AC-SYS
AC-SYSTEMS, Manfriani: AC-Structure, the Landing Gear, Propulsion System (Propellers, Piston Engines, Turbine Engines),....
AC-SYSTEMS, Manfriani: AC-Structure, the Landing Gear, Propulsion System (Propellers, Piston Engines, Turbine Engines),....
In a tension test, what does the constant E mean?
E is the normal modulus of elasticity or Young's modulus.
In an AC wing, the cross sectional area of the web carry all the
The web carries most of the
The torsion loading on a trapezoidal wing section with no sweepback is maximum at
The local aerodynamic heating at stagnation point is
The hottes part of an AC flying at supersonic speed is
At low speeds the load factor is limited by
At high speed, the limit is set by
- What are the load factor limits for the Pilatus-21 turboprop trainer?
- For the Lockheed Martin F-22 Raptor jet fighter?
- +8/-4
- +9.5/-3.5
- How is the load factor increment when encountering vertical gust defined?
- What does this equation tell us?
- \(delta n=delta L/(m*g)=Kg*(rho*U*V*CLalpha)/(2*m*g/S)\)
- The AC sensitivity to gust is a function of wing loading (2 m g/S), lift curve slope (cL alpha-->depends on wing aspect ration), flight altitude and airspeed.
How do AC actually react to vertical gust?
They accelerate upwards, so the net gust velocity is reduced.
How do Airworthiness specifications define the gust velocities to determine load factors due to turbulence?
They define the standard gust velocities (in equivalent airspeed).
The AC weight influences loads on the AC structure. What kind of loads are these?
- Fuselage bending
- Wing bending
- Shear loads
- Tailplane loads
How do airworthiness regulations limit the minimum MLW for the PC-12?
95% of the MTOW--> the AC must be able to land immediately in case of engine failure and cannot dump fuel in emergency.
For multi engine AC equipped with a fuel jettison system, a lower MLW can be specified.
- What is the MZFW critical for?
- What is the MRW critical for?
- The MZFW is critical for wing bending and shear at the limit load factor-> fuel weight alleviates these loads
- The MRW is critical for rolling, towing, braking loads on ground
- What weight does the MZFW limit?
- What does it mean to keep the MZFW balanced with the MTOW?
- The payload weight that can be carried. The MZFW should be as high as possible, while staying in balance with the MTOW.
- This means that the wing bending and shear at the limit load factor should be approximately the same at MTOW (with full tanks) and at MZFW.
What defines the zoning system in the ATA-100 specification?
The zoning system is specified to simplify the location of parts in modern large transport AC.
How many hierarchies of AC zones are defined?
3: Major zones, Intermediate zones, Minor zones
- What do odd numbers in the AC zoning system indicate?
- What do even numbers indicate?
- Odd numbers mark the zones of the left side
- Even numbers zones on the right side of the AC, as seen by the pilot.
What can the skin of the AC's fuselage be made of?
Fabric, plywood (Sperrholz), thin metal sheet or synthetic materials.
Layout of a tricycle undercarriage:
How much should the pitch angle Θ be at which the tail touches the ground?
10-15°
How much should the roll angle be for large transport AC and for light AC?
- at least 8° for large transport AC
- 15° for light AC
Which two limit angles define the required length of the main landing gear legs?
Ground pitch and roll angles
What loads should be on the nosewheel of the AC?
8-15% of the AC weight
For a general aviation or commuter AC, what maximum value of lift can be assumed for design and testing?
2/3 of the AC weight
On which value does the vertical speed at touchdown depend?
On the efficiency of the shock absorber.
What is the safety margin of a stroke?
2.5cm
What is a typical value for the internal gas pressure p0 of AC oleo shock absorbers under static load?
1800psi (12.5 MPa)
What is shimmy and why can it occur?
Shimmy is high speed vibration of the nose wheel and can occur because of the wheel imbalance, tire wear or free play in the wheel suspension mechanism. A hydraulic damper (shimmy damper) must be fitted.
What is the typical lifetime of a wheel?
300-400 landings
Propeller efficiency:
A short take off AC with a low cruise speed will certainly benefit from a...
...large propeller providing high efficiency and high thrusts at low speeds.
For a fast AC,....
....a smaller propeller diameter will be more suitable.
Which peak efficiencies reach modern variable pitch propellers?
0.85-0.9
What describes feathering?
In the event of an engine failure, propeller pitch must be set to a value that reduces drag to a minimum. (90°)
Fuel Systems:
Fuel tanks:
How much weighs the fuel mass in an AC (except for small general aviation AC)?
25-50% of the MTOM
Fuel pumps:
What are transfer pumps?
Transfer pumps move fuel between tanks to maintain mass balance and to supply fuel to the collector ranks from where fuel is drawn to feed the engines.
What are booster pumps?
Also called engine feed pumps. They are used to boost (antreiben) the fuel pressure in the engine feed lines.
Another reason is to prevent cavitation (vapour bubbles) at high levels.
Valves:
What is the function of shut-off valves?
Shutting off fuel when required, preventing fuel transfer from one rank to another.
What are the functions of refuel and defuel valves?
Filling the tanks to allow flow from the refueling gallery to the fuel tanks, or from the tanks to the refueling gallery during defueling (emptying the tanks for maintenance)
Cross-feed valves?
Shifting fuel from one side of the AC to the other side.
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Cartes-fiches
How are booster pumps cooled and lubricated?
By the fuel, similarly to transfer pumps, and can run dry for a long time.
What are refuel and defuel valves?
Refuel and defuel valves are used when filling the tanks to allow flow from the refuelling gallery to the fuel tanks, or from the tanks to the refuelling gallery during (emptying the tanks for maintenance).
What are Cross-feed valves?
They are used when fuel must be shifted from one side of the AC to the other.
What are fuel vent valves?
They are used to allow air to escape from the fuel tanks during the refuelling process: they may also be used to vent excess fuel from the tanks in flight.
In the PC-12 fuel system: Why is pressurised fuel from the low pressure engine driven pump returned to the wings?
To provide motive flow to the transfer ejector pump and the delivery ejector pump.
In the A380: How are the collector cells always kept full?
By the ejector pumps transferring fuel from the associated inner tank.
On which systems are nowadays most fuel quantity measurements based on?
On capacitance and ultrasonic probes.
- What kind of level does the fuel level information have concerning safety?
- What does that mean?
- The fuel level information is safety critical.
- This means that there is usually a dual redundant computer architecture.
How is the fuel level measurement system designed concerning failure?
The system is designed to fail with graceful degradation, that is to degrade gently in accuracy while informing the crew.
What is fuel tank inerting?
Reduction of oxygen content in the fuel tank air gaps.
- How are left and right wing tanks considered concerning safety?
- And why?
- Left and right wing tanks are usually considered to be non-hazardous.
- The fuel contained within is much cooler and the fuel does not suffer from the proximity of hot AC components.
- How is the fuel tank inerting system sometimes called?
- How is the quantity of oxygen in the air gap reduced.
- Flammability Reduction System (FRS)
- The fuel tank receives nitrogen-enriched air from the On-Board Inert Gas Generating System (OBIGGS). This reduces the quantity of oxygen in the air gap to less than 12%.
What is the purpose of AC hydraulic systems?
The purpose is to control movable parts transforming hydraulic energy (pressure and volume displacement) into mechanical energy (force and stroke).
Which are the four stages of hydraulic systems?
- Generation
- Control
- Transfer
- Utilization
How can hydraulic pumps be generated?
- Mechanically driven by engines or APU through a gearbox
- By electric motors
- compressed air turbines
- external ram air turbines
- manually operated by crew
How are hydraulic pumps equipped?
With valves and feedback circuits to stabilize the delivery pressure.
- What is a standardised hydraulic pressure for civil AC?
- For military AC?
- 3000psi (200bar)
- 4000psi (275bar)
How is hydraulic fluid cooled?
The fuel/hydraulic heat exchanger is used. This ensures that cooling on the ground is available.
Once the AC is in flight, further air/fluid cooling may be provided.
What does the pump case drain line do?
When a pump is running off load, all the heat generated by its inefficiencies is carried away by the pump case drain line.
What are hydraulic filters made of?
Resin bonded paper (Hartpapier) supported by arrangements of metal tubes and wire mesh.
- Which materials are normally used for hydraulic delivery lines (high pressure)?
- Which material is used for low pressure lines (return lines)?
- Stainless steel or aluminium and T6
- Lower quality aluminium alloys
What does the cut-off point need to ensure?
The cut-off point must ensure sufficient volume for the remaining systems under all conditions. (Hydraulic reservoirs)
How are hydraulic pumps usually driven?
Usually mechanically driven by engine through an accessory gearbox.
What is the common hydraulic pump type in aircraft?
Variable delivery, constant pressure.
- What are accumulators?
- What are the two main reasons to use accumulators in an AC?
- Accumulators are pressurised fluid storage units.
- Used for short emergency operation of branches or single components
- damping of pressure fluctuations (ripple accumulators)
- Where is the accumulator located for short emergency operations?
- Where is the acccumulator located in the case of damping of pressure fluctutations?
- The accumulator is installed on a hydraulic line very close to the actuator to be operated, with a check valve to prevent flow to be absorbed by other components.
- Sometimes the accumulator is located near the pump or in the area where more regular pressure is required.
How can the position of an actuator be signaled to the pilot?
This can be achieved by connecting a continuous position sensor such as a potentiometer, or by using microswitches at each end of travel to power a lamp or magnetic indicator.
What does the directional control valve do?
The directional control valve can be seen as a switch to operate an actuator, because it controls the hydraulic flow direction.
What is a restrictor valve?
A Restrictor valve is similar to a check valve; it allows full flow in one direction and a restricted amount of flow in the reverse direction. It is typically used to slow down the return stroke of an actuator.
What does the relief or bypass valve do?
A relief or bypass valve prevents over-pressurization, by opening a line to the reservoir and bleeding fluid from the system until normal pressure is reached.
What does the pressure reducing valve?
Sometimes a branch of the hydraulic system must operate at a lower pressure. This branch can be connected to the main system by a pressure reducing valve. The pressure in the low pressure sub-system pilots the valve spool; when the pressure increases, the spool moves to the right an reduces the opening from the high pressure inlet, and vice versa.
What are shuttle valves?
In case of main hydraulic pump failure an actuator, or a branch of hydraulic system, must be pressurised by an auxiliary or emergency unit, typically an accumulator. A shuttle valve allows selecting the emergency unit in case the pressure in the main system is reduced.
What is a sequence valve?
A sequence valve controls the sequence of operation between two branches of a system. It is often found in the landing gear system, to control the sequence between door opening and landing gear extension or retraction.
What are electro-hydraulic servo valves?
Electro-hydraulic servo valves are control valves driven by a low intensity electric signal. They allows precise actuator control at high frequency response and are typically used in hydraulic flight control systems.
Name the three main parts of a two stage nozzle-flapper servo-valve.
- Electrical torque motor,
- hydraulic amplifier,
- and valve spool assembly
What does the hydraulic power pack provide?
consists of an electrically driven, variable displacement hydraulic pump, an integral filter with automatic by-pass and visual by-pass indication, pressure regulation and failure protection systems, and an integral reservoir pressurised with nitrogen from a pressure cylinder to prevent pump cavitation.
What does the nitrogen charged accumulator in the hydraulic power pack?
The nitrogen charged accumulator compensates fluid thermal expansion and maintains system pressure in case of power pack failure. With normal system pressure leakage rates, it can keep the landing gear in the retracted position for 200 minutes. After this time, the landing gear may start extending due to loss of system pressure.
Why is the hydraulic system a fail safe system concerning the landing gear?
This system is fail safe in the sense that a complete hydraulic failure will eventually cause the landing gear to extend. A hand pump located at the rear of the center console is used to assist in free fall emergency landing gear deployment after failure of the main system.
How many independent sources exist in the B737 hydraulic system?
In the Boeing 737 system, hydraulic power is provided by three independent sources: System A, System B, and standby system.
How are in the B737 the hydraulic Systems A and B reservoirs pressurised?
The system A and System B reservoirs are pressurised by bleed air from the main pneumatic manifold. The standby reservoir is also pressurised, and is maintained at the full level by an interconnecting balance line to the system B reservoir.
