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Engine Sizing & SelectionCopyright 2006 by Don Edberg

Summary

Engine Sizing & Arrangement Introduction Performance Requirements Engine Geometric Characteristics &

Placement

Airframe Integrators Motto

Blame it on propulsion. Barnaby Wainfan, NGC El Segundo

Propulsion IntegrationExtremely Important

A small shortfall in performance can addup to millions of dollars in increased fuelcosts

Airframe supplier may have to paypenalties for shortfalls

Engine Choices

Types Piston engine with propeller Turbine engine with prop = Turboprop Turbojet Turbofan (low or high bypass ratio) Pulsejet Ramjet Rocket

Piston Engine

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Piston Engines

Inexpensive Best fuel economy Relatively heavy Vibration issues with intermittent

combustion process Performance decrease with altitude

Solved with turbocharger or supercharger

Turbojet Engine

Burner

Compressor Turbine

NozzleShaft

Inlet Diffuser

Afterburners

AKA reheat Pour fuel into rear of engine and burn it Get more thrust Get empty tanks fast (higher SFC)

Turbojet With Afterburner

Burner

High-Pressure

CompressorHigh-Pressure

Turbine

Afterburner Flameholders Nozzle

Afterburner

Low-Pressure

Compressor

Low-Pressure

TurbineInlet

Afterburner Fuel Injectors

Low Bypass Ratio Turbofan

FanBurner

High-Pressure

Compressor

High-Pressure

Turbine

Low-Pressure Turbine

Afterburner

NozzleLow-Pressure Compressor

Bypass Duct

Bypass ratio = 0.2 - 1.0, TSL/Weng = 6 10,TSFCDry = 0.8 - 1.3, TSFCWET = 2.2 - 2.7

High Bypass Ratio TurbofanFan

Burner

CompressorHigh-Pressure Turbine

Low-Pressure Turbine

Nozzle

Bypass ratio = 2.0 - 8.0, TSL/Weng = 4 6, TSFC = 0.5 - 0.7

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Pulsejet Engine Rocket-Powered Aircraft

Limitations

Performance of all engines limited bythermodynamics

Exhaust temperature must not damageengine

Usually run lean using excess air forcooling

Turbines use active blade cooling Thrust determined by mass flow, density

drops with altitude (no issue for rocket)

Thrust vs. Speed & Altitude(left: dry; right: afterburning)

0

1000

2000

3000

4000

5000

6000

7000

8000

0 0.2 0.4 0.6 0.8 1 1.2

Mach Number, M

Th

rust,

T, lb

s Sea Level

10,000 ft

20,000 ft

30,000 ft

40,000 ft

50,000 ft

0

1000

2000

3000

4000

5000

6000

7000

8000

0 0.2 0.4 0.6 0.8 1 1.2

Mach Number, M

Th

rust,

T, lb

s

Sea Level

10,000 ft

20,000 ft

30,000 ft

40,000 ft

50,000 ft

Figures of Merit

HP per pound (higher is better) Specific fuel consumption

SFC in terms of HP or thrust per weight of fuel Typically in terms of lb thrust/(lb fuel/h)

watch units for range & endurance calcs Equivalent for propped engines (delivered

power per fuel weight) Lower is better

Power Available vs. PowerRequired, Prop Aircraft

0

50

100

150

200

250

0 20 40 60 80 100 120 140 160

True Airspeed, knots

Po

we

r A

va

ila

ble

an

d P

ow

er

Re

qu

ire

d,

ho

rse

po

we

r

Power Available

Power Required

VmaxV for minimum

Power Required

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Power Available vs. PowerRequired, Jet Aircraft

Engine Selection

Criteria: Cruise speed Cost Economy (fuel, maintenance, etc.) Redundancy etc.Engine data on Internet, AV Week Source

issue, Janes, etc.

Requirements Size Engines

X(X)Maximum SpeedXSpecific Excess PowerXSustained TurnXXMinimum Rate of ClimbXXTakeoff Length

MilitaryCivilItem

Constraint diagram provides required T/W Estimated weight W provides T = (T/W)W

Engine Selection

Rubber engine Use an engine deck for performance

prediction (ref: AIAA competition history) High cost of engine development

Existing engines Search information sources for off-the-shelf

engines with sufficient performance No engine development costs Already in maintenance stores?

Scaling An Existing Engine

L = Lactual(SF)0.4

D = Dactual(SF)0.5

W = Wactual(SF)1.1

SF = scale factor(Raymer 10.1 - 10.3)

Engine GeometricCharacteristics (Raymer)

Non-afterburning and afterburning sizing dataequations 10.4 - 10.15, Raymer

Diameter, engine length,weight, SFC all arefunctions of takeoff thrust T and Mach no. M

Other inlets and ducts as neededBoundary layer divertersAfterburners?Add to your aircraft drawing

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Engine Nacelle Drawing Integration with Airframe

Thrust or power level picks or scalesengine

Inlet air duct must be sized for airflow inALL conditions

Fuel lines Cooling Engine-driven accessories Installation and removal clearances,

mounting structure

Engine Placement Choices

Under wing on pylon (traditional) Aft fuselage side-mounted engines (DC-9,

717) Center fuselage engines (DC-10, 727) Over wing (Honda jet) Other configurations (White Knight, etc.)

Engine Placement Trades Locate nacelle(s) to be above or below

wing wake Consider structural weight of pylons, etc. FOD ingestion, etc. Weight & balance considerations Wing location, fuselage upsweep, etc. Service & maintenance

Local Flow Effects

Angle nacelle for local flow direction(calculate upwash or downwash asneeded)

Example: B-717 engines at rear of fuseare angled upward

Upwash Downwash

Inlets

Very important to engine performance Must provide enough air in all conditions Must diffuse (slow down) air to M = 0.4 ~

0.5 Want as much pressure recovery as

possible (best >90%) Geometry affects drag of aircraft

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Inlet Types

NACA duct (for aux air) Conical (SR-71) Normal shock or Pitot

(airliners) 2-D Ramp (F-14, F-15) Inlet applicability

summarized in RaymerFig. 10.13

Normal Shock Inlet

Geometry in Raymer Fig. 10.7 Lip radius very important No shock if subsonic Rotate front face or entire engine to

account for up/downwash from wing

Other Inlets

May be used for subsonic or supersonic Often use variable geometry

Adjust geometry so shock is swallowed orminimized

Mechanism must be reliable Isentropic flow desired, but typically get

some oblique shocks Raymer Figs. 10.8 to 10.11

Location of Inlets/Nacelles

Many choices (Raymer Fig. 10.14) Nose, chin, side, over/under wing,

over/under fuselage, wing LE, etc. Want clean air to be ingested Minimizing length minimizes losses CG considerations OEI control (one engine inoperative)

S-duct vs. Straight(L-1011 vs. MD-11)

Internal separation in S-duct vs. structural weightissues with pylon mount

Servicing buried engine must be more difficult

Inlet Design

Capture area estimated using mass flow Estimate area using Raymer Fig. 10.16 If mass flow not known, rule of thumb is:

mass flow = 26[D(ft)]2 = 127[D(m)]2where D is front face diameter.

Better to use isentropic compressible flowper Raymer equations 10.16, 10.17, 10.19

!

m

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Boundary-Layer Air Need to avoid BL air for

better performance Use a diverter (Fig.

10.21) Diverter must be

integrated with inletlocation

Diverter must workeffectively at all angles ofattack

Space for BL air to bypassengine

F-35 Has No BL Diverters

Nozzle Integration

Nozzle must (or should) expand exhaustgases and accelerate them

Depends on mass flow: often use variable-area nozzle

Affects drag Lots of info Raymer pp. 257-8 Cooling also required, Fig. 10.24

Installed Jet Thrust Manufacturer data uses perfect inlet, exhaust,

etc. Losses due to:

actual inlet, air bleed, power extraction, actualexhaust nozzle, air temperature

Aerodynamic losses:drag of inlets, nozzles, trim drag due to changein thrust

Brandt suggests: installed T = 0.8Tmfr,installed SFC = SFCmfr/0.8

May be offset by engine improvements

Engines Mounted on Fuselage

Propellers

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PropellerTypes

One-Bladed Propeller

PropBladeAngles

Propellers

Props Helical speed = (Vtip2 +V2)1/2= (2R2 +V2)1/2

Inflow angle changes with velocity sovariable pitch props used for maximumefficiency

Propeller Blade Angles

VariablePitch

Propeller

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Prop Efficiency

Efficiency typically depends on advanceratio J and power coefficient Cp

J = V/nD Cp = P/n3D5

Can get propeller maps and find sweetspot

Corrections for fixed pitch Raymer Fig.13.13

PropellerEfficiency

Chart

Advance Ratio J

Pow

er c

oeffi

cien

t

Propeller efficiency dependson:Power levelRPMBlade pitch Dimensionless numbers areadvance ratio J and powercoefficient CpChoose pitch and RPM formax efficiency (eta)

Prop Configurations

Pusher allows shorter fuselage = less drag Pusher reduces efficiency because of

disturbed airflow over prop (= noise) Longer landing gear required

Other Propeller Notes

Wing-mounted engines require larger tailsfor OEI control

Rubber piston engine equations inRaymer Table 10.3, 10.4

Cooling vitally important

Fuel Considerations

Fuel System Tanks contain fuel Types = discrete, bladder, integral Volume depends on required fuel volume

(approx. density is 7.5 gal/ft3) Density varies with temperature (Raymer Table

10.5) Stow in wing or fuselage or tail or all Fuel CG must average near aircraft CG Calculate CG movement, show on CG plot

(Raymer Fig. 10.27) Pumps needed in certain cases

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CG Travel Diagram Valuable Info in Raymer App. E

Contains curves from engine decks Based on Mattingly et al Aircraft Engine

Design (good ref.)