aircraft systems - lec 3

7/29/2019 Aircraft Systems - Lec 3

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Resource personAli Khalid

Department of Aviation Management & Technology

Superior University, Lahore.


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By the end of this session , you will be able to:

Understand the basics of flight control system

Understand the aircraft actuation system.

Know about the advanced actuation concepts.


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All set for systems? now lets start then

Flight Control System Flight controls have advanced considerably throughout

the years.

In the earliest biplanes flown by the pioneers flight

control was achieved by warping wings and controlsurfaces by means of wires attached to the flyingcontrols in the cockpit.


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Figure shows the multiplicity of rigging and control wires onan early monoplane.

The use of articulated flight control surfaces followed soonafter but the use of wires and pulleys to connect the flightcontrol surfaces to the pilots controls persisted for many yearsuntil advances in aircraft performance rendered the technique

inadequate for all but the simplest aircraft.


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When top speeds advanced into the transonic regionthe need for more complex and more sophisticatedmethods became obvious.

They were needed first for high-speed fighter aircraftand then with larger aircraft when jet propulsionbecame more widespread.

The higher speeds resulted in higher loads on the flightcontrol surfaces which made the aircraft very difficult tofly physically.


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To overcome the higher loadings powered surfacesbegan to be used with hydraulically powered actuatorsboosting the efforts of the pilot to reduce the physical

effort required.

This brought another problem: that of feel.


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A further complication of increasing top speeds wasaerodynamically related effects. The tendency of manyhigh-performance aircraft to experience roll/yaw

coupled oscillations commonly called dutch roll ledto the introduction of yaw dampers and other auto-stabilization systems.

The implementation of yaw dampers and auto-stabilization systems introduced electronics into flight

control. Autopilots had used both electrical and air-driven

means to provide an automatic capability of flying theaircraft, thereby reducing crew workload.


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While there are many identical features between the fighterand commercial airliner examples given above, there are alsomany key differences.

The greatest difference relates to the size of the controlsurfaces in relation to the overall size of the vehicle.

The fighter control surfaces are much greaterthan thecorresponding control surfaces on an airliner.

This reflects its prime requirements ofmanoeuvrabilityand

high performance at virtually any cost. The commercial airliner has much more modest control

requirements; it spends a far greater proportion offlying time in the cruise mode so fuel economy ratherthan ultimate performance is a prime target.


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The pilots manual inputs to the flight controls are madeby moving the cockpit control column or rudder pedalsin accordance with the universal convention.

And you know what the universal convention right???


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There are presently two main methods of connectingthe pilots controls to the rest of the flight controlsystem. These are:

Push-pull control rod systems. Cable and pulley systems.

An example of each of these types will be described

and used as a means of introducing some of themajor components which are essential for the flight

control function.


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The key element in the flight control system,increasingly so with the advent offly-by wire and activecontrol units, is thepower actuation.

Actuation has always been important to the ability ofthe flight control system to attain its specified

performance.

The development of analogue and digital multiple-control lane technology has put the actuation central toperformance and integrity issues.


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An analog or analogue signal is any continuous signalfor which the time varying feature (variable) of thesignal is a representation of some other time varying

quantity. for example, as resistance increases voltagedrops down as shown in the figure.


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In digital signal a continuous quantity is represented bya discrete function which can only take on one of afinite number of values. For example


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Simple mechanical actuation.

Mechanical actuation with simple electromechanical

features.

Multiple-redundant electromechanical actuation withanalogue control inputs and feedback.


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The attributes of mechanical actuation arestraightforward; the system demands a controlmovement and the actuator satisfies that demand with

a power-assisted mechanical response. The BAE SYSTEMS Hawk 200 is a good example of a

system where straightforward mechanical actuation isused for most of the flight control surfaces.


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For most applications the mechanical actuator is able toaccept hydraulic power from two identical/redundanthydraulic systems.

The obvious benefit of this arrangement is that fullcontrol is retained following loss of fluid or a failure ineither hydraulic system.

This is important even in a simple system as the loss of

one or more actuators and associated control surfacescan severely affect aircraft handling.

The actuators themselves have a simple reversion modefollowing failure, that is to centre automatically under

the influence of aerodynamic forces.


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Mechanical actuation may also be used for spoilerswhere these are mechanically rather than electricallycontrolled. In this case the failure mode is aerodynamic

closure, that is the airflow forces the control surface tothe closed position where it can subsequently have noadverse effect upon aircraft handling.


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The majority of modern aircraft use electrical signallingand hydraulically powered (electro hydraulic) actuatorsfor a wide range of applications with varying degrees of

redundancy.

The demands for electro hydraulic actuators fall intotwo categories: simple demand signals or auto-

stabilization inputs.


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Simple electrical demand signals are inputs from thepilots that are signalled by electrical means. For certainnon-critical flight control surfaces it may be easier,

cheaper and lighter to utilize an electrical link.

An example of this is the airbrake actuator used on theBae system 146; simplex electrical signalling is used and

in the case of failure the reversion mode is aerodynamicclosure.


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In general, those systems which extensively use simplexelectrical signalling do so for auto stabilization.

In these systems the electrical demand is a stabilization

signal derived within a computer unit. The simplest form of auto stabilization is the yaw

damper which damps out the cyclic cross-coupledoscillations which occur in roll and yaw known as dutch

roll.


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Modern flight control systems are rapidly movingtowards fly-by-wire solutions as the benefits to berealized by using such a system are considerable.

These benefits include a reduction in weight,improvement in handling performance andcrew/passenger comfort.

Concorde was the first aircraft to pioneer thesetechniques in the civil field using a flight control system

jointly developed by GEC (now BAE SYSTEMS).


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In the civil field the Airbus A320/A330/A340 and theBoeing 777 are introducing modern state-of-the-artsystems into service.


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A simplified block schematic diagram of a multiple-redundant electro hydraulic actuator is shown


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1. The solenoid valve is energized to supply hydraulicpower to the actuator, usually from two of the aircrafthydraulic systems.

2. Control demands from the flight control computersare fed to the servo valves.

3. The servo valves control the position of the first stagevalves that are mechanically summed before applying

demands to the control valves.

4. The control valves modulate the position of the controlram.


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LVDT measures the difference between 1st stageactuator and the control ram, these signals are fedback to FCS.


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The actuation implementations described so far have allbeen mechanical or electro hydraulic in function using

servo valves. There are a number of advanced actuation concepts

under development that may supplant the existingelectro hydraulic actuator. These may be categorized as:

Direct drive actuation. The Electro Mechanical Actuator (EMA). The Electro Hydrostatic Actuator (EHA).


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In the electro hydraulic actuator a servo valve requires arelatively small electrical drive signal, typically in theorder of 10 15 mA.

In the direct drive actuator the aim is to use an electricaldrive with sufficient power to obviate the need for theservo valve/first stage valve.

The main power spool is directly driven by torquemotors requiring a higher signal current, hence the termdirect drive.


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The electromechanical actuator or EMA replaces theelectrical signalling and power actuation of the electro-hydraulic actuator with an electric motor and gearbox

assembly applying the motive force to move the ram.


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In the EHA an electric motor in each actuator drives aself-contained hydraulic system comprising pump andreservoir which provides the motive force to power the

control surface to the demanded position. Once the control surface attains the demanded position

the system locks up and no further power is requiredwhile that control position is held.

This has significant potential for use in high-power/sustained load applications such as a fore-planeor stabilizer actuator, whereas an EMA would require

power to hold the control surface position.


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A common feature of all three new actuator conceptsoutlined above is the use of microprocessors to improvecontrol and performance.

The introduction of digital control in the actuator alsopermits the consideration of direct digital interfacing to

digital flight control computers by means ofdata buses(ARINC 429/ARINC 629/1553B).


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DIRECT DRIVES

They emphasizeconcentration upon thecontinued use of aircrafthydraulics as the powersource, including theaccommodation of system

pressures up to 8,000 psi.

EMA, EHA

On the other hand lendthemselves to a greateruse ofelectrical powerderiving from the

all-electric aircraft concept.


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Shows the generic example of the main control loops asthey apply to aircraft flight control, flight guidance andflight management


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The inner loop provided by the FBW system and thepilots controls effectively control the attitude of theaircraft.

The next outer loop is that affected by the AutopilotFlight Director System (AFDS) that controls the aircrafttrajectory, that is, where the aircraft flies. Inputs to thisloop are by means of the mode and datum selections onthe Flight Control Unit (FCU) or equivalent control

panel. Finally, the Flight Management System (FMS) controls

where the aircraft flies on the mission; for a civiltransport aircraft this is the aircraft route.


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aircraft systems - lec 3

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