyond HP P/O usual extraction

levels. High potential to satisfy increasing demand of customer for mechanical power

offtake.

2. Lower fuel burn for low-range missions,

3. Reduced brake wear (due to low idle setting at Ground)

Angled Drive shaft


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4. Lower noise during Approach,

5. Wider operability range,

6. The IPC base working line should be raised; this gave a key advantage on High By-Pass

Ratio (HBPR) engines.

On the other side the main drawbacks with an IP P/O system are:

1. New air system (SASV) required due to an insufficient IP PR at idle, resulting in a problem

to seal IP Turbine rim,

2. Optimization of the VFSG/electrical system causing conflicts between the minimum IP

shaft speed limit and max idle thrust,

3. Concerns over FMECA associated with switched air system valve SASV (failure case

difficult to accommodate),

4. EIS maturity and lack of experience at the start of the Trent 1000 Programme

To assess these issues and design a certifiable engine, a huge number of parameters are involved

and many new solutions have to be investigated. The aim of this report is to define what are the key

parameters and features of the IP P/O system, how they are influenced, what are the systems and

stakeholders involved and, qualitatively, how they are influenced. This is done in order to explain and

better understand the decisions made during the design and development of the T1000 P/O system,

why certain solutions have been selected and other rejected, which areas of P/O design require

particular attention and what are the lessons learnt.

Figure 1.1-4 Trent 1000 Engine cut-off


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1.2 Load requirements

1.2.1 Interaction with the aircraft

The Trent 1000 has been designed and optimized to power the new Boeing 787. In order to

achieve the task of improving overall efficiency of the aircraft, Boeing made a decision towards the

“More Electric Concept”. This culminates in a requirement for the engine manufacturer to provide a

significantly larger amount of electric power via the external gearbox. For instance, the T1000 has

to provide power to a novel aircraft-mounted electric Environmental Control System (ECS), which

would have been existed in the form of a traditional bleed air system on recent Trent engines. This

system now comprises two fuselage mounted air conditioning and pressurization packs that are

driven by electric motors. This means that the P/O system has to provide the additional power

required by the Air Conditioning Packs. The main difference for a More Electric Engine is that

bleed air is only used for engine internal use (engine secondary air system) and to guarantee the

operability of the engine over a wider range (Handling Bleed Valves, HBV‟s).

One Engine Driven Hydraulic Pump (EDP) and two Variable Phase Starter Generators (VFSG‟s)

are mounted on each engine in order to provide the required power for the ECS packs. In

conjunction with the other core driven accessories (oil pump, fuel pump and oil breather) this

requires the P/O system to deliver a significant amount of power to the EGB during normal

operation. This was the first estimation of the power required from the aircraft. The “More Electric

Aircraft” was a very new concept and there was a small margin of confidence with the actual

requirements of such a configuration. The power requirements have largely evolved during the

development of the Trent 1000 programme, leading to a maximum peak value of more than

1100hp to be delivered by a single engine.

1.2.2 Normal conditions

The typical power extraction levels have been identified for different flight phases for a 3000nm flight

for a B787-8, with 223 and 371 passengers. Data are property of Rolls Royce plc.

Data removed

1.2.3 Max conditions / peak levels

The max load conditions are defined as maximum loads and hold time. Typical hydraulic pump loads

are less than one 10th of the VFSG power, except during certain conditions occurring at high engine

power when loads up to 120% of the nominal load can occur.

The engine and the power offtake system must be designed to accommodate the transient loads

defined in Errore. L'origine riferimento non è stata trovata. without suffering mechanical damage,

reduced drive train components‟ life and without threatening the engine stability. The worse situation


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for all conditions is the direct short (DS, i.e. short circuit). A 5 second sustained overload is required

for the system to recognise and isolate this fault, during this period the electric system has to provide

the maximum power required. Data are property of Rolls Royce plc.

1.2.4 Transient loads

The power offtake system has to cope with shocks and transitory load due to:

Normal load variation, i.e. activation of wing ice protection system or high load condition for

the A/C packs (aircraft on ground during a hot day)

Abnormal loads, i.e. failure of a part of the electric system

Starter.

The hydraulic pump and the fuel pump also require variable amounts of power but their variation is

one order of magnitude smaller than the electric demand fluctuation.

For transient step loads a dynamic factor applies (Ref. 1)

The system must stand shocks and rapid changes of load without diminishing the predicted life and

reliability. This can ultimately lead to an increase in the system‟s weight. Further work should be done

as part of an improvement task in order to reduce the rate of change of loads and avoid predictable

shocks.

From the performance point of view the engine shall demonstrate uniform Engine-to-Engine and

Control-to-Engine acceleration and deceleration rates with any combinations of normal level of bleed

and power extraction with an acceptable deviation from a reference value of acceleration rate.

The difference between the normal load and the peak load has a much deeper impact on idle setting

and performance: if a power load signal would be available than it would be possible to optimize the

control system for best SFC when there is no load. It would also remove problems with load peaks

because it would be possible to “prepare the engine” for such loads. Without this kind of control the

engine has to be designed and the idle condition be settled considering the worst-case load step,

which is usually an almost impulsive peak load.

Figure 1.2-5 Step load characteristics used to model transient loads

Base Load:

Ground

Dual engine

Single engine

Step height:

Sustained

Ground

Dual engine

Single engine Transient step load

Peak

Sustained

Ground

Dual engine

Single engine Base Load+Transient step load

Base Load

Fixed slope


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In order to assess the operability and the transient behaviour of the engine the time variation law of

the applied load has to be defined. The wave shape used for the Trent 1000 is shown in Figure 1.2-5.

There is no Environment Control System (ECS) bleed on the Trent 1000 engine; the VFSG‟s have to

provide the electric power for two A/C units installed on the aircraft also during one engine inoperative

(OEI) condition, there is therefore a need for a robust operability with high power load.

As the P/O level increases, its influence on the whole engine becomes more and more important

and the engine has to be optimized around the offtake level to reach the best performance. For

instance having a robust load signal would allow R-R to lower the idle speed and increase it only

when needed, e.g. only raise it when the anti-icing system is turned at idle condition.

On the Trent 1000 design, some conservatism has been taken because of the novelty of the P/O

system. Introducing other new devices, systems or control logic would have resulted in an

excessive risk level due to new failure modes. To improve reliability and robustness of the system

the EEC doesn‟t receive a control signal from the aircraft. The pressure ratio P30/P26 is used by

the EEC since this can be related to the extracted power. For future engines a monitoring system

should be desired if margin for max P/O is required to maintain a low minimum idle thrust.

Requirements for a better control of idle are extensively discussed between Performance, Fluid

System and IPTs but requirements for the design of the P/O system may be raised by other crucial

engine systems to define how to interact with the P/O.

1.2.5 Maximum P/O level for an IP/HP/LP system

There is not a unique and precise limit for the power that can be extracted with an HP or an IP P/O

system; everything depends on a trade off study and on the performance required by the customer as

operability, min and max thrust, overhaul intervals etc. The constrains of the system can be related

with the P/O level but this is not an easy problem and the answer depends on core size,

requirements, transient behaviour etc. The best thing to do this is express the extracted power as

percentage of the turbine power in a specific condition (e.g. MTO) to have an idea of how much the

core is affected by the power extraction. To compare P/O effects on surge margin at different

conditions one has to use the operative turbine power.

For instance, a binding condition could be the generator speed or the HPC/IPC surge margin but

there are also the air system related issues and other constrains that are not uniquely defined. Which

of these is the relevant design constraint depends on requirements and operative condition of the

engine. The next chapters are aimed at describing what the differences between IP P/O and HP P/O

are in different conditions like idle and throttle response.

For both the systems, IP and HP P/O, we can roughly say that the maximum power that can be

extracted can be expressed as a percentage of the turbine power. The percentage value is almost

independent on the shaft used for extraction.

It is not possible to define the limits for either system in a simple and straightforward way. Obtaining a

more accurate answer requires use of a performance model able to predict the stability margins for

each system.

A smart parameter to relate with the P/O is the core turbine power or the shaft turbine power.

During the preliminary design is fundamental to understand the relationship between P/O demand,

engine size, stability and other engine functions or constraints in order to work out an optimum

solution for the power offtake system, i.e IP or HP powered, to obtain a robust engine.


19

1.2.6 Requirements:

1. Maintain margin on efficiencies and sizing to avoid shortfalls and to allow refinement of the

design during EDP: More Electric Engine is a relatively new concept (see both cases T1000

and XWB)

2. Design for max load considering both steady state and transient, consider normal and

exceptional overloads

3. Modify VFSG control system and transient behaviour, 1.9 transient factor for step loads will no

longer be acceptable: it has a too strong impact on idle setting. Consider for custom design of

generators.

4. Performance need to push for a “load state” and “upcoming load” signals, reduce engine

weight and allow to redefine idle

5. Hydraulic loads are much smaller and more constant than electrical loads.

6. Consider shocks resulting from sudden application of load during transient operation and

during starting crank. These include switching anti-icing devices, faults on aircraft systems and

other conditions reported in Ref. 1.

7. Satisfy engine to engine and control to engine acceleration and deceleration rate

8. Compressor stability has to facilitate all IP P/O levels and scenarios at all engine conditions.


20

1.3 Description of key whole engine systems and attributes affected by

the definition of the IP P/O system

This paragraph is aimed at giving an introduction to the following issues, as these showed of being

the most meaningful and representative of the impact of the IP P/O system on the whole engine:

o Accessories speed range

o NMix variation

o Air/Oil system

o Thrust Response

o Thrust Asymmetry

o Icing

o Bleed Valve pressure ratios

o VSV overclosure

1.3.1 Accessories speed range

Taking IP power offtake reduces N2RTH24 and IPC pressure ratio as well, thus decreasing the

resulting thrust. During cold days the IP shaft speed falls below the minimum NI drop off speed

required by the generators. Adding this new boundary the minimum idle thrust would increase. The

IDG speed ratio has a fundamental importance in minimizing thrust especially during cold days as

explained below.

The first thing to consider for the IP P/O system is that the typical speed ratio (SR=Nmax/Nmin) of the

IP shaft and that of the Boeing 787 VFSGs are almost the same over the whole engine operating

cycle. The speed range is a fundamental parameter for sizing and design of the electrical generators,

for the Trent 1000 the VFSG speed range has been decided by Boeing. Considering the actual idle

and red-line speeds it can be noticed that the HP speed range is quire narrower than the IP speed

range. This means that the accessories with an IP P/O system have to operate over a wider speed

range.

The compressor design influences the aforementioned engine speed ranges and the minimum

achievable thrust. This design is the result of a trade-off study involving High power, idle and starting

performance.

The RDS speed is, of course, connected with N3 in an engine with an HP P/O and with NI in an

engine with IP P/O. While for an HP P/O system the minimum RDS drop-off speed lies below the

scheduled idle speed at almost all day temperatures, for an IP P/O this value depends on the AGB-IP

shaft gear ratio that is limited on the other side by the IP red-line speed and VFSG maximum

frequency.

The speed ratio of the shaft used to extract the power must always be smaller than the VFSG‟s SR.

Since the IP SR is larger than the HP shaft‟s one, the IP P/O will experience more binding conditions

when increased operability is required with the same VFSGs.

VFSG frequency is fixed but the lower limit can be exceeded for a short period up to a transient

redline frequency, for lower frequencies or longer transients the generator goes offline..

Low idle thrust is affected by the N2 speed limit that depends on:

o The VFSG‟s operating range,

o The IP shaft operating speed range that actually is larger than the HP‟s one,


21

o IGB speed ratio SR=N2_Redline/N2_idle with N2_Redline

On actual engine the IP SR is slightly smaller than the VFSG‟s one, this because some margin is

required to tolerate overspeed and sub-idle speeds. On the first T1000 engines the Generator

frequency range was shifted towards larger N2. By modifying the gear ratio between the NI spool and

the generator through an SAGB redesign this range of speeds had been moved towards smaller

values of N2 which would allow for a reduction in the idle setting.

The Figure 1.3-6 represents the qualitative change in minimum thrust at idle for the Trent 1000 at

different temperatures and with two different speed ratios. An overclosure of 5deg has been

accounted for the VSV to improve the IPC operability; the reduction of thrust with this modification is

not negligible but the IPC efficiency is also affected. For more details refer to Par 2.6.10 Operability /

VSV control.

Reduction in idle thrust can be obtained taking IP power offtake; this diminishes and

reduces IPC PR as well, thus decreasing the resulting thrust. During cold days, if the Trent 1000 idle

was not controlled in , the IP shaft speed would fall below the minimum VFSG drop off

speed. It is because of this that during cool day in Figure 1.3-6 the engine thrust rises: the knee of the

curve indicates where the engine control swaps to logic. The position of this point

depends on the transmission ratio between the IP shaft (i.e. ) and the VFSG shaft. The IP speed

ratio has a fundamental importance in minimizing thrust, especially during cold days and the VFSG

frequency range is the main limit for it.

On the other side, extracting power from the HP shaft raises both IPC and HPC working lines, at a

constant idle speed the resulting thrust will increase (Ref. 3). To meet the requirements the

(reduced N3 speed) must be reduced. Despite the variation in N3 this does not affect the VFSG‟s

operability and hence the thrust can be reduced below the design value without affecting operation of

the accessories. This can be done because reducing idle N3, allows to maintain the VFSG away from

Des.

load

-60 -50 -40 -30 -20 -10 0 10 20 30 40 DTAMB

FN (lb)

IP8 ESS - VSV closure Speed Ratio 100%

ESS - VSV closure Speed Ratio 110%

Figure 1.3-6 Trent 1000 idle thrust estimation Vs. day temperature with different IP

shaft SR


22

its drop-off speed because the generator SR1 is still larger than N3 SR. The binding condition for an

engine with an HP P/O system and high power extraction is likely to become the IPC stability margin

which in turn may require a bigger core size or a redesign of the PR split between the compressors.

This can take away the advantage of the hypothetical benefit from a larger speed margin

With an IP P/O, the speed of the electrical generators and P/O limitations can ultimately limit Ground

Idle reduction, the recovery though generators with a wider speed range is the subject of a current

R&T programme.

1.3.2 NMix variation

Considering the situation of having Low and High idle scheduled through corrected NMix rather than

allows to define a value for NMix that meets the compressor stability requirements and another

that meets the minimum Air System pressure ratio.

The effects of altitude, Mach number, day temperature and power offtake on the idle setting

parameters have to be assessed (Ref. 11).

Day temperature has not a relevant effect on HPC and IPC working line or on WRTP/corr_NMix

relationship that are instead affected by Mach number and altitude. These effects are due to

beneficial influence of a higher Reynolds number for the compressor/turbine efficiency and, assuming

the turbine being not choked, to the variation of the work split.

The main parameter affecting working lines and WRTP/NMix on both IP and HP P/O is the power

offtake level, its effect at idle is still more pronounced taking a larger amount of the total Turbine

power (as widely explained in Effects of the IP P/O system on thermodynamic aspects).

Analysis of the idle thrust for the Trent1000 at different altitudes, Mach number and day temperatures

showed that the highest thrust was reached at the sea level static condition (higher mass flow), the

NMix idle schedule can be defined at SLS but altitude and Mach number dependency is needed in the

schedule in order to cover the entire flight envelope. If the power offtake level signal is not available

design for worst condition is required (consistently with aircraft manufacturer specifications) (Ref. 11).

The minimum corrected NMix limiter could be set at the point of minimum required compressor

stability but, on the Trent 1000, this constraint hides behind the minimum generator speed.

With an IP P/O system at a constant corrected NMix as power offtake is gradually increased N2

speed will decrease linearly (see Figure 1.3-7) while N3 will increase. Consequently, the minimum

corrected NMix schedule is defined to ensure that when maximum power offtake is being demanded

the engine operates on the intersection between the minimum N2 limiter (due to generators) and the

minimum corrected NMix (for core stability). This has the additional benefit of maintaining high levels

of N2 speed when no power offtake is demanded which will allow the engine to cope with a sudden

load addition and the subsequent N2 dip maintaining N2 speed above the minimum generator

frequency. On the other side this limit affects idle thrust control and fuel burn (see previous

paragraph).

For an HP P/O system the N2 and N3 lines converges as the power extracted increases. Indeed the

HP shaft speed is reduced while the IP shaft accelerates to maintain constant NMix.

1 Speed Ratio


23

1.3.3 Air/Oil Systems

The main differences between IP P/O and HP P/O effects on the air system, their causes and

Trent1000 solutions will be discussed in detail later in the Air System paragraph. The pressure ratios

that need to be established in order to ensure no hot gas ingestion occurs in the turbine when

operating at idle are the and the ratio. Where:

With an HP P/O system the variation range of these parameters is such that the secondary air

system‟s functionalities are not negatively affected by power extraction. For a given P30/P24 when

the level of power offtake increases, P26 increases which improves rim sealing.

With an IP P/O system the situation is the opposite: the main parameters that will influence the above

air system pressure ratios at a corrected NMix are IP power offtake, Mach number (Mn) and altitude.

The P/O and Mn are the most important: during flight idle condition at high Mn with constant NMix,

since turbines are not choked at idle and work split depends on flight conditions, there will be a higher

HP PR. While this results in a reduction of the IP PR: the worst condition exists at high IP P/O (Ref.

11).

An additional performance requirement that needs to be met by the air /oil system on the Trent 1000

is to maintain a minimum air system offtake pressure (e.g. a minimum P26). This is fundamental to

Figure 1.3-7 N3 and N2 variation with IP P/O for constant NMix

Power offtake sweep - LI

0.00 1 2 34 5 6 7 8

Power offtake

Sh

aft

Sp

eed

[%

]

N2 M2.3.1

N3 M2.3.1

NMixRT20 M2.3.1

NMix

N2

N3


24

ensure that the bearing chambers buffers are correctly pressurised and pressure drops across oil

seals are such that oil leaks do not occur.

The pressure drop limit depends on the sealing technology used on the current Trent 1000: oil seals

are unable to withstand negative pressure drops, especially at low speed.

Low P26 on Trent 1000 can affect oil scavenge capability, reducing pressure difference between

bearing chamber and scavenge pump. Breather efficiency can be affected by this situation.

1.3.4 Thrust Response

Since idle schedule with IP P/O requires different settings than with HP P/O it is recommended that

once Performance have set the preliminary minimum limiters (P26/P44, N2_idle, NMix, P30 and

others listed in Table 2.4-1) are defined, transient performance team tests the Acceleration Control

Unit (ACU) schedule to check whether the acceleration thrust response requirements are met. The

transient behaviour with an IP P/O system is quite different from those with HP P/O since the binding

conditions are different: the worst case for the HP power extraction system is with no power offtake

since the IP shaft in this condition has a very low rotating speed. With the IP P/O the IP shaft is slower

when a high level of power is extracted. In both cases the idle thrust increases when a larger amount

of power is extracted from the core. This might lead to an iterative process by which the Low Idle and

High Idle settings as well as the ACU loop are redefined to ensure both the thrust response

requirements and the compressor stability requirements are met.

1.3.5 Thrust Asymmetry

As said above, for an IP P/O system the idle NMix schedule is currently set as a constant NMix, which

means N2 decreases with increasing power offtake. If engine load is greater than a threshold amount

the engine will be operating on the Minimum N2 loop in order to maintain generator speed and NMix

will increase. Modifying the shaft speed will change the „starting point‟ on the Acceleration schedule

and the two engines will start from different points. In order to minimize thrust asymmetry the

acceleration of the engines should be scheduled using the same parameters used to define Low and

High Idle and ensuring that the acceleration phase starts from the same point for all the engines. If the

idle is settled using NMixRTHT24 the ACU (Acceleration Control Unit) has to use the loop based on

Nmix.Dot Vs. NMixRTHT24. This is possible and gives good results only if idle conditions of all

engines are defined using the same parameters and the min limiter, otherwise it would lead to

different starting point with different transient behaviour. It is thus important that all the engines are

controlled with the same schedule when on the ground, in this case the resulting thrust asymmetry will

depend only on the engine deterioration and engine-to-engine variation.

For engines with HP P/O systems controlled in constant NMix there is no such kind of problem since

switching to minN3 logic is not required not even for high levels of power extraction


25

1.3.6 Icing

High bypass engines like the T1000 have a very low fan PR at idle, this corresponds to a small

temperature rise across the fan that can result in ice growth on ESS and VIGVs. With future IP P/O

engines this problem will become even more evident since this system allows for higher BP ratios with

lower pressure ratios (i.e. bigger fan for same core size, as explained in Par.2.6.8). HP P/O limits the

achievable bypass ratio since the IPC has to maintain a larger surge margin. This means that with the

same hardware the IPC pressure ratio has to be reduced if an HP P/O system is installed on the

engine to reduce the working line and increase the surge margin. In order to obtain an acceptable

cycle efficiency the fan PR cannot become too small as there are no improvement margins for the

HPC PR with current designs whilst with an IP P/O this pressure shortfall can be recovered through

the IPC.

1.3.7 Bleed valve PR

It is possible that with an IP P/O system once the idle settings have been defined the resulting

pressure ratios across the IP8 bleed valves at a number of flight conditions is not high enough to force

them to open once commanded because of P26 being too low. These cases will need to be assessed

in detail by the HBV system owner to determine whether or not bleed valve opening capability at idle

is required at these conditions (Ref. 19).

On HP P/O engines the lowest P26 at idle is obtained without P/O. If idle with no power extraction is

considered as the design condition for the bleed valve system then the engine will not suffer for this.

1.3.8 VSV overclosure

The VSV overclosure is aimed at reducing the IPC PR when a high value of this parameter is not

required or is detrimental for the engine performance.

1.4 Mechanical aspects

1.4.1 Shaft critical speed

On the first design of the Trent 1000 a certain seto of gear ratios have been used (Ref. 4) in order to

achieve the starting speed ratio SR=NI

NH requested by the performance team without excessively

compromise the layout and the design of the central bearing house. The following rotation speed have

been obtained:

The speed difference between the IP and HP radial drive shaft cannot be undertaken. This speed

difference, in conjunction with the max rotation speed of the two shafts and gears, could create

problems related to:


26

Critical shaft speed

SAGB/IGB oil and Windage heat

Bearing lubrication and minimum load

Coupling device control (only early T1000)

Coupling device complete disengagement and heat (only early T1000)

The last two points only exist if a coupling device is used to start the engine. The image below shows

the layout of the IGB/SAGB with a hydraulic coupling device. Note that the design of the centre

bearing house requires modifications (1inch lengthier) to create the space for the HPC gear, required

as part of the start system.

The mechanical instability of a rotating shaft depends on the so-called Whirling Constant which is

proportional to 2

L

. To have a rough idea of the effective importance of the rotational speed on the

RDS we can compare the whirling constants of different engines.

The absolute value of the whirling constent alone doesn‟t identifies the problem, however the mutual

interaction of the two shafts and the effective dynamic behaviour needs to be analyzed for each

configuration in order to avoid resonance or other unexpected loads. This is one of the requirements

for the mechanical design of the P/O system, irrespective of IP or HP driven.

1.4.2 SAGB/IGB heat management and Windage

The windage effect is energy transfer to or from a fluid as a result of drag either from viscous effects

or by form drag from, for example, surface protrusions. This energy transfer is manifested in two

ways:

o A change in the whirl velocity of the fluid

o A change in the static enthalpy (or static temperature) of the fluid.

Windage power is proportional to 3shaftspeed

5_ radiusdisc

It is strongly influenced by protrusions and swirl velocity of air in the cavity.

Since the windage effect is proportional to35 rpmdiameter , reducing the diameters of the gears or

their width has a very important effect on the generated heat. Redesign of local layout, i.e. gear type

and size, could be considered in order to achieve this task.

Note that rotating components see a relative gas velocity so will not see the Total temperature in the

non-rotating (measured) frame of reference.

Free disk windage 58.2Relog491.0mC


27

535.0 r

erWindagepowCm

Consider that in this formula Re depends on the teeth dimensions.

“Heat to Oil" within the IGB and SAGB was one of the largest concerns with the HP/IP dual drive

solution on the early T1000 engines. The main risks were oil degradation and coking caused by high

HTO and ineffective oil scavenging due to accumulation of oil lumps around the scavenge ports. The

extra heating of a second set of bevel gears and set of bearings is evaluated based upon Trent 500

experience in an internal memo by N.Fomison and on Ref. 32. The overall design requirement was to

improve the effectiveness of the IGB scavenge system and to generate no more windage heating

than the Trent 500. On actual Trent 1000 engines the higher heat generated is mainly due to the

bigger gears, bearings and contact force. To minimize the windage effects and drain the hot oil

directly to the scavenge oil system shrouds have been designed around the gears.

1.4.3 Effects of higher torque

The Trent 1000 has experienced problems due to VFSG failures. This happened early in the

programme and raised reliability concerns. Taking power from a shaft at lower rpm means more

torque and transient loads with very a high impact energy (which compromises the mechanical life of:

bearings, gears, shafts…). This needs to be considered by the design of the IP P/O system. Another

not negligible problem related with the failure of a VFSG, or the failure of another accessory, is the

containment of debris. In event of a failure of an accessory, for instance a pump seizes, the “slingshot

effect” on the gear train is stronger with an IP P/O system due to the higher involved torque.

1.4.4 Resonance during surge events

The most important mechanical excitation for the IP P/O system is a high power surge event. During

this situation the IP system excites the P/O drive train with a very high torque with a frequency that is

close to the drive train‟s natural frequency. The first natural frequency of the HP system is many

hundreds of Hz instead that of the IP. This is because the IP system is less stiff and heavier, hence

the gear train responds with non negligible deformations. On a Development engine an IGB bevel

gear broke because these loads had not been considered. Another important aspect is that when the

engine surges, the HP speed doesn‟t change significantly, whereas the IP speed changes a lot. This

is why the net pressure (and air density) in the HPC educes while the IPC pressure (and density)

rises. With larger VFSG‟s inertia and two generators per engine, high power surge can exert a

significantly higher running torque on the system, which must be considered in the design. Hamilton

Sundstrand‟s dynamic modelling of the Trent 1000 drivetrain showed large dynamic torques occurring

when the engine is subjected to high power IP surge. A safety factor, compared to the nominal

maximum load, has been found on the Trent 1000. The effects of surge can be more pronounced in

the internal gearbox than the transfer gearbox (Ref. 30, Ref. 31) .


28

1.4.5 Sustained Torsional Oscillations (STO)

There is also an issue related with to vibrations caused by the electric generators. The generator

produces a constant frequency electrical output. In order to do this the VFSGs are controlled by an

electronic device that uses the PMA signal as input. When the input speed changes the effect of this

system is a variation of the torque required by the VFSGs, which in turn results in a change of N2.

The properties of this closed loop control system, due to the interaction of torque variation and IP P/O

system mechanical behaviour, could lead to undamped torsional oscillations. This is likely to result in

a problem for the generator in achieving a steady state condition for the required torque (Ref. 29). As

stated above the first torsional mode of the IPC, IPT and shaft is very close to the frequency of the

VFSG control signal.

Boeing are using the Permanent Magnet Generator PMG output voltage as a speed signal, which is a

3 phase electric signal. Its frequency is three times the IP shaft rotational speed, e.g. - 60% N2 =

9,000rpm VFSG speed = 150Hz x 3 = 450Hz signal).

Torsional oscillations (STO) have been measured by the testing team as a variation in the frequency

of the PMG signal, for Ground idle:

55%N2, 400Hz PMG signal

+/- 2Hz Signal oscillation: 398 – 402Hz, Hz4

Frequency of oscillation = 25Hz (Natural frequency of mechanical system)

Resultant VFSG shaft torque equivalent to +/- kWW 80

These values were taken from a Boeing presentation. It is possible to compute the allowable steady

state torque AllowableM (being MW where30

rpm ) as:

MMM alNoAllowable min

Assuming an idle condition with the system rated at 250kW @ 60%N2 (i.e. 250kW are required by the

A/C systems), is a very conservative assumption since the maximum extraction is higher than 750kW

per engine.

1.5 Main drives to move from an HP P/O system to an IP P/O system

1.5.1 Introduction

In the first part of this paragraph a comparison between different engines and their P/O levels will be

carried out. The engines considered are other Trent family engines and the General Electric GEnx. In

the second part the XWB approach to the P/O system will be illustrated in order to understand the

problems involved with an HP P/O system when high power levels are required from the accessory

gearbox.


29

1.5.2 Power extraction

One of the challenges for the Trent 1000 is to deal with a large percentage of turbine power extracted

from the core as mechanical power offtake. The power required is almost three times higher than that

on Trent 900, 800 and 500, and even greater than the power extracted from the XWB (Ref. 23,Ref.

27) . A more exhaustive comparison is available in Ref. 16.

The upper offtake limit, in terms of percentage of the turbine power in a specific condition (e.g. MTO),

is independent of the system (HP or IP) that we use as power source. Nevertheless the choice of the

IP shaft instead of the HP shaft affects the behaviour of the whole engine, changes the specific detail

of the underlying problem and the way these problems need to be resolved.

An extraction of small part of the turbine power can be tolerated by the system. If this amount is

doubled it may be tolerated by mean of smart control devices. A power extraction of four times the

initial value cannot be accepted. Another way to describe the P/O is by means of the power extracted

as percentage of the core turbine power, i.e. IPT+HPT power.

Comparing the power levels extracted from the engine expressed as percentage of the turbine power

(i.e. HPT for Trent engines and GEnx; IPT for Trent 1000) it can be seen that the power extracted

from the GEnx is much lower than that extracted from the Trent 1000. The GEnx HPC PR is roughly

equal to 23 while T1000 HPC PR is 4.5 with approximately the same air mass flow. This means that

the GE‟s HPT is designed to generate a larger amount of power than the Trent 1000 IPT and can

better deal with a certain level of power extraction. To meet this competitive disadvantage the

concept of taking the power from the IP shaft was developed. This approach recognised that

extracting power from the IP system of a three shaft engine has the unique characteristic that both,

IPC and HPC, move towards more stable operating regions, enabling high power extraction while

retaining optimum core sizing for thermal efficiency (Ref. 3).

.

The P/O requirement is one of the most important criteria to determine if an HP-driven system is

acceptable for an engine, accurate prediction is quite complex during the preliminary stage of the

Figure 1.5-8 Power offtake levels of different engines

0

1

2

3

4

5

6

GEnx Trent 1000 Engine 1 Engine 2 Engine 3

% s

haft

po

wer

Max nominal rated Horse Power extraction as % of shaft power at 35K cruise

270% threat relative to experience

65% threat relative to GEnx


30

aircraft design. A certain safety margin has to be considered in the design of the selected system, in

order to accommodate an increase of the power needed by the aircraft during the whole flight

envelope.

Airbus has not adopted the “More Electric Engine” philosophy. This means that on the Trent XWB

there is an air bleed system for the Environment Control Unit (ECU), and the mechanical power

required from the AGB is smaller than on the Trent 1000.

A comparison of power levels extracted from the core should be checked against the non-dimensional

HPC inlet mass flow =30

3026

P

TW @ ISA day, SLS, MTO. This values for different Rolls-Royce engines

are considered Propertary Data.

Comparing the three engines with HP P/O, the worst cases for HPC & IPC and respective surge

margin losses due to a fixed power extraction are (Ref. 23):

o Trent XWB HPC: due to the higher initial working line: the HPT capacity that is defined by

SFC performance.

o Trent 500 IPC: due to the smaller core size.

A very important issue that comes with the More Electric Engine Concept is that the max amount of

power extracted in cruise (short circuit transient load) is 80% higher than the normal extraction (static

load). In conventional engines more or less 1-1.5% of the shaft power is extracted from the HP shaft

and to do this whilst maintaining the requires stability margin 3-4 IP BV and 3 HP BV are needed. On

a More Electric Engine the power extracted is more than 5% of the shaft power: up to four times more

than traditional designs. Taking this amount of power from the HP shaft would require a higher

operational flexibility for the engine i.e. more HBVs and a more complex control logic.

On the GEnx, the maximum power extraction is almost 3% of the shaft power. As stated above the

HPC PR is equal to 23 and this engine has got a more powerful HPT. Nevertheless this layout

requires a Compressor Discharge Pressure (CDP) bleed from the HP10 to protect the engine from

surge. Operation of this valve, in addition to increasing fuel consumption, generates a lot of noise and

results in raising both direct operative cost, due to worse SFC, and indirect operative cost due to air

traffic regulation.

Figure 1.5-9 P/O levels Vs. P/O architectures


31

2 PART TWO - A COMPARISON BETWEEN THE IP AND THE HP P/O SYSTEM


32

2.1 Engine weight

2.1.1 Differences between IP & HP P/O systems

Without a double-shaft drive on the T1000, the weight penalty due to the power offtake configuration

is only linked to the additional hardware required for the Switched Air System. This additional weight

is “the smaller evil” when comparing power extraction between IP and HP P/O. The additional

hardware required by an HP P/O with a very high level of power extraction, such that required by the

787-9, can be bigger than that associated with the Switched Air System. For instance, an additional

set of bleed valves on both, IP and HP compressor, or bigger core may be required to facilitate high

levels of HP P/O. This configuration would result in additional weight and worse cruise performance.

In principle, there are is direct reason why an IP P/O system should be heavier than an equivalent HP

P/O system. The hardware required to perform the main function (transfer power from the core to the

accessories) is exactly the same.

Considering what stated above the following factors can affect the system weight:

Bearings

Gears

Shaft

Stiffeners

Lubrication system

Differences between these components exist between IP & HP P/O as the torque transmitted to

generate the same amount of power in a system with 25-30% reduced shaft speed is roughly 30-40%

higher. The IP shaft is designed against buckling and the increase in torque due to the P/O system is

quite small so this is not important for the weight.

The actual increase of weight is attributed to the secondary effects of the individual system. A

secondary effect is e.g. the requirement to perform accessory functions (i.e. starting the engine, don‟t

threat the engine stability….) and other assembly features necessary to operate the system (engine

case stiffener, mounting system…).On the Trent 1000 the redesign of the drive train was not

accompanied by a full redesign of the adjacent areas, like e.g. mounting features for SAGB and IGB.

When the clutch was removed on Pack A engines, no other parts of the engine were changed to

recover the full weight penalty. The current design has therefore not been optimized. Knowing the

required dimension of the SAGB without a clutch, a complete redesign of this area could have

resulted in a layout with less interference with the Fan Duct. In the current engine design this feature

has not been changed significantly from the original design.

The IP shaft is designed against buckling and the increase in torque due to the P/O system is quite

small (max 5% shaft torque) so this is not important for the weight.

A short list of operational functions of the P/O system is reported and their impact on the engine

weight discussed.

2.1.2 Extracting power from the core

Increasing the system weight when the general level of power is increased is unavoidable from a

mechanical viewpoint: bigger gear, shafts, bearings and a more capable lubrication system are

required, irrespective of whether the power is extracted from the IP shaft or the HP shaft


33

2.1.3 Starting the engine

The forecasted increase in weight for the first engine design with the coupling device in the SAGB

ranged between 155 and 255lbs. Most of this mass was due to the clutch, its control system, the

double RDS, gears, bearings and larger casing to house all these components (SAGB sump and fan

air duct).

Because this complex system is no longer necessary, the increase in weight relative to an HP P/O

necessary for the “start the engine” function is zero.

2.1.4 Engine stability

The secondary switched air system is required by the Trent 1000 because of insufficient flows at

ground idle operation. The additional hardware required to switch the Secondary Air System feed

from IP8 to HP3 results in an estimated weight penalty between 55 and 155lbs per engine. The

155lbs weight difference is unlikely because the weight penalty associated with an HP power offtake

system with high power demands have not been taken into account. The indirect effects due to

control components and other necessary elements depend on the requirements of the engine.

Assuming that an HP P/O system (for a three-shaft engine) is able to cope with a 1000Hp extraction,

it would require a complex and heavy stability bleed system (on the GEnx there is also a Compressor

Discharge Pressure valve and, being a two-shaft configuration the GEnx accounted for a more

powerful HP turbine).

The increase in weight necessary to “establish stability margin” on the Trent 1000 has to be

accounted for considering the weigh penalty required to satisfy the same requirement with an HP P/O

system.

2.1.5 Engine weight estimation

An estimation of the weight that could be saved on a redesigned Trent 1000 compared to a Pack A

engine is reported in Ref. 16 redesigning or removing some components of the IP P/O system


34

2.2 Fuel burn

2.2.1 How the IP P/O system affects the fuel burn

The saving in fuel burn with respect to a 3 shaft engine with HP P/O system can be attributed to:

o IP drive configuration moves the operating points of the engine. The working points of the

compressors have changed, allowing the compressors to rematch (see Par.2.5.2).This has an

impact on the efficiencies and can possibly improve the engine fuel flow at some conditions.

o As described in the operability section, the IP power offtake improves the IPC stability by

dropping off the working line. This gives the opportunity to set up a different handling bleed

valve schedule in order to have less bleed valves open during certain flight segments, such as

hold and descent.

Using the IP P/O system on the B787s allows to reduce the HBV opening time during ground idle or

flight idle descent operations relative to an equivalent engine with HP P/O(see Par.0).This has a

significant effect on the SFC and on the fuel burn. With the original engine design, i.e. with the starting

coupling system, the beneficial effect of the different HBV schedule on the fuel burn is partially

mitigated by the weight increase, as discussed in the previous chapter.

On one of the earliest Trent 1000 engines the idle fuel burn was 40% high relative to the Boeing spec.

This shortfall was mainly due to differences in IP Compressor efficiency, Turbine efficiency, Handling

Bleed Valve schedule and minimum idle setting: excessively optimistic assumptions have been made

during the preliminary study due to the novelty of the engine configuration.

Fuel consumption at idle can be reduced by modifying the gear ratio between the N2 spool and the

generator, through a redesign of the SAGB with range of speeds set to lower N2. It has been

estimated that this would allow for a reduction of the minimum N2 limiter. These modifications have

shown only a little improvement of the block fuel.

2.2.2 Flight conditions

The effects of the IP P/O system on the fuel burn are much more evident for short range missions

where idle, taxi and descent (i.e. those conditions where the IP P/O allow to shut the HBV and the HP

P/O requires air bleed) occupy a larger fraction of the total mission. For a 500nm mission (considered

for Japanese operators), the former studies underlined a 6% benefit on the fuel burn of the RB262-58

(with IP P/O) over the RB262-51 (with HP P/O).

For the baseline aircraft (3000nm mission) the advantage is eroded as the length of time at hold and

descent constitutes a smaller percentage of the total flight cycle and the improvement is

predominantly associated with the ability of the engine to achieve a higher overall pressure ratio.

For instance, with an IP power offtake system, there is no IP handling bleed open at hold whereas,

with a HP offtake system, at least one IP8 bleed valve has to be constantly kept open. This has a

non-negligible impact on the resulting block fuel.


35

2.2.3 Impact on engine performance

During normal conditions, with all the HBV closed, HP and IP P/O systems affect engine performance

and SFC in the same way. The influence the IP P/O system is considerable on those flight envelopes

that would require an engine with HP P/O to command HBVs open for a higher percentage of flight

time.

An engine with IP P/O system has the key advantage that air bleed is not required during low power

conditions (or is required for a much shorter time). Since fuel consumption is negatively influenced by

air bleed, the IP P/O advantage results in a smaller block fuel burn during approach and idle. For

short flights, where time spent in these conditions is a larger percentage of the total flight time, the

benefits due to the P/O system are more evident.

Figure 2.2-10 Effects of weight increase on fuel burn

0.0

0 1000 2000 3000 4000 5000 6000 7000 8000

Range [nm]

IP wt = HP wt + 355lb/eng



IP wt = HP wt

Block Fuel Saving


36

2.3 Starting

2.3.1 Introduction to the coupling device and Mark 1 SAGB

The Power Offtake system is also used to crank the engine during the starting procedure. In order to

reduce weight and both engine and system dimensions the core shaft used to drive the accessory

gearbox is also used during the aforementioned procedure. At the beginning of the Trent 1000

programme Rolls-Royce had no experience with the starting of an engine driving only the IP

compressor and test results from a demonstrator engine discouraged the engineers since very long

time was needed to reach a sufficient HP shaft speed (Ref. 5) . The main concern was about how to

mitigate this problem in order to achieve an acceptable starting time with this system. A double

starting system, driving both HP and IP shafts, was introduced in the original Trent 1000 layout. Later,

due to reliability issues and the weight penalty of this solution, the development team attempted to

start the engine driving only the IP shaft. This situation arose when the team faced a clutch failure

which made it impossible to start the engine as intended. The requirements for the power to drive the

IP shaft were unchanged and the same VFSGs were used.

The positive result of this test was a great surprise for all the people involved in the project because

the engine was not designed to start in this configuration. Driving the HP compressor via the IP shaft

is a very inefficient way to obtain the sufficient pressure in the combustion chamber to attempt to light

the combustor. Through the compressors, from the IPC inlet to the HPC outlet, there are many bleed

areas and many sources of losses, thus a big part of the energy transmitted from the IPC to the fluid

is wasted. Because of this, people were surprised in seeing both the shafts reaching the green-light

rotation speed in a relatively short time, comparable with the HP-drive starting time. Rolls Royce

continued the development process of this solution, deleted the clutch and accepted a small shortfall

in starting performance. The engine‟s weight was considerably reduced which eliminated all the

drawbacks linked with the increase in size, relative to an HP P/O system.

2.3.2 Problems with coupling devices

In starting mode each VFSG supplies 300ft-lb of torque to the external gearbox, but when the engine

is running the power is taken from the IP shaft by the IGB. The IGB consists in a couple of conical

gears, that transmit the torque to the SAGB, which reduces the rotational speed by means of a couple

of bevel gears, transmitted through the fan air duct to the transfer gearbox and to the accessory drive

gearbox. In the AGB the torque is used to drive generators, pumps and other accessories. During the

starting procedure the power‟s path is in the opposite direction, with the same hardware used. It was

evident that the problem consisted in designing a light system that could drive two shafts during the

starting procedure, disengage from the HP shaft and, post the cranking procedure, transfer power

only from the IP system.


37

In order to optimize weight, the hardware must be reduced to a minimum requirement. Connecting the

HP and IP shafts together with an optimized gear ratio results in a more effective starting (due to

better compressor aerodynamic matching). Moreover it allows the engine to be started using only one

electric starter. This was the solution used to realize the “double power path” from the IGB to the

SAGB, using only one shaft delivering power from the SAGB to the TGB.

2.3.3 Differences between IP and HP starting system

The Trent 1000 is unique in the Trent family of engines due to its IP driven start system. All previous

starting experience has been with an air starter that drives the HP shaft whereas the Trent 1000 has 2

Variable Frequency Starter Generators (VFSG), which are mechanically connected to the IP shaft.

There are fundamental differences between IP driven engines and HP driven engines and these

differences change, depending on whether the engine is in flight or ground operation. The compressor

connected to the starter has to generate the airflow through the core, this has an impact on the

compressor operability.

When the HPC is driven, 26P can become smaller than the ambient pressure because both, fan and

IPC, work as turbines extracting work from the airflow instead of doing work on it. Because of the low

PS26, the IPC interstage bleed valves take air inside the compressor rather than bleeding it off thus

modifying the system stability. The limit for “fuel on” is on WRTP30 and 30P , thus resulting in the HPC

to work in an off-design conditions to create the satisfactory condition to attempt ignition. The HPC

has to provide the required outlet massflow and pressure, starting from an inlet pressure (P26)

smaller than the ambient pressure, which results in a very high pressure ratio while the airflow is not

substantial. The HPC is working with AmbPP26 as inlet condition and with combustor and turbines

Figure 2.3-11 Layout of the IP P/O with low speed mechanical clutch


38

acting as downstream blockage. During this procedure, working very far away from its design point,

the compressor is very prone to stall. The goal is to flow as much air as possible to keep the risk of

stall as small as possible. During the initial phase of cranking the HP shaft usually has to be turned

slowly, in order to avoid stall. When the IP system begins to turn and reach a minimum N2, then the

HP spool can be accelerated.

If there is an IP P/O system, the IPC condition during the starting procedure is very different. The inlet

pressure is slightly lower than the ambient pressure and the IPC can rely on the VSVs to improve the

working conditions on the front stages (Trent 1000 has 1 VIGV and 2 VSVs), moreover the IPC is

designed to work over a wider range of “off design” conditions. Consequently the IPC is not in stall

during cranking while in this phase the HPC works as a turbine2 and can run without mechanical

restrictions (i.e. there is no mechanical power extraction from the HP shaft).

During sub-idle running a different off-design condition exists in the last stages of the compressors.

The core is sized for cruise condition during which the pressure increase inside the compressor is

higher, and thus the decrease in air volume is quite substantial. During the starting procedure the PR

is quite low and the axial air speed increases across the compressors, which can lead to a speed

mismatch if the HP spool rotates too slowly. The IP P/O system is more prone to this kind of “off

design” condition because the HP shaft is very slow during the starting procedure. This situation can

easily be handled in an IP P/O system by using bleed valves to reduce the air massflow and its

velocity.

This conditions change at the ignition point, when both compressors begin to work as compressors

(during cranking the HPC works as a turbine) and the turbines begin to produce shaft power. From

this moment the compressors cannot be stalled, however surge could still occur. A minimum HP shaft

speed is required by an HP driven engine to prevent compressor from surge. No lower limit exists for

an IP driven engine, this is because the HPC is never stalled during cranking because it works as a

turbine.

Previous investigations have established that it is possible to start a three shaft engine by spinning

only the IP shaft (Ref. 4) . However, trials on industrial RB211 engines at Ansty (Ref. 6) demonstrated

very slow engine start times. Driving the IP spool by itself, or coupling of the IP and HP spools, during

the starting procedure was not considered a viable method for starting previous Trent engines since it

would have resulted in a too deep redesign of the engine. Previous Trent engines have an HP P/O

system, compressors and turbines are designed to handle different amount of power and have a

different matching range from those required by an IP P/O system. The IPC design PR of the Trent

1000 is quite larger if compared with that of an industrial engine (like that used during the

aforementioned Ansty test). This means that on the T1000 the bigger IPC is able to transfer a much

higher power to the air flow while a smaller HPC represents a weaker downstream blockage during

cranking.

2 Note that to define the effect of the rotors in a certain condition (whether a compressor is making work on the fluid or is

extracting work from it) the only important thing to consider are the flow characteristics; the pressure changes are only a

consequence. The direction of the torque generated by the compressor depends on the blade angles and not on the

pressure ratio.


39

A Trent 1000‟s peculiarity is the

VIGV‟s schedule during cranking:

while on other Trent engines to

improve IPC starting performance the

VIGVs are scheduled with the

maximum allowable closure (same

schedule as cruise, see Figure 2.3-21

) to improve IPC starting

performance, on the Trent 1000 a

different schedule with reduced

VIGVs closure (underclosure) is used.

Modifying the circumferential speed of

the incoming flow to a lower degree

makes the first stages of the IPC

running into stall but, the whole

compressor opposes a smaller resistance and can therefore accelerate faster. This results in a

positive effect which allows the IP compressor working closer to its design point with a higher

efficiency, hence obtaining a higher-energy outlet flow. Driving the IPC with appropriate power can

thus produce an outlet fluid with the ability to accelerate the HP spool up to the minimum speed

needed to light the engine.

The starting procedures and times for HP and IP P/O:

o In the HP power offtake configuration, as the starter air valve opens, the starter air motor will

provide power to rotate to HP shaft. Engine cranking will take an amount of time depending on

starter air pressure and ambient temperature. Fuel and ignition will be switched ON as the HP

and IP shaft speeds have reached a cranking speed (values depend on design features), to

avoid engine stall. The starter air valve will close when N3 reaches a threshold value, once the

engine dead crank speed is reached, and the engine will reach idle within next seconds.

o In the original IP power offtake configuration, the T1000 generators crank both IP and HP

shafts during starting. There is a delay to ensure the coupling device is engaged before

rotating the shafts. Fuel and ignition are switched ON at the N3 threshold speed and the start

power coupling automatically disengages once the engine has reached a point where it is

more beneficial to only motor the IP shaft. In a dual starter configuration, engine tests on the

Trent 1000 have reached idle within less than 35sec on average.

o Recent Trent 1000 engines without coupling device yield starting times of around 55sec. The

delivered mechanical starter power ranges between 80-120kW per VFSG Figure 2.3-15

shows starting time requirements for the Trent 1000. The requirements for the engine can be

divided into 3 areas:

1. Certification requirements

2. Customer requirements

3. Internal (R-R) requirement

WRTP24

VIGV

closure

51.8deg

40.8deg

Cranking schedule

High speed

schedule

Figure 2.3-12 VIGVs schedule for T1000, Cranking Vs High

speed

WRTP24

VIGV

closure

51.8deg

40.8deg

Cranking schedule

High speed

schedule


40

Theoretically, driving only the IP shaft would lock a normal IPC into stall due to the HPC acting as a

blockage. The Trent 1000‟s IPC capacity is designed larger to achieve a greater stall margin. The

compressor is able to perform a starting procedure with IP cranking, within acceptable starting time,

although requires more time if compared with the coupling device. When the IPC is cranked during

starting and the HP shaft is almost still, part of the HPC is stalled and has a very low efficiency. The

main losses are due to the presence of the combustor and to the absence of sealing air (the

secondary air system is not pressurized yet) which increases bleeds. The time required to overcome

this situation is roughly 30-35secs.

The time to fuel ON and time to idle are nearly divided by two when both the IP and the HP shafts are

cranked during engine start.

Figure 2.3-13 Starting procedure with IP P/O and dual starter (with coupling device).


41

During cold days the oil viscosity can become a limiting parameter for starting capability. In this

situation the IP P/O system has a disadvantage against the HP P/O system. The IP system is heavier

(angular momentum around the engine axis) than the HP system and is held in position by three

bearings (two roller bearings and one ball bearing) instead of two like the HP shaft (one ball and one

roller bearing). This increases the friction present in the system at all temperature condition and,

consequently, either the cranking time or the power requirement for the VFSGs.

2.3.4 Windmill - Relight capability

RR agrees with Boeing that engine in-flight starting performance should be at least as good as the

Trent 800 and shall use all practical means to achieve such performance. The final in-flight starting

envelope will be determined by Aircraft flight testing and mutually agreed in the Specification.

On the actual configuration, without coupling device, in-flight relight performance is slightly worse than

on early Trent 1000 with coupling device but still better than an HP P/O system. The IP & HP shafts

stabilize their speeds at different values depending on the type of system used: the shaft that

transmits power will settle to a lower speed. The IP shaft will spin slower with IP P/O than with the HP

P/O system, the opposite is true for the HP shaft.

During in-flight relight (windmill procedure) both the compressors work as turbines but with the cross

section annulus getting smaller through the compressors and the pressure decreasing across the

compressors, the axial speed increases. This modifies the speed triangle and pushes the compressor

in an off-design condition. This effect is much more evident and important for the HPC in the course of

0

0 35 00 0

N3%

N2%

N1%

P30

PS26

TGT

Shaft speeds %, P26, P30 [psi] TGT [K]

Time [s]

Figure 2.3-14 Starting procedure with IP shaft crank


42

the relight procedure. The situation with an IP P/O system, slower IPC and faster HPC, is much better

than the situation with the HP P/O. This gives the Trent 1000 an advantage over conventional engines

based on HP P/O systems.

Using a coupling device would improve the performance of the IPC during windmill. The intermediate

pressure compressor works less efficiently as windmill device compared with the HPC, due to a

relatively smaller Mach number seen during this phase.

Connecting the two shafts together would produce a stronger windmill effect compared with the actual

Trent 1000. The consequence would be that less energy is used to drive the AGB and more pressure

is available in the combustor chamber. This would have a direct effect on the relight capability,

especially at high altitude, relaxing a bounding parameter for the combustion chamber.

2.3.5 Hot start

Between ignition and idle, there is the reversion of compressors‟ roles: when thermal power is

available at IP turbine and HP turbine the latter accelerates faster than the former. This is reflected on

compressors‟ behaviours. As this happens the HPC is beginning to work properly as a compressor

and increases his flow demand, acting no longer as blockage for the IPC and removing the risk of stall

in its rear stages.

How does TGT (in terms of engine stations44T , LPT NGVs inlet temperature) affect this process? TGT

in this context is not used to monitor the LPT status but is used as a measure for the thermal state of

the engine when it is shut down. TGT can also be used to introduce in the ECC logic the information

about the hardware temperature since when thermal power is made available to the turbine the HPC

begins working properly; in this condition the engine behaviour changes from cold to hot days. The

Turbine Gas Temperature (TGT) signal can be used in the ECC to account for the Hardware

temperature during the starting procedure. The thermal expansion of shafts, drums, disks, casings etc

can be related with this temperature. These thermal effects change the clearance between the

abradable sealing rings and the compressor‟s blade tip; the phenomenon is called “Casing

Asymmetric Cooling” (CAC). The compressor surge line is very much affected by this situation. CAC

limits the relight capability because increasing locally the tip clearance, it promotes IPC surging. This

phenomenon is common to all engines and is very evident when the engine has been shut for 1-3

hours. Only after about 8 hours the TGT condition depends only on the external (ambient)

temperature.

Based on this, the surge threat can be expressed in terms of max TGT. When the engine has been

shut down only for a short time the TGT is higher than the allowed value, and a venting crank period

prior to relighting of the engine is required in order to cool the relevant engine components down. The

TGT cooling crank increases significantly the starting time during hot day conditions.

With an IP P/O system the HPC compressor is far away from the surge line during cranking

procedure (it is actually working as a turbine and not as a compressor), the limit is then on the IPC.

Being at the cold end of the engine the IPC is much less sensitive to thermal variations. This allows to

increase the max TGT limit for light up, and also incorporates the vent phase into the normal crank

procedure without requiring additional time (Ref. 7) . This results in a benefit every time the engine

has to be shut down and turned on in a short period, like for short-range missions.


43

Starting time is a major concern for the B787 launch customer (ANA). The Trent 1000 is much less

sensitive to high TGT and reduces the cooling-crank period to save both time and power required to

drive the spool. On Package B engines there is a proposal to raise the maximum fuel-on TGT limit.

2.3.6 Requirements for the P/O system:

1. The drive train has to be designed in order to satisfy the Starting Time Requirements from the

customer. Boeing requirements are reported in Figure 2.3-15 and all other requirements are

listed in Ref. 17

2. AGB/IP shaft gear ratio affects the cranking capability, especially during cold days. This needs

to be considered when designing the P/O drive train.

3. The power transmission system has to facilitate starting power demands.

4. External dimensions of the SAGB shall not affect the fan air duct shape

5. RDS length and bore have to be optimized considering structural issues (power transmitted

and instability) as well as integration issues (interaction with fan duct design, SAGB stiffness

and weight)

6. Consider debris and oil contamination effects on drive train life and required torque during

starting

7. Accommodate for FMECA for starting requirements with one VFSG inoperative, windmill

relight and starting capability with VSV failure

2.3.7 Requirements for stakeholders:

1. A larger IPC with higher PR is required to ensure enough energy is transferred to a sufficient

air mass flow during the starting procedure. This influences the power split between HP and IP

systems

2. IPC must maintain a positive stability margin and PR to start the engine.

3. Consider usage of blisk3 or bling4 for first HPC or last IPC stages to increase available space

in the IGB area and reduce limits on usable gears.

4. Consider pressure levels during cranking in the assessment of air system, oil system,

compressor stability and sealing.

5. Include transient behaviour during cranking and ignition in the assessment of air system, oil

system, compressor stability and sealing.

3 Bladed Disk

4 Integrally bladed ring or bling, see Rolls-Royce Metal Matrix Composite research website.


44

Figure 2.3-15 Starting time requirement, temperature envelope


45

2.4 Idle setting in the context of IP P/O system

2.4.1 Introduction

The idle setting is influenced by, and influences the P/O system in both configurations: HP and IP.

This paragraph will illustrate what parameters are considered critical for older engines with HP P/O,

and for the T 1000.

The limits imposed by these values in order to control the engine depend largely on the P/O design.

The trade study to set the idle schedule ended up in a requirement to manage:

Min speed (VFSG designed for an HP P/O system)

Switched air pressure

Idle thrust

The main problem is to define a control schedule that satisfies all the operability boundaries, from

VFSG load to thrust asymmetry during slam accelerations, optimizing engine thrust and fuel

consumption.

A key parameter to monitor is the rotational speed of the core shafts, the shafts‟ speed variation with

IP and HP P/O are opposite. Whilst boundary conditions are completely different from the former

Trent family engines, on the Trent 1000 an appropriate trade study has been performed in order to

establish which parameter can give the best control law. This study is detailed in Ref. 11 and Ref. 12.

2.4.2 The boundaries for the idle setting

The idle schedule depends on a number parameters utilizing directly or indirectly the shaft rotational

speed. From the Functional Mapping Session on Idle Setting (6th July 2011), the following limiting

conditions, involving the P/O design, have arisen:

Function Paragraph Function Paragraph

Air System - Disc Rim Sealing 2.8.2 Fuel pump capability 1.3.1

Air System - Bearing Chamber

Sealing 2.8.3 Power generation (e.g. VFSGs) 1.3.1

Air System - Bearing Load

Management

Errore.

L'origine

riferimento

non è

stata

trovata.

Oil Feed (via CF) 1.3.3

Turbine Blade Cooling

Errore.

L'origine

riferimento

non è

stata

trovata.

Oil Scavenge 1.4.2

Pneumatics System Actuation 2.8.1 Bearing Skidding 4.1.5


46

VSV Actuation 2.6.10,

1.3.8 Avoid Vibration / Resonance 0

TCC Valve Actuation 2.8.1 Drive Train Loading 0

Cold Start Capability 2.3 Aircraft Hydraulic Systems (Hydraulic

Pumps) 1.3.1

Engine Accel Capability 1.3.4

2.6 Compressor Stability 2.6

Avoid Turbine blade airflow

separation Oil feed (via Jets) 4.1.4

Zone ventilation Systems

Errore.

L'origine

riferimento

non è

stata

trovata.

Table 2.4-1 Limiting conditions for idle setting directly related to the IP P/O system

The P/O system design needs to consider its influence on the parameters listed in Table 2.4-1. The

description of how the IP P/O system affects these parameters/functions is described in the

paragraphs reported in the aforementioned table. There are some systems that limit the idle setting

and others that don‟t constitute a limit but are affected by the idle setting.


47

2.5 Effects of the IP P/O system on thermodynamic aspects

2.5.1 Introduction

On the Trent 1000 up to 30% of the power produced by the IP Turbine can be transmitted to the

Electrical generators when operating at idle (Ref. 11). This is a significant amount of the overall

turbine power and will therefore have a significant effect on engine matching.

To compare the performance and the different effects of the two types of power offtake systems on

the thermodynamic cycle of the engine appropriate parameters need to be defined (Ref. 3). During

the following description the pressure ratio across the two compressors ( 2430

PP ) and the level of

power offtake (defined as a fraction of the total gas generator shaft power for a specified condition)

will be kept constant. The shift of the compressor operating point is defined as the variation of the

corrected inlet/outlet mass flow.

HPC outlet non-dimensional mass flow 30

303030

P

TWWRTP

IPC inlet non-dimensional mass flow 024

0242424

P

TWWRTP

To compare the effects of the two systems, a PR Vs. non-dimensional inlet mass flow chart can be

used.

Figure 2.5-16 Typical HP compressor map with constant speed and constant efficiency iso-

lines

The following assumptions have been made in the following chapter:

Gas generator inlet conditions (station 024) are constant,

No P/O

% Compr.Eff.

Surge line HPC PR

Corr. Mass flow

4

7 97%

95%

93%

80%

90%

70%


48

24

30

PP

is kept constant

Fixed polytrophic exponents for compressor ( 3C ) and turbine ( 5.4T)

Mass flow rate is computed considering the HPT NGVs are choked, .40

404const

P

TW so an

increase in TET produces a decrease of the in core flow proportional to i.e. +5% TET

- =-1.025 -2.5% mass flow

2.5.2 Effects on Core Matching

The working lines of a core driving a free turbine5 are independent from the applied load and are

defined only by the capacity of the turbine. For an aero engine, where the LPT doesn‟t acts as free

turbine but contributes to the compression of the airflow, the LPT load influences the core matching,

but to a small extent (Ref. 36). This is partially due to the fact that the LP system matching

requirements are much less strict than that for the core.

One of the main features of the IP power offtake system is its positive effect on the IPC stability

margin, especially with high levels of extracted power; i.e. the opposite of the HP P/O effect. This is

mainly due to two reasons:

1. The HP and the IP compressor/turbine systems have very different constraints and are

influenced differently by the TET;

2. The HP compressor is aerodynamically more stable than the IPC (smaller speed range,

smaller range of working conditions, directly affected by the combustion chamber)

With an IP P/O system, during low power operation, the unrestricted HPC tries to pull more air

through the core than the mechanically loaded IPC will pass. This situation is aerodynamically much

more stable than the more common situation where the IPC tries to push more air into the HP system

than it can swallow. Indeed, the IPC working line is primarily determined by the flow requirements of

the HPC, thus determining the IPC flow, PR and IPT expansion. The HPC working line on the other

hand is determined by the HPT capacity and , which determine the mass flow.

Considering the characteristics of the airflow at the inlet of the IPC and at the outlet of the HPC the

following considerations apply for our simplified model without bleeds:

The pressure at the IPC inlet and 24

30P

P are given as constants, thus the total pressure at

the HPC outlet (P30) is constant for all the cases in our analysis,

The axial velocity is kept almost constant across the engine, considering inlet end outlet flow

with almost zero swirl speed, which gives a constant static pressure ,

The polytrophic exponent and the pressure ratio are fixed. This also keeps the temperature

ratio across the compressors unchanged,

5 A free turbine, or power turbine, is a turbine that is not used to drive air compression but to generate

mechanical power


49

Changing the amount of power extracted from the engine, the only thing that can modify the

non-dimensional core inlet mass flow WRTP24 is therefore the mass rate W worked by the

compressors.

Reducing the power offtake increases the mass flow while increasing it reduces the worked air, no

matter where the power is taken from.

This mechanism becomes more evident if we use a black-box diagram as shown below.

Case 1: Keeping the turbine power, the

inlet conditions and 024

030

PP

constant,

the only thing that can change with the

P/O level is 24W .

Case 2: keeping constant the P/O level

and changing the turbine power the air

mass flow W will change as 40T i.e. as

the turbine power.

In the event that HP or IP turbines are

choked the turbine power does not

depend on the non-dimensional mass flow

(that is always the same with the HPT

nozzle capacity being fixed as design

parameter) but depends on the pressure

ratio that, in turn, depends on the work

done by the compressor. The following

equations apply to the HP system (1st

approx):

W

T024-T24

P024 – P24

IPC HPC

Turbine

power

W

T030-T30

P030 – P30

Power

offtake

HPT PR=const

Stage

PR

P3/Pamb

LPT

IPT

HPT

HPT

choked

IPT

choked

LPT

choked

Figure 2.5-17 Pressure conditions for turbines

Low Idle

IPT PR=const


50

The required turbine temperature drop 042040 TT , in conjunction with the TET (i.e. 040T ) and the HPT

polytrophic efficiency mH , determines the HP turbine PR when the IPT is not choked.

When the IPT is choked the HPT is restricted to operate at a fixed non-dimensional point with the

values of , and all fixed. This can be demonstrated using the following equations:

;

and are fixed because they are turbine design properties. Polytrophic transformation inside

the turbine:

The last two equations can be written as:

;

The same applies to the IPT when the LPT is choked: the turbine expansion ratio is determined by

capacity of turbines and the compressor matching is a consequence of this.

In other situations, when the sonic condition isn‟t reached anywhere in the IPT, the working point (i.e.

HPT PR) is defined by the capacity of the downstream turbine (Ref. 36 pg.349) . This fact is

graphically explained in Figure 2.5-27, which underlines how the power split changes widely over the

operating range. While at high power the pressure drop (and thus the temperature ratio and the

equivalent power) of the HPT and IPT are almost the same, at low power the HPT has the major part

of the core power since the pressure ratio across it is bigger than across IPT or LPT.

Starting from high power conditions and reducing the thrust, the pressure drop across the turbines

starts to decrease, firstly across the LPT, than across the IPT but not across the HPT until low idle is

settled, as this turbine is choked for a wider range of conditions.

The fact that the HPC outlet flow conditions are independent from the shaft used to extract power is

very important for high speed conditions where the HPC working line is almost vertical in a PR Vs.

WRTP40 chart, i.e. the HPC inlet is almost choked. This means that in these cases any configuration

of power offtake will have the same effect on the HPC stability margin moving the working point

towards the surge line (see par. 2.6.3).

In our model the thermodynamic changes of the working gas from station 40 to station 44 are

independent from the shaft used as power source. On the other hand, choosing the IP or the HP shaft

as the source of P/O modifies the matching of the engine components. The rematching of the

compressors and its effect on the stability margins depends also on the boundary conditions of the

two systems. These conditions are defined by the properties of the components that surround the

HPT (combustor, NGV, IPT) or the IPT (HPT and IPT).

The effect of increasing TET in order to maintain the same when a certain P/O level is

applied are discussed hereinafter. Both HP&IP turbines experience an increase in available power

irrespective of an IP or HP power offtake system. The resulting changes in N3, HPC PR etc. depend


51

on the power required by the components mounted on the HP shaft (i.e. only HPC or HPC+P/O). On

the other hand the IP turbine power does not depend on the power that is extracted from the gas path

by the HPT, for a given TET. An increase in , obtained by burning more fuel, increases the power

available at the HPT and raises the HPC WL. The IPT “sees” the downstream flow conditions, i.e.

power from combustion minus the power extracted by the HPT. This happens because the HPT

always extracts part of the extra power from the gas flow. Since the power split across the turbines

depends on theirs capacities (which are fixed), when increases (e.g. resulting in +Y% HPT

power), roughly the same % power increase applies to the IP and LP turbines. The core turbine power

has to be increased by the amount needed to deal with the P/O level. The compression system then

re-balances to reach a new matching condition. For example with an HP power offtake the HPC

power falls and the IPC power rises. In Figure 2.5-28 is reported a graphical representation of the

power split variation in the core when either, IP or HP P/O system is used. The opposite happens with

IP power. Which is better for SFC depends on the efficiencies of the components at the new operating

points and on the power in those spools. It does not depend on the initial power levels of the turbines.

Setting in order to provide the additional power to the desired turbine, i.e. that mounted on the

P/O shaft, modifies the power of the other turbines and compressors depending on which shaft is

used for P/O.

This is only a rough, quantitative example but gives the idea of the different boundaries that apply on

IP and HP systems.


52

2.6 How the different P/O systems affect the surge margin

2.6.1 Introduction

HP P/O: any mechanical load on the HP shaft reduces the power available for the HPC, which

reduces the HPC corrected mass flow rate and the PR. This generates an increase of the IPC PR.

The HPC acts as a blockage for the IPC pushing it towards the surge line (high PR and small

mass flow). The reduced mass flow has a negative effect on the HPC performance, which works

far away from its design point. The extent of this shift depends on the work split between the two

compressors. As a crude guideline, 60% of the total core power is in the IP system at high power

conditions. At low idle the percentage of core power in the IP system is much smaller (Figure

2.5-27). With an HP P/O system both, working lines change; the HPC WL approaches the surge

Power offtake

= XX kW Hyp: core power split

IP/HP system is 60/40

XX kW in the core

engine

+Y% HPT

+Y% HPC

HP P/O

+Y% IPT

power

-Z% IPC

+Y% LPT

+Y% HPT

power +Y% IPT +Y% LPT

IP P/O

+Y% Fan

-J% HPC

+Y% Fan +Y% IPC

Figure 2.5-18 P/O effects on turbines' power


53

line due to a drop in IPC exit flow function WRTP26 (this is a direct effect of the power extraction),

whilst the IPC‟s line rises due to the HPC acting as a blockage.

IP P/O: generally it moves the IP working line towards a smaller corrected mass flow and pressure

ratio, forcing the HP compressor to work harder, i.e. higher PR and higher WRTP. The working

line of the HPC is unmodified, but a higher PR and mass flow rate will lead to a shift of the actual

operating range away from the normal design point with a conventional HP P/O. If no appropriate

action is taken, this could reduce the components efficiencies. With the IP power offtake system,

the working line of the IPC drops due to power extraction. The HPC working line remains the

same but the working point shifts towards higher non-dimensional mass flows.

Regarding the power split between IP and HP system, for a fixed amount of power offtake, the largest

shift in PR, as a percentage of the design condition, happens for the layout with smaller power in the

extraction shaft. This means that the percentage of the turbine power drained from this shaft will

increase as the turbine power get smaller. A smaller system responds more sensitive to a change in

P/O.

2.6.2 Definition of the available Surge Margin

The level of IP Compressor surge margin is defined as

Line Working

Line WorkingLine Surge

PR

PR - PR 100 (%)Margin Surge

Where LineSurgePR _

is the compressor surge line pressure ratio (24

26P

P ) and

LineWorkingPR _ is the compressor working line pressure ratio at sea level static with no

handling/customer bleed or power offtake.

All surge margins are expressed relative to a sea level static, steady state, working line with no bleed

and no power offtake. The surge margin requirements are calculated by summing the individual

threats and taking the square root of the sum of the squared tolerances, i.e.:

2b a Threats ofSummation

Where

o a is the summation of threats applied to the surge margin.

o b² is the root of the sum of the squares of the tolerance band applied to each of the

threats.


54

To investigate the stability margin of the compressor we consider (Ref. 3) :

A typical 60/40 work split for HP and IP,

P/O level as a fixed percentage of max total core power,

W and TET adjusted to maintain the turbine capacity constant, in all practical situations

@ NGV (HPT choked)

OPR=HPC_PR * IPC_PR is constant.

Extraction of power from the core shaft, either IP or HP, changes directly the working line of the

compressor mounted on that shaft. The effects on the other compressor changes from HP to IP P/O:

they depend on rematching of both compressors.

This behaviour is better explained in the next two paragraphs.

2.6.3 HPC Stability

Extracting mechanical power from a shaft reduces the power available to the compressor, which

results in reduced PR and air mass flow. Regarding the HPC, the inlet pressure is also modified by

both power offtake concepts, HP and IP P/O.

The HPC non dimensional inlet mass flow (in our analysis this is the same as the IPC outlet) is

reduced by taking HP power offtake, which results in a reduced HPC stability margin. The WRTP26

reduction is due to two effects: reduction in W and increase in . From the polytrophic compression

relations it follows:

Figure 2.6-19 Residual surge margin and threats to WL and surge line

Engine variation Surge line

Deterioration

Thermal effects

Residual margin

Vibration

Transient excursion

Power offtake

Thermal effects

Deterioration

Engine variation

Flight condition Working line

Inlet flow

PR


55

With

The HPC inlet non dimensional mass flow is

With , the following relation stands:

With constant and depending on cruise conditions. When power is extracted from the core

rises while W decreases independently on where the power is taken from.

With the HP P/O system, since is larger than with the IP P/O, the HPC works at smaller corrected

mass flows when the power extraction level is increased. On the other hand, the working line rises as

a direct effect of the additional HPT power resulting from the increase. With the HP P/O system,

and fixed , the IP has to provide for the larger part of the OPR but not as large as to allow

the HPC to lower its working line. It can be said that the effect of WRTP26 on this line is stronger than

that due to the reduction of the HPC PR: this disparity pushes the HPC towards the surge line.

At low power conditions there is a double penalty because the operative line and the surge line are

convergent in this region, resulting in a lower surge margin.

No P/O

Figure 2.6-20 Effects on the HPC working line: IP P/O and HP P/O

Surge line

IP P/O

WRTP26

IP P/O Range HP P/O Range

Normal range

Effect of HP P/O

HP PR Stability margin

Effect of IP P/O


56

With the IP P/O system the working line of the HPC is not modified but the compressor is pushed to

work at higher WRTP26 on its map. The HPC works on its original working line (that one with no P/O)

in a region where this line gets more parallel to the stability-margin line thus reducing the surge risk.

Obviously this beneficial effect becomes inconsistent if the working point moves too far away from the

design point. Increasing the power taken from the IP shaft will shift the HPC mass flow range out of

the normal operative range. If the actual operative range moves too far away from the design point

the efficiencies of the components (turbines, compressors, combustor...) begin to decrease and the

HPC working line bends towards the surge line. There is a maximum corrected max flow for the HPC

computed in the “max climb” design condition. Use of IP P/O can push the operative point beyond

this value. This issue should be recognized and fixed at the design stage.

A typical behaviour of the HPC with IP P/O is that the working line is no longer straight but has a loop-

shape, which means that the minimum inlet non-dimensional mass flow no longer occurs at idle

condition but it occurs in the middle of the power range. This is a typical example of how the different

boundaries (see above) modify the behaviour of the system. There is no need for bleed valves to

stabilize the HPC with the IP P/O system; indeed a higher PR and non dimensional flow are required

for the HPC stability.

When the power is taken from the HP system the

IPC operative line rises, the bleed valves have to be

scheduled in order to protect the engine from surge.

The IPC working line needs to be reduced in order

to establish a certain gap with the surge line and

maintain the HBV closed during cruise with all the

P/O level conditions. Otherwise there will be an

important penalty in SFC. For an engine with the

HP P/O system satisfying this requirements is more

complicated that for an engine with the IP P/O

system. To lower the working line, for a given core

size and mass flow, the installed hardware needs to

be “under exploited” reducing the working PR to

increase the stability margin.

If any component in the core is negatively

influenced (i.e. its efficiency is reduced because it

doesn‟t work in the neighbourhood of its design

point) this will always result in higher fuel consumption. Higher fuel flow generates higher TGT that, in

turn, pushes up the HPC working line.

In our model, doubling the amount of power extracted will amplify the characteristics outlined above.

With 10% of power extracted, the mass flow range of the HPC with IP P/O is no longer compatible

with the original design range, requiring the HPC to be completely redesigned. With the HP P/O

system and high P/O levels the negative effects on the HPC are even stronger, making this solution

unviable unless a very large amount of air is extracted from the IPC.

Deterioration of engine components has always a negative effect on the compressors‟ stability since it

introduces losses and reduces efficiencies, eroding the available surge margin.

Idle WITHOUT P/O

Idle WITH

IP P/O

Loopshape

HPC

PR

WRTP26

Surge line

Figure 2.6-21 Loop shaping of the

HPC with IP P/O system


57

2.6.4 P/O effects on HPC worst case assessment

Unlike the IPC, the HP compressor stability stack-up with either HP or IP P/O system should

always include the effect of high power offtake when accounting for the threats applied to the

working line. The worst maneuvers in the above idle engine operating range have been identified

using experience of the previous Trent engines. The worst case stability for slam acceleration is

based on transient engine modelling and verified by test.

After accounting for the threats, the XWB HP compressor residual surge margin has been assessed

at each of these critical points for both IP and HP power offtake configurations. The final HPC stack-

ups with IP and HP power offtake from the Trent 1700 trade study using the respective handling bleed

valve schedules are reported in Ref. 21. In fact, as the HPC does not operate at the same point on its

map depending on IP or HP power offtake, the bleed valve schedules must be set to accommodate

for this difference and compensate for the threats. The shortfalls associated with increased bleed

requirement for the HP P/O system were discussed in a previous paragraph. It can be said that the

HP compressor efficiency is likely to vary quite a lot from IP to HP offtake. This difference cannot be

captured in the stack-ups. The IP offtake configuration gives better HPC stability than the HP offtake

option when the same bleed valve schedule is used. For further information regarding HPC stability

and stack-ups see Ref. 21.

2.6.5 IPC stability

The boundary conditions for the IP system are slightly different since between the compressor and

the turbine there is not only the combustor but also the HP system. This system can represent a

blockage or can promote the flow through the IPC whether the HPC required flow is smaller or

bigger than that delivered by the IPC. Being in one situation or another depends also on the P/O

system layout.

With an HP P/O system the IPC working line changes unlike the HPC WL with the IP P/O. When

power is extracted from the HP shaft, the specific power of the turbines is increased as a result of

the increase in turbine inlet temperature temperature. This means that the IPC specific power also

raises, i.e. pressure ratio at a flow increases. The intermediate pressure compressor tries to

accelerate in order to deliver a higher pressure ratio. If the compressor is operated on its standard

working line, this results in a larger WRTP24. The HPT cannot swallow all the air that the IPC tries

to compress since its capacity is fixed:

is fixed because it is the HPT capacity, is fixed for hypothesis and is almost

constant because it depends only on the combustor losses. Since has to be increased in order

do deal with the power offtake, the final result is that decreases. This blockage pushes the IPC

to work at lower WRTP24 with an increased PR, i.e. with reduced stall margin.


58

With IP P/O, the IP compressor working line is lowered by the P/O system because a smaller

pressure ratio is produced by the compressor whilst the non dimensional mass flow WRTP24 is less

affected than WRTP26 is affected by the HP P/O. The IPC non dimensional inlet mass flow is

influenced only by the change of W while remain constant, since they depend on

ambient/cruise conditions.

The mass flow rate, as stated above, is determined by the HPT capacity, OPR and .

Using IP power offtake has the effect of moving away the IP compressor from surge during normal

operation conditions, on the other hand at low power conditions, due to P/O effects and the low PR,

the IPC and IPT work at very low speeds with reduced efficiency.

The working line of the IPC without P/O converges towards the surge line for low WRTP and PR

values. The compressor is designed in order to maintain a safety margin during all the normal

conditions without P/O (Figure 2.5-16). When IP P/O is used, the IP WL changes: it drops down and

becomes steeper, i.e. more parallel to the safety line labelled as “WL no bleeds IP P/O” line in Figure

2.6-34.

Theoretically, with the IP P/O system, the HBVs are not required during cruise operation with high

P/O levels, but are necessary in different flight conditions, i.e. transient manoeuvres with small P/O

levels. With the IP P/O system, the engine takes an advantage. Since a lower than zero P/O stability

margin is required the IP compressor can produce a PR equal to 7.8:1 (Trent 1000) instead of 6:1

(previous HP P/O engines) with the same aerodynamic design. The Trent 1000 architecture allows

the IP compressor to be worked aerodynamically harder than on previous Trent engines without the

need for large air bleed at low power conditions. This results in an improvement in fuel burn on short

haul operations.

Surge

WL no bleeds

HP P/O

Safety

line

WL IP bleeds

HP P/O Nondim. inlet

massflow

WL no bleeds

no P/O

IPC PR

WL no bleeds

IP P/O

Figure 2.6-22 Changes on IPC working line with IP or HP P/O system, HBV effect is

shown for Hp P/O


59

Taking power from the IP shaft, as stated above, modifies in a convenient way the IPC working line

since it increases its steepness. Consequently the available surge margin increases as the extraction

level gets higher. This allows for a reduced base surge margin and it is possible to obtain a larger

pressure ratio at high power settings. In this way it is possible to exploit the hardware at its best,

reducing the component weight at a given performance.

2.6.6 P/O effects on IPC worst case assesment

Data removed

2.6.7 LP power offtake

The LP P/O provides some interesting features. Some of these are:

It doesn‟t change the core matching,

LPT is very efficient, best place to take power off, it is “designed” right for this purpose,

Possibility of using the fan as windmill generator to obtain emergency power for the aircraft,

The LP P/O concept has also a series of drawbacks for using it on a civil aero engine:

The LP shaft has a very wide speed range (@ idle 20% of design N1),currently not compliant with

VFSG speed range,

High power offtake levels at low idle are problematic. In order to maintain a min N1 speed more

fuel must be burned which also increases the HP and IP speed resulting in a higher thrust. The

core needs to be oversized to cope with this situation.

Cannot be used to start the engine. Need for an additional mechanical drive/starter.

2.6.8 Core size

The design of new engines is going towards smaller core sizes optimized for cruise conditions in

order to improve efficiency and reduce engine weight. A conventional HP P/O system would struggle

to deliver the amount of power required by the Boeing 787. A consequence of high P/O levels would

be a continuous increase of the core‟s dimensions thus leading to a heavier engine. This will

considerably worsen the cruise SFC. On the RB262-51 (former name of the Trent 1000, preliminary

study, configuration 51), adopting a HP P/O system, the high power demand was one of the

controlling parameters for the minimum achievable engine core size. This situation is further

aggravated by the requirement to achieve low thrust during the descent phase of the flight.

This problem was solved on the Trent 1000 by using the IP P/O system.

Figure Errore. Per applicare Heading 2 al testo da visualizzare in questo punto, utilizzare la scheda

Home.-23 Effects of the switched air system on the HPC and IPC stability margin


60

2.6.9 Operability / HBV control

To rematch the core and control the engine stability, two systems are used on the Trent 1000:

Handling Bleed Valves and Variable Stator Vanes. The main difference between T1000 and other

engines is how and when these systems are used. The HBVs are used on both compressors; on the

other hand the VSVs are used only on the IPC. With an IP P/O system this allows to use the angle of

the VSVs as an additional parameter to control the compressor mounted on the shaft used for power

extraction.

The main differences between an IP P/O and HP P/O, with regards to the HBV operation, are:

o With an HP power offtake configuration, the first IP8 handling bleed valve must open earlier

than with an IP P/O for both steady state and transient schedules, to accommodate for the

higher threats due to power offtake. This will cause fuel burn penalties as the IP handling

bleed valve will be opened more often at certain flight conditions like taxing and descents

(report PTR127002)

o The HP3 steady state handling bleed valve schedule has been kept identical for both offtake

configurations. Although reduced with HP power offtake, the residual surge margin remains

positive at all stack-up cases.

o Transiently, the HP handling bleed valve HP3.1 will be closing at a lower speed with an HP

offtake configuration to accommodate for the higher power offtake threats. This is due to the

working line excursion, which operates at a lower power setting, and to the nature of the surge

line, which drops as bleed valves open.

From the performance point of view the big work is on schedule optimization, taking into account

the requirements on bleed stations and flow rates that, as on the Trent 1000, could be influenced

by the P/O strategy. To do this, precise information about how the IP P/O system works has to be

collected from flight tests over the whole flight envelope.

2.6.10 Operability / VSV control

The control of the VSV incidence angle is required to:

obtain optimum SFC,

relax the N2 speed margin,

improve low idle performance,

improve IPC surge margin

improve IPC off design performance

reduce IPC noise.

Other characteristics that are influenced by the VSV schedule are: VSV actuator load, air system

behaviour and blade/vane stress which all need to remain within acceptable limits. Extracting power

from the IP shaft and using it to start the engine modifies the control law as well as the VSV schedule.

IP P/O drops the working line on the IP compressor, as the HPC is driving more freely (no more

blockage) and improves surge margin at all flows. On the other side, the HPC reduces the operating

range (HPC works at higher flows) improving low power surge margin.

The operational requirements for the Handling Bleed Valves are:


61

Ensure sufficient residual surge margin at all

flight/power conditions considering both steady state &

transient.

Reduce SFC

Increase the operability range of the engine

allowing for a lower thrust at low idle condition on

ground

The over-closure of the VSV in the first stage of the

IPC during steady state low idle shall be defined as a

function of altitude and shall directly raise or lower the

VSV low speed stop (LSS). The VSV‟s control will take

the minimum of either the modified LSS or the basic

steady state schedule; otherwise, at high N2 speeds,

engine fuel flow will increase to maintain its pressure limits, driving N2 speed higher and larger VSV

malschedules, causing engine instability.

These are scheduled at different angles throughout the power range in order to achieve the optimum

velocity triangles on the blades and, for same N2 (i.e. same in Figure 2.6-25), modify the work

done by the compressor that is proportional to .

VSV Schedule

-10

0

10

20

30

40

50

60

50 60 70 80 90 100

N2RTHT24

VS

V A

ng

le [

deg

]

VSV Schedule

Figure 2.6-24 VSV open loop

schedule

High speed

Cruise


62

The engine Variable Stator Vanes are scheduled with an open loop control system, i.e. no feedback

from the engine is actually used in the control logic. The selection of a certain VSV position is a

function of N2 speed and IP Compressor inlet temperature 24T (Ref. 11).

15.28824

%2242

T

NRTHTN

Where: T24= total temperature @ IPC inlet

N2= IP spool speed

- If N2<threshold value: cruise schedule

Figure 2.6-25 IPC speed triangle control with VSV

Low N2

Design

High N2

Design

Stator

Rotor

Rotor

Stator

Modified

(overclosed VSV)

Design

Design

Modified

(underclosed VSV)


63

- If N2>threshold value: high-speed

schedule

- If between: interpolation

The optimal VSV schedule is chosen as a compromise

between compressor efficiency and available surge

margin. To improve the efficiency of this system and

optimize idle fuel consumption the main problem of the

open-loop VSV control is overcome. As sentenced

above, the schedule has to be chosen as a compromise,

considering the minimum required margin for the sudden

application of the highest possible load. Because of this

the compressor cannot work in a high-efficiency point. In

particular, for idle operation the VSVs are scheduled

predominantly to maximize surge margin (same applied

load subtracts more stability margin because of the

higher %turbine power value) hence resulting in severe

efficiency penalties and low levels of compressor outlet

pressure.

The most desirable idle operating point is one where the

engine is simultaneously running at the minimum compressor outlet pressure AND at the minimum

speed. The VSV closed loop control scheme basically attempts to accomplish this by using the

existing fuel control loops which are part of the Control System. The compressor VSVs are used as a

second variable to modulate the relationship between pressure rise and speed. It is now possible to

use a reference working line and, for a givenambP , P24 and T24, define the minimum P26/P24 and

N2RTHT24.

The control system detects the actual operating point and, knowing the reference working line,

modifies the VSV LSS position in order to minimize the distance between the current operating point

and the reference working line.

The overclosure of VSV‟s allows increasing the IP shaft speed range (as Redline/idle speed). This

has demonstrated a satisfactory low idle thrust. This system is however constrained by the

accessories‟ speed range.

The HP P/O system doesn‟t need to deal with this problem because the higher T40 required to

facilitate the P/O is directly used by the HPT. Delivering the required power in an IP P/O system, the

increase in fluid energy (higher T40) is not felt only by the IPT but also by the HPT. This results in an

increase in generated thrust. See also Par.2.5.2.

2.6.11 Cruise conditions

The VSV schedule during cruise condition is not directly affected by the power offtake system but is

influenced by the engine idle control logic that is different from those on the previous Trent engines.

For a typical flight descent profile the engine operates at high idle with relatively low corrected IP

speeds (engine running to shaft speed control). However, when the engine is controlled to one of its

Engine

Pamb

Calculate minimum required compressor corrected outlet

pressure and minimum required compressor corrected speed

Calculate reference compressor operating line

Engine

Variable Stator Vane actuator

P20 T20

Compare measured operating point with reference working line

Compressor speed

Compressor outlet pressure


64

pressure limits, due to the aircraft operating at high altitude, low Mach number and high IP offtake,

this will push the engine steady state operating points to higher corrected N2 speeds. It is important

that the VSV‟s are constrained to the Minimum of either the nominal steady state VSV schedule or the

Overclosed LSS. Otherwise, if the VSV‟s remain on the overclosed LSS, the engine control would

continue to raise fuel flow in an attempt to maintain its pressure limit, resulting in high values of N2

speed and ever increasing levels of VSV malschedule.

2.6.12 Effect of P/O on core size for increased BPR

As seen in previous chapters the IP P/O system modifies the IP working line, making it steeper

(almost parallel to the surge line) and reduce the PR when power is extracted from the core. Taking

power from the IP shaft allows the IPC working line to be raised to higher PR in zero-load conditions

since in normal operation the stability margin will increase. This results in a key advantage for a

turbofan with an high bypass ratio (HBPR). Indeed, maintaining the same core size and increasing the

fan diameter or reducing the core dimension for the same fan size (i.e. increasing the BPR to obtain a

higher propulsion efficiency) results in a higher bypass/core mass flow ratio and a lower pressure ratio

across the fan. This is clear if, considering the same core (i.e. same LPT power), the flow rate worked

by the fan is increased. The HBPR philosophy consists in accelerating a bigger amount of air to a

lower speed, this is achieved by increasing the bypass ratio and obtaining a smaller PR across the

fan (this means a lower 125P and 125T with risk of ice on the ESS, see Ref. 24). Thus, in order to

increase the BPR the contribution of the fan to the OPR is reduced and this loss in pressure has to be

recovered in the core engine in order to maintain a high efficiency of the thermal cycle.

The HPC is already working at its best and on modern engines there is no appreciable margin for

improvement in that direction. The IP P/O system allows recovery of the loss in the fan PR within the

IPC, resulting in a more efficient and lighter fan (20 fan blades are sufficient to provide the required

PR) without affecting the core performance. As stated in the IPC stability paragraph, with IP P/O the

engine takes an advantage by a higher compressor PR with the same aerodynamic design and same

dimensions. Actually a different PR means different loads that require a modified structure to

withstand the cycles.

This aspect is very important for future engines: airlines and aircraft manufacturers are given lower

limits for the operative cost and the environmental impact of their airplanes. The general trend is to

reduce the cruise speed in order to diminish the drag on the engine. Increasing the engine efficiency

by using higher bypass ratios is compatible with the reduced cruise speed. Owning the know-how and

the experience in the IP P/O system and be capable of delivering an effective solution in a short time

can give R-R an advantage over other competitors.


65

High BPR Low fan

RPM

Low fan delta T

@root

Bigger IPC (move up both surge

and operative line)

- Under exploiting the hardware - More weight - More fuel cons. @ idle - More complexity

Is it possible?

- Design for worst case

- Less surge margin

- Bleeding schedule

- Cruise conditions?

- SFC?

HP P/O has very negative effect on

the IPC working line and stability

margin. Not a viable solution (many

drawbacks)

Key

advantages for

the IP P/O

system

- Modify the shape of the working

line making it more parallel to

the surge line

- HBV not required

- Cruise condition is ok

- Optimize the exploitation of my

hardware

Sonic cond.

@ tip Lower fan PR

ECC Anti ice

Core effects?

Performances?

Differences?

Higher IPC PR

How?

Rise WL

Figure 2.6-26 Flowchart for HBR requirements


66

2.6.13 Requirements for P/O system

1. VSVs and HBVs schedules required to facilitate for engine operation with IP P/O (min win

logic to cover the whole operation envelope)

2. Compressor rematching is required to facilitate engine operation with IP P/O.

2.6.14 Requirements for stakeholders

1. Consider variation of tip clearance at idle/high power for the engine model used to generate

the engine spec.

2. Consider loss in efficiency with VSV overclosure, it is not negligible.

3. Consider the need for a switched air system.


67

2.8 Secondary Air System

Data removed

For further details about the Switched Air System the reads is redirected to the following reports:

“Trent 1000 - Analysis of Switched Air System Experiments on Engine10001/2b”, E Jeanne,

DNS134745

“Trent 1000 - Instrumentation Specification for HP3/IP8 switched AirSystem”, E Willocq,

DHC218469

“Trent 1000 Package C1 - Air Systems Product System Definition Document”, J C Ellans, FSG49860

“Trent 1000 performance requirements for controls –SASV fault detection and

accommodation”; Marc Pons Perez; PTR 109059; March 2009

“Trent 1000 – Acceptability of Bleed Valves Delivered for Flight Compliance Programme”; D

Hulme; PTR 109627; Apr 2009

2.8.1 Controls of Handling Bleed Valves with IP P/O system

The Handling Bleed Valve Schedule for IP Power offtake configurations is very different from the one

for an HP P/O engine (see Par. 2.5 for further information regarding compressor stability).

In order to accommodate large amounts of power extraction it is not possible to maintain a single HBV

schedule for all P/O levels. To aid engine operability, the Trent 1000 was designed with an additional

control parameter: P26 static pressure.

It was identified that the ratio of P30 to PS26 was a good indication of the IP power offtake level in the

engine. P26 Static was used in preference to P26 Total because the range of the P30 to P26 ratio

was much smaller than the range of P30 to PS26 ratio.

A “Phase Advance logic” has also been implemented: this logic considers also N2RTHT24.dot to

modify the valves actuation speed during fast transients.

The bleed logic was designed to take advantage of these additional control parameters whilst

maintaining as much commonality to previous designs as possible.

The main constrains to HBVs schedule for all engines are:

SFC

Noise

For the Trent 1000 two new limits are set:

HPC stability margin

Maximum allowed TGT.

The EEC contains two engine HBV schedules within its software, one based on shaft speed and the

second, as stated above, on P30Q26S. On the Trent 1000, due to its particular architecture, the HBV

schedule is more or less the opposite than on a normal engine.

The default schedule for controlling the HBV is those based on N2RTHT24 where T24 is the IPC inlet

temperature. The EEC compares the two schedules and selects the schedule that requires the lower

number of open HBVs. If both the schedules demand the same number of valves to be open than the

N2 schedule is chosen.


68

When conditions with high VFSG load occur then a very high P30s/P26s is registered. This will try to

close more bleeding valves than the N2-based schedule to protect HPC surge margin. The valves will

then be controlled to Pressure Ratio Schedule (Ref. 19).

During starting of the Trent 1000 and when engine is not running, the valves are held open by the

spring pressure.

As stated in Par.2.5.2, the critical condition for the stability of the IPC for this engine is at low thrust

conditions with a small amount of power offtake. When only a small percentage of the design

maximum power is taken from the IP shaft the beneficial effects of the IP P/O system on the IPC

stability are not strong enough to obtain a satisfactory stall margin. In order to maintain an acceptable

stability margin it could be necessary to command one or more IP HBVs open. As shown in Figure

2.8-42, it is clear that when an IP HBV is opened the HPC working line is negatively affected and it

approaches the surge line. At low power conditions the HPC working point is lower than the surge line

and the aforementioned effect does not represent a serious stability issue. On the other hand, when

the HPC surge margin is reduced (i.e. with high level of P/O) and IP bleed is harmful for the HPC,

opening the IP HBV is not required for IPC stability. It can be said that the engine is able to maintain a

considerable stability margin during all conditions above idle (in order to deal with unexpected and

quick changes of power offtake) with a reduced use of the HBVs.

Low idle with low P/O level forces the engine to maintain, particularly in low idle condition, a power

level higher than that actually required. Having an input signal that communicates information about

the rate of change of the required power would be very useful in diminishing the fuel burn during low

power conditions. Additional improvements should result from modulating the rate of change of the

electrical, and thus mechanical, loads. The first benefit is on fuel burn at low speed conditions and the

other is on gear train weight. Reducing the mechanical shocks on gear teeth due to sudden

overloads, starting and transients would allow gears, bearings and shafts to be resized, thus reducing

weight. The effect shown in Figure 2.8-42 does not apply only to IPC and HPC when some amount of

air is extracted from the station 026, it applies also to front and rear stages of the same compressor

when the air is taken from an intermediate stage. On Pack B engines the IPC drum is pressurized

using IP5 bleed instead of IP8, as on Pack A T1000. The IP8 bleed is therefore diminished. This

reduces the performance of rear stages of the IPC, (Talk with Arthur)

The effect of the Switched air System has an impact on core matching since the bleed point is moved

forward (IP8) at high power or backward (HP3) at low power conditions. This modifies the working

point of the first portion of the HPC


69

2.8.2 Sealing, IPT issue

The idle thrust requirements have to be met considering also second air system‟s requirements. To

seal the IPT rim these requirements can be stated as:

o 15.144

26

STATICP

Pto avoid gas ingestion into the IPT drum &

o 2.144

26

STATICP

Pto avoid revert air system flow.

Figure 2.8-28 shows the values for a SLS/ISA Day/165hp IP Offtake. IPT rim sealing with no

Switched Air System would mean unacceptable idle thrust with a shaft speed ratio equal 2.1 / 2.2

At ISA-60 and with higher level of power offtake the idle thrust level would increase even more. The

power extraction in Figure 2.8-28 is equal to 165Hp and is more or less 18% of the maximum

required power from the engine at ground idle. Having an insufficient IPT sealing air pressure has

important consequences on the engine operability and integrity:

Net ingestion of gas in the LPT drum causes:

o Higher temperature of the turbine disk,

o Reduced expected life,

o Blade tip clearance problems.

Full reversal of the IP8 air system affects:

o The temperature and the integrity of shafts, bearings, seals and oil,

Figure 2.8-27 Effect of bleed air on compressor stages

Front

Stages

Rear

Stages

Bleed

Front Stage

Surge Line Bleed Cl

Bleed

Op

D

A

B

C

Front Stage WL

A

B

C

D

Rear Stage Surge

Line

Bleed Cl

Rear Stage WL

Bleed OP


70

o Risk of fire in oil/air mixed areas,

o Reduction of IGB/TBH buffer efficiency,

o Surge/stall of the compressor,

o Thermal dimensional control ineffective.

Maintaining a PR bigger than the minimum required to avoid both net ingestion and full reversal of the

IP air system using IP8 bleed air would result in an unacceptable level of thrust exceeding of 80% the

customer‟s requirement. This makes necessary to design a switched air system on the Trent 1000.

The chart in Figure 2.8-44 shows the results obtained from the Trent 1700 performance model

regarding the hot gas ingestion issue. The red line represents the secondary air flow into the HPC

drum, as percentage of W26, when changes. The blue line represents the same thing but

considering the IPT rear rim, where IP8 is used as sealing air. A negative value of %W26 means that

net hot gas ingestion occurs at the considered station (HPC drum or IPT rear rim) for the considered

. For pressure ratios comprehended between 1.125 and 1.25 no net ingestion occurs but

local ingestion could affect component‟s life.


71

Figure 2.8-28 Effect of min P26/P44 on resulting thrust at low idle

2.8.3 Bearing chamber issue

Data Removed

2.8.4 ESS anti-icing system

The Trent 1000 baseline design incorporates an ESS anti-ice system to prevent ice formation in the

ESS region (Figure 2.8-45). The requirement for ESS anti icing was fully recognised on the Trent

1000. The Trent 900 and other Trent engines do not have an active ESS anti-icing system since ice

growth is not necessarily affecting the engine performance or safety. With the introduction of the new

Low Speed Swept Fans and increased BPR the Trent 1000 ESS section has moved further inboard

than on other engines. The delivery pressure and temperature of the flow entering the core are

becoming too low to ensure ice-free conditions on the ESS section. This happens because of the

fan‟s boundary condition is on the fan tip speed, which must be subsonic in cruise condition. The

T1000 has a very high BPR, this means a larger fan and taller blades for the same diameter of the

ECC section. A lower NL to ensure supersonic-free operation at the fan tip results in a lower relative

speed between air and blades near the fan root. Since the fan has to spin slower, a smaller increase

in pressure and temperature will be available for the air near the root of the fan blades. Therefore, the

Figure 2.8-29 Cooling air flow as function of P26/P44

-0.5

0.0

0.5

1.0

1.5

2.0

0.95 1 1.05 1.1 1.15 1.2 1.25 1.3

IP8 System Pressure Ratio (P26/P44)

Flo

w (

%W

26

)

Net Ingestion at IPT Rear

Rim

No Net Ingestion

Local Ingestion Full Reversal

of System

IPT rear

rim flow

HPC drum

flow

No Local

Ingestion

T5

00

& T

ren

t 170

0 5

0%

Co

nfid

en

ce

Idle

T70

0 &

T90

0

Idle

Tre

nt 1

70

0 8

0%

Co

nfid

en

ce

Idle


72

Trent 1000 uses IP8 air to supply the ESS vanes. Using air from the IP compressor the ESS air

system improves the stability at low power/low offtake conditions (power offtake drops the working

line while at high power condition the surge margin is usually larger), reducing the necessity to use

HBV bleeding.

o The ESS anti-icing system takes air from the I compressor and therefore drops its working

line. The design point of the ESS bleed system is in the same order of magnitude than the IP

HBV (Ref. 19) . The surge margin benefit coming from the ESS anti-icing is slightly offset as

re-entry of air into the gas path can possibly cause a surge line effect. The IP compressor

stability assessment was therefore made for the critical case where the ESS anti-ice valves

failed closed.

o For the HP compressor, the ESS anti-ice offtake acts as an IP handling bleed valve, and

therefore raises its working line. The HP compressor stability assessment was made for the

worst case, which is with ESS anti-icing flowing.

2.8.5 Reliability – FMECA issues

Data removed

Figure 2.8-30 Trent 1000 ESS anti iceing system


73

3 PART THREE – ANALISYS OF THE IP P/O SYSTEM


74

3.1 System engineering

3.1.1 System engineering and system thinking

When a new system has to be designed, or an existing one has to be improved it is fundamental to

approach the problem from the correct point of view.

The first thing to define the concept of “system”:

System = All Parts + Interactions + Context + Functions

The system is an assembly of components showing the properties of the whole rather than that of a

single part. These properties are characteristic of the system and can be used to predict its behaviour

under different conditions.

The base of System engineering is the so called “system thinking” which consists in seeing the “big

picture” and different connection between aspects of the system (Ref. 37)

System engineering provides a systematic approach to create solutions from requirements, taking into

account context and purposes: the requirements flow down from a level to the other allowing for risk

identification.

When a customer requires a new system, the requirements are defined for the whole system. These

requirements have to be interpreted and transformed into product requirements, sub-system

requirements and part requirements.

The definition of the solution starts from the smaller scale (components) and is based on the derived

component requirements. It is evident that a good interpretation and flowing-down of customer

requirements is a key step in the product definition process.

To properly apply the system thinking procedure one has to think about:

Whole system,

Purpose (what is the system for?),

Structure of the system and integration,

Sensitivity and robustness,

Point of view.

This task can be achieved by balancing broad overview and selection of the right details. Two typical

issues arising during the product design must be avoided:

“Fix here and move there”, solve a problem in one component and create difficulties in

another part,

“Fix now and move later”, underestimate a problem or trying to eliminate it at a certain design

level can modify requirements for the lower level of the project and create an issue even

bigger than the former one.

The power offtake system is a typical example of a system that is larger than the sum of its

components: the interaction with its engine environment and the context where the system operates

are as important as the definition of the system performance, and can affect the operability of the


75

solution delivered to the customer. The system affects and is affected by the whole engine

environment whereas all metrics that constrain the system can change with time.

Understanding the changes of a system and where they descend from is a fundamental step that can

compromise the whole product, leading to critical behaviour, or issues that are impossible or very

difficult to fix during the more advanced design stages. It can be said that Systems Engineering is

(Ref. 37):

An interdisciplinary approach, focused on defining the needs and requirements for a product

or service. It is especially relevant early in the development cycle

Design synthesis and system validation considering the whole problem across the complete

lifecycle

Systems Engineering considers both technical and business needs of all customers and

stakeholders

3.1.2 Emergent properties

The term “emergent behaviour” refers to a characteristic behaviour of a system that is not shown

during the development process, or during the first part of its operating life. It arise with time and

usage.

All systems have emergent behaviours: some of these are beneficial and some detrimental for the

system performance, i.e., desirable or undesirable.

The skill of the system thinking is to optimize the desirable features, and to minimize the unwanted.

Since a system behaves with properties different from those of the components some emergent

characteristics cannot directly be attributed to the individual parts. The system thinking process

Functional

Systems

Engineering

Power

Systems

Design


76

enables the designer to properly influence these properties and change the right set of components‟

characteristics. In order to reach this goal the designer has to:

Integrate “late lifecycle issues” in the design lifecycle,

Try to understand and prevent the problems involved, remembering that it is almost inevitable

to avoid all unforeseen issues,

Ensure that design attributes have been addressed.

Usually, iteration between requirements and solution is required. This process is shown in the

flowchart reported in the following diagram:

As can be seen from the previous charts the definition of the parts affected by the system is essential

for the further design step of the project because it defines the boundaries and the limits of the

system. Applying System Engineering tools to a complex system results in the so called “W diagram”

shown below. It represents the correct procedure to obtain a product consistent with customer

requirements ensuring traceability and correct flow down of requirements in order to define all, from

top level to component, characteristics

From the analysis carried out in the first part of this report it emerges that the IP P/O system, while

helps to resolve key problems like engine operability, it arises some issues that consist in conflicting

requirements, e.g. need for a high and low idle thrust. Overcome this issues without

deteriorate the engine/system performance somewhere else require the best application of system

thinking.

Stakeholder

needs Requirements Solution

Evidence

Understand and elicit

requirements as

purpose/functionality

Communicate

functionalities, what are

the targets, know how

reach them

Decompose solution in

sub-systems, remain

focused on purpose

Figure 3.1-31 Solution definition procedure, logic scheme


77

Analyze

stakeholder

Systematic

textural analysis

Viewpoint

analysis

Functional

modelling

Sensitivity

Function

analysis and

FMECA Quality

Function

Diagram

Figure 3.1-33 Solution definition using System Engineering, preliminary design tools

Environment

Enterprise (Project/Programme)

System

Sub - system

Component

Customer Requirements

(Boeing) Project Policy

Regulatory Requirements

Customer Requirements

(Airline)

Business Requirement

Document

Requirement Document Product/Supply Chain/Services/Sales

Requirement Documents

Requirement Document Product/Supply Chain/Services/Sales

Definition Documents

Business Definition

Document Evidence

Evidence

Satisfies Direct flow

Sub - system Requirement

Documents Evidence

Sub - system Definition

Documents

Component Requirement

Documents Evidence

Component Definition

Documents

Direct flow where there is no ‘derivation’ of a solution required at this level

Derived solution parameters = requirements at level below

Evidence Product System Product System Requirements

Document

Product System Definition Document Requirements could be flowed

into a Product System e.g. Turbine Case Cooling PSRD from any level

EE Product System Product System

Requirements Document

Product System Definition Document Requirements could be flowed

into a Product System e.g. Turbine Case Cooling PSRD from any level

Evidence

Figure 3.1-32 Requirements flowdown diagram


78

3.2 Define requirements

3.2.1 Summary

During the requirement elicitation process reported in the next paragraphs it has been recognized that

for the P/O system there is a small set of direct requirements arising from a very small list of

stakeholders. The P/O functionalities are very limited and hardly can be changed or other

functionalities can be added since the boundaries of the system are very narrow and settled by the

power extraction level required. The only thing that can be slightly changed is the system‟s

architecture considering integration and reliability problems. The general requirement for the P/O

system that could arise from other systems is “Optimize the impact of the P/O system” and the

majority of the times this can be conveniently achieved only changing the system architecture, as

shifting from HP P/O to IP P/O with all the consequent issues discussed in

Figure 3.1-34 The "W diagram"


79

Summary

The progressive increase of power extracted from the engine for the electrical aircraft has raised the

issue of the competitive position of 2-shaft vs. 3-shaft engines and, within the 3-shaft architecture, the

effects of using a different shaft to drive the accessory gearbox.

















PART ONE – Understanding of the IP Power Offtake system.

From the integration point of view the P/O system is taken as a datum since, once that the

architecture has been decided, its effect on other systems depends only on the amount of power

extracted which is a requirement arising from the airframer and cannot be changed. The other engine

systems, due to their greater design flexibility, are optimized in order to mitigate the power offtake

impact on the engine performance and operability.

Because of this, the “requirements flow” is larger from the P/O system towards other systems rather

than from other systems to the P/O.

3.2.2 Requirements of the IP P/O system

Usually, the customers do not provide a complete set of requirements. The available list is often

inappropriate for direct use, thus some work has to be done to produce a proper requirements

document. Within Rolls Royce six techniques are used to explicit a full set of design requirements.

The first technique is a basic stakeholder requirements elicitation, and it is carried out in five steps

that use different techiques:


80

o Systematic Textural Analysis

o Viewpoint Analysis

o Functional Modelling

o Sensitivity Analysis

o QFD 1

The relevance given to each technique depends on the specific system, on its complexity and its

novelty.

3.2.3 Context Diagram and definition of boundaries

The first step in understanding the requirements is to think about the boundaries of the considered

system, the interfaces (logical and physical), i.e. what is in and what is outside the project‟s

accountability.

The power offtake system has been defined from an engine integration point of view, as the entirely of

hardware extracting the power from the primary flow and transmitting the power with the purpose of

driving all accessories. This definition has been reviewed and agreed within the Trent 1000 FSE

team.

Particular attention has to be taken when considering the functionalities of the hardware linked with

the Operational requirement “drive the accessories”. Indeed part of the P/O system consists of

components that are shared with other systems and only a portion of the component‟s functionalities

are related with the P/O. For instance, with regards to the IPT disk, the function that involves P/O

system is the functional requirement of “transferring momentum from the blades to the shaft”. On the

other hand this piece of hardware has other functionalities, e.g. “House the blades” or “Guarantee tip

clearance”, that involves other systems and don‟t depend on the IP P/O system.

Thus the following components and functions are part of, or related to the P/O system on the Trent

1000:

IP turbine

AssumedThose features that

while not basic are

taken for granted and

not worth mentioning

CUSTOMER SATISFACTION

CU

ST

OM

ER

EX

CIT

EM

EN

T

High

Low

Low High

BasicThe fundamentalfeatures and aspectsso obvious that theyare not worth mentioning

DelightWhen deliver allthe expressed,Basic andAssumed requirements

ExpressedFeatures and aspectsverbalised bythe customer

Systemic Textual

Analysis

Systemic Textual

Analysis

Viewpoint

Analysis

Viewpoint

Analysis

Functional

Modelling

Functional

Modelling

Sensitivity

Analysis

Sensitivity

Analysis

QFD 1QFD 1


81

IP shaft

IGB gears and bearings

RDS

SAGB

ADS

TGB

AGB

Figure 3.2-35 Power Offtake System physical boundaries and interfaces (cyan balloons)

Power

Offtake

system

Gas path

Oil System

Sealing

system Accessories

Air system

IPC

System

Power

Offtake

Lubrication

Load spectrum

Starting Pwr

Power

Starting

Power Scavenge

Air flow

Sealing

Heat

Debris

Power

Figure 3.2-36 IP power offtake context diagram


82

From To Label Description

Figure 3.2-37 Engine context diagram for systems related to the IP P/O system


83

IP P/O System Oil System Mechanical

Pwr

The Oil System provide for the engine lubrication (except VFSGs,

independent lub. syst.). The Oil pump and the breather are driven by

the AGB.

IP P/O System Oil System Heat Lubrication system has the task of cooling down the engine's

components (heat due to friction, windage, conduction, convection)

IP P/O System IP System Working point Extracting power from one shaft allow the compressors to rematch in

different ways. Changing PR, NI, NH, core massflow.

IP P/O System HP System Working point Extracting power from one shaft allow the compressors to rematch in


IP P/O System EECS Electrical

power PMA

The EECS is powered by an independent generator (PMA) driven by

the AGB and used as NI monitor.

IP P/O System EDP Mechanical

Pwr

EDPs is driven by the AGB

IP P/O System VFSGs Mechanical

Pwr

VFSGs are the main source of load for the P/O system and are driven

by the AGB

IP P/O System Accessorie

s Drain

System

Fluids Leak Is possible that during operation some pressurized fluid leaks across

seals and is collected by an apposite drain system

IP System IP P/O

System

Mechanical

Pwr

The mechanical power is provided to the P/O system by the IP shaft

that takes the power from the IP turbine

IP System Secondary

air system

Airflow IP6 air is used as secondary air flow, characterized by flow rate,

pressure and temperature that change depending on operating

conditions

IP System Vibration

monitoring

Vibrations An accelerometer and other instrumentation are mounted on the SAGB

to provide information about the health condition of the gear train

IP System EHMS Data Shaft speed, pressure, temperature are measured at different stations

and data used for engine control

Secondary Air

System

Oil System Pressure

distribution

The secondary air system sets pressure and temperatures inside the

drums all around bearing chambers and seals affecting their

functioning

Secondary Air

System

HP System Working point Bleeding air from the HPC for the SAS is like opening an HP HBV

Secondary Air

System

IP System Working point Bleeding air from the IPC for the SAS is like opening an IP HBV

Oil System EHMS Data The Oil Debris Signal Conditioner (ODSC) inform the EHMS if there are

debris in the lubrication system

Oil System IP P/O

System

Lubricant The oil system provide the lubricant necessary to lubricate and cool

down the P/O system

Oil System Accessorie

s Drain

System

Oil Leak Some oil could leak from the Oil pump or other components of the

system, scavenge circuit provide for collecting

EDP Aircraft Hydraulic Pwr The EDP is the high-pressure hydraulic pump that delivers hydraulic

power to the aircraft

VFSGs Aircraft Electrical Pwr VFSGs are generators that deliver electrical power to the aircraft

VFSGs VFSGs

Cooling

System

Heat Each VFSG has an its own lubrication circuit with the same tasks as

the oil system. Heat is due mainly to the conversion of mechanical

power into electrical power.

VFSGs Cooling

System

FOHE Oil The hot oil is ducted from the VFSGs to the FOHE

VFSGs Cooling

System

FOHE Heat The hot oil is ducted from the VFSGs to the FOHE

FOHE Fuel Heat FOHE is responsible for heat transfer from VFSG's oil to fuel


84

System

Fuel System FOHE Fuel Fuel is provided by the fuel system to the FOHE to cool the VFSG's oil

Fuel System Accessorie

s Drain

System

Fuel Leak Overflow in the fuel drain collectors tank or leakage in the fuel pump

pad are collected by the drain system

Fuel System VSVs Hydraulic Pwr The hydraulic power necessary to actuate the VSVs is provided by the

HMU that use high pressure fuel as working fluid

Fuel System VSVs Control The HMU is responsible for VSV control and is a component of the fuel

system

Fuel System Combustio

n System

Fuel The fuel is delivered from the fuel system to the FSN and the

combustion chamber

ECCS ESS anti

icing

Control The ESS anti icing system is controlled by the ECCS, his effect is the

same as opening a HBV.

ECCS HBVs Control The HBVs are controlled by the ECCS using either a NMix or HPC PR

schedule (use those who require less open HBVs)

ECCS Fuel

System

Control The fuel system is controlled by the ECCS using a FADEC logic

ECCS Secondary

air system

Control The secondary air system is controlled by the ECCS using a P26

schedule

ECCS Aircraft Engine status The information about the engine status are sent from the ECS to the

flight deck

HBVs IP System Working point The HBVs are used appositely to modify the compressors working line

thus increasing operability

HBVs HP System Working point The HBVs are used appositely to modify the compressors working line

thus increasing operability

HBVs Environme

nt

Noise The HBVs extract high pressure air that is discharged into the fan air

duct, this increase significantly the noise level

Combustion

system

EHMS Data Data for the combustion system are obtained indirectly (using P30,

T30, fuel flow, T40 (measured by T44 and TGT/TET ratio),P42)

Combustion

system

IP System Thermal Pwr The combustion system provide the thermodynamic energy to the IP

turbine

Combustion

system

HP System Thermal Pwr The combustion system provide the thermodynamic energy to the HP

turbine

Combustion

system

Environme

nt

Waste gas A mixture of air, NOx, Smoke and combustion products are released

into the atmosphere

EHMS EECS Temperatures The Engine Monitoring Unit conllects information from the engine

sensors and transmits usefull parameters to the EECS

EHMS EECS Pressures The Engine Monitoring Unit conllects information from the engine


EHMS EECS Shaft Speed The Engine Monitoring Unit conllects information from the engine


HP System Secondary

air system

Airflow When SASV is commanded open HP3 air is used to feed the

secondary air system

IP P/O System Oil System Mechanical

Pwr

The Oil System provide for the engine lubrication (except VFSGs,

independent lub. syst.). The Oil pump and the breather are driven by

the AGB.

IP P/O System Oil System Heat Lubrication system has the task of cooling down the engine's

components (heat due to friction, windage, conduction, convection)

IP P/O System IP System Working point Extracting power from one shaft allow the compressors to rematch in


IP P/O System HP System Working point Extracting power from one shaft allow the compressors to rematch in



85

Table Errore. Per applicare Heading 2 al testo da visualizzare in questo punto, utilizzare la scheda

Home.-2 Engine Context diagram legenda (Function DIctionary)

3.2.4 Stakeholder analysis

A stakeholder is a person, a group of people or another subject that is affected by a subject matter, or

that can influence it. The stakeholder‟s influence on project execution, the level at which a stakeholder

is affected by the project itself, its position regarding the project, its interest in it, what can go wrong

with negatively affected stakeholders are all points that have to be considered.

In order to capture the stakeholder‟s requirements the questions under consideration are:

“what is needed to enable an IP power offtake system to be developed?” (“bottom up”

approach),

“what do all the stakeholders want?”, ( “top down” approach).

Both approaches have strengths and weaknesses: indeed the bottom up method is easier but can fail

to capture key requirements while the latter is more complicated but addresses the limitations of the

former.

To help us identifying the stakeholders we can try to answer 4 questions: o To whom

–Who will accept servicing of the product – throughout its life?

o For whom

–Who is the product for – throughout its life?

o From whom

–Who will provide the solution creator, i.e. RR, with a service/product?

o Through whom

–Who will allow the product/service to be „exercised‟?

The list of stakeholders reported in Table 3.2-3 is a top-level analysis of the subjercts that have to be

involved into the design of the power offtake system to successfully integrate it with the whole engine.

For the IP P/O system, while the hardware is easily identifiable and can be located in a restricted area

of the engine, the functional impact on other systems is so broad and relevant that it affects the other

teams thinking to achieve solutions. Whilst teams accountable for the systems influenced by power

extraction have to take into account the P/O effects, the power extraction system is only marginally

affected by other systems‟ solutions.

While there were not many changes of requirements with regard to the basic function of gears, shafts

and bearings during the Trent 1000 and Trent XWB EDP the issues related to the variability of the

power extraction level have turned out to be of high importance. It is because of this that the

interaction with Airframe manufacturer, Airline operator and suppliers of accessories is very important

and strongly recommended. This is particularly important during the first stage of the design process

in order to have the best possible understanding of the effective loads. The importance of the

interactions with other design teams stems from the fact that the higher the power extracted the

stronger the influence on the whole engine


86

Name/ Function Degree of influence on

project execution

Degree affected by project 1-5

(1=min, 5= max)

Internal

1. Design 1.1. Performance 1.2. Transmission 1.3. WED 1.4. Oil system 1.5. Air system 1.6. Electric and power

management 1.7. Architecture EPACS 1.8. Engineering for services 1.9. Fans 1.10. Compressors 1.11. Turbines 1.12. Fire & Vent 1.13. Installation & Controls 1.14. Containment 1.15. FMECA 1.16. Building and Assembly

4

5

4

3

4

3

4

2

2

2

4

3

3

4

3

3

4

5

4

4

4

4

2

4

3

4

4

3

4

4

3

3

2. Testing

2.1. Development engineering

2.2. Validation

3

4

4

4

3. Manufacturing & Repair 4 5

External

1. Suppliers

1.1. Gears

1.2. Bearing

1.3. Shafts

1.4. Accessories

2. Airlines

2.1. Mission

2.2. Engineering for service

3. Airframe manufacturer

4. Certification agency

5. Environment

3

3

2

5

4

3

5

3

2

4

4

4

5

5

4

2

2

2

Competitors 3 3

3.2.5 Systemic Textural Analysis

This is a technique that looks at the requirements that have been expressed, and starts the process of

structuring them. Structured Textual Analysis starts by separating the requirements into the

categories:

o Operational requirements

o Functional requirements

Table 3.2-3 DfPE Project Stakeholder Analysis


87

o Non-functional requirements

This procedure is very useful when there is input documentation from customers that is considered

incomplete, or when the requirements come from a range of customers.

Applying this analysis to the power offtake system allows to identify only a small part of the system

requirements because the customers define only top level functions and characteristics that flow

down in this system. The spreadsheet that contains the aforementioned process is reported in the

SysTexturalAnalysis.xls spreadsheet and reported below.

Identify key product attributes (What are the target as weight, reliability, power requirements

etc?)

The design loads are reported in Errore. L'origine riferimento non è stata trovata. and

Table 3.2-5,

There are no specific requirements on the system‟s weight, the engine should have a dry

weight of less than 11.920lbs

The VFSGs frequency range for steady state conditions, the lower limit can be shifted to a

lower value for no longer than X sec.

The PMA must be able to power the EECS at sub-idle condition

There are no explicit limits for the Engine Driven Pump

The engine mean time to shop visit shall, as a goal, be no less than 10.000 Engine Flight

Hours (EFH) or 8.000 cycles, whichever comes first (Ref. 1, 3.21.3.1).

For ETOPS requirements, the In-Flight shout down rate should be less than 0.01 per 1000

EFH considering all causes (Ref. 1, 3.21.3.1)

The design objective shall be that the engine have a useful on-wing life without repair or

replacement of major components in excess of the hours stated in Table 3.2-4.

Table 3.2-5 Max transient loads (removed)

3.2.6 Viewpoint Analysis

This technique exposes the basic requirements. It is important to capture these, as they are

often not explicitly expressed because, whilst they might raise low technical interest, they

are an enormous source of customer dissatisfaction if they are not met (partly because they are so

basic).

Viewpoint Analysis is a tool that can help a multi-disciplinary team to identify and structure the

functional and non-functional Basic Customer Requirements of a system, i.e. those characteristics

and features that are so obvious that are not specified into the customer documentation. Indeed, the

customer will never provide a complete, consistent and un-ambiguous list of requirements, it is often

up to the System Engineer to derive such a document.

Table 3.2-4 Mature shop visit lives (removed)


88

The cons of this procedure are that it does not provide much insight into “new” / emerging functional

requirements (that take a system type to the next level).

The following set of critical scenarios has been considered during the requirements elicitation

process:

Low idle, small P/O

Low idle, high P/O

Flight idle, high altitude

Surge

Accessory failure

Negative g

Backlash/STO interaction

The same list of use cases has to be taken into account during the detail design of sub-systems and

components.

3.2.7 Functional Modelling

The next technique, which should also be used as a key methodology for bridging between

requirement and solution, is Functional Modelling. Generally this technique is powerful for

identifying interfaces and dependencies between requirements, but it can also identify functions or

requirements that have not been asked for or assumed. Functional modelling is used to produce a

readable and maintainable model of an existing or planned product or system that highlights the

functionality and the inputs and outputs to those functions.

This analysis starts with a Context Diagram in Figure 3.2-52 that defines:

Relationship with super-systems,

Relationship with other parallel systems,

Relationship with the environment

The Function Flow Diagram has the same aim as the Context Diagram reported in Figure 3.2-52 , i.e.

represent relationships and flows between entities, but it focuses on the functions of the system.

The bricks that constitute the FD are:

boxes = entities, external

circles = system functions

arrows = physical quantities, energy flows, data flows control signals


89

The major parameters considered for the Functional modelling are:

Power extracted as % of turbine power

N2 (Min & speed range)

IPC working point

o WRTP24

o P026/P024

o IPC efficiency

P26/P44

IPT capacity WRTP44

IPT efficiency

Max Oil temperature

VFSG control system

STO, load overshoot

The minor parameters considered for the Functional modelling are:

Power extracted as % max core power

NMix

HPC working point

Extract power

1

Transfer power

2

Deliver power

3 Lubricat

e

Working

Line IPC

Working

Line HPC

EDP Oil sys PMA

Power

Power

IP Compresso

r

HP Compresso

r

Mechanical

power

Detect

Debris

Mount accessori

es 4 Interfaces

Crank the

engine

5

Power

Power

Offtake

System

IP

Turbine

Lubrican

t

Manage HTO

6

Heat

VFSGs

Protect from acc.

Failure 7

Seal. air scav. path

8

Contain vibration

s 9

Power

Heat

Air

Define

max

load

Staring

condition

Acc.

Failur

e

loads

Support

Vibration

Figure 3.2-38 IP P/O system Function Flow Diagram


90

o WRTP26

o P030/P026

o HPC efficiency

HPT capacity

HPT efficiency

BPR

Idle thrust

While requirements flow down from product characteristics towards component characteristics, the

boundaries can arise from a lower level or from related systems: the system‟s performance is

determined by the limits of its components and by the limits of the other systems involved. It is useful

to start from the outputs and consider what inputs they need, and then think about the elementary

functions of the system.

For the power offtake system the requirements derived from the system analysis (just looking into the

system) don‟t necessarily highlight the main problem. Moreover, they are not very different from the

requirements for an HP P/O system, hence the owned know-how can be used considering the

peculiarities explained in Par. 2.3, 0 and 2.1. The main issues related with the power offtake system,

those that require a deeper investigation in the first design stage, are related to the system interaction

with the whole engine as highlighted in Figure 3.2-53

3.2.8 Sensitivity Analysis

The Sensitivity Analysis, directly using the Functional Modelling output, looks at the issues that

would be in the interest of the customer. When using this technique it needs to be clear that the

identified functions are the ones whose value customers or stakeholders are prepared to pay for or

invest in. Sensitivity analysis provides a “black box” view of the effect the effect of variation in

“flows” (inputs to functions) on the functionality of the system this tool can be used in conjunction

with the FMECA to study how a function can fail. This analysis has not been reported in the

present report.

3.2.9 Quality Functional Deployment

QFD1 is used to integrate together all the techniques described in the previous paragraphs. QFD1 is

an essential technique to ensure robust flow down of requirements through functional definitions. It

enables a check whether there are functions/qualities definitions to address all requirements, and that

all functions address a requirement.

If the level of novelty of the system is not sufficiently high or an existing system only requires an

upgrade, that is our case, functions and stakeholders are known and maybe most of the procedure

has already been performed. There are two possibilities with relative drawbacks:

o Re-doing the analysis from scratch (which can be a significant effort), and may merely

duplicate what was done before (or even miss some bits out) and hence add no value,


91

o Picking up the analysis from a previous time, and failing to recognise the novel and different

issues or functions required. In this case the best thing is elicit a list of the differences between

the new and the old system and analyze what requirements can be affected, and how, by this

differences

The second option has been considered: the analysis is focused on the differences between the IP

and HP P/O systems and on those fields where a shortfall or a poor understanding has been

identified in Chapter 1.

When the problem is truly mew, and the situation is one where there is an attempt to “change” the

market by “pushing” a new capability that customers are not really familiar with, then the

techniques described above can go wrong on the first iteration. The best practice is to select the

technique that is supposed to be the most sensible, and guess scenarios or use case vignettes. An

idea of a potential solution can be used to extract a definition of functionality, and then

requirements. This then gives a clearer definition of what the capability is. After this, it is possible to

look at stakeholders and their needs in a more informed manner, during a second iteration for

requirements.

The techniques descript in the previous paragraphs produce a heterogeneous set of requirements

that cannot be directly used. To guarantee correct flow-down and traceability of requirements

through the design process it is necessary to sort them into three categories depending on their

impact on the project. This procedure is the same applied during the Systemic Textural Analysis

but it is more detailed and applies to the entire set of requirements, and not only to those directly

expressed by the customers. In order to obtain a complete and structured set, the requirements

have to be divided in:

1. Operational Requirements: define the major purpose of a product/system (i.e. what

it fundamentally does – its capability) together with the key overarching constraints

2. Functional requirements: specify what the product/system has to do in order to

satisfy the Operational Requirements. A functional requirement is a verb or verb

phrase. A phrase can have a verb but not be a function. A good test to check if a

requirement is a Functional Requirement is try to answer the question “does this

system do this or has to be this?” E.g. for a civil aircraft they include:

a. navigate

b. control flight

c. store passengers

d. control cabin environment

e. communicate with other aircraft and ATC

f. Etc.

3. Non-functional requirements: are constraints on the system/product. They define

how a system is to be built in terms of a specific technology. They are solution-based

requirements from customer, legislation or other stakeholder. They fall into three

categories:

a. Non-functional Performance, associated with corresponding Functional

Requirement and define “how well a particular function has to perform”


92

b. Non-functional System, define constraints that affect the whole system and

include:

i. Physical Attributes, e.g. style, size, weight etc.

ii. The –ilities, e.g. reliability, maintainability, interoperability, deployability

etc

iii. System Performance, e.g. cost, speed, manoeuvrability etc.

iv. Contractual/commercial requirements, e.g. “the system must be ready

for trials by a particular date”.

c. Non-functional Implementation, define how a system is to be built in terms of a

specific technology. They are solution-based requirements from the customer

or legislation.

The output of this procedure cannot be used into the QFD1, another step is required to start this

analysis. The Functional Requirements have to be organized in “function trees” in order to identify

functions and the related sub-functions. This allows to choose the level of detail used to analyze

each function: high, if all the sub-functions are reported into the QFD1 chart, or low, if only the top-

level function is used.

The Functional Requirements fit into the “HOW” box in the QFD chart (Figure 3.2-55).

Operational and Non Functional System requirements fit into the “WHAT” box.

Non-Functional Performance requirements fit into the “HOW MUCH” box.

The results of the aforementioned procedure are reported hereinafter.

Figure 3.2-39 QFD1 structure


93

Comments

Requirement Target Value Comments

Safety Must not invalidate existing safety

methods & strategies

Reliability

IFSD rate <XX 1/fhr, Hazardous event rate

<YY 1/fhr,Minor faults must be detected

within XXXcycles

Limited overhaul required, Simple

failure mode, easy fault detection

Satisfy life requirements System integrity and reliability for the

whole design life

Wear, fretting, fatigue, Must not

invalidate existing safety

method/strategy (P/O->compr.)

Provide Maintainability Location on engine, space around of XXmReparability, Quick operational

function test

Contain Life Cycle Cost Manufacturing, transport, assembling,

overhaul

Weight

There are no express requirements about

the system‟s weight, the engine should

have a dry weight less than 11.920lbs

EfficiencyPower @ AGB should be > XX% of core

extracted power

Engine operability

Fan flutter (affected by bleeding),

compressor stability, pressure

changesHandle geometrical

variations and

misalignments

misalignment of SAGB/TGB

Facilitate reqired Idle

performanceThrust<XX , Fuel burn<YYgph

Facilitate noise

containment

HBVs commanded closed for noise-

critical conditions

Producibility

Fillet radius>R, Sheet thickness>t,

Surface hardness<HBR, Gear teeth

dimensions>f etc

State-of-art technilogies, R&D,

Technology maturity

Physical integrationInner compressor disc radius<rc, RDS

ext bore<XXcm,

Match accessory speed

ranges

XXX<VFSGfreq<XXX,

XXrpm<Hyd.pump,Yyrpm

Dynamic stabilityRDS Margin of Stability: XX<MoS<YY;

Drivetrain MoS=...Contain vibration

generation

Vib @ IGB < XX in/sec, @ SAGB<YY

in/sec, @ AGB<zz in/sec.

Non Functional System Requirements: (requirements that apply to the whole system)

Systemic Textural Analysis

Operational Requirement: (purpose for the system)

Requirements

Drive accessories

Start the engine


94

Comments

Stakeholders:

Non Functional

Implementation

Requirement:

(constraints on the solution

imposed by the

stakeholder)

Functional

Requirement:

(verb - noun statement

of functions needed to

meet operational

requirement)

Non Functional

Performance

Requirement:

(performance

expected from each

function)

System attributes listed in

the “What‟s” tab will form

the What‟s of a QFD,

whereas the Functional

requirements of the

“How‟s” tab will form the

How‟s of a QFD.

T S&DTurbine Shaft, Bearings,

GearboxTransfer power Load spectrum,

VFSG, Hydraulic pumps,

etcDeliver power

Accessories'

speed, accessories

power demand

Manage Design loadsCope with loads of

XX, YY

Subfunction of "transfer

power"

Manage non-typical

loads

Cope with STO of

XX

Cope with backlash

of XX


power"

VFSG, Shafts, bearings,

gearsCrank engine X kW for Y mins

Manage surge loads

Surge

characterization -

surge load of XX

Surge duration of

XXms

Surge frequency


power"

Manage failure loadsSubfunction of "transfer

power"

Oil, Gears, Bearings, etc Manage HTOHTO not to exceed

XXkW

Oil SystemProvide power to EDP

(oil-> P/O)XX hp sub function of "deliver power"

Mount AccessoriesInterface

requirements

Systemic Textural Analysis

Requirements (Non Functional Requirements can be a systems attribute, a functional

performance target, or a solution constraint)


95

House EDPSubfunction of "mount

accessories"

Understand HTO

contribution

Manage HTO (P/O-

>oil)see Manage HTO

VFSGs, Shafts, etc Start Engine

Engine must start

on cold days -

temp<XXC

Engine start in

<Xxseconds

Ensure sufficient shaft

speed to deliver oil where

required during all

conditions

Provide minimum oil

pump input speed

min pump speed at

an engine shaft

speed

Subfunction of "deliver power"

Provide oil scavenge

path

Lubricate (P/O->oil)

Manage HTO (P/O-

>oil)

Remove debris(P/O-

>oil)

Air System

Thermal management

for sealing air from

SAS

Subfunction of "manage HTO"

Manage PO impact

(P/O -> Perf)

Maintain foreseable effects on

the air system

Provide sealing air

scavenge path

Air flow XX kg/hr

@ YYdegC

Performance

VFSGs, shafts, VSVs,

Controls etc

Start Engine

See starting time

envelope as

function of

temperature

Compressor, turbines, air

intake, bearings,

Allow Windmill

capability

In flight relight

performance req'sSubfunction of "Start Engine"

Ensure engine

operability (PO-->Perf.)

Engine must provide sufficient

flow characteristics to feed air

bleed systems

Protect engine from

hazardous events (PO--

>Perf.)

Subfunction of operability

Protect engine from

hazardous events (PO-

->Fan.)

Ensure sufficient fan flutter

margin for engine operability


96

Fan

Manage non-typical

loads

Load

characteriziation

for FOD

(internal+regulati

on)

FMECA

Shear necks, debris

collector, oil, oil filters

Protect engine from

accessory failure

Torque for

accessory failure =

xx Nm, heat

generated after

accessory

failure=yy%des.hea

t...

CompressorEnsure engine

operability

(Performance--

>compr.)

Manage surge loadsSurge loads

characterizationAlready considered

Contain vibrations Vibrations <YYPartially moved to whole

system (wear/fretting)

Protect engine from

hazardous events

(wed-->perf.)

Protect engine from

hazardous events

(perf-->compr.)

Turbine

Extract power from

the primary airflow

Max power to

extract=XXkw,

stage PR<YY,

speer range: mm -

MM rpm

Deliver power to the

IP shaft

Max power to

extract=XXkw,

stage PR<YY,

speer range: mm -

MM rpm

Subfunction of transfer power

Generate required

power over all P/O

pwr&speed range Turbine integration (LPT-

->IPT)


97

EPACS

VFSGs, PMA, EEC,

ingniters

Start the engine

(EPACS-->airframer)

Power required by

the engine to be

lighted over the

whole envelope

Manage failure loads

Aricraft system

failure derived peak

load=xxkW,

duration=Yyms

Subfunction of transfer

power

Extract power from the

primary airflow (effects

on compressors and

turbine)

Aricraft system

failure derived peak

load=xxkW,

duration=Yyms

Deliver power

Aircraft AC

frequency range=

360-800Hz

Ensure aircraft architecture

and engine speed range are

compatible

Manage PO (EPACS--

>performance)

Figure 3.2-40 IP P/O system Functional Reqirements Tree (Red boxes are top-level functions)


98

CorrelationsValue

Strong 9

Medium 3

Weak 1

305

265

235

212

177

70

93

35

94

275

75

82

21

175

292

188

204

135

2114 Score307 117 169 169 177 219Technical Importance 273 293 353 333 191 181 151

1 1

9 1 1 19 3 3 1 3 1

9 9 1 1 1 1

3

Match accessory speed ranges 3 1 9 3

3 9 1 3 3 3Physical integration 4 3 3 9 9

3

Producibility 4 1 3 3

3 9 9 9 9 3Facilitate noise containment 4 9 1 3 9 3 3

3 9 3 3

1 1

3 3 1 13 1 3 3 3 1

3 1 1 1 1 3

1

Facilitate reqired Idle performance 5 9 1 3

1 1 1 1 1 1Handle geometrical variations and misalignments 1 3 3 3 3

1

Engine operability 2 9 9 9

1 3 1 1 1 1Efficiency 3 3 3 3 3 1 3

1 1 1 1

1 9

1 1 9 93 3 1 3 1 3

3 3 1 3 3 1

3

Weight 5 9 3 9

3 1 1 1 1 3Contain Life Cycle Cost 2 9 3 3 9

1

Provide Maintainability 1 3 3 9

3 9 1 1 1 1Satisfy life requirements 3 1 3 3 3 1 3

1 3 1 1

9 3

9 3 1 13 1 1 1 9 1

3 3 9 3 9 1

1

Reliability 2 3 1 1

9 3 1 1 1 1Safety 3 3 9 9 9

9

Requirement 4 1 9 9

3 9 3 1 1 1Non Functional System Requirements: (requirements that apply to the w hole system)5 1 3 9 3 1 3

1 1 3 1

3 9 93 9 3 1 1 1Operational Requirement: (purpose for the system) 5 1 9 3

1 9 1 39 9 9 9 1 3

1

System Attributes, Needs, Requirements,

Requirements 5

Ho

use a

ccesso

ries

Pro

vid

e a

ccesso

ries' in

terf

ace

Co

ld s

tart

Allo

w W

ind

mill cap

ab

ilit

y

Extr

act

po

wer

fro

m t

he g

as p

ath

Man

ag

e D

esig

n lo

ad

s

Man

ag

e n

on

-typ

ical lo

ad

s

Deliver

po

wer

Cra

nk t

he e

ng

ine

Co

nta

in h

eat

gen

era

tio

n

Man

ag

e s

ealin

g a

ir-r

ela

ted

heat

Pro

tect

en

gin

e a

gain

st

accesso

ry's

failu

re

Pro

vid

e s

ealin

g a

ir s

caven

ge p

ath

33 1

P

Fu

ncti

on

al R

eq

's

P P P P W

W PP P P O W P

W O P P

W

O P W PP P W O W

P W W

Rela

tive Im

po

rtan

ce o

r W

eig

ht

(1-5

)

O O P O P

O

P P

P O O O W W W W

P P O

WW P W W P O

WSubfunctions W W W P P

W

W No influenceW W P

P Positive InfluenceW W W P

O Negative Influence W P W

P W

P

P

Figure 3.2-41 IP P/O system QFD1 results


99

1) Drive accessories 305

2) Physical integration 292

3) Engine operability 275

4) Start the engine 265

5) Safety 235

6) Reliability 212

7) Dynamic stability 204

8) Match accessory speed ranges 188

9) Satisfy life requirements 177

10) Producibility 175

11) Contain vibration generation 135

12) Efficiency 94

13) Contain Life Cycle Cost 93

14) Facilitate reqired Idle performance 82

15) Handle geometrical variations and misalignments 75

16) Provide Maintainability 70

17) Weight 35

18) Facilitate noise containment 21

Pos# System attribute Rate

Figure 3.2-42 QFD1 results: System attributes rating

1) Manage non-typical loads 353

2) Deliver power 333

3) Protect engine against accessory's failure 307

4) Manage Design loads 293

5) Allow Windmill capability 219

6) Crank the engine 191

7) Contain heat generation 181

8) Cold start 177

9) House accessories 169

10) Provide accessories' interface 169

11) Manage sealing air-related heat 151

12) Provide sealing air scavenge path 117

Pos# Functional requirements Rate

Figure 3.2-43 QFD1 results: Functional requirements rating


100

4 PART FOUR - REVIEW OF THE TRENT 1000 IP P/O SYSTEM


101

4.1 Stakeholders discussion

4.1.1 Introduction

The following paragraph is aimed at describing how the most important engine systems, considered

as stakeholders, are affected by the IP P/O system. The description focuses on the aspects that that

are considered the most important, discussing the mechanism and the rationale behind this aspects.

A list of critical parameters is reported for each stakeholder. Note that though some effects are small

and negligible on the Trent 1000, their importance could increase significantly for new engines if the

values of the controlling parameter differ from those in the Trent 1000

4.1.2 Compressors

Description

Compressors are deeply affected by the IP P/O system as explained in Effects of the IP P/O system

on thermodynamic aspects. The starting procedure on the Trent 1000 affects the compressor design

since a new scenario has to be considered while evaluating the compressors‟ operability.

How compressors are affected by the IP P/O

The influence of the P/O system is due to the division of the turbine power between the accessories

and the IP compressor. The P/O modifies the power available to the IP compressor causing core

rematching and change of pressure distribution. This modifies the compressors‟ pressure ratios,

corrected inlet mass flows and shaft speeds. Regarding the starting procedure, the pressure

distribution is reversed across the compressors with P26>P30, and the HPC acts as a turbine and

runs into stall for a long part of the cranking procedure. The IPC‟s ability to start in a short time and at

low N2, requires sufficiently high P26 and WRTP26.

Critical parameters

The most important parameters that affect the compressors‟ behaviour is the power extracted as % of

the turbine power.

The main parameters affected by the changes in compressors‟ behaviour during normal operation

are:

P30/P26

P26 (and all the ratios involved)

N2RTHT24

WRTP24

the stall margin and drag for IPC cranking

VSV schedule


102

4.1.3 Turbines

Description

Turbines are not directly affected by the P/O system in since HPT and IPT are usually choked during

the main part of the operational envelope, as explained in Effects of the IP P/O system on

thermodynamic aspects.

How Turbines are affected by the IP P/O system

The starting point to size a turbine is the specific power required based, on the aerodynamic

parameters which deliver the work at the required level of performance. If the required specific power

goes up due to power offtake load, the stage efficiency should drop for a heavily loaded stage,

requiring another stage and the redesign of the turbine.

An increase in P/O level from the IP shaft, affects the turbine also via variation of cooling and sealing

air characteristics. Considered are also effects of rematching and the effects of SAS on the IPT

blades cooling air.

No action has been taken to take into account changes of mechanical loads due to P/O level variation

or dynamic behaviour (e.g. STO): the P/O effect has been considered only as extra power to deliver

to the IP shaft over a certain range of conditions.

Critical parameters

The parameters affected by the presence of the IP P/O system are the same as those modified by a

larger compressor:

Annulus mean radius,

Blade tip to root ratio,

Blade number,

Blade speed.

The limiting parameters for the specific stage load level and thus for the shaft power are:

Flow turning angle within a single stage,

Peak Mn,

Diffusion rate on blade suction surface,

LPT inlet flow characteristics,

Stage load.

The critical parameters for the secondary air system are:

P44/P26,

P43/P27.

4.1.4 Air and Oil systems

Description

The P/O system changes the pressure distribution inside the engine, which affects bearing loads and

their clearances. The P/O system, in turn, can be affected by changes in air bleed temperature:

different working temperatures for gears and bearings can require modifications of the detailed design


103

and of the oil/air flows. If the temperature difference is significant then modification of the lubrication

system and heat-to-oil management could be required.

At low engine power levels, the SASV feeds the secondary air system with HP3 air, the same stage is

used over the whole engine operation to provide the air necessary to cool the IPT disk and blades.

When HP HBVs are commanded open (total of 4 bleeds from HP3: 2 HBVs, 1 SASV, and internal air

system bleed) up to the 10% of the core airflow can be extracted. If too much air is taken this can

reduce the stage pressure at HP3 and modify the pressure difference across the secondary air

system. On actual Trent 1000, there is not enough air bleed from HP3, so the small pressure variation

is likely to be harmless for the considered systems.

The IP P/O system has a particular effect on the sealing systems of all bearing chambers: increasing

P/O level increases bearing loads and reduces the buffer pressure. The first effect is a direct effect of

the larger forces between the gears‟ teeth, the second effect is a consequence of the PS26 drop

when the IP P/O level increases. This is exactly the opposite of what happens in an HP P/O engine,

where PS26 increases with the power extraction. The design condition to avoid that the source

pressure drops lower than sink pressure is where high power is extracted. The EDP for an HP P/O

engine is conducted with no P/O, i.e. the worst air system configuration. For the Trent 1000, to obtain

the worst condition; the power extraction has to be kept to the maximum.

Negative G acceleration has a negative effect on the engine since the oil system is based on gravity

effects, from the oil tank to scavenge and venting lines. When the gravity is reversed the scavenge

flow stops while the feeding system is still pumping oil inside the gearbox until the feed line is filled

with oil. Something similar happens also in the oil tank and the pump is no longer fed with oil but

starts pumping air into the oil system increasing the pressure in the bearing chambers.

The robustness of the oil system has to be checked against negative-G requirements which arise

from Airframer and Certification Agency. If the result of this assessment is negative, i.e. the oil system

is not able to operate at negative G and provide for this capability would result in an unacceptable

effort, mitigation has to be provided by the P/O system design. This can produce a requirement in

terms of operation (time and load) without lubricant and with non standard pressures in gearboxes

and bearing housings.

How Air and Oil systems are affected by the IP P/O system

Data removed

Critical parameters

The working characteristics of both compressors are important since air and oil system rely on P26

(IP8), P27 (HP3) and P30 (HP6). The oil pressure, oil massflow and HTO performance depend

directly on shaft speed. The oil pressure affects the oil sealing capability.

The impact of the P/O system on air and oil systems is through these pressures and shaft speed

changes. The most important but less evident features that are on the Trent 1000 (e.g. buffer

pressure and turbine rim seals) depend directly on this matter.

The main parameters affecting the Air system, which are related to the P/O system are:

1. Compressors‟ (HPC and IPC) delivery pressures

2. Secondary air system pressures

3. Secondary air system temperatures


104

4. Shaft speeds.

4.1.5 Transmission, Structures and Drives

Description

T, S & D are the Stakeholder responsible for the hardware design and integration of the Power

Offtake system. Increased power offtake capability of the drive train, decreased weight, minimized

windage are some aspects that have to be considered by the T, S & D department.

The mechanical problems of the P/O system are not directly related with taking the power from the IP

shaft instead of the HP, but are likely to depend on secondary effects of the two architectures and on

the accessories‟ control system, behaviour, and configuration.

The loads resulting from compressor deep surge are larger on the IP system than on the HP system;

the same observation can be made for the speed gradient: usually, after a deep surge, the IP shaft

speed is almost zero. The deceleration of the spool is huge and loads are very high since the inertia is

much bigger than in an HP P/O system. These loads are a design constraint since they cannot be

influenced or changed, and the mechanical system has to be able to withstand them. It is also

because of this that all problems related to resonance or natural frequencies don‟t constitute a threat

for the system integrity: the system is usually somehow “oversized”. The HP spool has a lower inertia

compared to the IP shaft, smaller speed variation during surge, a smaller speed range during normal

engine operation and smaller torque have to be transferred through the drivetrain.

The length of the Radial Drive Shaft (RDS) is usually set by the radial dimension of the fan air duct.

The Trent 1000 has a very high BPR, which requires a larger fan duct and a longer RDS. Another

issue that restricts the range of viable solutions is the necessity of allocating the bearings inside the

IGB considering lubrication, HTO, assemblability and effects on structure performance. Natural

frequencies of the drive train should not change very much from an HP to an IP P/O system. The

spool inertia of both HP and IP systems are so high that the elastic characteristics of the drive train

can have only a very small influence on their dynamic behaviour.

Mounting the P/O bevel gear on a stub secured to the IP shaft instead that directly on the IP shaft has

a beneficial effect from the mechanic prospective: a “bumper-like” system is created. This feature

decouples the oscillations of the two systems, i.e. the IP shaft with compressor/turbine and the P/O

drive train. The IP shaft is very thin compared to the much stiffer stub shaft and compressor drum,

and can be considered as a torsional spring with two masses at both ends. Because of this, the drive

train is only slightly affected by oscillations of the IP system.

The SAGB‟s radial dimension is a concern for the fan air duct because it can cause aerodynamic

interference. When attempting to reduce this dimension, new problems arise from the RDS since the

shaft‟s length becomes critical for the dynamic behaviour. This is all due to an aerodynamic

requirement. The minimum required distance between the splitter and the fan OGVs is fixed and the

splitter length is proportional to its width that, in turn, depends on the RDS outer diameter. The RDS

location bearing in the SAGB cannot be excessively moved towards the centre of the engine and

must provide a sufficiently rigid support for the shaft. During normal operation, the relative

displacement between the core fairings and the fan case ranges between X and Ymm. The drive train


105

has to be designed to tolerate this misalignment. This is a very stringent requirement in conjunction

with all the other characteristics required by the drivetrain: large misalignment in the barrel splines

under long time means a huge amount of heat produced inside the RDS assembly and larger gap

between the RDS and its case. Because of the increased amount of power extracted, and the

aforementioned lower speed and higher torque, the heat generated by friction or windage on the Trent

1000 is larger than on other Trent engines.

Another set of requirements for the AGB is related to maintainability and assemblability, which can

limit integration and new layouts for the accessories. This issue is common on IP and HP P/O

systems.

An important difference between IP and HP P/O system is the management ball bearing skidding for

the HP shaft. Historically the HP ball bearing in the IGB is exposed to aerodynamic and pressure

loads, from turbine/compressor interaction, and from mechanical loads (P/O drive train). The

aerodynamic loads at idle are very small and near to zero. The reason why this has been accepted on

other Trent engines is that a small mechanical load was always present in the HP P/O system,

preventing the bearing from skidding

How T, S & D are affected by the IP P/O system

The bevel gear that rotates with the IP shaft is not mounted directly on the torque-carrying section of

the IP shaft but on a stub, which is coupled with the IP ball bearing and then secured on the IP shaft

in order to reduce the gear teeth misalignment. Bringing the loads as close as possible to the bearing

in the IGB is an important requirement for the IP P/O system because the transferred torque is bigger

compared to an HP P/O system with the same power

With an IP P/O system, since the gears work at lower rotational speed but with a higher torque, the

teeth‟s width has to be increased in order to meet the life requirements. This affects the design of the

whole IGB area including

The bearing position and the gap between the last IP stage and the first HP stage

The radius of the IP8 blades roots.

Bearing design and size.

Critical parameters Power peaks during load transients caused by VFSG control system need to be considered by the

design of the IP P/O system to guarantee the drive train‟s life and mechanical integrity. There will be a

considerable amount of extra weight due to this requirement. A similar behaviour can be seen when a

brittle shear neck breaks: a very high load is impulsively removed, this create a “slingshot effect” that

is harmful for mechanical parts.

Transient overshoot factor,

STO frequency,

Minimum P/O load,

Maximum P/O load,

Loads related to accessory failure,

Loads related to shear neck failure,

IGB – SAGB displacement and misalignment,

IGB bevel gears axial movement,

IP shaft – AGB gear ratio,


106

RDS and ADS whirling constants,

4.1.6 FMECA

The Failure Mode Effects And Criticality Analysis, also known as FMECA, is the main tool to satisfy

the requirements of EASA Certification Specification (Engines CS-E 510 Safety Analysis) with respect

to the engine turbo-machinery. The Safety Analysis of the Engine Control System is excluded from

the scope of this procedure. A FMECA is produced for two main purposes:

1. To assist in the design process in order to identify and eliminate unsafe or unreliable designs

2. As part of a process to produce a Safety Analysis for product certification.

There are four main activities involved in producing a FMECA:

Identification and Design Understanding

Failure Mode and Effects Analysis

Criticality Analysis and Classification

Reporting and Feedback

The first point of the list is implicit to the first part of this report (


107

Summary

The progressive increase of power extracted from the engine for the electrical aircraft has

raised the issue of the competitive position of 2-shaft vs. 3-shaft engines and, within the

3-shaft architecture, the effects of using a different shaft to drive the accessory gearbox.

















PART ONE – Understanding of the IP Power Offtake system). The most important activities

are the second and the third, while the fourth is used during the design and will not be reviewed here.

Failure mode and effects analysis

For each part of the engine, all potential failure mechanisms should be recorded. The resulting

System Failure Modes are entered into FMECA tables. For each System Failure Mode all possible

effects of the failure should be considered and recorded. The effects should be described at a system

level, at an engine level and at an aircraft level. The analyst needs to address any effects propagating

over interfaces. Consideration must be given to the continued safe flight and landing of the aircraft.

Criticality Analysis and Classification

For each failure, the effects need to be classified at Engine level. The categories are given in the

regulations. It is important that the severity classification reflects the failure effect exactly.

A judgement or estimation needs to be made concerning the rate at which each effect might occur.

This needs to be estimated based on whatever evidence or experience is available. Failure rate

bands are given in the regulations. Advice on failure classification and rate estimation is given in the

ALG section in the RRQMS (Rolls Royce Quality Management System). When a FMECA is used to


108

answer the requirements for Safety Analysis, that FMECA is executive in classification of Critical

Parts.

The FMECA should be developed throughout the design process. In stage 1 the FMECA should

concentrate on novel features of the design. Increase in detail should be added throughout stage 2. A

FMECA statement is required for each gate review

Before the engine development programme starts, an estimate of the likelihood of detection (in the

programme) should be made for each failure mode / effect. This combined with the severity and the

estimated rate of occurrence in service should be used to calculate a Risk Priority Number or RPN.

Failure modes / effects with large RPNs should be highlighted to the Engine Project for further

mitigation. A list of high risk items or list of items in RPN order should be produced.

Full FMECA System/Functional FMECA and, subsequently, part FMECA detailing all

failure mechanisms, propagations and consequences. The first issue should

be available before first engine test bed run.

Summary FMECA Summary of all Major and Hazardous Effects is normally a document used for

certification and therefore has to be agreed by the Certifying Authority. The

first issue is required for Engine Certification. Includes assumptions about the

aircraft and its operation, maintenance and instrumentation.

Within the present report the Summary FMECA performed for the Trent 1000 Pack B engines has

been reviewed to underline the main features related to the presence of the IP power offtake system.

As underlined in the previous paragraphs, from the engine prospective, the main differences between

HP and IP P/O system can arise from the following systems or components:

IGB;

Air System;

Oil system.

Data removed


109

4.2 Requirements – Functions correlation

In this paragraph, the functional requirements will be checked against the implementation solutions

that have been adopted on the Trent 1000. A short summary of the lessons learnt from each solution

is reported in the last column.

The information reported in the following table has been obtained from interviews with senior

engineers working in the Trent 1000 programme. Technical reports and from the Lessons Learnt Log

on the R-R Intranet Capability website have also been used as data source. For a proper design of

the drive train, in addition to the aforementioned recommendations, all the lessons learnt listed in

“Definition checklist for Gearboxes”, DNS 131800, C. Childs must be considered.

Data removed

4.3 ERMS report

The acronym ERMS stands for Event Recording and Monitoring System. An essential element of a

development process for all projects is the recording of significant events during engine build, test and

strip that may have implications for the reliability of in-service engines. These events must be formally

considered for corrective action and a database maintained giving traceability of what action has been

carried out. This process is a subset of the total workflow needed to completely define a new or

modified Rolls-Royce product within the context of Create customer solutions. In order to maintain the

coherency of the safety and reliability process the entire product lifecycle is covered. The ERMS

process should run through the entire Engine Development Programme until Entry Into Service, at

which point all problems should be raised and dealt with via the Resolve Customer Issues process.

At EIS+2 years the ERMS database should be closed and archived. At EIS all open ERMS events

should be addressed and driven towards closure by EIS+2. At EIS+2 all residual ERMS events should

be migrated to the RCI process to allow formal closure of the ERMS database and archiving of the

data.

An ERMS event is defined as an event during build, test or strip of development engines or

components which would be considered undesirable on a production or service basis and which

represents a potential reliability concern

A Category assigned to an event denoted by a letter that indicates a level

of reliability according to likely effects that fault/failure may produce.

4.3.1 ERMS Reliability categories

These categories are used to classify the impact of the arisen/detected problem on engine

performance and to express the severity of its consequences. They range from major issue that can

result in a safety concern to minor issues that have no effects on operability or reliability of the engine.

A Events that could have directly or indirectly cause an IFSD

B Events where the commanded thrust is not achieved or


110

uncommanded thrust changes are observed.

C Events where the in-flight restart capability is degraded.

D Events which could result in an unscheduled engine removal.

E Events which could cause serious service disruption in terms of

delays, cancellations or diversions

F Loss of dual lane redundancy of the FADEC system

X Other, non-ETOPS affecting events. These may be further sub-

divided as given below as X1 or X2.

X1 Non-ETOPS Customer irritant

X2 Non-ETOPS

4.3.2 ERMS Status Categories

Investigate The Event is still being looked into by the Problem Owner.

Repeat An event with the same root cause as an existing event on the database

Problem Report/A-Sheet Following the investigation, a design solution has been requested.

Hold Although the Event has been investigated, there is no clear understanding of

how to address it, and unless there is a recurrence, or additional evidence from

elsewhere, the Event cannot be taken any further.

Reject The Event has been rejected. No further updates are required.

Ex-committee An event that can be closed/rejected etc. when the appropriate out of meeting

action has been carried out.

Closed The Event has been understood and addressed, and the solution proven, if

applicable (PCB 3 or 4, as agreed within the project, dependent upon

modification incorporation policy. Refer to GQP C.1.4).

In addition, there is a temporary status of 'Undecided' that can be applied to new Events before they

are discussed at the Committee Meeting and given a status.

4.3.3 Trent 1000 ERMS log

The analysis of the Trent1000 ERMS was focused on capturing those elements related with the

Power Offtake hardware. Investigate the issues that are plausibly related to the IP P/O system is likely

http://www.infocentre.rolls-royce.com/rrqms/level_3/GQP_C_1_4_.htm


111

to result in an excessive burden since the ERMS database counts more than 2500 entries

considering both opened and closed procedure. The indirect effects of the P/O system can potentially

affect the whole engine by mean of complicate chains of events, as seen in the Chepter 0

. This makes a systematic research of cause-effect relationships almost impossible. During the first

step the research was aimed at individuating the “open” ERMS records regarding the P/O hardware

or directly related to it searching within the following components:

Engine – generic VFSGs

Self Sealing Coupling IPT module

LP/IP Bearing and support assembly Compressor intermediate case

LP/IP location bearing assembly IGB driving gear/IP location bearing

assembly

Inner Air/Oil static seal IGB driving gear/HP Location bearing

assembly

IP IGB driven gear IP IGB drive quill shaft

HP IGB driven gear IPT case group assembly

HP/IP bearing support HP/IP bearings

External Gearbox (AGB+TGB) IGB upper housing group assembly

Intermediate Gearbox IP driving gear Intermediate Gearbox IP driven shaft

PMA Accessories‟ cooling system

Intermediate case vibration transducer

The results of this research are listed in Table Errore. L'origine riferimento non è stata trovata..

This table reports the ERMS directly related with the IP power offtake system that are still open on the

15th of October 2011. In order to deliver the engine solution to the customers and seen the imminent

EIS the most of these records are supposed to be closed within the updated closure dates

------------------Data removed ------------------


112

4.4 Lessons Learnt Log

-----------------Data removed----------------------

5 APPENDIX 1 – IDLE CONTROL PARAMETERS

On the Trent family the idle schedules are based on (Ref. 1):

o Minimum T20/288.15

N3 i.e. HP reduced rotation speed, primary schedule used to define Low

Idle and High Idle. Also ensures HP electrical generators do not drop offline on Trent 700,

Trent 800, Trent 500, Trent 900

o Minimum N1 – Ensures the IP Compressor inlet conditions are aligned with those

demonstrated during the Certification icing tests, for all Trent engines,

o Minimum T30 – Avoids presence of liquid water at combustor inlet conditions, for all Trent

engines

o Minimum P30 – Ensures cabin bleed pressure requirements are met (Trent 700, Trent 800,

Trent 500, Trent 900) and protects combustor stability, for all Trent engines

o Minimum Fuel Flow / P30 – Protects the engine lean stability limit by ensuring operation with

healthy levels of Air-to-Fuel ratio, for all Trent engines.

o Minimum PS26 – Protects Air/Oil system by sealing the Front Bearing Housing to prevent oil

leaks and ensure enough HP Compressor drum flow. Also protects HP Compressor stability at

the top left hand corner of the envelope on Trent 1000 only,

o Minimum corrected NMix – Avoids excessive thrust asymmetry during slam acceleration

manoeuvres and is the primary schedule used to define Low Idle and High Idle, only on the

Trent 1000,

o Minimum N2 – Ensures the electrical generators do not drop offline, on the Trent 1000 only,

The idle schedule is controlled only by some of the aforementioned parameters. The remaining

quantities are extracted as function of the controlling variables. NMix is a measure of the core shaft

speed and depends on the angular momentum of the IP and HP spools. Since this parameter is

very

1

2

3

2

2

K

NNKNMix (%)

Where: .N 100%

N 100%2

3

2 constJ

JK

HP

IP

N2= IP Shaft Speed (%)

N3= HP Shaft Speed (%)

JI = IP Spool Inertia (lb.ft2)


113

JH = HP Spool axial Inertia (lb.ft2)

100% N3 = 100% HP Shaft Speed (rpm)

100% N2 = 100% IP Shaft Speed, (rpm)

Note that K~1.289 for Trent 1000 (Ref. 12). NmixDot is the time derivate of NMix and is used as the

acceleration control feedback parameter and the set point is taken from a schedule versus Nmix . The

ACU (Acceleration Control Unit) parameters are:

696.14/20 NMDTDP20P

NMixDot Vs.

15.288/20 NMRTHT20

T

NMix

Where:

P24: Engine Inlet Total Pressure (psi)

T24: Engine Inlet Total Temperature (K)

Increasing IPT size to lower the IPC working line (and PR at idle), as a mean to reduce idle thrust and

fuel burn needs to be checked against the requirements for the IP P/O system. Advantages gained

lowering the IPC operating point are counterbalanced by the need for a higher speed for the IP/HP

shaft (

Ref. 41, Ref. 42).


114

6 APPENDIX 2 – SYS-ML MODEL OF THE IP P/O SYSTEM

6.1 SysML language

The Systems Modelling Language (SysML) is a general-purpose modelling language for systems

engineering applications. It supports the specification, analysis, design, verification and validation

of a broad range of systems and systems-of-systems.

Systems Engineering (SE) relies on modelling and simulation methods to validate requirements or

to evaluate the system. Therefore modelling is common practice in SE to deliver functional

specifications, data flow description, or system structure definitions through the use of modelling

techniques such as:

Data Flow Diagram (DFD) to define the data going through a system, and any processing

required

Functional Flow Block Diagram (FFBD), similar to the UML6 activity diagram

Specifications produced with SE often result from a document-based approach, producing a large

amount of documents with various types of diagrams or notations, sometimes used in an

inconsistent manner.

SysML offers the alternative of using a model-based approach, usually referred as the “Model-

Based Systems Engineering”, to deliver a consistent set of system views, stored and managed in a

repository. Modelling the system in such a way helps to manage complexity and avoid ambiguities

or discrepancies since each element that constitutes the system can be defined only once and

then used in different diagrams.

SysML defines the following diagrams:

The requirements diagrams

Structure diagrams

o The Block Definition Diagram (BDD)

o The Internal Block Diagram (IBD)

o The Parametric Diagram,

o The Package Diagram

Dynamic diagrams

o The activity diagram

o The sequence, state chart, and use case diagrams

Only some types of diagram have been produced for the IP P/O system. They are reported in the

next paragraphs alongside a short description of their content.

6.2 Requirements

Systems Engineering uses requirements to formalize the stakeholders‟ needs, which will be

realized as functionality and constraints, satisfied by the delivered application or system. In order to

6 Forerunner of the SysML language


115

create a consistent set of requirements the procedure described in Par.3.2 can be used, this set

may then be organized using SysML. A model based approach makes use of requirements

through dependency associations with elements from the model such as use cases, blocks, or test

cases, establishing the model traceability.

6.3 Block Definition Diagram (BDD)

The BDD is a kind of “bill of sub-system” or components that constitutes the IP P/O system. It

provides a black box representation of the hardware.

6.4 Internal Block Diagram (IBD)

The Internal Block Diagram or IBD provides the white box or internal view of a system block, and is

usually derived from the Block Definition Diagram (BDD) to represent the final assembly of all

blocks within the main system block. Composite blocks from the BDD are instantiated on the IBD

as parts. These parts are assembled through connectors, linking them directly or via their ports

(standard ports with exposed interfaces and/or flow ports).

6.5 Activity diagram

The Activity Diagram represents steps of a process, often making use of “input and output pins”

that respectively correspond to the element type required as the input of an activity or action, and

the element generated as an output. If an action or activity corresponds to a block operation, it is

possible to ensure that the types of the input and output of this activity are consistent with the block

operation signature.

6.6 Use case diagrams

A Use Case Diagram can be used to describe the functionality of a system in a horizontal way.

That is, rather than merely representing the details of individual features of your system, UCDs can

be used to show all of its available functionality. It is important to note, though, that UCDs are

fundamentally different from sequence diagrams or flow charts because they do not make any

attempt to represent the order or number of times that the systems actions and sub-actions should

be executed. The use case diagram usually relies on a Use Case Analysis, for the Trent 1000 IP

P/O system this diagrams are obtained starting from the Viewpoint Analysis carried out during the

requirements elicitation process. A use case diagram can represent the system and the actors

from a top-level point, showing just a link between the parts, or it can show what are the quantities

involved into the aforementioned interaction.


116

7 APPENDIX 3 - FLUID COUPLING DEVICE

Base principles

The fluid coupling device operates by switching on a flow of oil to the IP side of the impeller as the

starter motors begin to rotate the engine. The centrifugal energy of the rotating oil transmits torque

from the IP side of the impellor (lower part in Figure 6.6-44) to the HP side (upper side of the same

figure) and the flow of oil is switched off draining the fluid from the toroidal circuit once the engine is

started. During windmill operations, the flow of oil to the impeller is switched on again and the

transmission of torque is now from the HP to the IP system (the HP system, due to the higher Mn is

more efficient during windmill relight).

The two main characteristics of this system are that:

o There are no moving parts suitable for wear or mechanical damage,

o The device act ad a damper, reducing vibrations and transmitted peak loads.

Figure 6.6-44 Fluid coupling device

This kind of device will always transfer load and generate heat, the total amount depends, other than

on the geometric characteristics, on the working fluid density and on the relative speed of the two

halves. These characteristics are, in turn, related to the transmitted power and the efficiency of the

device, indeed the heat generated can be interpreted in different ways:

)(

1 .

outin

inCoupl

T

WQ


117

Where: Coupl

is the coupling device efficiency,

inW is the inlet power

T is the torque, equal between in and out

in and out are the inlet and outlet rotation speeds.

Since the torque is constant among the two shafts the power transmitted and that lost will be a

function of the relative speed outin as stated above.

Now is possible to define a series of non-dimensional parameters:

Speed ratio 1in

outSR

Slip Factor SRSin

outin 1

Torque coefficient: 52 D

TC

inM

Where: D is a characteristic dimension of the coupling device, assumed being

the PCD (Pitch Circular Diameter)

is the density of the working fluid

It can easily be seen as SRW

W

in

out being T=const.

Using the aforementioned parameters the rate of heat generated by the fluid clutch can be written as

)1(53 SRCDQ Min

This expression underlines the importance of some key parameters involved in heat generation, i.e.

power loss. These parameters are driving shaft speed and the coupling size D. the fluid density and

the slip factor are also present but with unitary exponent.

Using a 1-D model that does not consider the number of vanes can be obtained that:

----Data removed-----

Where: k is the internal flow loss coefficient, accounts for friction and

imperfections,

d is the internal diameter of the impellor

This model has been shown to be reasonably reliable by comparison with experimental data and CFD

analysis if the following set of input parameters is used (Ref. 8, Ref. 9):

o 4.0D

d

o 5.0

1 SRk if

15.0

5.00

SR

SR

o Approximately 20 vanes.


118

The main difference between the methods to calculate the thermal power generation for an oil-filled

and a “drained” (air filled) one is that in the former the density of the working fluid can be considered

constant and the equations illustrated above can be directly used, for the second there is the need for

an additional equation to relate pressure, density and temperature.

Consider also that the air density, being the pressure inside the vanes dependant on the external

pressure too, depends on where the coupling device is located. High pressure carters are to be

avoided if low heat generation is desired.

It is also important that the air within the coupling device should, as far as possible, be dry of oil to

prevent any increase in effective density.

High speed fluid coupling

The device for the engagement and disengagement of the HP and IP radial drive shafts has been

subject of numerous investigations (see references in Ref. 4). This is a new device for modern aero

engines but similar pieces of equipment have been used in the past to engage piston engine

superchargers (RR Merlin & Griffon single friction clutch plate and Daimler Benz DB601 double fluid

clutch). More recently the F135 powerplant for the JSF incorporates a multi-stage friction plate clutch

to synchronize the drive shaft speeds for the 28,000-horse power lift fan.

On the Trent 1000 the clutch is required to transmit 200ft.lb toque during normal starting conditions

and a basic design specification has been created which identifies the overall operation requirements.

The preferred clutch system for the RB262-58 is a fluid coupling device similar to that found in

automatic car transmissions and is the focus of a detailed design study within Transmissions and

Structures Engineering.

Figure 6.6-45 SAGB with breather, high speed fluid coupling device

The advantages of this device over other clutches are:

Torque transmission is accomplished without the use of sliding or contacting surfaces. Simple

on/off control through oil flow switching, no high-powered actuators required.


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Soft re-engagement of HP and IP shafts during windmill operations. Automatically maintains

bearing and gear teeth loading on HP shaft without the induction of a separate mechanical

loading device.

There are a number of concerns with the fluid coupling.

During cruise the IPRDS rotates at 28,300 rpm whilst the HPRDS rotates at 18,300 rpm. The

impellor continues to pump air (approximately 1/700th the density of oil), which is a potentially

useful attribute as this maintains a load on the HP bearings and gear teeth. However, the

differential speed slippage materializes as an estimated 10 horse power of heating within the

SAGB which will be removed by the application of impingement oil cooling over the external

surfaces of the coupling.

At present there are also unassessed risks associated with vibration effects, particularly the

unknown characteristics of the coupling during oil filling and emptying. A succession of

meetings has uncovered 70 risks (Ref. 13) of which the majority will be covered by

conventional design procedures.

Two rig tests and a Trent 900 demonstrator engine test were planned to deal with vibration, heat

generation and transient operating concerns.

Slow Speed Fluid Coupling

An alternative arrangement to that shown in Figure 6.6-46 is a proposal for a slow speed fluid clutch:

the two halves of the impellor are mounted onto the Angle Drive Shaft rather than the Radial Drive

Shaft, see Figure 6.6-46. This has the effect of dropping rotational speeds at MTO down to 15,000

and 10,000 rpm for the IP and HP impellors respectively, which matches more closely the speed limits

associated with the available test rig. Due to the slower speed, the coupling is required to transmit a

higher toque hence the characteristic dimensions of the coupling have to be increased from

approximately 8 inch to 10 inch outer diameter with a bowl diameter increase from 50mm to 70mm

diameter. This arrangement is proposed to mitigate risks concerning high cycle fatigue, acoustic

vibration and the predictability of fluid flow patterns as these attributes will be fully tested on the test

rigs. The low speed fluid clutch will give a higher level of confidence of the operating characteristics of

the clutch, both full and empty of oil, in advance of first engine run. The slow speed clutch requires an

extra HP layshaft gear within the SAGB, which effectively utilizes the space of the oil breather, which

is likely to return to its original position on the external gearbox for other reasons. The introduction of

the HP layshaft gear establishes a second gear ratio into the overall ratio between HP and IP shafts.

This allows the HP pinion gear to be reduced in diameter hence further reducing heat to oil within the

IGB.


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Figure 6.6-46 SAGB, low speed fluid coupling device

Wet Friction Plate Clutch

This is intended as the back-up design should the risks associated with the fluid coupling become

insurmountable or impractical to solve. Clutch plate technology is envisaged to be typical of present

day high performance motorbikes combined with oil pressurized piston actuator. A single plate friction

clutch of 8-inch diameter requires some 8000 lbs of end load in order to transmit 200ft.lb toque. A

multi-plate clutch requires a lower end load dependant upon the number of friction surfaces but will

require a method of maintaining plate separation whilst in the open condition during cruise. Concerns

relate to debris generation within the oil system together with FMECA issues regarding rubbing

surfaces in a hot oil environment (Ref. 14).

7.1.1 Accommodation of Failure cases

Considering the coupling device primary functions “transmit torque to HP shaft during start

procedure”, “Let IP and HP shafts be independent during normal operation” and “Allow engaging

during windmill relight” there are two main failure cases:

1. Coupling device remains disengaged when it should be engaged,

2. Coupling device remains engaged when should be disengaged

These failure cases have to be detected both during flight and on ground and an adequate strategy

has to be implemented within the control system to accommodate for these situations. The control

logic implemented on the Trent 1000 uses the shaft speed ratio N3/NI as a reference, considering it

equal to the starting gear ratio (2.23) when the clutch is engaged and different from this value in all

other situations.

The ECU will abort the starting procedure if a failure is detected on ground. In flight, engagement of

the clutch is not necessary for windmill relight, while a failed disengagement (if the system can not

reach his fail safe position “open”) will not result in an aborted starting procedure. This will result in a


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progressive deterioration of the coupling system until it burns, the heat released would not threaten

the engine integrity but could damage the SAGB.

7.1.2 Additional requirements for coupling devices

1. Consider power required to engage and control the coupling system

2. Consider heat generated by the coupling device when in “disengaged position”

3. Vibration effects on the system:

a. Disengaged

b. Engaged

c. Engaging transiently (filling the oil coupling device)

4. For multiple disk clutch provide a device that separates the disks when the clutch is not

engaged.

5. Accommodate for fail close / fail open and consequences on the component and on the

engine.


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References

Ref. 1 D715Z004-01 Specification for Trent 1000 Commercial Turbofan Engine on 787 Aircraft

Ref. 2 Boeing 7E7 / Rolls Royce Coordination memo 04-40-6001; rev. BA2; Oct. 2004

Ref. 3 “Power offtake effects on Multi-spool Gas Turbine Engines”; Arthur Rowe

Ref. 4 “Rb262-58 (Trent 1000) HP/IP P/O drive system – Concept design Report”; Alan Maguire;

Technical report-TIA42553.

Ref. 5 “Starting of three shaft aero engines by IP drive”, J. Towe, Technical report – PDR95040.

Ref. 6 “IP starting – The Ansty experience and implications for aero engine starting”; A. Rowe;

Technical report - PTR90627

Ref. 7 “Trent 1000 TGT ground starting”; D. Elysee; CE Highspots 13/07/2011

Ref. 8 “ An Approximate Model for the Characteristics of a Fluid Coupling”; N. Fomison; FSG

49039; Iss. 2, Jan 2005

Ref. 9 “ A CFD Investigation of Fluid Flow and Heat Transfer in a Fluid Coupling”; Z. Sun;

Thermo-Fluids UTC Report TFSUTC/2004/4, May 2004.

Ref. 10 “Trent 1000 - Package B VSV Schedule Development and Performance Effect Of

Production Schedule”; Dave Brown; PTR109906; June 2010

Ref. 11 “Trent 1000 – Idle Performance”; M. Pons Perez, Technical report - PTR109631

Ref. 12 “Trent 1000 Ground Idle Performance Requirements for Controls”; Peter Gillard;

Technical report PTR 109018

Ref. 13 “IP Power offtake presentation pack April 2004”; R. Clough and A. Swift.

Ref. 14 “Trent 7E7 HP-IP Coupling Design Rationale version 3”; D. Knott; Technical report.

Ref. 15 “IP power offtake on Trent 1700”; presentation pack.

Ref. 16 “Trent 1700 trade study – IP Vs. HP P/O”; C. Fouche M. Roquelle; Technical report – TIA442901. Ref. 17 “Functional Operability Starting Strategy on Trent 1000”; D.T.Hayes ; DNS 154945 Ref. 18 “Trent 1000 IPC stability audit prior to IPC aero rig test and first engine run”; B. Zewde; Technical report – PTR109049 Ref. 19 “Trent 1000 Performance requirements for control – Handling Bleed Valves for M2.0

cert software and Secondary Air System Bleed Logic”; B. Zewde, Technical report – PTR 109026

Ref. 20 “R-R Trent XWB Power offtake and speed range status”; Stephane Demoulin; Airbus

presentation pack.

Ref. 21 “Trent 1000 HPC stability audit – First Flight Readiness”; M. Dann; Technical report –

PTR109402

Ref. 22 “Trent 1000 IPC stability audit post IPC aero rig 823/1”; B. Zewde; Technical report – TIA

455307

Ref. 23 “A350 Trent XWB Power-off-Take”; N. Feuillard J.L.Rivot; Technical report (Airbus).

Ref. 24 “Trent 1000 ESS Anti-icing system worst case design points”; B. Zewde; Technical

report - PTR 109009

Ref. 25 “Trent 1000 Engine Secondary Power Extraction” , N/A, Internal Memo

Ref. 26 “Horse power extraction limits Boeing 7E7 / Rolls-Royce coordination memo”; B. Kaku;

coordination memo 04-40-6001

Ref. 27 “Trent 1000 IP Power offtake concept for Boeing 787”; G. Knight; internal memo

Ref. 28 “Starter / Generator and Drive Train Dynamic Loading Characteristics”; D.Luxton;

Lessons learnt log; Source Reference: DEI 1183

Ref. 29 “Dynamic factors on stepped electrical loads”; M Mountney / D Scothern; Lessons learnt

log


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Ref. 30 “High surge torques on IP driven power offtakes”; M Mountney / D Scothern; Lessons

learnt log

Ref. 31 “Transmission and structures, Gearboxes, Lesson 636”; Lesson Learnt log

Ref. 32 Capability Intranet – Fluid systems

Ref. 33 “Trent 1000, Sub Systems Requirements Document for the secondary air system”; C.

P. Gravett; Technical report - FSG49004.

Ref. 34 “Air System Familiarization”; Paul Ferra; Internal report

Ref. 35 “Guide to Transient Performance”; A. Rowe; Performance Engineers Guide

Ref. 36 “Gas turbine theory”; H. Cohen H. Saravanamuttoo; Prentice Hall ed.

Ref. 37 Capability Intranet - System engineering

Ref. 38 Rolls Royce Quality Management Process; Intranet Capability

Ref. 39 “Product systems Integration and System Engineering”; Tony Bell; 15/06/09 Internal

meeting on product system integration

Ref. 40 "Trent 1000 Summary of oil and fuel system development problems and solutions";

R.E.Leese; Technical report FSG49755; Oct. 2009

Ref. 41 “Trent 1000: Study of Turbine Capacity Design Space Including an Assessment of the

Influence of Post Engine 4/3 Analysis Factors”, D J Sherwood, PTR109429

Ref. 42 "IPT Capacity for IPC Operability", Paul Richardson, PTR109333

Ref. 43 "Bleed Valve Status for 787-8 Package B Noise Certification", E. Spalton, Rolls-Royce

ECM 10-00-7078 RR0

Ref. 44 "Summary FMECA for Trent 1000 Package B Engine Cooling Air System", M. Krohn,

DNS 156082

Ref. 45 "Summary FMECA for Trent 1000 Package B. External Gearbox", P. Russel, DNS 156086

Ref. 46 "Summary FMECA for Trent 1000 Package B Intercase and Internal Gearbox", S.

Morton, DNS 156118

Ref. 47 "Summary FMECA for Trent 1000 Package B. Step-Aside Gearbox." P. Russel, DNS

157135

Ref. 48 “Trent 1000 Performance requirements for control – Handling Bleed Valves for M2.0

cert software, M2.2.2 flight software and Secondary Air System Bleed Logic”, Braky Zewde,

PTR109026 & PTR109235, Jul 2007

Ref. 49 “Trent 1000 - Package B VSV Schedule Development and Performance Effect Of

Production Schedule”, Dave Brown, PTR109906, June 2010

Ref. 50 “Trent 1000 Performance Requirements for Control – VSV ATF Software M1.0.1”; Mark

Stockwell, PTR 109039, Dec.2005

Ref. 51 “Trent 1000 performance requirements for controls –SASV fault detection and

accommodation”; Marc Pons Perez; PTR 109059; March 2009

Ref. 52 “Trent 1000 – Acceptability of Bleed Valves Delivered for Flight Compliance

Programme”; D Hulme; PTR 109627; Apr 2009

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