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Airframe system tutorial answer (Topic 3 ~ 14) 2010

Topic 3: Pumps 1i) A positive displacement pump is a pump that moves a fixed volume of fluid per revolution provided that there is not volume control incorporated into the pump. 2) The difference between a fixed and variable displacement pump is that the variable displacement pump allows the user to control the amount of fluid it moves per revolution, but the fixed displacement can only move a certain amount of fluid per revolution, regardless of speed or pressure. 3i) A single-acting hand pump generally works on the basis of a piston. As the user pulls the lever up, the piston creates a vacuum within the cylinder, sucking the fluid from the tank, and a check valve prevents the piston from sucking fluid from the pressure line. As the user pushes the lever down, the piston pushes the fluid through the pressure line as a check valve prevent fluid from flowing back to the tank.

3ii) A double-acting hand pump has works on the basis of a piston. The cylinder has 2 inlets and 2 outlets, 1 inlet and outlet for each side of the piston. As the user pulls the lever up, the primary side of the piston creates a vacuum, which sucks fluid from the tank, sucking of fluid from the pressure is prevented by the means of a check valve. At the same time, the secondary side of the piston pushes the fluid from the cylinder, allowing the fluid to move through the pressure line, and fluid flow back to the tank is prevented by the means of a check valve. When the user pushes the lever down, the primary side of the piston will pump fluid through the pressure line and the secondary side of the piston sucks fluid from the tank instead. 4i) The external gear pump consists of 2 gears meshed together. Usually only one of the gears is rotated. As the primary gear (the one above) turns clockwise, the other turns anti-clockwise. As the gears rotate, the gears carry fluid between the teeth and the casing of the pump. As the teeth mesh together, the fluid is pushed to the pressure line. When the gears un-mesh, they create a vacuum, sucking fluid from the tank. 4ii) No, the capacity of gear pumps cannot be varied as they are fixed displacement pumps. 5) The rotary vane wheel is placed at the bottom of an oval shaped casing. As the vane wheel rotates, rods protrude out of the vane wheel to carry the fluid. Starting from just before the inlet, as the rods protrude, they create a vacuum, sucking fluid from the tank. As it rotates, more space for the rod to extend is created due to the eccentricity of the vane wheel in respect to the housing. As the fluid reaches the outlet, the space of the rod decreases; forcing the fluid to the pressure line through the outlet. 6i) In an inline swash plate design, the cylinder block in the pump is rotated by the drive shaft. The pistons fitted to the bores in the cylinder are connected to through the piston shoes and a retracting ring. The pistons follow the swash plate as the cylinder rotates, causing the pistons to reciprocate. The ports are arranged in the valve plate so that the pistons that pass the inlet are being pulled and pass the outlet as the pistons are pushed. The displacement of inline swash plate pumps can be fixed or variable by adjusting the swash plate angle. 6ii)

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

In a bent-axis design, the cylinder block in the pump is rotated by the drive shaft. The pistons fitted to the bores in the cylinder are connected to through the piston shoes and a retracting ring. The pistons follow the angle of them with respect to the swash plate as the cylinder rotates, causing the pistons to reciprocate. The ports are arranged in the valve plate so that the pistons that pass the inlet are being pulled and pass the outlet as the pistons are pushed. The displacement of a bent-axis multi-piston pump can be varied by adjusting the axis angle.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

Topic 4: Pressure control valves 1i) The function of the main relief valve is to limit the maximum pressure in the hydraulic system. ii) Cracking pressure refers to the fluid pressure where the relief valve begins to open to let fluid relief through the relief valve. iii) The main relief valve is usually installed at the pressure line, connected to the return line. 2i) A thermal relief valve is similar to a normal relief valve. These valves are used in parts of hydraulic systems where fluid pressure is trapped and may need to be relieved due to increase in pressure when there is a change in temperature. ii) The setting of a thermal relief valve would be lower than the main system relief valve.

3i) The function of an automatic pressure regulator or unloading valve is to unload the pump when the pump fluids are not required by the system. ii) An unloading valve is used at the pressure line, just before the check valve; and connected to the return line to allow the pump to unload fluids back to the tank. 4i) Upward force acting on piston = 1499 x 1 = 1499 lb/sq inch Downward force acting on piston = 1499 x 1/3 + 1000 = 1499.67 lb/sq inch Nothing will happen. 4ii) Upward force acting on piston = 1501 x 1= 1501 lb /sq inch Downward force acting on piston = 1501 x 1/3 + 1000 = 1500.33 lb /sq inch The piston will push the ball out, allowing the fluid to unload through the unloading valve. 5i) The pressure reducing valve is used to limit pressure in a certain branch of the hydraulic circuit to a pressure lower than the relief valve setting for the rest of the system.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

5ii)

Fluid enters at C and exit through E at the reduced pressure at A. A hydraulic force is transmitted through D to exert a force on B. When this hydraulic force exceeds the spring force, the reservoir return valve B is pushed to the right, allowing the poppet spring (F) to seat the poppet. This prevents the system fluid from going out through the reduced pressure port E to build up a further pressure. 6i) A sequence valve is used to allow an action to be performed by the hydraulic system before another action is allowed. ii)

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

As fluid flows from B, It pushes the wheel-well door actuator. Before the actuator is fully activated, the fluid flow stops at Sequence valve B. When the wheel-well door actuator is fully activated, it pushes the sequence valve, allowing the fluid to flow to the main gear actuator, allowing the landing gear to extend after the door is fully opened. When the fluid flows the other way from A, the sequence valves will only allow the wheel-well door to close when the landing gear is fully retracted. 7i) A sequence valve is activated by a mechanical contact and the priority valve is activated by hydraulic pressure instead. ii)

As fluid flows from B, there is not enough pressure in priority valve A, so the wheel-well door actuator activates first. When the wheel-well actuator is fully extended, pressure builds up, casing the priority valve B to allow fluid to flow to the main gear actuator, causing it to retract in order to extend the landing gear. When fluid flows from B, the priority valves will only allow the landing gear to be fully retracted before the wheel-well door actuator can activate.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

Topic 5 –Directional control valves 1i) The function of a selector valve is to control the direction of flow of the fluid in a hydraulic system. ii) Plug type: Looks like a rotary wheel with channels in them; for low pressure application Poppet type: Contains poppet and a shaft to open and close them; for high pressure and leak free application Spool type: Spools with a shaft to control the position to change flow direction; for high pressure application Piston type: A shaft with pistons that blocks or allows flow; for high pressure application 2) An open-centre directional control valve directs fluid through the centre of the valve back to the tank when no units are being actuated. A closed-centre valve stops the flow of fluid when in neutral position. 3a) In neutral position, poppet 3 is opened. Fluid flows in from P and through poppet 3 and back to the return line. b) In up position, poppet 2 and 5 are opened. The fluid flows from the inlet of the valve and through poppet 2 to the actuator at B, extending the actuator. Fluid then flows out from A side of the actuator and through poppet 5 back to the return line. c) In the down position, poppet 4 and 1 are opened. Fluid flows from the inlet of the valve and through poppet 4 to the actuator, causing the actuator to retract. Fluid then flows out from the B side of the actuator, flowing through poppet 1 and back to the return line. 4)

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

Off position: No fluid flow. Gear down: Fluid flow from pressure line to down line; Up line to return line Gear up: Fluid flow from pressure line to up line; down line to return line

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

5)

Neutral position: From P to R. Up position: From P to A and B to R. Down position: From P to B and A to R, through the centre of the shaft. (See dotted line) 6)

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

Position A: From 1 to 2. Pressure from 3 is not enough to overcome the spring. Position B: From 3 to 2 when pressure from 3 overcomes the spring pressure. Flow from 1 is stopped.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

Topic 6 – Flow control valves 1i) An orifice valve is used to restrict flow from both directions. ii) A variable restrictor valve is used to control flow from both direction, allows user to restrict or even stop the flow. 2i) Valve Orifice check valve Check valve

Similarity Allows full flow in one direction Allows full flow in one direction

Difference Restrict flows in opposite direction only Stops flow in opposite direction

Valve Orifice check valve

Similarity Allows full flow in one direction

Metering check valve

Allows full flow in on e direction

Difference Restrict flow in opposite direction. Restriction is fixed Restrict flow in opposite direction. Restriction can be adjusted.

ii)

3) The function of the flow equalizer is to balance flow going to two actuators. (To extend/retract 2 landing gears at the same time)

4) It is used to stop flow to a part of the hydraulic system when a serious leak occurs.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

5) Quantity measuring fuses and pressure sensing fuses.

Quantity sensing: Piston closes after a certain amount of fluid pass through. To open the piston, fluid must flow from opposite direction. Quantity sensing fuses can only stop flow in one direction.

Pressure sensing: The spring overcomes the pressure of the fluid as the fluid pressure drops and the pistons move to the position where it blocks fluid flow.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

Topic 7- Hydraulic Actuators 1) A linear actuator transforms fluid energy to mechanical energy in linear form. 2i) Single acting hydraulic cylinder has fluid inlet on the piston side, a spring on the rod end side of the piston returns the actuator to fully retract position. ii) Translate fluid energy to linear mechanical energy, but has ports at both piston and rod end side. Allows control in bother direction. iii) The double ended linear actuators are normally used in mechanism where equal force/speed is required in both directions of piston rod travel. iv) Used in most flight control systems, tandem hydraulic cylinder allows the output force to be doubled with no change in piston area. They are usually used in places where there is insufficient radial space for larger cylinder actuators. 3) A hydraulic motor is used to translate fluid energy to rotary mechanical energy. 4i) Torque depends on pressure. ii) Rpm depends on fluid flow. 5) Hydraulic gear motor has the same construction of a gear pump. Instead of the gears rotating to move the fluid, the fluid moves the gears. 6) Hydraulic motors can vary speed without loss of efficiency, change direction without stopping first and able to stall without damage. 7i) Servo actuators are used for direct and positive movement when controlled intermediate units are required. ii) In this scenario, the control column is pulled back, to move the elevator to the pitch up position. When the control column is pulled back, the linkage moves the spool in the control valve to right. This allows fluid to flow from the pressure line to the rod end of the actuator, retracting

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

the rod. But as the rod is retracted, the control valve body moves with the load, towards the left, which blocks the pump flow. The cylinder then stops in the intermediate position.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

Topic 8 – Hydraulic Accumulators 1) The three types of accumulators are: diaphragm type, bladder type and piston type. 2) The first reason is that piston type hydraulic accumulators take up less space. The second reason is that diaphragm and bladder accumulators tend to fail in a sudden manner due to rupturing of the bag or diaphragm, which is critical on aircrafts. 3) In a close centre system, pressure is usually accumulated in a subsystem that requires it. In order to use reduce costs and/or reduce the size of pump needed, an accumulator is often used to help achieve the amount of the pressure required, which is usually higher than what the pump is capable of outputting. Such applications, for example, are usually found in landing gears, where hydraulic pressure is required for only a short period of time. 4a)

Note*: I am using a 2 linear actuator system to explain, unlike the question’s 4 linear actuators. In the above example, if in a case where both actuators are required at the same time, but the pump can only provide enough pressure for one to operate, the accumulator’s stored pressure can be put to use. When the selector valves of both actuators are activated, the accumulator together with the pump can produce pressure twice of what the pump can provide, thus allowing both linear actuators to be activated at the same time. 4b) i) Volume required to extend cylinder = (π/4) x 0.152 x 0.4 = 7.069 x 10-3 m3 = 7.069ℓ Volume supplied by pump in 3 seconds = (30/60) x 3 = 1.5ℓ

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

Working volume, Vw = 7.069 – 1.5 = 5.569ℓ ii) Time taken to charge accumulator = 5.569 / 0.5 = 11.138 seconds

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

4c) Min working pressure = Force / Area = 200x103 / (π/4) D2 = 200000 / (π/4) (0.15)2 = 11318 x 103 N/m2 = 113.18 bar (gauge) = 114.18 bar (abs)

4e)

Accumulator size =

V1 =

Vw 1 1 1.4

P ( ) P2

V1 =

P −( 1 ) P3

1 1.4

=

5.569 1

1

50 1.4 50 1.4 ( ) −( ) 114.18 150

5.569 = 56.723l 0.0981789

To get V2 and V3

P1V1 = P2V2 = P3V3 V2 =

P1V1 50(56.723) = = 24.839l P2 114.18

V3 =

P1V1 50(56.723) = = 18.908l P3 150

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

Topic 9 – Hydraulic Power Systems 1) Open systems have fluid flow but no appreciable pressure when actuating mechanisms are idle. Closed systems have direct fluid flow to the main manifold and builds up pressure in that portion that leads to the selector valves. 2) Selector valves Type – Open systems use open center selector valves and closed systems use closed center selector valves. Selector Valves Arrangement – Open systems have selector valves in series and closed systems have selector valves in parallel. Unloading valves – Open systems do not have unloading valves but closed system have them to unload pumps when not in use. Subsystems – Open systems can actuate one subsystem at one time without interference. Closed system have no such limit.

3) In a hydraulic system, pressure is generated due to the resistance of flow when the fluid is pumped. 4)

Pressure regulator – Also known as an unloading valve. It unloads the pump by allowing the system fluid to return to the tank when actuating mechanisms are not in use. System Relief valve – It limits the maximum pressure present in the system to protect the whole system. When system pressure hits the critical pressure adjusted, it allows system fluid to return to the tank. Selector Valves – The selector valves, especially the ones with an open center, allows system oil to return to the tank when the actuating mechanisms are not in use.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

5)

1 – Reservoir 2 – Hydraulic Pump 3 – Check Valve 4 – Filter 5 – Open Center Selector Valve 5.1 – Open Center Selector Valve 7 – Main Landing Gears 7.1 – Nose Landing Gear 7.2 – Flaps 8 – Hand Pump

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

6)

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

7)

As you can see in the above diagram, the pumps of System A provides for Flaps, Slats, Lower Rudders, Ground Spoilers, Nose Brake, Elevator, ailerons and Brakes. The pumps of System B provides for Stairs, Upper Rudder, Cargo Door, Brakes, Elevator and Ailerons. There is also a Standby system that can provide for either System A or B or even both at the same time. In multiple system provisions, multiple pumps that are driven by separate engines are used. The purpose is to ensure the smooth operation of the hydraulic systems in the event that one of the pump or engine fails. In the above diagram, both system A and B provide for their different users and some critical users are shared. Each system contains 2 pumps. The reason for 2 pumps per system is such that in case one pump fails, the other pump can still provide hydraulic fluid flow for the hydraulic system. In case of a complete system failure, usually due to failure of both pumps of a system or even the failure of an engine, the standby system will then provide for the system that is down by taking over the duties.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

8i) Power transfer units are used to pressurize a hydraulic system from another hydraulic system without the exchange of any fluids. The Power Transfer Unit usually consists of a hydraulic motor driving a hydraulic pump. 8ii) The Ram Air Turbines are usually used in emergencies when all hydraulic power is lost. Ram Air Turbines can be extended into the air stream outside the aircraft so that the air flowing through the turbine blade can drive a hydraulic pump to provide enough hydraulic power to actuate systems needed to get the aircraft safely on the ground. 8iii) An Auxiliary Power Unit is a compact, self-contained unit that provides electric power and compressed air to drive hydraulic pumps during periods of ground activity or in flight if needed. The Auxiliary Power Unit consists of a small gas turbine engine with engine controls. Its compressor bleed system is connected to the airplane’s pneumatic systems and provides pneumatic power for airplane functions.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

Sub-topic 10 1i) Aircraft pneumatic systems are primarily used as emergency sources of pressure for many of the hydraulic actuated subsystems. 1ii) Landing ear retraction and extension, nose wheel centering, propeller brakes, main wheel brakes and passenger door retraction. 1iii) Advantages: •

Air is available as an unlimited supply.

The components in a pneumatic system are reasonably simple and lightweight.

Minimum weight as compressed air is lightweight and no return line is needed.

Pneumatic systems are relatively free from temperature problem.

No fire hazard and danger of explosion is low.

Contamination easily controlled through use of filters.

Disadvantage: Does not provide any form of lubrication. 2i)

In a single-stage reciprocating compressor, when the piston moves down, the check valves at the inlet of the pump opens due to vacuum created to allow air to flow into the piston chamber. As this happens, the check valves at the outlet of the compressor remains closed to prevent air from delivery manifold to be sucked in. As the piston moves up, the check valve at the outlet of the compressor opens, allowing compressed air to be supplied to the delivery line. While this happens, the check valve at the inlet remains closed to prevent air from flowing to the inlet.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

2ii)

1. Stage 1 piston draws air in as it moves down. 2. As stage 1 piston moves down, stage 2 piston move up to pump compressed air through outlet. 3. Stage 1 piston moves up, pumping air through intercooler for cooling and stage 2 piston moves down to draw air from intercooler. 4. The cycle repeats.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

3)

i) The shuttle valve allows the use of ground pneumatic pressure sources when the pump is not in use and the aircraft is on ground. ii) Unloading valves allows pumps to unload when systems are not in use at 3300psi. iii) Reduces the pressure that goes to passenger door, normal gear control, and nose steering and propeller brakes to 1000psi. iv) Removes most of the moisture from the pneumatic air leaving the pump. v) Removes the remaining moisture present in the pneumatic air not removed by the moisture separator. vi) Allows isolation of pneumatic system users in case of a contamination. vii) Provides pneumatic power by pumping air when aircraft is in flight. viii)

Provides emergency pneumatic power in case of compressor failure.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

Sub Topic 11 – Landing Gear and Brake systems 1) The landing gear is the primary shock absorbing element and supports the weight of the aircraft during ground maneuvers. It also provides braking and steering for ground maneuvering of the aircraft. 2) I)

Absorbs secondary landing shock

II)

Dampens vibrations when aircraft taxiing or being towed and absorbs impact on landing.

3i) Advantages: •

Good forward visibility

Cabin is level, easier for passengers to move within cockpit

Allows more forceful applications of brakes without nosing over

Disadvantages •

A lot heavier

More suitable for use on smooth road surfaces

3ii) Advantages •

Reduced drag in air and reduced weight

Suitable for rough field applications

Disadvantages •

Loss of forward visibility when maneuvering on ground

Less stable on ground and requires more skill when taxiing

4) 1. If the nose is extended and not locked, its weight assisted by the air stream will cause it to lock. 2. If there is not hydraulic power to extend the gear, weight of the gear and air stream will push nose gear into extended position. 5) The shimmy damper is a hydraulic snubbing unit that reduces the tendency of nose wheel to oscillate from side to side. It can also be modified to provide power steering as well as shimmy damper action. Wheel shimmy can be caused by uneven tire pressure, uneven tire wear or uneven running surface when taxiing.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

6) i)

The shock strut is a vertical member of the landing gear assembly. It supports the aircraft on the ground, protects the aircraft structure by absorbing shock on landing.

ii)

It allows up and down movement of the inner cylinder within the outer cylinder, but prevents rotational movement of the inner cylinder with respect to the outer cylinder.

iii)

A drag link is designed to support the shock strut and stabilizes the shock strut longitudinally.

iv)

It applies pressure to the centre pivot joint to prevent the link from pivoting at the joint except when the gear is retracted, thus preventing collapse of the gear during ground operation. It locks the main gear only in the down position.

v)

It is an additional safety device that prevents the collapse of the landing gear during ground operation.

vi)

Axles are what that connects wheels to the trunk which is pivoted to the lower end of the inner cylinder of the shock strut. For large aircrafts, in order to distribute weight and reduce the size of wheels, more gears and wheels are used. Additional wheels also prevent wheel loads from inducing high stress into the pavement structure.

7) The shock strut consists of 2 telescopic cylinders, the outer cylinder and the inner cylinder. The upper part of the outer cylinder is attached to the aircraft structure, or is an integral part of the trunnion, which is attached to the airframe. The inner cylinder, is the movable portion of the shock strut, has the lower part attached to the trunks, which are attached to the axles where aircraft wheels are fitted. Air/Gas and oil are charged through charging valves at the upper part of the outer cylinder. The valves are used to service the shock strut with nitrogen and hydraulic fluids which are used to absorb shock. The gas acts like a spring and the fluid acts like a damper. When the aircraft lands, the shock strut compressed, which causes the volume of the gas to decrease but increase in pressure. On the other hand, the volume of the hydraulic fluid does not change. Instead, the initial shock of landing is cushioned by the hydraulic fluid being forced through the metered opening in the piston. As the pressure and temperature of both the gas and the hydraulic fluid increase, vertical speed of the aircraft decrease. Pressure in both cylinders will increase until it is sufficient to stop vertical movement of the aircraft. At this point, the energy in the gas pressure is sufficient to recoil the aircraft upward. During recoil, which is when the strut begins to extend, limited fluids are controlled back to the inner cylinder through the metered opening in the piston. The shock strut will then extend until the gas pressure is just enough to support the weight of the aircraft. 8) For large aircrafts, in order to distribute weight and reduce the size of wheels, more gears and wheels are used. Additional wheels also prevent wheel loads from inducing high stress into the pavement structure. Another function is that multiple wheels provide for redundancy in case of a tire blow out.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

9)

Functions of aircraft tires include: •

Supporting the aircraft

Absorbing some shock during landing and taxiing

Providing gripping contacts and discharging static electricity.

*Extra information: Aircraft tires are filled with nitrogen because nitrogen molecules are bigger, thus gas in the tires escape at a lower rate. Nitrogen is also relatively inert and lack moisture or oxidative qualities, which in a way or another contribute to higher reliability of tires. Aircraft tires are usually inflated with nitrogen or helium in order to minimize expansion and contraction from extreme changes in ambient temperature and pressure experienced during flight. 10) Requirements of aircraft tires: •

Very high internal pressure (200psi)

Extremely heavy load for short durations

Facilitate stability in high crosswind conditions

Filled with Nitrogen or Helium to minimize expansion and contraction during flight

Equipped with heat fuses (Deflates tire in case of overheating to prevent explosion of tire)

Requirements of automotive tires: •

Low internal pressure (27 to 32psi for cars and 32 to 40psi for pickup trucks and SUVs)

Relatively constant load for long durations

Facilitate stability mainly for rain and maximum fuel economy

Can be filled with air. No strict restriction on inflation gas used.

Not equipped with heat fuses as tires do not overheat in normal operating conditions

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

11i)

1.

When the pilot operates the steering control to steer the aircraft, the steering control valve direct hydraulic fluid into either one of the steering cylinders.

2.

Fluid from the opposite of the cylinders is directed back to the reservoir through a pressure valve which holds constant pressure on the system to snub any shimmying.

3.

An accumulator in the line to the relief valve holds the pressure on the system when the steering control valve is in the neutral position.

4.

Relief valve is used to release pressure if a turning load becomes so great that it can cause structural damage.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

11ii)

Refer to Fig. 14-23: 1.

When the wheel is operated, the follow-up differential mechanism is upset, and this unit pivots and actuates the steering-control valve.

2.

The steering control valve directs hydraulic pressure to the appropriate steering cylinder and releases pressure from the other cylinder.

3.

When the nose wheel starts to rotate, the cable around the steering collar on top of the nose wheel strut moves and return the follow-up differential mechanism to a neutral position. This centers the steering-control valve and stops rotation of the nose wheel.

4.

If the cockpit control is moved by a large amount, the follow—up differential mechanism is deflected an equal amount, requiring that the cylinders turn the nose wheel a large amount to return the follow-up unit and steering control valve to a neutral position.

5.

A similar operation would occur if the control were moved a small amount. The nose wheel would move only a little before the follow-up differential mechanism returned to the neutral position.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

12) A Landing gear indicating system is used to provide a positive indication to the crew on the position of the landing gear and operation of locks. The system usually consists of a number of micro switches or proximity switches on the up-lock and down-locks connected to landing gear position indicators on the instrument panel. When a red light is indicated, the landing gear is not locked in selected position. (Even when landing gear is moving up or down) When a green light is indicated, the landing gear is down and locked. When no light is indicated, the landing gear is up and locked.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

Sub-topic 12 – Understand the Aircraft Brake Systems 1i) 1.

Reservoir – A storage tank that supplies the fluid to compensate for small leaks in the connecting lines or cylinders.

2.

Master cylinder – a foot-actuated, single-action reciprocating pump. Its purpose is to build up hydraulic fluid pressure in the brake system.

3.

Brake assembly – consists of the wheel and necessary lines, hoses, fittings and brake mechanism, which is usually a drum brake with actuating brake shoes.

1ii) This type of braking system is usually found in small aircraft. The reason is that the force required is low, so this system is suitable as the pressure built up is low. 2)

There’s always pressure from the pressure manifold, but when the pilot activates the brakes by pedals or other control methods, the pilot metering valves will then direct flow to the shuttle valves and then to the brakes. i)

Bypass check valve – The function of the bypass check valve is to only permit flow in one direction but not the other. In this case, the bypass check valve is to prevent loss of brake system pressure in the case of a hydraulic system failure.

ii)

Accumulator – The accumulator is used to store a reserved pressurized supply of fluid. In a brake system, it holds pressure for the brakes in case of a failure of the main hydraulic system.

iii)

Metering valves – The metering valve amplifies the input from the pedals into a more powerful pressure for the brakes.

iv)

Shuttle valves – Shuttle valves basically allows flow from two sources. In a brake system, shuttle valves are used to direct pressurized fluid from the emergency brake control valve to the brakes if the normal brake control valve system fails.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

Debooster valve: Hydraulic system pressure is normally too high for effective brake action, so a debooster valve is installed between the metering valves and the wheel cylinders. The function of a debooster valve is to reduce pressure and increase the volume of fluid going to the brakes.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

3a) In a power brake system, the pilot metering valve or the power brake valve is connected to the pedals. The pedals are connected tro the valve through a valve rod. 3b) i) When the pedals are depressed, the valve rod moves to the right, closing the return port and connects the pressure port to the brakes; this allows flow of hydraulic fluid to the brakes, putting pressure on the brakes. A passage through the valve rod allows the hydraulic fluid under pressure to enter a compensating chamber. Pressure at this end creates a return force tending to close the valve. This causes the metering rod to move slightly to the left, closing both the return port and the pressure port, holding the brake pressure. ii) When the pedals are released, the pressure in the compensating chamber and the force of the return spring moves the valve rod to the left, connecting the brake pressure line to the return line. This causes the pressure in the brake line to fall as fluid flows from the brakes to the return line.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

4) i)

When the brake pedal is depressed, the spool (component in red) moves to the left, allowing fluid from the system pressure manifold to flow to the brakes. This creates a pressure in the brakes, which causes the brakes to activate. As pressure builds up in the brakes, some of the fluid flows into the compensating chamber of the spool, which pushes the spool slightly to the right, holding the brake pressure.

ii)

When the brake pedal is released, the pressure in the compensating chamber of the spool and the spring force pushes the spool to the right, allowing fluid to flow from the brakes to the system return manifold. This releases the brake pressure in the pressure, disengaging the brakes.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

5)

When a total failure of hydraulic system occurs, the pilot can operate a pneumatic valve to use nitrogen to operate the brakes. When the pneumatic valve is opened, nitrogen from a bottle or container flows to a air/fluid transfer tube. The nitrogen will then exert a pressure to the fluid, which then exerts pressure of the fluid that flows from the tube to the shuttle valves. At this point of time, the shuttle valve then allows the fluid to flow into the brakes to apply brake pressure. 6) i)

The wheel speed sensor is a small AC tachometer generator, as the wheel spins; it produces an output voltage which is directly proportional to the speed of the wheel.

ii)

The antiskid control box contains a computer and electrical circuitry to interpret the signal from the wheel-speed sensors, compare them with pre-programmed values tailored to the aircraft it is installed in and then send appropriate signals to the antiskid control valves to prevent a skid from developing.

iii)

When a skid develops, a signal from the control box is sent to the control valve solenoid to lower brake pressure by dumping fluid back into the reservoir.

iv)

The squat switch is used to sense if there is aircraft weight on the wheels. Together with the wheel speed sensor, both the squat switch and the wheel speed sensor must be activated before the skid control valves permit brake application. This is to prevent the wheels from being locked when they contact the runway.

7) i) The main function of the control box in the air is to ensure that the brakes do not engage unless the weight of the aircraft is on the wheels. ii)

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

On the ground, its main function is to prevent the wheels from locking and cause skidding by controlling the brakes. When the wheels lock, it releases the brake completely to allow the wheels to turn again; when wheel sliding action just begun, it reduces the brake pressure to allow the wheels to rotate faster.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

8)

In the air: 1.

When aircraft is in the air, the squat switch is closed.

2.

This causes lock-wheel detector to send signal through amplifier to anti-skid control valve.

3.

Anti-skid control valve will then open the passages to return manifold.

4.

This prevents the application of brake pressure, which is also known as touchdown protection.

On the ground 1.

When aircraft touches down, the squat switch opens to locked-wheel arming circuit.

2.

When wheel speed sensor senses wheel speed reach 15 to 20mph, it sends signal to locked wheel detector.

3.

Locked wheel detector then sends signal to antiskid valve to allow full brake pressure.

4.

When wheel decelerates faster than preprogrammed values, skid detector sends signal to amplifier.

5.

This signal then goes to the anti-skid control valve to reduce brake pressure.

6.

At the same time, a signal is sent to the modulator which assesses the amount of current flowing to anti-skid solenoid valve.

7.

This will maintain a pressure slightly less than that which causes a skid.

8.

When all wheels are turning at less than 15 to 20 mph, the locked wheel arming circuit becomes inoperative.

9.

This gives pilot full control for low speed taxiing and parking.

10. If hydroplaning or skids on ice and slows down to less than 10mph, locked wheel detector sends signal to valve. 11. This signal will then cause valve to allow fluids to return to return valve until wheel speed increases.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

9) The 2 conditions are: 1.

When wheel speed is less than 6mph slower than aircraft speed

2.

When speed differences of all wheels are very small, within 5mph

10)

1.

Flapper

2.

Permanent Magnet

3.

Nozzles

4.

Filter

5.

Fixed Orifices

6.

Second Stage Spool

1.

When skid develops, a signal from the control box is sent to the skid control valve solenoid.

2.

This signal energizes the coils on the armature of the flapper valve, causing the flapper to move left.

3.

This will cause the fluids to flow more to the right of the flapper, causing more pressure drop. In the cause, P1 will be higher than P2.

4.

The spool valve thus moves to the right, causing the pressure line to be blocked, while the brake pressure line connects to the return line.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

11) i)

Normal skid control comes when wheel rotation slows but has not stopped. When slowing down takes place, wheel sliding action just begun. In this situation, skid control valve removes some brake pressure from the wheel. This allows wheel to rotate faster to stop the sliding.

ii)

Locked wheel skid control causes brake to fully release when the wheels lock. A locked wheel occurs when there is a lack of tire friction with the surface, It occurs when normal skid control fail to prevent wheel from reaching full skid. To relieve locked wheel skid, pressure is bled off longer than in normal skid function. Doing so allow wheel to have time to regain speed. Lock wheel skid control is not in use when aircraft speed is less than 15 to 20 mph.

iii)

Touchdown protection prevents application of brakes during landing approach. This prevents the wheels from being locked when they contact the ground. The wheels have to start rotating before they carry full weight of aircraft. Before brakes can be applied, squat switch must detect weight of aircraft is on wheels and when speed of wheels are over 15 to 20 mph.

iv)

Fail-safe protection circuit monitors the operation of skid control system. It automatically set brake system to full manual mode in case of a system failure. When this happens, a warning light is turned on.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

Sub topic 13: Understand the principles of Flight Control

1) Flight control surfaces are used to provide stability and control during flight.

2) Ailerons: Provides roll control of the aircraft. (Aircraft movement about longitudinal axis) Elevators: Provides pitch control of the aircraft. (Aircraft movement about lateral axis) Rudders: Provides yaw control of the aircraft. (Aircraft movement about vertical axis)

3) The two main functions of secondary flight controls are: i)

Modifying the amount of lift the primary controls produce

ii)

Change the amount of force needed to operate the primary controls

Primary flight controls includes: i)

Ailerons

ii)

Elevators

iii)

Rudders

Secondary flight controls that modify lift include: i)

Flaps

ii)

Slats

iii)

Spoilers

iv)

Speed brakes

Secondary flight controls that change operating forces include: i)

Trim tabs

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

4) i)

Flaps change the chamber of the wing and modifying the lift. Flaps are found near the trailing edge of the wings.

ii)

Slats allow air from high pressure area beneath the leading edge of the wing to be directed along the top of the wing. This reduces the possibility of stalling at low speeds. They are located at the leading edge of the wing.

iii)

Speed brakes are large panels to control the speed of the aircraft. They are located between the leading and trailing edge of the wing.

5) i)

Plain flaps are sections of the trailing edge of the wing, inboard of the ailerons. They are about the same size as the aileron and are hinged so that they can be deflected. The effects of plain flaps are generally minimal and are seldom found on modern aircrafts. ii)

Split flaps are usually lower part of the trailing edge that hinge downwards. The effects of split flaps are generally minimal and are seldom found on modern aircrafts. iii)

Slotted flaps are similar to plain flaps, but when they extend, a gap develops between the wing and the flap. This allows air to flow smoothly through the gap and helps hold the airflow on the surface. This increases the lift of the wing with the flap extended. Slotted flaps are the most popular on airplanes today.

iv)

When extending, fowler flaps slides backwards in the first portion of its extension. This increases lift by a great amount with a small increase in drag. As the extension increases, the flap deflects downward, increasing drag with little increase in lift.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

v)

The leading edge flap, also known as Krueger flap, when retracted form part of the leading edge of the wing. When extended, the flap moves forward and downward to increase the camber of the wing and increases lift at low flight speeds.

6) Trim controls are secondary flight controls that give pilot finer control over flight or to ease workload. The most common control is elevator trim control, which allows the pilot to control the elevator through a wheel or other devices instead of constantly maintain an forward or backward pressure of the control column to maintain a specific pitch attitude. A trim tab is usually connected via a cable, linkage and gearing system to a trim wheel in the cockpit. When a trim tab is employed, it is moved into the slipstream opposite to the control surface’s desired deflection. To trim an elevator to hold the nose down, the trim tam will actually rise up into the slipstream. This increases pressure on top of the trim tab surface caused by raising it will then deflect the entire elevator slab down slightly, causing the tail to rise and the nose to move down.

7) Spoilers are also called lift dumpers. When spoilers are operated symmetrically, it spoils the lift on the wing during landing. When it is operated asymmetrically, it makes the aircraft roll during flight, acting like an aileron.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

Sub-topic 14: Flight Control System 1) i)

Cables are fabricated from carbon fiber or corrosion resistant steel wire and are generally used for aircraft control.

ii)

Pulleys are used in aircraft control systems to change the direction of a cable.

iii)

Turnbuckles are used to adjust the tension of the control cables.

iv)

A Push-pull rod is used between bell cranks and from bell cranks to torque arm to transmit the force and motion from one to the other.

v)

A quadrant is often employed at the base of a control column or control stick to impart force and motion to a cable system.

vi)

A torque tube is a hollow shaft by which the linear motion of a cable or push-pull tube is changed to rotary motion.

vii)

Cable guards are installed in the flanges of pulley brackets to prevent the cable from jumping out of the pulley.

viii) Fairleads are used to guide control cables where they pass through a structure member. The other functions of a fairlead are to vibration dampening of the cable, maintain cable alignment. And to seal openings in bulkheads. 2) The mechanical control systems used in a aircraft are: •

Push-pull rod system

Cable and pulley system

Chain and sprocket system

3) Steel cables used in aircraft mechanical transmission are constructed with 7 strands which are helically twisted and each strand consists of a number of wires which are helically twisted.

4) i)

For 3.2mm x 7 x 7, it means that the cable has a circumscribed circle diameter of 3.2mm, consisting of 7 strands and each strand consisting of 7 helically twisted wires.

ii)

For 3.2mm x 7 x 7, it means that the cable has a circumscribed circle diameter of 3.2mm, consisting of 7 strands and each strand consisting of 19 helically twisted wires.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

5) 6) •

Cables are lighter than push-pull rod.

Cable system requires less connection, resulting in less backlash and more accurate transmission.

Cable system shows signs before failing whereas the push-pull rod can fail without warning.

Cable tension deceases as the aluminum aircraft fuselage contracts more than steel during cold weather. Push-pull rod have the same material as the aircraft structure.

7) When ambient temperature decreases, the tension decreases. To remedy this, a cable tension regulator is used. A cable tension regulator consists of 2 quadrant halves mounted on common axles, spring loaded cross head mounted on guide.

8) •

Cables can only transmit pull forces, while push-pull rods can transmit both push and pull forces.

Push-pull rods do not require adjustment of tension, unlike cable systems.

9) i)

When the control wheel is moved to the left, the right ailerons move down (increase lift) and the left ailerons move up (decrease lift). This results in a roll to the left.

ii)

When the control column is moved forward (away from the pilot), the elevator moves down. This increases the upward force causing the aircraft to nose down.

iii)

When the right pedal is pushed forward, the rudder deflects to the right, causing the aircraft to turn right.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

10a)

When the pilot moves the control column to increase aircraft pitch, the bottom part of the control lever pivots at the servo unit will move to the left, while the top part of the control lever moves to the right, and accordingly commence moving the elevators up. This causes the control valve piston to move to the right and allow hydraulic fluids to flow to the left side of the actuating cylinder. The piston rod, which is secured to the aircraft structure, will cause the whole servo-unit and the control lever to move to the left until the control valve piston moves back to its original position. This produces greater control effort and hence the pilot is assisted in making further upward movement of the elevators.

10b)

In the power-operated system the pilot’s control is connected to the control lever only, while the servo-unit is directly connected to the flight control surface. When the pilot moves the control column to increase aircraft pitch, the bottom part of the control lever pivots at the servo unit will move to the left, while the top part of the control lever moves to the right. This causes the control valve piston to move to the right and allow hydraulic fluids to flow to the left side of the actuating cylinder. The rod, which is secured to the aircraft structure, will cause the whole servo-unit and the control lever to move to the left until the control valve piston moves back to its original position. The effort required by the pilot to move the control column is simply that needed to move the control lever and control valve piston.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

In the power-operated type hydraulic transmission system, the pilot does not feel any of the air load effect on the flight control surface.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

11) An artificial feel unit is used to simulate air load. It is incorporated at a point between the pilot’s controls and connected to the servo unit control lever. The artificial feel unit simulates air load smaller than actual air load making it easier to control the aircraft. 12) •

Main components in fly by wire systems consist of computers, sensors and wires.

Fly by wire carry signal from pilot’s control through electrical wires.

Movements of control column and rudder pedals are measured by electrical transducer

Hydraulic actuator units are controlled by computers

Fly by wire systems require less maintenance.

Fly by wire systems are lighter.

Fly by wire systems are safe from sudden breakage of mechanical links are signals are electrical.

Fly by wire systems are less bulky and more flexible to be installed as electrical wires are used.

Fly by wire systems can respond more flexibly to changing aerodynamic conditions.

13)

14) A yaw damper is an automatic flight control device that senses the Dutch roll and applies corrective rudder action to prevent it. It moves the rudder an amount proportional to the rate of yaw, but in the opposite direction. 15) Aircraft rigging is the final adjustment and alignment of the various component parts of an aircraft and its control surfaces so that all the surfaces have exactly correct angles in order that the aircraft and the systems give a proper aerodynamic response and exhibit design characteristics. 16) Aircraft rigging involves checking and adjustment of the cables and linkages in the flight control transmissions to ensure proper system operation. 17) The purpose of aircraft rigging is to ensure that the flight control surfaces provide the proper aerodynamic response and effect when used according to design characteristics. 18) If an aircraft is out of rig, it is impossible to perform any hand-off flights on it, for example, using of autopilot as the flight control surfaces are unable to behave to design characteristics.

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Airframe system tutorial answer (Topic 3 ~ 14) 2010

19) Aircraft symmetry check is the procedure in which the measurement on the distances from the reference point on the aircraft central axis to the reference points on the adjustable components which are within the tolerance given in the maintenance manual. In short, aircraft symmetry check is to ensure that the aircraft is in correct alignment. 20)

1.

The aircraft is first leveled.

2.

Then measurement of the distances from reference points on the central axis and the reference points on the adjustable components is done.

3.

Show in the diagram, alignment points are numbered 1 to 6, from left to right.

4.

Measurements of 1R to 3R must be equal to 1L to 3L within the allowed tolerances given in the maintenance manual.

5.

Similarly, 2R to 5R must be equal to 2L to 5L and etcetera. If any of the measurements are not within the tolerances, the alignment must be adjusted.

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Airframe System Tutorial Answers