Flying Controls-engineering

Page Mod 11.9 Issue 1

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FLYING CONTROLS

PRIMARY FLIGHT CONTROLS

Aircraft theory of flight has already been discussed in Module 11.1. We shall now look at how the Aircraft are equipped with moveable aerofoil surfaces that provide control in flight. Controls are normally divided into Primary and Secondary controls. The primary flight controls are: Ailerons Elevators Rudders Spoilers

Because of the need of aircraft to operate over extremely wide speed ranges and weights, it is necessary to have other secondary or auxiliary controls. These consist of: Trim controls High Lift Devices Speed Brakes and Lift Dump

Page 1 Mod 11.9 Issue 1

Note: There is some variation of opinion as to whether spoilers are considered to be primary controls. The EASA 66 syllabus includes them as primary controls, so that is how these notes will define them. Both types of controls are illustrated in the following diagram.

Typical Aircraft Flight Controls

Figure 1


AILERONS

Ailerons are primary flight controls that provide lateral roll control of the aircraft. They control aircraft movement about the longitudinal axis. Ailerons are normally mounted on the trailing edge of the wing near to the wing tip.

Inboard and Outboard Ailerons

Figure 2

Some large turbine aircraft employ two sets of ailerons. One set are in the conventional position near the wingtip, the other set is in the mid-wing position or outboard of the flaps. At low speeds both sets of ailerons operate to give maximum control. At higher speeds hydraulic isolate valves will cut power to the outer ailerons so that only the inboard ailerons operate. If the outer ailerons are operated at high speeds, the stress on the wing tips may twist the leading edge of the wing downwards and produce “aileron reversal”.

ELEVATORS

Elevators are primary flight controls that control the movement of the aircraft about the lateral axis (pitch). Elevators are normally attached to hinges on the rear spar of the horizontal stabiliser. Fig 11.1 shows the typical location for elevators.


RUDDERS

The rudder is the flight control surface that controls aircraft movement about the vertical or normal axis. Rudders for small aircraft are normally single structural units operated by a single control system. Rudders for larger transport aircraft vary in basic structural and operational design. They may comprise two or more operational segments; each controlled by different operating systems to provide a level of redundancy.

Rudder Figure 3

SPOILERS

Spoilers are secondary control surfaces used to reduce or spoil the lift on a wing. They normally consist of multiple flat panels located on the upper surface of the wings. The diagram below shows the more common configuration.


Operation of Spoilers on a Typical AircraftFigure 4

The spoilers lay flush with the upper surface of the wing and are hinged at the forward edge. When the spoilers are operated, the surface raises and reduces the lift. The spoilers may be used for different purposes.


Mod 11.9 Issue 1

TRIM CONTROLS

The majority of aircraft at some time during a flight develop a tendency to deviate from a straight and level attitude. This may be caused by a fuel state change, a speed change, a change in position of the aircraft's load, or in flap and undercarriage positions. The pilot can counter this tendency by continuously applying a correcting force to the controls - an operation, which, if maintained for any length of time, would be both fatiguing and difficult to maintain. The tendency to deviate is therefore corrected by making minor trim adjustments to the control surfaces. Once an aircraft has been trimmed back to a 'balanced' flight condition, no further effort is required by the pilot until further deviation develops.

FIXED AND ADJUSTABLE TRIM TABS

Fixed Trim Tabs

A fixed trim tab is normally a piece of sheet metal attached to the trailing edge of a control surface. It is adjusted on the ground by bending to an appropriate position that give zero control forces when in the cruise. Finding the correct position is by trial and error.

CONTROLLABLE TRIM TABS

Controllable Trim TabFigure 5

A controllable trim tab is adjusted by mechanical means from the flight deck, usually with an indication of its position being displayed to the pilot. Most aircraft have trim on the pitch control and more advanced aircraft have trim on all three axes. Whilst the controls in the cockpit are by lever, switch etc., the actuation can be by mechanical, electrical or hydraulic means.

SERVO TABS


Servo Tab Figure 6

Sometimes referred to as the flight tabs, the servo tabs are used primarily on large control surfaces, often found on larger, older aircraft. This tab is operated directly by the primary controls of the aircraft. In response to the pilot's input, only the tab moves. The force of the airflow on the servo tab then moves the primary control surface. This tab is used to reduce the effort required to move the controls on a large aircraft.

BALANCE TABS

Balance Tab Figure 7

A balance tab is linked to the aircraft in such a manner that a movement of the main control surface will give an opposite movement to the tab. Thus the balance tab will help in moving the main surface, therefore reducing the effort required. This type of tab will normally be found fitted to aircraft where the controls are found to be rather heavy during initial flight-testing.


ANTI-BALANCE TABS

These anti-balance tabs operate in the same way, mechanically, as balance tabs. The tab itself is connected to the operating mechanism so that it operates in the reverse way to the balance tab. The effect this has is to add a loading to the pilot’s pitch control, making it appear heavier. These tabs can often be found fitted to ‘stabilators’, which are very powerful and need extra ‘feel’ to prevent the pilot over-stressing the airframe.

SPRING TABS

The spring tabs, like some servo tabs, are usually found on large aircraft that require considerable force to move a control surface. The purpose of the spring tab is to provide a boost, thereby aiding the movement of a control surface. Although similar to servo tabs, spring tabs are progressive in their operation so that there is little assistance at slow speeds but much assistance at high speeds.

Spring TabFigure 8


FULLY POWERED FLYING CONTROL TRIM SYSTEM

As fully powered flying controls are irreversible, i.e. all loads (reactions) are fed via mountings to structure; trim tabs would be ineffective.

To overcome this, electric trim struts or actuators are used within the input system. These actuators commonly reposition the "null" position of a self-centring spring device to hold the control-input system in a new neutral position. Thus the main control surface will be held deflected and the aircraft trimmed.

TYPICAL TRIM SYSTEM

The following is a typical trim system as used on a fully powered flight control system.

RUDDER TRIM SYSTEM

In a typical rudder trim system for a powered system, trim commands from the trim switch causes an actuator to extend or retract, which rotates the feel and centring mechanism. This provides a new zero force pedal position corresponding to the trimmed rudder position. The trim switch is spring loaded to return to neutral. Both positive and negative elements of the circuit are switched to prevent a trim runaway should one set of switch contacts become short-circuited. The trim indicator is driven electrically by a transmitter in the rudder trim actuator. The indicator shows up to 17 units of left or right trim. Each unit represents approximately one degree of rudder trim.

AILERON TRIM SYSTEM

In a typical aileron trim system for a powered system, trim commands from the trim switches causes the actuator to extend or retract, which repositions the feel and centring mechanism null detent. The trim switches must be operated simultaneously to provide an electrical input to the actuator, as both positive and negative elements of the circuit are switched to prevent a trim runaway should one set of switch contacts become short circuited. The available aileron trim provides 15 degrees aileron travel in both directions from neutral.


TAILPLANE TRIM SYSTEM

For trimming the aircraft longitudinally (about the lateral axis) the elevators are not trimmed. Instead the angle of incidence of the whole tailplane is altered. Raising the leading edge of the tailplane will increase lift over the tailplane, which imparts a nose-down attitude to the aircraft or vice versa.

This is done by mounting the forward end of the tailplane on a screw jack. Depending on the system the screw jack is rotated by two hydraulic or electric motors via a gearbox. Movement is induced by a lever in the flight deck, which operates solenoid selector valves or an electric control circuit to operate the motors. Over-travel is prevented by micro-switch.

Reasons for fitting to transport aircraft:

1. All aircraft benefit from having as large a range of useable centre of gravity as possible. This gives flexibility in cargo loading and allows for fuel usage in a swept wing.

2. Aircraft benefit from a wide speed range. Very simply, when an aircraft is trimmed at a particular speed, a reduction in speed calls for "up" elevator and an increase in speed calls for "down" elevator. This would cause extra drag.

3. The need to compensate for centre of pressure changes due to slat/flap extension, gear extension.

4. To reduce trim drag to a minimum to give the optimum performance in cruise.


HIGH LIFT DEVICES

FLAPS

These devices have two primary aims, to provide extra lift during take-off and to provide greater lift as well as high drag during landing. The types of flap used on different aircraft depends on the type of aircraft, the method of aircraft operation and other variables. For example, a single engined light aircraft might only have some form of simple trailing edge flap, whilst a large airliner like the Boeing 777 has complex, triple slotted flaps.

Types of Flap System

Figure 12

Flaps are fitted to most aircraft and are usually one of the types shown, together with the maximum increases of lift over the 'clean' configuration. As the complexity increases to improve performance, there is a proportional increase in weight, maintenance and cost.


Whilst the term 'flaps' is used, it is taken as meaning trailing edge flaps, and the term 'leading edge flaps 'refers to those fitted to the leading edges of the wings of most large aircraft.

The methods of operation of flaps, are numerous. They can vary from simple, mechanical push rods or cables actuated, via a lever in the cockpit, by the pilot, to complicated, multiple flaps that are electrically selected on the flight deck and hydraulically or electrically powered.

Most flap systems have a number of positions, which can be selected at various times. As an example, five positions could be as follows;

00 - flaps up 250 - landing, first position

80 - take-off, first position 400 - landing, second position

150 - take-off, second position

These would all be selected by movement of a lever in the cockpit, which will have 'detents' at the various positions. This movement will, as can be seen in the illustration, be transferred to the control valve and on to the motor, which moves the actuators.

Flap Mechanism Figure 13


Other high lift devices can be found on the leading edges of the wings and include slats, drooped leading edges and Krueger flaps. All of these devices are aimed at smoothing the airflow over the leading edges of the wings when they are at a high angle of attack, thereby maintaining, or increasing lift when the wing would normally be stalled.

SLATS

Slats are separate small aerofoils, which can be fixed or retractable. Their purpose is to control the air passing over the top of the wing at slow speeds. On larger aircraft, the retractable slats have their extension interconnected with the trailing edge flaps.

This can be seen in the illustration, which not only shows the operation of the slats through three different positions, 'stowed', 'active' and 'open', but their association with the four positions of the trailing edge flaps.

Fixed slats are usually found on light aircraft, where the complications of weight, cost etc, can be balanced by the limitation of slightly higher drag than a 'clean' wing.

Leading and Trailing Edge Flap Settings Figure 14


DROOPED LEADING EDGES

Drooped leading edges are a different design, but are aiming at the same effect, that of smoothing the air over the top of the wing. They operate in much the same way as most high lift devices, by screw jack operation with the motive power for the jacks coming from the hydraulic system.

KRUEGER FLAPS

Krueger flaps are, again, a different design for the same effect. These are usually found fitted to the leading edges of the wing at the inboard sections where the effect of 'slats' or 'drooped leading edges' are not as efficient.

Figure 15

Krueger (left) and Drooped (right) Leading Edge Flaps


LIFT DUMP AND SPEED BRAKES

LIFT DUMPERS

These devices are used to spoil lift from the wing after touchdown. This ensures that the aircraft's weight is fully on its landing gear, which enables the brakes to work at 100% for the full landing run. If this did not happen, the aircraft would tend to 'float' or ‘bounce’ at touchdown, making the brakes inefficient and the risk of skidding much greater.

Lift dumpers are nearly always flat, rectangular panels, hinged at their leading edge and powered by hydraulics. They can usually be found on the top of the wing, and located about the maximum thickness, where their deployment would destroy the maximum lift from the wing.

To ensure that they deploy at the correct time and also without the need for the pilot to select them, at a very busy time, there is a simple system to deploy them automatically. A set of switches are fitted to the landing gear which 'make' and indicate weight-on-wheels to several systems, once the aircraft is completely on the ground. By giving the pilot a "lift dumper arming" button, he can arm the system, in flight, and know that it will deploy the lift dumpers at the correct time.

SPEED BRAKES

The use of speed brakes is similar regardless of the aircraft type. If the aircraft is a sailplane it is so streamlined that it requires high drag when descending and landing in unprepared fields. A large 400 seat airliner needs to be able to follow Air Traffic Control instructions to descend and maintain certain speeds and a military jet fighter needs to have very high drag on approach, permitting the engines to accelerate quickly if the landing is aborted.

All types of speed brake use a variation of the same principle, to put panels of varying shapes into the airflow, to increase the drag. Some are able to modulate, (vary the amount of drag to suit the situation), whilst others are just 'IN' or 'OUT'. Some airliners use the same surfaces on the top of the wing to carry out more than one operation, such as speed brakes when in flight and needing drag; roll control to augment (or replace) ailerons; or as lift dumpers to be used after landing.


Light aircraft rarely need speed brakes because of their generally high drag designs. A reduction in power will produce a satisfactory slowing down of the aircraft. Streamlined sailplanes, however, usually have vertical panels that project from the wing, top and bottom, which produce large amounts of drag, enabling steep, slow and safe approaches when landing.

Military jets have a different need for drag, not only as mentioned during the approach to landing, but during combat and other operations where fast application of drag with a quick reduction in speed can have a life saving effect.

Speed Brake InstallationFigure 16


SYSTEM OPERATION

MANUAL OPERATION

POWERED FLIGHT CONTROLS (P.F.C.U’S)

In large modern aircraft that fly at high speeds, the air loads on the flying control surfaces far exceed the ability of the pilot to move them manually. To overcome this problem hydraulic pressure is used to move the control surfaces, a POWERED FLYING CONTROL UNIT or BOOSTER being used to convert hydraulic pressure into a force exerted on the control surface.

In its simplest form, a P.F.C.U. consists of a hydraulic jack, the body of which is fixed to the aircraft structure and the ram, via a linkage to the control surface.

To control the P.F.C.U. a servo valve (control valve) is mounted on the jack. The servo valve, which is connected to the pilot's controls by a system of cables and/or pushrods, called the input system, directs fluid to either side of the jack piston and directs the fluid from the other side to return. This flow of fluid will displace the jack ram and as this is connected to the control surface via an output system of pushrods or cables, the control surface is moved.


Figure. 17

INPUT SYSTEMS

Generally the input system of the powered flying control system is mainly a cable system with the related quadrants, pulleys and fairleads with the connections to the control column and the PFCU input lever by push rods. To guard against loss of control due to cable breaks the cable system is duplicated. All duplicated runs are routed separately through the aircraft to avoid one incident damaging both control runs. The cable systems meet at a common input lever to the PFCU'S.


Input SystemsFigure 21

MECHANICAL & ELECTRICAL FLIGHT CONTROL SYSTEM

MECHANICAL CONTROLS


Most aircraft use conventional mechanical controls to move the flight controls. These will normally consist of cables, chains and control tubes. Many examples of this type of system have been described and illustrated previously. The ailerons and elevators on this type of system would normally be operated by a conventional control column and control wheel. Operation of this is instinctive to the pilot, the control wheel being rotated to the left to bank left and right to bank right. Pushing the control column forwards causes the aircraft to dive and pulling back causes the aircraft to climb. A typical control wheel and other cockpit controls is illustrated.

Figure 33

FLY BY WIRE


INTRODUCTION

Fly by wire is used on some aircraft to operate the controls. Instead of a conventional mechanical link between the pilot’s controls to the control surfaces or powered control servo valves, the link is by an electrical or fibre-optic cable. The abbreviations Fly by wire (FBW) or Fly by Optical Wire (FBOW) are used.

PRINCIPLES OF FBW

FBW is a control system that receives inputs directly by electrical signals. The flying control actuators are electro-hydraulic design converting electrical signals into movement of a hydraulic ram.

Many systems on the aircraft use electrical signals to automatically control the flight path. It is a natural development to integrate the pilot’s input with these automatic controls. Correcting signals can be sent directly to the control actuator as well as those sent by the pilot.

PRINCIPLES OF FBOW

An optical fibre cable consists of multiple glass fibres, each about as thick as a human hair. The cable can carry pulses of light without amplification and without electromagnetic interference. One fibre can carry over 9,000 simultaneous signals.

Fibre optics transmits information using:

A light source modulated with information

A fibre optic transmission medium (cable)

An optical receiver to de-modulate the information

ADVANTAGES OF FBOW OVER FBW

Increased amount of information can be passed

Increased speed of transmission

Lighter in weight

OTHER INPUTS TO POWERED FLYING CONTROL UNIT

The pilot is the main controller of the aircraft controls. There are, however, other inputs as follows:

Auto-stabilisation (variable incidence tailplane)


Datum shift caused by operation of landing gear. The system can automatically make an input to the PFCU when the gear is lowered or raised.

Mach trim will deflect the tailplane or elevators to compensate for changes in aircraft attitude at high Mach Numbers due to rearward movement of the centre of pressure

Autopilot will be interfaced directly with the PFCU’s.

Terrain Following Radar (TFR) – The system can process information on radar or radio height to the PFCU.

Inertial Navigation System (INS)

Instrument Landing System (ILS) – programmed automatic landing sequences can be fed directly into the control system.

Airspeed – The aircraft engines can also be controlled to give fully automatic programmable airspeed.

Position of other controls, including secondary controls such as flaps and LE flaps and slats.

777 FLIGHT CONTROLS - INTRODUCTION

GENERAL

The flight controls keep the aeroplane at the desired attitude during flight. They consist of movable surfaces on the wing and the empennage. The flight controls change the lift of the wing and the empennage.

There are two types of flight controls: the primary flight control system and the high lift control system.

777 PRIMARY FLIGHT CONTROL SYSTEM

The primary flight control system (PFCS) uses a fly-by-wire control system with digital and analogue electronic equipment. It receives commands from the flight crew and the autopilot and causes the control surfaces to move.

The PFCS controls the attitude of the airplane during flight. The control surfaces operated by the PFCS are:

One aileron on each wing

One flaperon on each wing

Seven spoilers on each wing

One horizontal stabiliser

One elevator on each side of the horizontal stabiliser

One tabbed rudder.


HIGH LIFT CONTROL SYSTEM

The high lift control system (HLCS) uses a fly-by-wire control system with digital electronic equipment. It receives commands from the flight crew and causes the flaps and slats to move.

Operation of the HLCS increases the wing lift so the aeroplane can takeoff and land at lower speed and higher weight. The high lift devices operated by the HLCS are:

Seven leading edge slats on each wing

One Krueger flap on each wing

One single slotted outboard flap on each wing

One double slotted inboard flap on each wing.

Operation of the HLCS also causes the ailerons and the flaperons to move. They droop on both wings when the high lift devices extend.

BENEFITS OF THE FLY-BY-WIRE SYSTEM

The fly-by-wire design of the flight controls permits:

A more efficient structure design

Increased fuel economy

A smaller vertical fin

A smaller horizontal stabiliser

Reduced weight

Improved controls and protections.

ABBREVIATIONS AND ACRONYMS

ACE· actuator control electronics

ACMS· aeroplane condition monitoring system

ADIRS· air data inertial reference system

ADIRU· air data inertial reference unit

ADM· air data module

AFDC· autopilot flight director computer

AFDS· autopilot flight director system

AIMS· aeroplane information management system

ARINC· Aeronautical Radio, Inc.

BAP· bank angle protection

B/D· backdrive


CMCS· central maintenance computing system

CPU· central processing unit

EDIU· engine data interface unit

EHS· electro-hydraulic servo valve

EICAS· engine indication and crew alerting system

FCDC· flight controls direct current

FMCS· flight management computer system

FSEU· flap/slat electronics unit

HLCS· high lift control system

LIB· left inboard

LOB· left outboard

LVDT· linear variable differential transformer

MCP· mode control panel

MFD· multi functional display

PCU· power control unit

PDU· power drive unit

PFC· primary flight computer

PFCS· primary flight control system

PMG· permanent magnet generator

PSA· power supply assembly

PSEU· proximity sensor electronic unit

RIB· right inboard

ROB· right outboard

RVDT· rotary variable differential transformer

SAARU· secondary attitude air data reference unit

SOL· solenoid

SOV· shutoff valve

STCM· stabiliser trim control module

TAC· thrust asymmetry compensation

WEU· warning electronic unit

WOW· weight on wheels


PRIMARY FLIGHT CONTROL SYSTEM - INTRODUCTION

Purpose

The primary flight control system (PFCS) controls the aeroplane flight attitude in relation to the three basic axes:

Longitudinal

Lateral

Vertical.

777 Primary Flight Controls

Figure 51


Roll Control

The roll control uses the ailerons, flaperons, and spoilers to control the aeroplane attitude about the longitudinal axis. During a bank of the aeroplane, the aileron and flaperon on one wing move in an opposite direction from the aileron and flaperon on the other wing. The spoilers move up only on the down wing and do not move on the up wing.Pitch Control

The pitch control uses the horizontal stabiliser and the elevator to control the aeroplane attitude about the lateral axis. The stabiliser controls long term pitch changes. The elevator supplies short term pitch control.Yaw Control

The yaw control uses the rudder to control the aeroplane attitude about the vertical axis. The rudder has a tab, which moves to increase the effectiveness of the rudder.Speedbrakes

The PFCS also includes the speedbrakes. In addition to roll control, the spoilers also act as speedbrakes in the air and on the ground. They deploy on both wings to increase drag and to decrease the amount of lift the wings supply.

PFCS – GENERAL DESCRIPTION

The pilots or the autopilot commands control the PFCS. The pilots can override the autopilot.

MANUAL OPERATION

Position transducers change the pilots' manual commands of the control wheel, the control columns, the rudder pedals, and the speedbrake lever to analogue electrical signals. These signals go to the four actuator control electronics (ACEs). The ACEs change the signals to digital format and send them to the three primary flight computers (PFCs).

The PFCs have interfaces with the aeroplane systems through the three flight controls ARINC 629 buses. In addition to command signals from the ACEs, the PFCs also receive data from:

The airplane information management system (AIMS)

The air data inertial reference unit (ADIRU)

The secondary attitude air data reference unit (SAARU).

The PFCs calculate the flight control commands based on control laws and flight envelope protection functions. The control laws supply stability augmentation in the pitch and yaw axes and flight envelope protections in all three axes. The digital command signals from the PFCs go to the ACEs.


The ACEs change these command signals to analogue format and send them to the power control units (PCUs) and the stabiliser trim control modules (STCMs). The ACEs and the PCUs form control loops, which control the surfaces based on the PFCs commands.

One, two or three PCUs operate each control surface. One PCU controls each spoiler, two PCUs control each aileron, flaperon, and elevator, and three PCUs control the rudder. The PCUs contain a hydraulic actuator, an electrohydraulic servo valve, and a position feedback transducer.

When commanded, the servo valve causes the hydraulic actuator to move the control surface. The position transducer sends a position feedback signal to the ACEs. The ACEs then stop the PCU command when the position feedback signal equals the commanded position.

Two STCMs control hydraulic power to the motors and brakes of the horizontal stabilizer.


Flying Controls-engineering

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