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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What is another reason for the booster pumps?
Another reason is to prevent cavitation at high altitudes. Cavitation is a phenomenon in which the combination of low pressure, relatively high fuel temperature and high engine demand causes the formation of vapour bubbles in the fuel.
What are the consequences of cavitation?
The effect is a drastic reduction in fuel flow to the engine. This can cause a flame-out in the same way as aeration.
How are booster pumps cooled and lubricated?
By the fuel, similarly to transfer pumps, and can run dry for a long time.
What are fuel dump valves?
They open when excess fuel must be dumped from the AC tanks in an in-flight emergency.
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.
How are crossfeed, isolation and LP valves operated to garantee reliability?
They are operated by twin electrical actuators for better reliability.
How can each engine be shut-off?
By a low pressure isolation valve.
How is fuel transported from the centre tank to the inner tank?
By transfer pumps.
- What is the minimum fuel weight in the inner tanks?
- What happens after the inner tanks reached 4000kg?
- 4000kg fuel
- The trim tank tranfers fuel to the inner tanks, by transfer pumps.
What happens if the inner tanks are emptied down to 3500kg fuel?
Fuel from the outer tanks flow to the inner tanks by gravity.
What are most fuel quantity measurements based on nowadays?
They are based on capacitance and ultrasonic probes.
How are the left, centre and right wing tanks categorized concering safety?
- The centre wing fuel tank is categorised as hazardous and requires inerting (replace flammable oxygen with non-flammable inert gas) due to proximity to the external heat sources (i.e. air conditioning packs).
- Left and right wing tanks are usually considered as non-hazardous, primarily as the fuel contained within is much cooler and the fuel does not suffer from proximity of hot AC components.
What is the fuel tank inerting system?
Sometimes also called a Flammable Reduction System (FRS), 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%.
How are movable parts in the Aircraft System controlled?
They are controlled by the aircraft hydraulic system by transforming hydraulic energy (pressure and volume displacement) into mechanical energy (force and stroke).
In which four stages can hydraulic systems be subdivided?
- Generation: Pumps pressurize a fluid
- Control: Valves and other devices control the energy delivered, or pressure
- Transfer: Pipes distribute energy to all necessary areas of the vehicle
- Utilization: Actuators operate the final conversion to mechanical energy
What are the main advantages of hydraulic systems?
- excellent power to weight ratio;
- high power transmission efficiency;
- flexibility in installation;
- resistance to overload conditions;
- high reliability;
- fast response;
- precise position and load control.
Name some functions that can be performed by actuators or hydraulic motors.
- Primary and secondary flight control actuator
- Landing gear extension and retraction
- Landing gear doors operation
- Wheel braking
- Nosewheel steering
- Cargo door operation
- ...
- How is hydraulic energy generated?
- How are pumps operated?
- By pumps
- Mechanically driven by engines or APU through a gearbox (Getriebe),
- by electric motors,
- by compressed air turbines,
- by external ram air turbines,
- manually by the crew
Hydraulic reservoir: What does the cut-off point?
The cut-off point must ensure sufficient volume for the remaining systems under all conditions.
- What are accumulators?
- Why are they useful?
- Accumulators are pressurised fluid storage units (Druckspeicher).
- Short emergency operation of branches or single components of the hydraulic system.
- Damping of pressure fluctuations (ripple accumulators)
What does a directional control valve?
A directional control valve can be seen as a switch to operate an actuator, because it controls the hydraulic flow direction.
What does a check valve?
The check valves are the most commonly used in hydraulic systems. They allow flow in one direction only. They may be installed independently in a line, or they may be incorporated as an integral part of a sequence, counterbalance, or pressure-reducing valve.
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 is a relief or bypass valve?
A relief or bypass valve prevents over-pressurization, by opening a line to the reservoir and bleeding fluid (Ablassflüssigkeit) from the system until normal pressure is reached.
What is a 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.
What is the purpose of a shuttle valve?
A shuttle valve allows selecting the emergency unit in case the pressure in the main system is reduced. 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.
- What is the purpose of a sequence valve?
- Where is it often found in the AC System?
- A sequence valve controls the sequence of operation between two branches of a system.
- In the landing gear system, to control the sequence between door opening and landing gear extension or retraction.
What are Electro-hydraulic servo valves?
They are control valves driven by a low intensity electric signal. They allow precise actuator control at high frequency response.
(Typically used in hydraulic flight control systems)
Name the three independent hydraulic systems of the B737.
System A: driven by electrical and engine 1 driven pumps
System B: driven by electrical and engine 2 driven pumps
STBY system: driven by one electrical driven motor
Name the three continuously operating hydraulic systems of an A320.
Yellow (System 3): pressurised by a pump mechanically driven by No. 2 Engine and also by an electric motor driven pump
Green (System 1): pressurized by a pump mechanically driven by No. 1 Engine
Blue (System 2): pressurized by an electric motor-driven pump that delivers up to 23 litres per minute. A Ram Air Turbine (RAT) can provide up to 78 litres per minute at 150 bar in emergency conditions.
In the event of low hydraulic pressure in an A320 what does the priority valve do?
In the event of low hydraulic pressure, the priority valve maintains pressure to essential systems by cutting the supply to heavy load users.
- What is the bi-directional power transfer unit (PTU)?
- When will the PTU operate?
- The PTU enables the Green or the Yellow systems to power each other without the transfer of fluid.
- In case of an engine failure, the PTU will automatically operate when the differential pressure between the systems is greater than 500psi. On ground, while operating the Yellow system using the electric motor driven pump, the PTU will also allow the Green system to be pressurised.
When does the RAT extend?
The RAT extends automatically in flight in the event of failure of both engines and the APU.
In an engine, what causes the turbine rotors to revolve?
The resulting mixture of fuel and air is burned at almost constant pressure and expands in the turbine. This causes the turbine rotors to revolve.
The more air an engine can compress and use,...
...the greater is the power or thrust that it can produce.
A minor AC system failure:
In load analysis, the system of reference for the equilibrium of loads is
The ultimate stress:
Gust loads can be reduced by design if
The diameter of an oleo shock absorber depends on:
