Aeronautical Institute of Bangladesh (AIB)
Aeronautical Institute of Bangladesh (AIB)
Aircraft Flying Control System
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 1
Aeronautical Institute of Bangladesh (AIB) SECTION-1 FLIGHT CONTROL SURFACES 1. To familiar with the introduction of Flight Control System 1.1 The principle use of flight control is to maneuvers the ac from one flight condition to another, working against stability which tries to maintain the original flight conditions. A very stable aircraft is therefore, likely to be heavy and slow to respond to control movement. Usually the control surface does not directly cause the change of flight condition. This change the attitude of the aircraft, which cause changes in the size or direction of the main aerodynamics forces e.g. raising the elevator causes a nose up, tail down pitching movement which increases the angle of attack of the wings, which causes an increase in lift force and the aircraft climbs (if the airspeed remain unchanged). By hinging control surfaces behind main aerofoil surfaces and deflecting them upward and downward, the camber of the main aerofoil surface is effectively changed. This change of camber changes the effective chord line and hence the angle of attack of the whole main aerofoil surface is changed. This change of angle of attack changes the resultant lift force produced by the main aerofoil surface thus allowing the aircraft to manoeuvre. A control surface movement causes the aircraft to rotate about the aircraft C. G. by applying a movement equal to the change of lift. Force X distance from the C.G. The size of this moment (and therefore the speed of the aircraft movement) depends on:
1.2
a.
Distance of control surfaces from the aircraft C.G.
b.
Magnitude of control surface deflection
c.
Air density
d.
Aircraft air speed
e.
Control surface area
Aircraft control surfaces are categorized under 3 groups, they are: a.
Main or Primary group.
b.
Secondary group.
c.
Auxiliary group.
2. Understand the purpose of Flight Control Surfaces Purpose: The purpose of the flying control in to enable the aircraft to be maneuvered about major axis.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 2
Aeronautical Institute of Bangladesh (AIB)
3. Related Terminologies 3.1 Three Major Axis: The major axis of the aircraft are the 3 imaginary lines (each are) passing through the centre of gravity. a.
Longitudinal Axis: This is am imaginary line running force and aft of the fuselage through the centre of gravity.
b.
Lateral axis: This is an imaginary line running span wise through the centre of gravity.
c.
Vertical or Normal Axis: This is an imaginary line running vertically through the centre of gravity.
Fig 3-1 Diagram on 3 Axes 3.2 Centre of Gravity: This is a point on the aircraft through which the total weight of the aircraft acts. 3.3 Centre of Pressure: This is a point on the aircraft through which the total pressure (lift) of an aircraft acts. 3.4 Control surfaces: aircraft flying control surfaces are moveable aero foils used to change the altitude of the aircraft during flying. 4
Understand aircraft Flight Control Surfaces The aircraft control surfaces are divided into 3 groups as bellows
4.2 Primary Group of Control Surfaces 4.11.
Aileron
4.1.2
Elevator
4.1.3
Rudder
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 3
Aeronautical Institute of Bangladesh (AIB) 4.3 Secondary Group of Control Surfaces 4.2.1 Tabs: Tabs are small hinged aerofoil surfaces attached at the trailing edge of the primary control surface. 4.3.1
Types of tab: There are five types of tabs. They are
4.3.1.1 Balance tab 4.3.1.2 Spring tab 4.3.1.3 Servo tab 4.3.1.4 Trim tab i. Fixed trim tab ii. Controllable trim tab 4.3.1.5 Trim balance tab. Note. There is a special tab called anti-balanced tab for special purpose. 4.4 Auxiliary group of control surfaces: Classification of control surface: There are two types of auxiliary group of control surfaces. They are, 1. Lift increasing device 2. Lift decreasing device 1. Lift augmenting devices: there are three lift augmenting devices. They are, a. Flap (trailing edge flap) b. Leading edge flap c. Slat (leading edge device) 2. Lift spoiling devices: There are three lift spoiling devices. They are, a. Flight spoiler b. Roll spoiler c. Ground Spoiler d. Speed Brakes
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 4
Aeronautical Institute of Bangladesh (AIB) SECTION-2 PRIMARY GROUP OF CONTROL SURFACES 5. Understand the aileron control 5.1 Ailerons: These are the control surfaces attached at the trailing edge of each wing near wing tip and used to control the aircraft about its longitudinal axis. The lateral control is done by using ailerons is known as roll control. 5.2 Principle of aileron operation: The ailerons are rigged in such a way that if one aileron deflected upward the others aileron in opposite wing deflected downward and vice versa. 5.3 Roll Control: The ailerons are moved by means of a control wheel in the cockpit. If the control wheel is moved to left, then the left aileron moved up the upcoming aileron spoils left so causing this wing to drop. At the same time the opposite aileron moves down and it improves the camber on its respective wing, causing it to increases lift have this wing will rise.
Fig. 5-1 To have effective roll control now a days in high speed modern wide body aircraft empty two sets of ailerons at the trailing edge of each wing. The ailerons are known as: I)
All speed ailerons – Both inboard ailerons
II)
Low speed ailerons – Both outboard ailerons
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 5
Aeronautical Institute of Bangladesh (AIB)
Fig. 5-2 Dual aileron control When aircraft speed increases the effectiveness of deflected aileron also increases due to the following reasons. I)
Increase of air load on the ailerons due to high speed.
II)
Use of roll spoilers due to which banking effectiveness increases. All speed and low speed ailerons: With increasing forward speed of aircraft, it becomes necessary to deflect the flying control less. This means that at high speed, very small control column movement in needed, and control column become so sensitive
Fig. 5-3 5.4 Adverse yaw & its preventive measure: Whenever aileron are operated for the purpose roll control, the up going aileron defect in to the zone low pressure area whilst the down going aileron defects into a zone of high pressure area. The aerodynamic load on the ailerons is unequal. Thus producing a couple about the normal axis, tending to ‘YAW’ the area plane towards down going aileron side, which is not wanted by the operator? This is known as “Adverse yaw” (unwanted yawing). This adverse yaw is prevented by the following configuration change: 5.4.1
Differential aileron deflection: Whenever cockpit input is operated to have the roll control, the up going aileron is deflected more than the down going one to equalise the
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 6
Aeronautical Institute of Bangladesh (AIB) aerodynamic reaction. This is mechanically arranged system to achieve this differential control, generally employed in different aircraft.
Fig. 5-4 Diff. Ail. Control 5.4.2
Frise-type aileron: So moved after an aerodynamic expert L.G. FRISE in 1920’s.
This arrangement in build with an extended nose of such dimension that when aileron is deflected up the nose of the aileron protrudes below the wing surface (high pressure zone) thus drag is increased on this wing and equalizes the aerodynamic reaction on both the wings. Both of the arrangement function on the same principle and opposes the unwanted yaw due to unequal forces.
Fig.5-5 Frise type aileron control
5.5 Aileron lock out mechanism A gust lock on an aircraft is a mechanism that locks control surfaces in place preventing random movement and possible damage of the surface from wind while parked. Gust locks may be internal or external.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 7
Aeronautical Institute of Bangladesh (AIB) Understand the Elevators Control 6. Elevator: Elevators are the primary control surfaces attached at the trailing edge of horizontal stabilizers. Elevators are operated from the cockpit with the help of control stick. 6.1 Principles of Elevator Operation: When input is given from the cockpit both the elevators will be deflected up or down simultaneously depend upon selection. 6.2 Pitch Control: The longitudinal movement of the aircraft about laterel axis is known as pitch control. The actual displacement being identified by the dissection of the aircraft nose which is subsequently deflected such as pitch-up (climbs) pitch down (dive). Whenever control stick in the cockpit moved forward both the elevators are deflected down. Thus increasing tail plane comber, hence increasing lift and tail up so nose down. Whenever the control stick is moved rearward, both the elevators deflected up and spoils tail plane lift causing the tail to drop down so aircraft nose moves up.
Fig. 6-1 Pitch Control 6.3 Moveable Horizontal Stabilizer (M.H.S): A medium to large modern aero plane all moving tail arrangements are installed to have better output with minimum possible drag in controlling the pitch movement of the aircraft. The purpose of this arrangement is to achieve more effective output by less deflection of lager surface. To get required range of pitch movement by operating elevators, it is required to be deflected more so drag on the surface is increased. But to achieve some output of pitch movement with the help of M.H.S. the surface in required to be deflected very less, hence total drag on the surface is considerably minimized. The operation of M.H.S. is done the cockpit trimming wheel or by electrical switch. Generally, trim wheel is installed on the pedestal between two pilots and electrical switch is installed on the outer horn of each control wheel. The name of the switch is rockers as by operating each of the rocker switch M.H.S. is controlled electrohydraulically. 6.4
Mach Trim
During high speed flight, an aircraft is subject to certain changes in control and stability. One such change is the rearward movement of the wings centre of pressure causing a nose down pitching moment. This is commonly known as TUCK UNDER. At supersonic Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 8
Aeronautical Institute of Bangladesh (AIB) speed the use of trim tabs etc has little effect on the pitching movement of the aircraft. Mach trim can be achieved in two ways: one way is to effect the centre of gravity of the aircraft by using the fuel as the trimming medium. If the centre of gravity is to far forward giving a nose down pitching moment fuel can be transferred rearwards, thus restoring the aircraft to normal flight by raising the nose. This type of trim is normally carried out by flight crew, guided by instrumentation that indicates aircraft cg at all times. The more popular method of mach trimming is to use a mach trim system. From design studies, the point at which the pitch down conditions occur is known to the aircraft designers. Say for instance, it occurs in a certain aircraft at mach 0.85. By having sensors and drive motors coupled to the pitot statics system a signal will be generated to the mach trim system at mach 0.85. This then sends on signals to the automatic flight control system, which in turn will carry out small trim changes to the horizontal stabilizers or the all moving tail plane to counteract the tendency to tuck under. Understand the Rudder control 7. Rudder: Rudder is a vertically attached control surface hinged at the trailing edge of vertical stabilizers. Ruder is controlled by foot pedal in the cockpit. 7.1 Principle of Rudder operation: When input is provided from the cockpit to operate rudder, it moves towards right or left, depends upon selection. 7.2 Directional Control/Yawing Movement: The rudder is designed to provide yawing movement to the aero plane. Yawing movements is the turning of aircraft about vertical/normal axis. 7.3 Operation of Rudder: When input is given from the cockpit to move the aircraft nose towards right, then rudder will deflect towards right side hence comber nose will be created between the rudder and the fin, so air drag will increase aircraft tail will move to left hence nose to right and vise versa. Aircraft nose to post, yaw to post, and nose to starboard yaw to starboard
Fig. 7-1
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 9
Aeronautical Institute of Bangladesh (AIB) 7.1 Use of Rudder: 7.1.1
A small application during turning to help the aircraft to turn.
7.1.2 Application of rudder daring take-off and landing to keep the a/c straight. 7.1.3 Application of rudder on multi-engine aircraft to correct yawing caused by asymmetric power, if one engine fails. N.B: In each instance, the control surfaces are placed as per as possible away from the centre of pressure, so as to provide eficient leverage to alter position of aircraft easily. 7.5
Yaw Damper
A yaw damper is a device used on many aircraft (usually jets and turboprops) to damp (reduce) the rolling and yawing oscillations due to Dutch roll mode.[1] It involves yaw rate sensors and a processor that provides a signal to an actuator connected to the rudder. The use of the yaw damper helps to provide a better ride for passengers, and on some aircraft is a required piece of equipment to ensure that the aircraft stability remains within certification values. Dutch roll is a type of aircraft motion, consisting of an out-of-phase combination of "tailwagging" and rocking from side to side.
Fig. 7-2 7.6 Control Column and Rudder Bar: The control column and rudder bar in the cockpit allow the pilot to control the aircraft through the medium of the primary flying controls.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 10
Aeronautical Institute of Bangladesh (AIB) SECTION – 3 8. Understand the Secondary Flight Controls Tabs are the auxiliary control surfaces used to off-set the forces that tend to unbalance the primary control surfaces. a. Definition: Tabs are small aerofoil surfaces attached to the trailing edge of the primary control surfaces. b. Function: i) ii)
i) ii) iii) iv) v)
Tabs are operated to provide aerodynamic assistance to the primary control surfaces, when they are deflected. Particular types of tab also provide small correction to rectify minor flying fault. c. Principle of tab operation: Tab deflects opposite to primary control surface to which it is attached. d. Different types of tab: Balance tab Servo tab Spring tab Trim tab Trim balance tab
a. Balance Tab: Balance tab attached at the trading edge of primary control surfaces. The tab movement is geared mechanically in such a way that it moves opposite direction to the primary control surface movement when primary control surface is operated from cockpit thus, aerodynamic load primary on the tab assist the pilot to move the primary controls. Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 11
Aeronautical Institute of Bangladesh (AIB)
Fig-27-20 b.Servo Tabs: This type of tab generally attached with comperatively large type primary controls. In this arrangement primary control has no direct mechanical like with cockpit input; the cockpit mechanically connected with surbo tab only.
Fig-27-21 So, whenever cockpit inputs is provided to operate the primary control surfaces, servo tab gets deflect and this deflection of servo tab encounters aerodynamic load which will then force the main control surface to deflect and assisting the pilot. c. Spring tab: In this arrangement both primary control and tab are connected to cockpit control. The deflection of the spring tab is directly proportional to the aerodynamic load imposed upon the primary control surface; therefore at low speeds the spring tab remains in neutral position and primary controlled is direct manually controlled surface. At high speed, however when the aerodynamic load is great tab function as an aid in moving the primary control surface. The rod is spring loaded. The linkage is connected in a such a way that the movement of the primary control in one direction causes the spring tab to be deflected in the opposite direction. The more is the air load on the primary control the more is the winding of the spring of the surface starts moving. The higher the speed of the aircraft more in the angular movement of the tab in compressor to primary surface to assist the pilot. At high speed corresponding high aerodynamic load we have the assistance of spring tab.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 12
Aeronautical Institute of Bangladesh (AIB)
d. Trim Tab: Whenever the position of e.g. e.p change in level flight, the aircraft altitude in disturbed. Trim tab is used to rectify this disturbance, hence to allow the aircraft to be flown in a “hands off” condition to relieve the work load on the pilot. Trim tab may be two types. i)
Fixed trim tabs.
ii)
Controllable trim tab.
i. Fixed Trim tab: This type of tabs remain fixed at be trailing edge of primary control surface. This type of tab can only be adjusted by ground crews with speed tool & control be adjusted in the air. Adjust went of this tab depends upon flight report.
ii. Controllable Trim tab: Controllable trim has independent control ream with which tab can be controllable to trim the altitude of the aircraft. Trim means to correct any tendency of aircraft to move toward an undesirable flight altitude. Movement of the tab in one direction causes a deflection of the control surface in the opposite direction. Most of the trim tabs installed on aircraft are mechanically operated from the cockpit through the individual cable system. However some aircraft have trim tabs that we operated by an electrical actuator (motor). The trim deflection will cause a displacement of the cockpit input from its neutral position, and this is because of the displacement of primary control surface from its neutral position.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 13
Aeronautical Institute of Bangladesh (AIB)
iii. Use of trim tabs A trim tab is usually operated under the following situations: In multi-engine aircraft to off-set the effect of yaw due to engine failure. a. To off-set the yawing effect of cross winds. b. Trim may enable a problem maneuver to be negotiated specially during off. c. Trim will enable movements of the c.p. or c.6. during flight to be catered. d. Trim will allow for individual flying characteristics due of possible structural imperfections & alteration deflection on behalf of the pilot. e. Trim-Balance Tab: This is combination of trim tab and balance tab this tab can be operated directly from the cockpit with the help of trim wheel to rectify small flying fault just like trim tab, about any axis of aircraft this same tab also get operated as a balance tab when this control surface in operated in control surface operation. After trimming the movement of the tab is to provide aerodynamic balance, will start from new neutral position. f.
Anti-Balance Tab: This anti-balance tab in incorporated with fully power operated control system. In the power operated control system this act as a artificial feel mechanism to provide feel to the pilot.
FIG This tab is attached at the trailing edge of primary control surface in such a way that whenever primary control surface is deflected, tab also deflected in the same direction with a greates degree than the primary control surface. So aerodynamic load action on the tab is directly felt in the cockpit, as it is mechanical, connected with the cockpit control.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 14
Aeronautical Institute of Bangladesh (AIB) SECTION-4
9.
Auxiliary Flight Controls 9.1
The secondary control surfaces are classified under two headings.
9.2
9.2.1
i)
Lift augmenting devices/lift increasing devices.
ii)
Lift damping devices/lift spoiling devices. Lift augmenting devices: Lift augmenting devices are:
a.
Wing trailing edge flaps
b.
Wing leading edge flaps
c.
Slates
Wing trailing edge flaps
These flaps are hinged or sliding surfaces mounted at the trailing edge of each wing, There may be single or two sets of flaps in board and out board at each wing. These flaps operation we controlled from the cockpit with flaps handle/lever.
9.2.2
Principle of flaps operation: Whenever input is provided from the cockpit both wing flaps are deflected downward simultaneously from the neutral position. Flaps do not deflect up.
9.2.3 Purpose: The purpose of flap operation is to ensure the lift producing characterstics wing are increased, hence delaying the stalling single of attack of wing in low speed combustion.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 15
Aeronautical Institute of Bangladesh (AIB) 9.2.4
Type of flaps: On the basic of performance/effectiveness, the flaps are classified as follows: g. Plain flaps: Normally they form the part of the wing trailing edge when not deflected, but once operated they effected down of increase the small aircraft.
b) Split flap: This flap is hinge at the bottom part of the wing near trailing edge allowing it to be lowered from fixed top surface when deflected, this type of flaps an generally used on low subsonic light aircraft.
c) Fowler flaps: the fowler clap is installed at the lower part of the wing trailing edge. When operated it slides near ward on track and tilts downward at the same time, thus increasing the wing comber and area to provide added lift without unduly increasing drag.
Fig-27-14 d) Slotted flaps: The slotted flap is like the fowler flap in operation. During operation it also moves downward and rearward away from the trailing edge of wing and create slot between the trailing edge of wing and leading edge of flap. So that air flows from under with the wing through the slot on the upper surface of the flap to change the turbulent flow in to laminar one to improve the efficiency of flap by increasing lift with a minimum drag.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 16
Aeronautical Institute of Bangladesh (AIB) 9.2.5
Leading Edge flaps:
These are installed at the leading edge of the wing when operated by cockpit control, they external forward from the leading edge, hence. They increase wing area and camber ness of the wing purpose is the increase the lift producing characteristics of the wing. Generally these flaps are operated at high angle of attack at low speeds so that stalling of the aircraft can be delayed.
9.2.5.1 Krueger Flap
9.2.5.2 Droop Flap
Fig-27-16
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 17
Aeronautical Institute of Bangladesh (AIB) 9.3 Slat: Slat is also a wing leading edge device, whenever operated they make a slot between the leading edge of the through and the slat. Hence high pressure air is being supplied through the slot with a venture effects the wing. Thus turbulence air is being removed / changed into laminar flow from the upper surface of the wing which is produced due to high angle of attack. Main purpose is to increase the lift producing characteristics of the wing.
9.3.1
Types of Slat
Types include:
Automatic - the slat lies flush with the wing leading edge until reduced aerodynamic forces allow it to extend by way of springs when needed.
Fixed - the slat is permanently extended. This is sometimes used on specialist lowspeed aircraft (these are referred to as slots) or when simplicity takes precedence over speed.
Controllable - the slat extension can be controlled by the pilot. This is commonly used on airliners.
Fixed slat: This fixed slat is attached at the leading edge of the wing. A calculated way slat is kept in between the slat and leading edge of the wing this fixed has got no connection with cockpit control.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 18
Aeronautical Institute of Bangladesh (AIB)
i)
a)
Operation: Whenever aircraft angle of attack is increase in a particular higher range, these slats comes into operation and in that position, the high pressure air supplied through the slot on the upper surface of the wing, hence turbulent air is removed to maintain the laminar airflow, thus increasing the lift producing characteristics of the wing of delayed the stalling of the aircraft.
Automatic Slat: Normally this auto remains flushed on the leading edge of the wing to maintain undisturbed airflow on the stagnation point. Whenever angle of attack increases and decreasing aerodynamic load on the leading edge as the wing approaches the stall this leading edge, thus creating a slot between leading edge of the wing and slat. So high pressure air through this venture moved on the upper surface of the wing and turbulent air removed which has forward due to high angle of attack. Hence lift producing characteristics increases and stalling of the aircraft delayed. This auto-slat also does not have any link with cockpit control.
b) Controllable Slat: This controllable slat also remain flush with the leading edge of the wing to maintain normal leading edge contour. This is the slat directly operated and controlled from the cockpit. Whenever angle of attack is required to be increased and stall indications comes on, like oral warning, control stick shaking, the pilot operates slat and safe the aircraft from stalling. When slat lever in the cockpit is put on, the slat goes ahead from wing leading edge and creates a slot between wing leading edge and slat, so high pressure air moves through the slot on the upper surface of the wing hence turbulent air is removed which occurred due to high angle of attack. Thus lift characteristics increased. Leading edge and slot now a days in modern aircraft are operated and controlled with hydraulic power by wing a survo-control unit in between. 9.3.2
Effects of Slats
The actual effects of the slat are
The slat effect: The velocities at the leading edge of the downstream element (main airfoil) are reduced due to the circulation of the upstream element (slat) thus reducing the pressure peaks of the downstream element.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 19
Aeronautical Institute of Bangladesh (AIB)
The circulation effect: The circulation of the downstream element increases the circulation of the upstream element thus improving its aerodynamic performance.
The dumping effect: The discharge velocity at the trailing edge of the slat is increased due to the circulation of the main airfoil thus alleviating separation problems or increasing lift.
Off the surface pressure recovery: The deceleration of the slat wake occurs in an efficient manner, out of contact with a wall.
Fresh boundary layer effect: Each new element starts out with a fresh boundary layer at
its leading
edge.
Thin
adverse gradients than thick ones.
boundary
layers
can
withstand
stronger
[3]
9.4 Lift Dumping Devices: Now a days due to the development of aviation technology aeroplane flies with a high speed and body become large enough, hence during landing aircraft momentum has become very high so, it has become increasing difficult to control the role of aircraft during landing. To minimize the landing role of these large aircraft, different devices are used. Lift dumping devices are those mainly operated during landing, to minimize the landing role. 9.4.1
Type of Lift Dumping Devices:
a) Flight spoiler b) Roll spoiler c) Ground spoiler d) Speed brake / Airbrake a. Flight spoiler: The flight spoiler are installed on the upper surface of each wing aft area. They are installed equal in number and size on each wing to maintain stability and balance of aircraft. Whenever input is provided from the cockpit control, spoilers are operated / deflected simultaneously on both wings, heavy drag is crated, hence aircraft speed and lifts are spoiled. Usually during landing these spoilers are used during approach or devib these spoilers are also used these spoilers are operated generally electro-hydraulically from the cockpit. their positions are accordingly indicated in the cockpit. b. Roll Spoiler: The roll spoilers are used to have effective roll control on large aircraft. The same flight spoilers on the wing surface are connected with the aileron control wheel in such a way that when ever control wheel is moved for roll control the spoilers deflected up on the wing in which aileron of deflected up, but in the opposite wing were aileron are deflected down spoilers of the wing do not deflect (remain flash). It means roll spoilers of both wing are connected with aileron control wheel in the cockpit. For roll control whenever control wheel in moved left, left wing aileron and spoiler on the left wing will deflect up and right wing aileron will go down and right wing spoilers will remain retracted (Remain flush with the wing surface). These roll spoiler generally do not have separate control expect aileron wheel in the cockpit Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 20
Aeronautical Institute of Bangladesh (AIB) and operates with hydraulic power. The same spoiler surfaces raise together on both the wing and used as flight spoiler whenever respective lever is selected from the cockpit.
c. Ground Spoiler: Ground spoilers are operated to have the effective control on aircraft landing run. These spoilers are also installed on the upper surface of each wing beside the flight spoilers.
9.4.2 Operation: During landing whenever flight spoilers are operated, they deflect up simultaneously both the wing of aircraft and ground spoilers yet remain flush with wing surface whenever aircraft touches the ground and aircraft weight transfers from wing to landing gears an air ground sensor get operated and round spoilers get power and deflected up without delay. Ground spoilers surfaces are also equal in number on each wing like flight spoilers to have aerodynamic balance during landing. For ground spoilers there is no separate control in the cockpit, flight and ground spoilers have same control lever. Ground spoiler deflected under the condition the cockpit control operated and micro switch installed on lading torque link get actuated. 9.5 Speed breaks/Air breaks: Speed breaks are just like flight spoilers, but only different that flight spoilers are installed on the upper surface of each wing and speed breaks are generally installed on the both side of the aft fuselage section and specific location depends upon manufacturers design criteria. The purpose of speed break is also to minimize the aircraft speed. These surfaces are Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 21
Aeronautical Institute of Bangladesh (AIB) deflected mainly during landing to minimize the landing run and sometimes used for the purpose of quick diving. The speed breaks are operated from the cockpit with a lever and may be electro-hydraulically or hydro electrically operated surface. 9.6 Position Indicators: The lift augmenting and spoiling devices are also coupled with position indicating devices in the cockpit. So that through these position indicating system pilot can get conformation about synchronizations made between the input and out put. In light aircraft, due to considerably lower aerodynamic loads these systems may be operated mechanically from the cockpit run a system of control cables, push-pull rod etc. 9.7
Synchronization: (Lift augmenting & Dumping Device.)
Failure of one side of a lift augmenting system (post or starboard) would probably lead to catastrophic result, especially when one considers that these systems are usually deployed during the aircrafts most vulnerable periods that is take-off and landing. This is due to the asymmetry on the aircraft brought about by the system function. To avail this specially in larger aircraft a synchronization system in employed. The purpose of this will be to ensure that if one element of the system ceases to function, even in an intermediate position, the other corresponding element on the opposite wing will stop immediately in the same position. This system will also ensure that halves will operate at the same speed.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 22
Aeronautical Institute of Bangladesh (AIB) SECTION – 5
10. Unusual Control surfaces (Duel Control) In some aircraft are designed with special type of control layout which do not come as normal control system.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 23
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 24
Aeronautical Institute of Bangladesh (AIB) SECTION - 6
11. Understand the control surface balance 11.1 As control surfaces are hinged at their leading edge, their CG remains behind the hinge point. To balance this and to assist pilot to move controls in the absence of powered or power assisted controls, the following forms or balancing are carried out on ac flight control surfaces. To have the accurate and effective functioning out put the control surfaces are required to be balanced. A practical application is the balance chamber found in B707 airlines and elevators. 11.2
Mass Balance
Fig. 8-1 The purpose of balancing the control surface is to reduce methods there are two methods used to balance the control surface. Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 25
Aeronautical Institute of Bangladesh (AIB) (1) Static Balance.
(2) Dynamic Balancing
11.2.1 Determine Static balance: Static balance is achieved by installing weight at the forward of the hinge line of the control surface, usually; static balance requires that the same of the weight at the forward of the hinge line is approximately equal to the weight aft of the hinge line. The weights may be fixed or removable. The methods are provided in the respective maintenance manual. 11.2.2 Dynamic Balance: (Aerodynamic balance) it is accomplished by designing the control surface as such the aerodynamic forces during flight will tend to balance moments occurred at the forward of the hinge line with the moments aft of the hinge line. This dynamic balance may be achieved by one or more of the following arrangements. a) Horn balance: Whenever control surface is deflected either side, the horn (extended surface) of the same surface is protruded to opposite side, hence some amount of aerodynamic balance can be gained.
Fig. 8-2 b) Graduate horn balance: By the arrangement of graduate horn balance “Snatching” of horn at high speed is minimized. c) Inset hinge: By the arrangement of control surface hinge behind the leading edge also aerodynamic balance can be achieved up to some extent. Whenever control surface is deflected either side, forward position of the control surface from the hinge point protrudes opposite to the main surface deflection.
Fig. 8-3 d) Internal seal / Sealed nose balance: A plate project forward from the nose the control surface. This plate or tongue, is joined to the main of the wing tail plane or fin Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 26
Aeronautical Institute of Bangladesh (AIB) by a lose fold of impermeable fabric (impermeable, which constitutes a seal between the region above and below the control, or on the two sides of control surface, incase of a rudder. The space above the tongue, incase of elevator is open to the air above the tail-plane and similarly the space below the tongue is open the air below the tail plane. When the elevator, be cause downwards, lift is created on the elevator, be cause reduced pressure on the upper surface, increased pressure on the lower surface. Specially the pressure just above the tongue is low and just below it is high, so that there is an upward force on the tongue which provides a nose-up hinge moment, which is the required balancing moment.
Fig. 8-4
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 27
Aeronautical Institute of Bangladesh (AIB) SECTION – 7
12. Mechanical Fittings 12.1
Cable Fittings
Cables may be equipped with several different types of fittings, such as terminals, thimbles, bushings, and shackles. Terminal fittings are generally of the swaged type. They are available in the threaded end, fork end, eye end, single shank ball end, and double shank ball end. The threaded end, fork end, and eye end terminals are used to connect the cable to a turnbuckle, bell crank, or other linkage in the system. The ball end terminals are used for attaching cables to quadrants and special connections where space is limited. The thimble, bushing, and shackle fittings may be used in place of some types of terminal fittings when facilities and supplies are limited and immediate replacement of the cable is necessary. 12.2
Turnbuckles
A turnbuckle assembly is a mechanical screw device consisting of two threaded terminals and a threaded barrel. [Figure 5-69] Turnbuckles are fitted in the cable assembly for the purpose of making minor adjustments in cable length and for adjusting cable tension. One of the terminals has right-hand threads and the other has left-hand threads. The barrel has matching rightand left-hand internal threads. The end of the barrel with the left-hand threads can usually be identified by a groove or knurl around that end of the barrel. When installing a turnbuckle in a control system, it is necessary to screw both of the terminals an equal number of turns into the barrel. It is also essential that all turnbuckle terminals be screwed into the barrel until not more than three threads are exposed on either side of the turnbuckle barrel. After a turnbuckle is properly adjusted, it must be safeties. The methods of safe tying turnbuckles are discussed later in this chapter. 12.3
Push-Pull Tube Linkage
Push-pull tubes are used as linkage in various types of mechanically operated systems. This type linkage eliminates the problem of varying tension and permits the transfer of either compression or tension stress through a single tube. A push-pull tube assembly consists of a hollow aluminum alloy or steel tube with an adjustable end fitting and a check nut at either end. [Figure 5-70] The check nuts secure the end fittings after the tube assembly has been adjusted to its correct length. Push-pull tubes are generally made in short lengths to prevent vibration and bending under compression loads.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 28
Aeronautical Institute of Bangladesh (AIB) 12.4
Cable Tension Regulators :
Cable Tension Regulators are utilized to maintain optimum rig tensions in wire rope cable control systems, irrespective of thermal or structural deflections of the airframe. The benefits on the aircraft are low breakout forces, lower stresses and system weight, together with longer cable life and reduced maintenance. Cable tension regulators are designed to custom engineered specifications and are approved and in-service on both civil and military aircraft. Triumph Controls - UK provides a variety of cable tension regulators. Quadrant type regulators are installed are installed at the pilot end or the control run end of a cable loop, and maintain optimum rig tension while transmitting control movements to the flying control surfaces. The accessory and in-line cable tension regulators install within the run length of the cable loop and will act to transmit pilot control loads while regulating the tension in the cables within a controlled load range for best pilot feel. These units are generally installed into flying control trim systems and engine controls or where installation is restricted. 12.5
Push/Pull Controls : Triumph Controls – UK
Triumph Controls - UK designs and develops a large range of customer specific push/pull controls using helix, flat wrap and wire rope cable controls, suitable for a wide variety of motion control applications. The developed control assemblies require minimal to no maintenance and are used in a vast range of applications throughout the aircraft industry. These product lines are available with both rigid casing and flexible casing for better design versatility and ease of installation. Along with the push/pull product line Triumph Controls UK provides control gearboxes which can be employed at the input or output end of a control cable assembly to transmit rotary to linear motion or linear to rotary motion utilizing a helical tooth gear wheel.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 29
Aeronautical Institute of Bangladesh (AIB) 12.6
Sprocket
A sprocket and roller chain A sprocket[1] or sprocket-wheel[2] is a profiled wheel with teeth, cogs,[3] or even sprockets[4] that mesh with a chain, track or other perforated or indented material.[5][6] The name 'sprocket' applies generally to any wheel upon which are radial projections that engage a chain passing over it. It is distinguished from a gear in that sprockets are never meshed together directly, and differs from a pulley in that sprockets have teeth and pulleys are smooth. Sprockets are used in bicycles, motorcycles, cars, tracked vehicles, and other machinery either to transmit rotary motion between two shafts where gears are unsuitable or to impart linear motion to a track, tape etc. Perhaps the commonest form of sprocket is found in the bicycle, in which the pedal shaft carries a large sprocket-wheel which drives a chain which in turn drives a small sprocket on the axle of the rear wheel. Early automobiles were also largely driven by sprocket and chain mechanism, a practice largely copied from bicycles. Sprockets are of various designs, a maximum of efficiency being claimed for each by its originator. Sprockets typically do not have a flange. Some sprockets used with timing belts have flanges to keep the timing belt centered. Sprockets and chains are also used for power transmission from one shaft to another where slippage is not admissible, sprocket chains being used instead of belts or ropes and sprocket-wheels instead of pulleys. They can be run at high speed and some forms of chain are so constructed as to be noiseless even at high speed.
Chain Sprocket A chain is made up of a series of links with the links held together with steel pins. This arrange makes a chain a strong, long lasting way of transmitting rotary motion from one gear wheel to another. Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 30
Aeronautical Institute of Bangladesh (AIB) 12.7
Cable and pully
A pulley is a wheel on an axle that is designed to support movement of a cable or belt along its circumference.[1] Pulleys are used in a variety of ways to lift loads, apply forces, and to transmit power.
Belt and pulley systems
Flat belt on a belt pulley
Belt and pulley system A belt and pulley system is characterized by two or more pulleys in common to a belt. This allows for mechanical power, torque, and speed to be transmitted across axles. If the pulleys are of differing diameters, a mechanical advantage is realized. A belt drive is analogous to that of a chain drive, however a belt sheave may be smooth (devoid of discrete interlocking members as would be found on a chain sprocket, spur gear, or timing belt) so that the mechanical advantage is approximately given by the ratio of the pitch diameter of the sheaves only, not fixed exactly by the ratio of teeth as with gears and sprockets. In the case of a drum-style pulley, without a groove or flanges, the pulley often is slightly convex to keep the flat belt centered. It is sometimes referred to as a crowned pulley. Though once widely used in factory line shafts, this type of pulley is still found driving the rotating brush in upright vacuum cleaners. Agricultural tractors built up to the early 1950s Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 31
Aeronautical Institute of Bangladesh (AIB) generally had a belt pulley. It had limited use as the tractor and equipment being powered needed to be stationary. It has thus been replaced by other mechanisms, such as power take-off and hydraulics.
12.7.1
How it works
The simplest theory of operation for a pulley system assumes that the pulleys and lines are weightless, and that there is no energy loss due to friction. It is also assumed that the lines do not stretch. In equilibrium, the forces on the moving block must sum to zero. In addition the tension in the rope must be the same for each of its parts. This means that the two parts of the rope supporting the moving block must each support one-half the load.
•
Fixed pulley
•
Diagram 1: The load F on the moving pulley is balanced by the tension in two parts of the rope supporting the pulley.
•
Movable pulley
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 32
Aeronautical Institute of Bangladesh (AIB)
•
Diagram 2: A movable pulley lifting the load W is supported by two rope parts with tension W/2. 12.7.2 Different types of pulley systems: •
• •
Fixed: A fixed pulley has an axle mounted in bearings attached to a supporting structure. A fixed pulley changes the direction of the force on a rope or belt that moves along its circumference. Mechanical advantage is gained by combining a fixed pulley with a movable pulley or another fixed pulley of a different diameter. Movable: A movable pulley has an axle in a movable block. A single movable pulley is supported by two parts of the same rope and has a mechanical advantage of two. Compound: A combination of fixed and a movable pulleys forms a block and tackle. A block and tackle can have several pulleys mounted on the fixed and moving axles, further increasing the mechanical advantage.
12.8 a. BELL CRANKS AND WALKING BEAMS.— Bell cranks and walking beams are levers used in rigid control systems to gain mechanical advantage. They are also used to change the direction of motion in the system when parts of the airframe structure do not permit a straight run. They are often used in push-pull tube systems to decrease the length of the individual tubes, and thus add rigidity to the system. A bell crank has two arms that form an angle of less than 180 degrees, with a pivot point where the two arms meet. The walking beam is a straight beam with a pivot point in the center. Bell cranks and walking beams are mounted in the structure in much the same way as pulley assemblies. Brackets or the structure itself may be used as the point of attachment for the shaft or bolt on which the unit is mounted. Examples of a The walking beam is a straight beam with a pivot point in the center. Bell cranks and walking beams are mounted in the structure in much the same way as pulley assemblies. Brackets or the structure itself may be used as the point of attachment for the shaft or bolt on which the unit is mounted. Examples of a BELL CRANKS.
Bellcrank b. IDLER ARMS.—Idler arms are levers with one end attached to the aircraft structure so it will pivot and the other end attached to push-pull tubes. Idler arms are used to support push-pull tubes and guide them through holes in structural members.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 33
Aeronautical Institute of Bangladesh (AIB) c. BUNGEE.—Bungee are tension devices used in some rigid systems that are subject to a degree of shock or overloading. They are similar to push-pull rods, and perform essentially the same function except that one of the fittings is spring-loaded in one or both directions. That is, a load may press so hard(compression) against the fittings that the bungee spring will yield and take up the load. This protects the rest of the rigid system against damage. The internal spring may also be mounted to resist tension rather than compression. An internal double-spring arrangement will result in a bungee that protects against both over tension and over compression. 13. Tensiometer Tensiometer is used to check the tension of cables in aircraft system.
13.1
Purpose.
13.2
Description.
Small pulleys on fixed spindles and a spring loaded pointer which caries a third pulley. Two scales are provided for the tension range of each of two cable size. The tension to apply to control cable, by adjustment of the turn buckles, is quoted in the aircraft air publication and ranges from 5 to 200 lbs. The tension which varies with the size of the control cable can check by one of three marks of tensiometer. a.
MK-3. Calibrated for 5 to 10 CWT cables. This marks has a tension range of 30 to 120 lbs. The outer pulley centers are 5 inch apart.
b.
MK-5. Calibrated for 25 to 35 CWT cables. This has a tension range of 60 to 200 lbs. The outer pulley centers are 6 inch apart. Note. It is emphasised that the correct type of tensiometer must be used when checking the tension of control cables.
13.3
Operating Instructions. The instrument is fitted where there is a clean run of cables. Ensure that the tensiometer is the correct mark and sue as follows:
a.
Pull the pointer over to its stop so that the spring is extended.
b.
Base the control cable under the fixed pulley on the right. Then pass the cable over the central pulley and finally spring it under the fixed pulley on the left.
c.
Ensure that the cable lies in the groove of each pulley and that the tensiometer hangs freely.
d.
If possible to ensure that the reading is correct, run the tensiometer along a few inches of the cable and tap the cable and tap the cable lightly so that the reading settle down.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 34
Aeronautical Institute of Bangladesh (AIB)
If the tension is correct the tensiometer must be removed from the cable before Note. the tension is adjusted. After adjusting the tension, replace the tensiometer on the cable and recheck the reading. 13.4 . Check for Accuracy. Before use the position of the spring anchorage pin which governs the tension of the spring should be verified, the pin is housed in an elongated hole and when correctly positioned, a circle is engraved round is so that if displacement occurs the pin cab be restored to its original position. The tensiometer should be checked for accuracy by the instrument section at regular three monthly in intervals. 14.
RIGGING EQUIPMENT
14.1
Introduction
Control surfaces should move a certain distance in either direction from the neutral position. These movements must be synchronized with the movement of the cockpit controls. The flight control system must be adjusted (rigged) to obtain these requirements. Generally speaking, the rigging consists of the following: a. Positioning the flight control system in neutral and temporarily locking it there with rig pins or blocks. b. Adjusting surface travel, system cable tension, linkages, and adjustable stops to the aircraft manufacturer’s specifications. When rigging flight control system, certain items of rigging equipment are needed. Primarily, this equipment consists of tensiometers, cable rigging tension charts, protractors, rigging fixtures, contour template and rulers. 14.2
Rigging Fixture.
2. Fixtures are the devices used to locate or clamp material during the process of fitting, machining or drilling. The term “Jigs and fixtures” is sometimes used and as a general rule the holding of the work in the jig. Fixtures are used in the form of blocks, clamps, angle plate and bolts etc. for the setting up of work on the bench, machine tools or jig.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 35
Aeronautical Institute of Bangladesh (AIB) 14.3
Straight Edges.
These are usually made of wood and are used in conjunction with a level to check the attitude of a surface. Straight edge has their two long edges accurately made straight and parallel to each other. When mot in use it should be supported by a rack or hung vertically to prevent it warping or bending. a. Accuracy Check. Depending on the size of the straight edge, parallelism may be checked by ordinary calipers or by micrometer or venire calipers. Warp or twisting can be check by sighting. To check a large straight edge for parallelism and straightness place the straight edge on a flat surface and draw two lines to represent the edges. Turn over the straight edge and replace so that one edge is touching one of the pencil lines. Draw a third line to the outside edge and remove the straight edge. If the straight edge is true, the three pencil lines will be parallel. b. Spirit Level. a spirit level is used to check the level of a surface. It comprises of a curved glass tube partially filled with liquid and sealed at each end. The glass tube with its curvature upper most is supported by a wooden block so that the bubble is central to reduce wear of the base a brass plate is secured to the base of the wooden block. c. Place a known straight edge well protected with rag in a vice, place a spirit level on the straight edge and adjust the straight edge unit the spirit level bubble is (zero) central. Mark with chalk the position of the spirit level base on the straight edge. Turn the spirit level round end for end and replace between the marks made on the straight edge. If the spirit level is true the bubble will again be in the centre of the glass tube. 14.4
Measuring Tape.
The steel tape is used for symmetry checks, such as, when measuring from the outer aileron hinge to the stern post, either side of the aircraft. Note: When using a steel tape over a long distance, pre-load the tape by use of spring balance to remove the rag in the tape. The load to apply will depend on the distance, but 5 lbs should be sufficient. 14.5
Plumb Bob.
The plumb bob sued for giving a true vertical line and for checking vertical members. It consists of a weight suspended by a thin corn trammel points consisting of two adjustable points on a beam or rod, are used for comparing distance which would be equal, specially where an obstruction prevents the of a steel rule.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 36
Aeronautical Institute of Bangladesh (AIB) SECTION – 8
FOKKER 28 AIRCRAFT 15.
FLIGHT CONTROLS ( Focker – 28 aircraft )
15.1. GENERAL The flight controls are provided to control the aircraft during flight and on the ground. For identification purposes, the flight controls can be divided in three sections: a.
The Primary Flight Controls, which are primarily needed to control the aircraft during flight;
1. Ailerons on the outboard trailing edge of each wing 2. Rudder on the trailing edge of the vertical stabilizer 3. Elevator on the trailing edge of the horizontal stabilizer. b.
The Supplementary Flight Controls, supplementing the primary flight controls;
1.
Trim systems. On the ailerons and rudder the trim systems are an integral part of the mechanical control system. Pitch trim is provided by means of the adjustable horizontal stabilizer. 2. Auto Flight system, controlling the aircraft by automatic means through the primary flight controls and the horizontal stabilizer. This system is discussed in chapter 22. c.
The Secondary Flight Controls, which are used to increase the aircraft capabilities:
1. Wing flaps on the trailing edge of each wing, between the ailerons and the fuselage.
2. Speed brakes on the fuselage tail section. 3. Lift dumpers on the upper surface of each wing inboard half, forward of the wing flaps. NOTE: Powered flight controls are another name often used for primary and supplementary Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 37
Aeronautical Institute of Bangladesh (AIB) flight controls. Gust locks are provided on all primary flight control systems. The gust lock handle is located at the left hand aft side of the pedestal. The handle locks in the "UP" position the flight controls. The handle itself is locked in both positions by means of a spring loaded trigger lock on top of the handle. 15.2 THE PRINCIPLE OF OPERATION OF THE PRIMARY AND SUPPLEMENTARY FLIGHT CONTROL SYSTEM (powered flight controls) 15.2.1 Introduction The ailerons, rudder, elevator and horizontal stabilizers are operated with The assistance Of hydraulic power to reduce the forces on the controls in the cockpit. The controls in the cockpit are of the conventional type being control wheels and columns for ailerons and elevator, and adjustable pedals and trim wheels for rudder and stabilizer. The above mentioned cockpit controls are cable connected to the various hydraulic control units. The hydraulic force is transmitted to these flight control systems by means of the hydraulic control units (actuators) in which it is converted into a mechanical force by operating a cylinder/piston assembly. Selection of pressure admittance into the desired cylinder chamber is by means of a spool type slide valve, called servo valve, which is directly or indirectly operated by the cockpit controls through the mechanical control system. 15.2.2 HYDRAULIC SAFETY As a matter of safety, the hydraulic control units are duplicated (dual) units, in which the two halves are basically identical. One half is connected to hydraulic system no.1 (utility system), the other half is connected to hydraulic system no.2 (flight control system). This ensures normal operation even when one of the halves or one of the hydraulic systems is depressurized. In the control units, the hydraulic systems are completely separated so no interflow is possible In the rudder, elevator and stabiliser systems the two halves are built as one unit which is mounted directly in front of the relevant surface. In the aileron system the two halves are built as single units, mounted one in front of each aileron. Since the aileron surfaces are mechanically interconnected, the safety principle is not affected because one single unit is capable of operating both ailerons. 16. MANUAL OPERATION In the case that both halves of a control unit are depressurized, operation of the flight controls is still possible. The primary flight controls are operated manually i.e. ailerons by means of control tabs, and rudder and elevator direct manually. The stabilizer may be operated with an electric motor which is controlled by a toggle switch on the pedestal. 16.1
STRUCTURE BUILD-UP
The structural build-up of the control units, although basically identical, differs as follows: a. The aileron units are of the single type, mounted horizontally in front of each aileron. Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 38
Aeronautical Institute of Bangladesh (AIB) b. The rudder unit is of the tandem type. The two halves operate tandem wise with the two pistons and servo valves mechanically connected. It is in front of the rudder. c. The elevator unit is of the parallel type, mounted horizontal in the horizontal stabilizer (bullet fairing). The two halves-operate side-by-side. d. The stabilizer unit is of a combined type in which a tandem servo valve assembly is used in combination with parallel (side-by-side) cylinder/ piston assemblies. The unit is mounted in the vertical stabilizer almost vertically. 16.2
FOLLOW-UP AND FEED-BACK SYSTEMS
In all control units, the cylinder and the servo valve are mounted within the same housing. In the aileron and rudder systems this housing is (pivot) connected to the particular surface pivot arm, whilst the piston in the cylinder is stationary since the piston rod is fixed to the aircraft structure. In this set-up, when the servo valve is operated with the cockpit controls the hydraulic pressure is selected into one of the cylinder chambers whilst the other chamber is connected to return. The piston is fixed, which results in a movement of the housing and surface. As long as the servo valve remains open the movement goes on, meaning that as long as the cockpit controls are operated the surface will deflect. The faster the cockpit controls are operated, the further the servo valve will be open, resulting in faster travelling of the housing and surface. The moment the cockpit controls are held stationary, the servo valve is held stationary but the hydraulic connections are still open, so the housing and surface are still moving, closing the connections and stopping the movement, From this it can be concluded that the travel of the servo valve within its housing is very small to prevent large follow-up movements. In the above described systems, during the movement the servo valve is ahead of the housing (that follows). These types of systems to close (null) the servo valve are called "follow-up" systems". In the elevator and stabilizer systems the housing is mounted to the aircraft structure, whilst the piston in the cylinder is connected to the particular surface. In this set-up, when the servo valve is operated, the piston is hydraulically moved, resulting in surface travel, which is actually the same idea as the one above, but the method to close (null) the servo valve is different because the housing is stationary. Also in these systems the servo valve is connected to the cockpit controls (by means of the mechanical control system) but with the aid of an input mechanism that is also connected to the flight control surface. When the cockpit controls are operated, the servo valve is displaced and the surface is deflected hydraulically. This movement now is mechanically fed back to the input mechanism where it tries to null the servo valve. As long as the controls in the cockpit are operated, the servo valve remains open and the surface is travelling until the controls are stopped. Then the servo valve is nulled (moved in the opposite direction) by the small further travel of the surface via the mechanical feed-back action on the input mechanism. In the latter systems, during the movement the servo valve is just kept open until the input movement stops. These types of systems to close (null) the servo valve are called "feedback" systems. Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 39
Aeronautical Institute of Bangladesh (AIB)
16.3
BOOSTED AND FULLY POWERED
A boosted flight control is a system in which the hydraulic pressure is used as assistance to the pilot. In these systems the cockpit controls are directly connected to the flight control surface with the hydraulic control unit operating in parallel to the mechanical connection in order to lower the steering forces. The pilots feel the air loads. This boosted system is used for elevator operation only. The boost ratio is 4:1. The ailerons, rudder and stabilizer are fully powered surfaces. This means that the surface is operated by means of hydraulic pressure only. Cockpit control operation results in displacement of the servo valve only, thus selecting pressure in the cylinder chambers. The pilots do not feel the air loads. When the surface is held in position (servo valve neutral), airloads are not capable of overcoming the hydraulic lock, which makes it impossible to move the surface by means of an external force, reason why no trim tabs are used on these surfaces. 17.1
ARTIFICIAL FEEL and TRIM SYSTEMS
Since the pilots do not feel the air loads on the fully powered primary flight controls (ailerons and rudder) these loads are simulated with artificial feel units that mainly consist of a spring unit which is compressed when the cockpit controls are operated. Releasing the controls will automatically bring back the entire system to the original position by the expanding spring. This position datum is not fixed but may be changed with the trim knobs on the pedestal, meaning that trimming is just altering the "NO LOAD" position of the total system (including the flight control surface) whilst the expanded spring of the feel/trim spring unit holds the system in this position. The stabilizer, although fully powered, is not provided with artificial feel because it is a trim system itself. The pilot "feels" with his elevator and trims these forces away with the stabilizer by which action the elevator must be released until it is in neutral again. 17.2
BY-PASS VALVE and INDICATION
As explained in the "hydraulic safety", the control unit is capable of operating when one half is depressurized. In this unit half, of course it must be possible that the hydraulic fluid flows from one side of the piston to the other upon movements by means of the other (pressurized) half or external forces. This to prevent a hydraulic lock. For this reason each control unit half is provided with a spring loaded by-pass valve that is normally held in the closed position as long as hydraulic pressure is present in the control unit half. Upon depressurization the by-pass valve is automatically opened thus interconnecting the two sides of the piston. The by-pass line is connected to return to prevent any build-up of pressure (although in most systems the pistons are balanced. This design assures uninterrupted operation in depressurized situations, independent of servo valve position. Operated by the by-pass valve is a micro-switch called by-pass switch, that basically serves to illuminate its relevant caution light on the flight control panel (middle aft pedestal) in depressurized situations. The aft row of four caution lights is for the system no.1 control unit halves, the forward row of four is for the system no.2 control unit halves.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 40
Aeronautical Institute of Bangladesh (AIB) The flight control central caution light on the secondary instrument panel serves as a repeater light in order to call pilot's attention to an unintentional depressurized control unit half. It is illuminated any time one or more caution lights on the flight control panel are on with the relevant master switch(es) in the "ON" position. Switching to "OFF" will extinguish the central caution light but leaves the panel caution light on. 17.3
HYDRAULIC PRESSURE CONTROL
The hydraulic pressure to the control unit halves is admitted via depressurization valves, called shut-off valves, which are solenoid operated, "de-energized open" valves. In other words, when the solenoid is energized the particular control unit half is depressurized. Under normal circumstances, all eight solenoids are de-energized basically resulting in pressurized control unit halves (with the exception of rudder no. 1 unit half which is normally depressurized). Manual control over the shut-off valves is by means of master switches on the flight control panel, one for each shut-off valve. Normally, all eight-switches are in the "ON" position. When a control unit half is to be depressurized, the relevant master switch must be placed in the "OFF" position (energizing the solenoid of the shut-off valve). This will illuminate the relevant caution light by the automatic action of the by-pass valve and by-pass switch. 18.1
DETENT ASSEMBLY and STICKING SWITCH
Automatic depressurization control is provided on all systems to cater for the possibility of a sticking servo valve. This could happen when dirt or foreign matter in the hydraulic fluid enters the servo valve sleeve and since the servo valves are nor provided with any seals but operate on metal-to-metal sealing with of course very close tolerances there is a chance that the servo valve sticks. Apart from the systems filters the ingress of dirt or foreign matter is prevented by a filter in each control unit half. When, in spite of these safety provisions the servo valve should stick, resulting in a locked (nulled servo valve) or a runaway (selected servo valve) flight control, the opposing force of the pilot on the relevant cockpit control will operate a detent assembly, mounted in the mechanical connection between the servo valve and the mechanical control system. This operation will trip a micro-switch, called sticking switch, with subsequent energizing of the relevant shut-off valve solenoid thus immediately and automatically depressurizing the particular control unit half. This automatic operation releases the hydraulic pressure that caused the trouble. To provide for permanent depressurization as long as the relevant master switches are in the "ON" position, in each "sticking" electrical circuit there is a relay with a held circuit that will be energized when the sticking switch is operated. For the control units with tandem servo valves (rudder and stabilizer), only one detent assembly with one sticking switch is mounted. Operation of the sticking switch must result in depressurization of the total unit (both halves because when one of the servo valves is stuck, the other half is fixed in the similar position. For the control units with single servo valves (ailerons an elevator), a detent assembly with sticking switch is mounted for each servo valve. Operation of a sticking switch results in depressurization of the relevant control unit half only. Located on the flight control panel is a reset button. This button is to be depressed when after pressure build-up in the hydraulic power systems one or more caution lights remain on. It could happen when flight controls have been operated without pressure, that in the aileron Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 41
Aeronautical Institute of Bangladesh (AIB) and stabilizer systems the sticking switches were tripped (this is impossible in rudder and elevator systems). Due to the hold circuit the units remain unpressurised (caution light on) after pressure build-up. Depressing the reset button will interrupt the hold circuit then.' NOTE: In the rudder system, when no.1 half of the control unit becomes pressurized before no.2 half (which is incorrect), no.1 rudder caution light remains on. Depressing the reset button will correct this. (Refer to the rudder control system for further information). 18.2
MAINTENANCE
The hydraulic control units can be inspected, removed, installed and rigged without affecting the rigging of the mechanical control systems. 18.3
AILERON CONTROL SYSTEM - GENERAL
The ailerons are fully hydraulic powered surfaces. The control system can be divided in three subsystems; the mechanical control system, the hydraulic control system and the electrical control and warning system. 19.1
The Mechanical Control System
The control wheels in the cockpit are connected to the servo valves in the hydraulic control units by means of the mechanical control system. The two control wheels are interconnected. Movement of these wheels will operate a cable system in the fuselage and the wings, being two independent circuits, connected by means of a tension regulator and cable quadrant assembly, located in the tunnel between the main wheel bays. In the wings, just forward of the inboard part of the ailerons, the cable system ends are attached to a cable quadrant which is connected to the servo valve in the hydraulic actuator (via a hydraulic coupling in the actuator, refer to the hydraulic control system). Operation of the mechanical control system will also compress a spring in a feel/trim spring unit, connected to arms on the tension regulator shaft, providing artificial feel and a neutral position in the system. Via the trim system it is possible to change the neutral position of this feel/trim spring unit thereby changing the neutral position of the total mechanical control system. To achieve this, the unit is connected to the aileron trim, knob on the pedestal via a cable system. This trim knob is provided with a trim position indicator. The ailerons are mechanically interconnected by a cable system, called the anti-up float cable, thus providing synchronization, the possibility of both ailerons operating on one hydraulic actuator, and preventing the aileron from up floating in depressurized conditions. A control tab is located in the trailing edge of each aileron which serves to cause aileron deflection by aerodynamic forces on the tab in case no hydraulic pressure is available to operate the surface. The ailerons and the control units are disengaged from the mechanical control system in this case through the operation of a hydraulic coupling thus enabling the ailerons to be deflected by control tab forces. Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 42
Aeronautical Institute of Bangladesh (AIB) In pressurized conditions the control tabs do not move with respect to the ailerons. In a depressurized situation the control tabs can be deflected by mechanical control system operation. This difference is caused by tab lock-out actuator automatic action (part of the aileron control unit) which repositions the control tab operating link such that when no hydraulic pressure is available, mechanical input motion will operate the link, causing tab deflection. 19.2
The Hydraulic Control System
The left hand control unit is operated on pressure from hydraulic system no. 1 (utility system), the right hand one on pressure from system no. 2 (flight control system). The control units consist of two parts each, the actuator which operates the aileron, and the tab 1°e -but actuator which controls the position of the control tab operating link% The aileron actuator housing is connected to the aileron pivot arm whilst the piston rod is connected to the wing rear spar. Pressure admittance is via solenoid operated valves, controlled and operated as described in "Principle of Operation". The connection between the mechanical control system and the servo valve in the hydraulic actuator is by means of a hydraulic coupling. When hydraulic pressure is present, the connection is made. In case of a sticking servo valve, a sleeve in this coupling can be moved slightly with respect to the servo valve by means of the mechanical control system. This action will trip the sticking switch with subsequent depressurization, uncoupling the servo valve, and repositioning the control tab operating link by tab lock-put actuator action. The unit remains unpressurised in this case due to a hold circuit in the electrical control system. Operation of the mechanical control system in unpressurised situations will not displace the servo valve, but only the sleeve in the hydraulic coupling so no direct movement of the hydraulic actuator is possible. Only the control tab will be operated causing aileron deflection and actuator housing movement. The reason that on these actuators a hydraulic coupling and not a detent assembly is used is that complete independent movements of the mechanic-al control system and-the control tabs must be possible in unpressurised situations. 19.3
The Electrical Control and Warning System
In general this electrical system is similar to the one described in "Principle of Operation". The sticking switches when operated will depressurize their respective control unit which will remain unpressurised by operation of a hold circuit to prevent opening and closing of the shut-off valves when the sticking switches are tripped upon mechanical control system operation. During pressure build-up in the hydraulic systems (engine start etc.) the system could be in this situation so the actuators will remain unpressurised. To pressurize the actuators and couple the units to the mechanical control system the reset (pushbutton) switch on the flight control panel must be pushed in. This interrupts the hold circuit and opens the shut-off valves so hydraulic pressure is admitted. Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 43
Aeronautical Institute of Bangladesh (AIB) Test switches on the maintenance test panel behind the co-pilot's seat enable functional testing of the sticking switch circuits. Repress rising is performed by operating the reset switch to interrupt the hold circuit. The caution lights are controlled as described in "Principle of Operation". 20. AILERON MECHANICAL CONTROL SYSTEM The control wheels in the cockpit are connected to the servo valves in the hydraulic actuators by means of the mechanical control system that consists of: 1. The control wheels and linkages in the cockpit, pushrod connected to 2. The aileron control/gust lock quadrant below the cockpit floor, providing the mechanical interconnection between the control wheels, locked in case the gust lock system is engaged, and connected by means of a single cable loop to. 3. The tension regulator in the tunnel between the main wheel bays which maintains the desired tension in the single cable loop and transmits the motion to dual wing cable loons by means of a cable quadrant, mounted on the forward end of the tension regulator shaft. The dual cable loops are connected to. 4. The aileron actuator input quadrants, being one for each aileron actuator, mounted on the wing rear spar by means of brackets and connected to the servo valve in the aileron actuator. The lower side of the quadrant is provided with the operating mechanism of the aileron control tab. 5. Trimming of the ailerons is performed by altering the neutral position of the mechanical control system thereby deflecting both ailerons. Rotation of the aileron trim control knob on the pedestal will achieve this. It is connected by means of a single cable loop and a chain to 6. The trim screw actuator in the tunnel between the main wheel bays to convert the rotational motion into a linear one. This vertical motion alters the position of the input rod in
trim
7. The feel/trim spring unit that determines the neutral position of the mechanical control system. Operation of this control system out of neutral will compress the spring in the unit, providing artificial feel. Release of the control wheels will automatically return the control system to the original (neutral) position by the spring force. To achieve this, the unit housing is connected to arms on the tension regulator shaft by means of two pushrods. Altering the vertical position of the trim input rod by means of the trim screw actuator will change the position of the complete spring unit and via the pushrods and arms change the neutral position of the mechanical control system. In order to provide a mechanical interconnection, each aileron upper pivot arm is pushrod connected to a quadrant, located above the already mentioned input quadrant (item 4), independently rotating on the same fixed shaft and connected to the quadrant in the opposite wing by means of a single cable loop, called the anti-up float cable.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 44
Aeronautical Institute of Bangladesh (AIB) 20.1. AILERON CONTROL WHEELS AND LINKAGES The aileron control wheels are mounted on a shaft at the head of the control columns. Input motion to the mechanical control system is by means of a chain-sprocket wheel on the control wheel shaft with two chains that are connected to two vertical cables which, on the lower end, are connected to a bell crank that is splined on a horizontal shaft at the lower end of the column. Also splined on this shaft is a lever that protrudes vertically out of the base of the column into the column torque tube. This lever is pushrod connected to the forward end of a bell crank that pivots about a vertical shaft and is mounted on the outboard side of the compartment below the cockpit floor. The pushrod is located in the center of rotation of the column torque tube, so no movement of the pushrod is possible by column fore and aft displacement. The aft end of the bell crank is pushrod connected to the operating lever of the control/gust lock quadrant. Rig pin holes are provided in the vertical lever at the base of the column, and in the bell cranks below the cockpit floor. The gust lock system in the control/gust lock quadrant may also be used for rigging purposes. Two adjustable stop bolts are provided in the head of the control columns, whilst the stop rotates with the control wheel. 20.2. AILERON CONTROL/GUSTLOCK QUADRANT The cable quadrant is spline mounted on a vertical, bearing supported shaft, and is provided with a lever to which the control wheel linkage pushrods are connected. The quadrant is provided with a gust lock hole that is used during gust lock operation and rigging of the aileron control system. 2o.3. AILERON CABLE TENSION REGULATOR It serves to maintain the desired cable tension in the cable circuit between the control/gust lock quadrant and the tension regulator. The regulator mechanism consists of two cable quadrants, pivoted at their center and connected to a spring-loaded yoke that slides over a fixed shaft, mounted to the regulator support. The yoke springs are on the opposite end attached to the regulator support. Should the cables tighten, and then the regulator springs are further compressed and if the cable slackens the springs act on the quadrants to release the tension by the required amount. The yoke slides over the shaft. However, when the ailerons are operated, the regulator unit action must be held or otherwise the regulator springs would absorb the control load. In this case an unequal force acts on the yoke that tends to tilt it, thus preventing any sliding movement over the shaft, thereby locking the quadrants and rotating the assembly without the quadrants changing their position with respect to each other. An indicator scale is provided to inspect and rig the cable tension. The scale is stationary fixed to the support, a Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 45
Aeronautical Institute of Bangladesh (AIB) dot mark is provided on the moving spring yoke. 20.4. AILERON ACTUATOR INPUT QUADRANT The dual cable loops are attached to each input quadrant (which is the lower one of the two, the upper one being used for aileron mechanical interconnection) that is connected to the aileron actuator servo valve. The bottom side of each input quadrant is provided with the so-called "power to manual reversion mechanism", operated by the tab lock-out actuator. This mechanism consists of a housing which is part of the quadrant with an out-of center pivoting dog link. The forked end of the dog link is connected to the tab operating link and the piston rod of the tab lock-out actuator. During normal pressurized aileron operating conditions, the piston in the tab lock-out actuator is hydraulically retracted and has pivoted the dog link such that the pivot point of the tab operating link is located in the center of rotation of the input quadrant. Rotation of this quadrant has no effect on the tab operating link so the tab remains lined up with the aileron in this case. When the relevant aileron hydraulic system becomes depressurized, the spring force extends the piston in the tab lock-out actuator that pivots the dog link to such a position that the pivot point of the tab operating link is located out of the center of rotation of the input quadrant. Rotation of this quadrant will move the tab operating link in this case so the tab will be deflected. (The hydraulic coupling has disconnected the servo valve from the input quadrant so the tab is the only one that is moved by this action. Aerodynamic forces take care of aileron deflection). Dog link pivoting is limited by the housing. The quadrants are provided with rig pin holes. 20.5. AILERON TRIM CONTROL KNOB A cable system connects the forward drum to the aft cable drum located in the tunnel between the wheel bays. The cables are supported by rollers. One rig pin hole is provided in the forward drum and one in the aft drum. The control knob is provided with a position indicator. Rotating the control knob causes aft cable drum rotation and screw actuator operation via the chain. The trim position indicator is rotated by a gear wheel on the control knob rod. 20.6. AILERON TRIM SCREW ACTUATOR The actuator consists of a housing, a screw shaft with a right-hand thread and an actuating shaft which is threaded on its inside bore to engage the screw shaft. The actuating shaft is splined externally to prevent it from rotating. Two ball thrust bearings with the screw shaft flange between them, are located and held by Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 46
Aeronautical Institute of Bangladesh (AIB) the protruding end of the body. A chain-sprocket wheel is splined to the end of the screw shaft, chain connected to the aft cable drum chain wheel. A chain tensioning mechanism is mounted on the upper actuator mounting. It consists of a threaded shaft with a horizontallymounted chain wheel, rotating on a fixed shaft. The lower part of this shaft is clamp connected to the protruding end of the actuator body. The threaded shaft end protrudes through a lug on the upper mounting. Chain tension is altered by nut adjustment. When the screw shaft is rotated in a given direction it will cause the splined actuating shaft to move either in or out. Inward movement is limited by the actuating shaft contacting the outermost thrust bearing, outward movement by a circlip round this shaft. 20.7. AILERON FEEL/TRIM SPRING UNIT The aileron feel/trim spring unit is vertically mounted in the tunnel between the main wheel bays. This unit serves to provide artificial feel and trim under all circumstances. The unit consists of a spring bushing, connected to two feel input arms on the tension regulator shaft, containing a spring, enclosed by two caps. The trim input rod extends through the spring bushing, its two flanges against the caps The trim input rod is on the upper end connected to the trim actuator actuation shaft that acts as a fixed point during control system movements, but can be relocated by trim system operation. The lower end of the trim input rod is supported in a sliding bearing, attached to the tunnel floor. Operation of the mechanical control system causes the two feel input arms to rotate, moving the feel unit bushing, thereby compressing the spring with one cap. This compression provides the artificial feel on the control wheel in thy cockpit. Rotation of the aileron trim control knob on the pedestal operated the trim screw actuator and alters the position of the trim input rod, sliding through the bushing and extending the spring until the opposite cap bottoms again in the bushing. 21.0. AILERON HYDRAULIC CONTROL SYSTEM The left hand aileron hydraulic control system is connected to the hydraulic system no. 1, the right hand one to system no. 2. Pressurizing or depressurizing of the aileron systems is performed by 1. Electrically operated shut-off valves, being one for each system. In de-energized condition of the solenoid the relevant aileron hydraulic system is pressurized. Two hydraulic units are connected to each system. 2. The aileron tab lock-out actuator, to relocate the pivot point of the control tab operating link in the depressurized situation in order to operate this link by input quadrant movements. In the pressurized situation this pivot point is located in the center of rotation of the input quadrant. To provide for the relocation when the system depressurizes, a spring is inSubject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 47
Aeronautical Institute of Bangladesh (AIB) corporated. A spring-loaded sequence valve is provided in order to relocate the operating link before the pressurization of 3. The aileron actuator in order to prevent the hydraulic coupling from being decoupled whilst the tab lock-out piston has not moved. This would make aileron operation impossible. For this reason, press-de admittance to the aileron actuator is via this sequence valve. The actuator housing is connected to the lower pivot arm of the aileron, whilst the piston rod eye end is attached to a lug on the wing rear spar, meaning that it is a follow-up type actuator as described in the "Principle of Operation". The hydraulic coupling serves to disconnect the servo valve from the lower quadrant in depressurized situations order to operate the control tab completely independent of the actuator. When the unit is pressurized, the sticking switch can be operated by the hydraulic coupling in the case of a sticking servo valve. 21.1.
ELECTRICALLY-OPERATED SHUT-OFF VALVES
Each aileron system is connected to its supply system via a solenoid operated shut-off valve. The two valves are located in the rear cargo compartment, mounted in the right hand forward upper part on a passengers compartment floor stringer and accessible after removal of a floor panel. They serve to depressurize their aileron system automatically when tae servo valve in the actuator has stuck, or intentionally by pilot's switching. Each valve consists of housing, accommodating a spring-loaded plunger type slide valve and a ball valve. A 28-V D.C. solenoid is mounted on the valve housing, its spring-loaded core pressing on the ball valve in de-energized conditions. The housing is provided with three connections marked PRESS., RET. and CYL. The CYL. port is connected to the aileron system. 21.2. AILERON TAB LOCK-OUT ACTUATOR The tab lock-out actuators are mounted on the outboard side of the aileron actuators. Each actuator serves to operate the dog link in the power to manual reversion mechanism. They consist of housing, accommodating a spring-loaded tab lock-out piston, a springloaded sequence valve, a pressure port, a return port and two ports for aileron actuator connection. The pressure port is provided with a filter. Hydraulic fluid enters the actuator via the filter and moves the sequence valve against the spring force. This action permits pressure to be admitted to the relevant aileron hydraulic actuator and the lock-out piston. This piston movement pivots the dog link in the tab lock-out mechanism. Depressurization of the aileron hydraulic system results in displacement of the sequence valve by a spring force. This action depressurizes the tab lock-out piston and the aileron actuator, providing aileron actuator uncoupling after control tab engagement.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 48
Aeronautical Institute of Bangladesh (AIB) 21.3. AILERON HYDRAULIC ACTUATOR Each actuator is mounted forward of the inboard side of the aileron, and consisting of a housing, attached to the aileron lower pivot arm, containing a servo valve with decouple and sticking switch, a spring loaded by-pass valve with micro switch and a cylinder/piston assembly. The piston rod end is attached to a lug on the wing rear spar. The servo valve (actually the de coupler sleeve) end is attached to the input quadrant. The supply pressure enters the actuator through a filter union is then admitted to the servo valve for selection purposes, to t hydraulic coupling to connect the servo valve to the input quadrant, and to the by-pass valve to close the cylinder by-pass lines. The pressure in the hydraulic coupling forces the pistons assembly into the sleeve and tries to force the servo valve end out of the sleeve. This last action is stopped by the recess in the pistons assembly, meaning that the ser-vo valve is now connected to the sleeve (and to the input quadrant) by the hydraulic fluid under pressure. Rotation of the input quadrant will displace the servo valve and pressure admittance to the cylinder with immediate follow-up action of the housing and aileron will take place. In the case of a sticking servo valve the sleeve can be moved with respect to the servo valve, forcing the pistons assembly into the servo valve or forcing the sleeve over the pistons assembly, dependent on the direction of movement but in both cases forcing fluid out of the coupling back into the pressure line by movement of the sleeve that trips the sticking switch in order to depressurize the failing actuator. A depressurized actuator enables the sleeve in the hydraulic coupling to be moved without any movement of the servo valve to provide for control tab deflections by the input quadrant. (The aileron and its actuator are then disconnected from the mechanical control system). 22.0. AILERON ELECTRICAL CONTROL AND WARNING SYSTEM 22.1 General The electrical system consists of two independent circuits; the control circuit and the warning (caution lights) circuit. The control system can also be divided into two independent identical circuits; a left hand on connected to 28-V D.C. bus 1 and a right hand one connected to 28-V D.C. bus 2. The circuit for the caution lights on the cockpit panel is connected to the A.C. dim bus 2. The circuit for the central caution light is connected to the A.C. dim bus 1. 22.2
The Control System
This system serves to operate the electrical (solenoid) operated shut-off valves in the aileron hydraulic systems. The left hand and right hand circuits are identical, so only one circuit will be explained. Normally, the shut-off valve solenoid is de-energized to provide for a pressurized system. Energizing can be performed in two ways:
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 49
Aeronautical Institute of Bangladesh (AIB) 1. By switching the master switch on the flight control panel to OFF. This depressurizes the relevant aileron hydraulic system. 2. By operation of the sticking switch when the servo valve sticks. This energizes the sticking relay with subsequent energizing of the solenoid. This relay is provided with a hold circuit, so subsequent opening and closing of the sticking switch will not influence the depressurized condition of the aileron hydraulic system. Before pressure build-up in the system (engine start) the sticking switch could be tripped because the aileron could have a different position from the mechanical control system (hydraulic coupling depressurized). This would create an energized sticking relay and a depressurized aileron system. When this happens, the reset button on the cockpit panel must be pushed in to interrupt the current to the sticking relay. This de-energizes the shut-off valve solenoid and pressurizing of the aileron system takes place. The hydraulic coupling lines up and the servo valve position determines the line-up of the aileron. This opens the sticking switch and the system remains pressurized. Sticking test switches on the test panel behind the co-pilot's seat enable functional testing of the sticking circuits. Operating a test switch provides an earth for the sticking relay so immediate depressurization must follow. Re-pressurizing is performed by depressing the reset switch. 22.3
The Warning System
This system serves to illuminate the caution lights on the cockpit panel and the flight control central caution light on the secondary panel in the case of an abnormal situation. Illumination of the caution lights is via an earth connection provided by the relevant by-pass valve operated switch in depressurized situations. When the relevant master switch (flight control panel) is in the "ON" position, the central caution light will be illuminated as well. Switching "OFF" will extinguish the central caution light. 23.0.
ELEVATOR CONTROL SYSTEM - GENERAL
The elevator is a hydraulically boosted surface. The control systems can be divided in three subsystems; the mechanical control system, the hydraulic control system and the electrical control and warning system. 23.1
The Mechanical Control System
The control columns in the cockpit are directly connected to the elevator pivot arm by means of the mechanical control system. The two columns are interconnected. Movement of these columns will directly operate the elevator via a cable system in the fuselage and vertical stabilizer, connected to two cable tension regulators in the bullet fairing of the horizontal stabilizer. Rotation of these regulators will, via a vertically mounted summing bar and pushrods, operate the elevator pivot arms. Summing bar operation will also displace the servo valves in the elevator booster, resulting in movement of the pistons. The piston rod ends are connected to the summing bar, so every mechanical operation of the elevator will directly be assisted by the hydraulic action.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 50
Aeronautical Institute of Bangladesh (AIB) 23.2
The Hydraulic Control System
The elevator boost control unit is a dual (parallel) unit, one half operating on pressure of system no. 1 (utility system), and the other half on pressure of system no. 2 (flight control system). Pressure admittance is via solenoid operated valves, controlled and operated as described in the "Principle of Operation". The connection between the mechanical control system and the servo valves in the boost control unit is by means of an idler bar in between the summing bar and the servo valves because of the feedback action that must follow when the input motion stops. The first very small movement of the mechanical control system operates servo valves. When the input movement stops, further movement of the pistons will centralize the servo valves via the idler bar, which has a stationary pivot point on the structure. The summing bar pivots on the idler bar resulting in a sort of wobbling motion. This would also happen when the boost control unit is completely depressurized, introducing "play" or "backlash" in the system. When this happens the idler bar is automatically locked by a backlash lock-out mechanism, consisting of two cylinder/piston assemblies operating a spring-loaded lock. As long as hydraulic pressure is present in either control unit half this pressure is also admitted to either cylinder, keeping the idler bar unlocked. When the unit becomes completely depressurized, the spring force will lock the idler bar. 23.3
The Electrical Control and Warning System
In general this electrical system is similar to the one described in "Principle of Operation". The sticking switches are mounted on the idler bar and can be tripped by the spring-loaded detents in this bar when a servo valve sticks. This results in energizing the solenoid of the relevant shut-off valve that depressurizes its respective control unit half. A hold circuit ensures a permanent depressurization. Test switches on the maintenance test panel behind the co-pilots seat enable functional testing of the sticking switch circuits. Repress rising is performed by operating the elevator switches to "OFF" and "ON". This to interrupt the hold circuit. 24.0. ELEVATOR MECHANICAL CONTROL SYSTEM The control columns in the cockpit are connected to the elevator pivot arms and the servo valves in the boost control unit by means of the mechanical control system that consists of:
1.
The control columns, rigidly interconnected by a torque tube below the cockpit floor, the arms of which are pushrod connected to 2. The input quadrants which serve to transmit input motion to the dual cable loops that are connected to 3. The cable tension regulators in the bullet fairing on the horizontal stabilizer which are pushrod connected to 4. The summing bar upper parts. The lower parts are pushrod connected to the elevator pivot arms via intermediate pivot arms, rigidly mounted on the shaft about which the tension regulators can rotate freely and independently. The summing bar pivot point is not stationary but is mounted in an idler bar which has a stationary point on a Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 51
Aeronautical Institute of Bangladesh (AIB) support. The reason for this is that the servo valves of the boost control unit are connected to the idler bar whilst the piston rod ends are connected to the summing bar. This permits servo valve operation and feedback which would not be possible with a stationary summing bar pivot point due to the fact that as a matter of necessity the servo valves would be connected to the summing bar as well as the piston rod ends that would result in a locked summing bar since the pistons are hydraulically locked as long as the servo valves are in the neutral position. They could not be moved out of neutral because of the locked pistons. 24.1. ELEVATOR CONTROL COLUMNS AND LINKAGES Two arms rotate with the torque tube and are pushrod connected to operating levers of the input cable quadrants which are independently rotating on the rudder quadrant torque tube. Each control column is provided with a rigging lug on the front side. 24.2. ELEVATOR CONTROL INPUT QUADRANTS The input quadrants are located below the cockpit floor. They are mounted on the rudder quadrant torque tube, but can independently rotate on it. The quadrants serve to transmit the control column input motion to the cable circuits. For that purpose they are pushrod connected to the column torque tube input arms. For rigging purposes they are provided with rigging holes. A stop bell crank moves with the right hand input quadrant, on the inboard side. Adjustable stop bolts are located at the end of the bell crank arms, whilst the fixed stop is mounted to the right hand bearing support. 24.3. ELEVATOR CABLE TENSION REGULATORS The tension regulators are mounted on and independently free to rotate about a shaft. This shaft is free to rotate in bearings and provided with intermediate pivot arms to transmit the summing bar motion to the elevator pivot arm. The tension regulators are connected to bell cranks, also free to rotate about the shaft with the opposite arm connected to the upper attachment point of the summing bar. The operation is described in the aileron system. Different from the aileron cable tension regulator is that each quadrant is provided with a slack absorber to provide quadrant movement during regulator operation in case slack is developed due to the length of the cable. The slack absorber consists of a slotted link, attached to the yoke end, and the quadrant, the bolt mounted through the slot, so the quadrant can move slightly with respect to the yoke. Attached with the quadrant bolt is a spring retainer, the spring being positioned in between the recess of the slotted link and the spring retainer, trying to force the quadrant half towards the yoke but restrained by the cable tensions. When slack in tee cable is developed, the cable tension decreases, the yoke tilts so now the slack absorber spring is able to rotate the quadrant thus keeping the cable tense.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 52
Aeronautical Institute of Bangladesh (AIB) 24.4. ELEVATOR SUMMING BAR ASSEMBLY (see elevator summing bar drawings) The elevator summing bar is located in the bullet fairing on top of the vertical stabilizer. It serves to provide an interconnection between the mechanical control system and the elevator, with a booster connection to decrease the forces on the control column under normal operating conditions. The assembly consists of two parts, the idler bar and the summing bar. The idler bar pivots at three points in supports (E), attached to the horizontal structure, in one line. The summing bar is attached to the idler bar with three pivot points in one line (B). The booster unit servo valves are attached to the idler bar by means of detent mechanisms and valve sticking micro switches. The booster piston rod ends are attached to the summing bar (C). Also attached to the summing bar are the tension regulator pushrods on the upper connections (A), and the elevator Pushrods on the lower connections (attached to the intermediate pivot arms) (D). When an input movement is applied, the booster piston rods initially remain stationary so the summing bar pivots about the piston rod end attachment points (C) and pivots the idler bar about its supports (E). This displaces the booster servo valves, the pistons are hydraulically actuated, the summing bar is moved by mechanical force and is hydraulically boosted, and pivoting about the servo valve summing bar connection (B). When the input motion stops, further piston motion will pivot the idler bar and servo valves back to the original position, resulting in hydraulically stopping the booster pistons. During this feedback action the summing bar Pivots around the upper pushrod pivot points (A). In case of unpressurised operations, however, this initial displacement of the servo valves would introduce a small in effective travel in the control system when the motion is started (backlash). To prevent this, the idler bar is locked by means of the spring-loaded backlash lock-out mechanism in that case. Input motion then pivots the summing bar around the locked idler bar exclusively (B). 25.0. ELEVATOR HYDRAULIC CONTROL SYSTEM The hydraulic control system consists mainly of the boost control unit, stationary mounted forward of the summing bar in the bullet fairing. It is a dual (parallel) unit, one half operating on pressure of system no.1 the other half on pressure of system nr. 2 constantly. Pressurizing or depressurizing of the control unit halves is performed by internally mounted solenoid operated shut-off valves, being one for each control unit half. In the de-energized condition of the solenoid the relevant half is pressurized. The boost control unit housing is stationary whilst the piston rod ends are connected to the summing bar. The servo valves are connected to the idler bar (by means of detent devices with sticking switches), meaning that it is a feedback type mechanism as described in the "Principle of Operation".
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 53
Aeronautical Institute of Bangladesh (AIB) Each control unit half is provided with a filter, a by-pass valve with micro-switch, a servo valve and an actuating piston, apart from the already mentioned solenoid operated shut-off valve. Two backlash lock-out cylinders are mounted on the control unit; each one connected to the pressure of its relevant control unit half, so pressurizing or depressurizing of these cylinders is controlled by the solenoid operated shut-off valves. The lock-out mechanism serves to lock the idler bar in the case of a depressurized control unit. When this bar was not locked in this case, the first movement of the mechanical control system was a displacement of the servo valves only, rotating the summing bar about the lowest pushrod Pivot points until the servo valves reached their extreme position after which the elevator started moving. Reversing the direction of motion would again first displace the servo valves only. This free movement is called "backlash" that is now Prevented by the spring-loaded backlash lock-out mechanism which locks the idler bar when the control unit is unpressurised. 25.1. ELEVATOR BOOST CONTROL UNIT See 32.0. for operation and location. 26.
ELEVATOR ELECTRICAL CONTROL AND WARNING SYSTEM
26.1 General The electrical system can be divided into two circuits; the control circuit for automatic or manual operation of the solenoid operated shut-off valves in the elevator hydraulic boost control unit, and the warning circuit for illumination of the caution lights in the case of an abnormal situation. The control circuit is connected to 28-V D.C. bus 1. The circuit for the caution lights on the flight control panel is connected to the A.C. dim bus 2. The circuit for the central caution light is connected to the A.C. dim bus 1. 26.2
The Control System
The system no. 1 half circuit is identical to the no. 2 half circuit so only one circuit will be explained. Normally, the shut-off valve solenoid is de-energized to provide for a pressurized control unit half. Energizing (resulting in depressurizing) can be performed in two ways:
1. 2.
By switching the master switch on the flight control panel to "OFF". By operation of the sticking switch when the servo valve sticks. This energizes the sticking relay with subsequent energizing of the solenoid. This relay is provided with a hold circuit so opening and closing of the sticking switch will not influence the depressurized condition of the control unit half. Sticking test switches on the test panel behind the co-pilots seat enable functional testing of the sticking switches circuits. Operating a test switch provides an earth for the sticking relay so immediate depressurization must follow. Repress rising is performed by switching the relevant Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 54
Aeronautical Institute of Bangladesh (AIB) master switch "OFF" and "ON" again. 26.3
The Warning System
This system serves to illuminate the caution lights on the flight control panel and the flight controls central caution light on the secondary instrument panel in the case of an abnormal situation. Illumination of the caution lights is via an earth connection provided by the relevant by-pass valve operated micro switch in depressurized situations. When the relevant master switch (flight control panel) is in the "ON" position, the central caution light will be illuminated as well. Switching "OFF" will extinguish the central caution light.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 55
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 56
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 57
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 58
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 59
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 60
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 61
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 62
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 63
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 64
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 65
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 66
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 67
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 68
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 69
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 70
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 71
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 72
Aeronautical Institute of Bangladesh (AIB)
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 73
Aeronautical Institute of Bangladesh (AIB)
Ref: a. Airframe and Power plant Mechanics General Handbook, AC 65-9A b. Airframe and Power plant Mechanics Power plant Handbook, AC 65-15A , c. Fokker-28 maintenance books, d.
Principles
of
flight,
JAA
ATPL
Training
and
also
from
Internet.
Subject Name
Subject Code:
Aircraft Flight Control System
8241
Issue Date Page 74
