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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Lernkarten
A constant speed propeller operates at the minimum pitch angle
How are the sections in the turbine engine called?
- Inlet
- Compressor
- Combustion chamber
- Turbine rotors
- Exhaust duct
- Nozzle
Jet engines: How is the power that the turbine extract from the hot gases used?
It is used to drive the compressor.
Why does the air need to be compressed in the engine?
Because the gases created by the fuel and air mixture burning under normal atmospheric pressure would not expand enough to do useful work. Air under pressure must be mixed with the fuel before the gases produced by combustion can be successfully employed to make a turbojet operate.
What is the difference between HPC and LPC?
The HPC is designed to operate at higher rotational speeds than the LPC.
In which station in the jet engine is the expansion of the combustion gases used to power the HPC?
The expansion of the combustion gases through the HPT, between station 4 and 4.5, provides power to drive the HPC.
How is the LPC powered?
The gases expand further through the LPT which power the LPC.
What happens in the afterburner?
In the afterburner more fuel is sprayed in the exhaust gases and burned to raise their temperature.
What is the purpose of the flameholder?
Station 6 is the flameholder, a device that creates a low speed turbulent flow area to prevent the combustion flame from being blown out.
Which conditions can the nozzle flow have?
- Perfectly expanded: p9=p0. Always when the exhaust flow is subsonic. This is the momentum balance. For subsonic or sonic exhaust flow, perfect expansion is only possible if A9 has a certain value, which depends on the upstream conditions in the engine.
- Overexpanded: p9<p0. This is the case for subsonic exhaust flow when the nozzle area A9 is larger than needed for perfect expansion.
- Underexpanded: p9>p0. This is the case for sonic or subsonic exhaust flow when the nozzle area A9 is smaller than needed for perfect expansion.
Why is the underexpanded (p9>p0) condition not good for airliner?
In such a case the size of the nozzle could be changed.
What is in the uninstalled engine (in static conditions) not accounted?
The skin friction and pressure drag of the nacelle. The influence of the nacelle and of the pylon on the static pressure at the exit plane of the control box is ignored. Engine nacelles and pylons affect the aircraft aerodynamic characteristics, causing interference drag effects.
How is the thermal efficiency defined?
The thermal efficiency of a jet engine is defined as the ratio of its mechanical output to the fuel thermal energy released in the combustion process. thermal efficiency=Ekin/(mfuel*HL)
What happens with the mechanical output if the nozzle is perfectly expanded?
If the nozzle is perfectly expanded, the mechanical output is equal to the increase in kinetic energy of the gas mass flow.
What happens if the nozzle is not perfectly expanded?
If the nozzle is not perfectly expanded, the mechanical output of the engine must take into account the effect of the imbalance between atmospheric pressure and exhaust pressure.
The pressure imbalance contributes to the engine thrust. This can be accounted by introducing the concept of effective exhaust velocity U9eff.
If only 30% of the fuel thermal energy is converted into kinetic energy of the exhaust, what happens to the rest?
A small fraction is used to overcome the internal friction in the engine and to drive accessories, such as the fuel pump and the oil pump.
What can be done to improve the thermal efficiency of such an engine?
We could lower the exhaust temperature by installing an additional turbine wheel to produce mechanical power. This power could be used to drive a propelle, a fan or a helicopter rotor.
What is the definition of the propulsive efficiency?
The propulsive efficiency is the fraction of the engine mechanical output which is converted into propulsive power.
Propulsive power is the product of installed engine thrust and flight speed.
What is the definition of specific thrust?
The specific thrust is the ratio of thrust to air mass flow. It is a primary figure of merit to define engine performance and thermal efficiency is also primary figure of merit for military engines.
What does it mean to maximise specific thrust?
It means to minimize the engine intake area. This is desirable because it reduces airframe drag, engine size and weight.
What is the disadvantage of maximizing the specific thrust? Why is the specific thrust for transport engines not as important as propulsive efficiency?
This also means that the exhaust speed is increasing and this has a negative effect on propulsive efficiency.
How is the specific fuel consumption defined?
For jet engines, the specific fuel consumption is defined as the ratio of fuel flow and thrust.
What is one of the most important engine design parameters in civil aviation?
The TSFC, it has a major influence on direct operating costs.
Propulsion system: What is the definition of the overall efficiency?
The overall efficiency of an AC propulsion system is the product of the thermal and propulsive efficiency. It is the fraction of the fuel thermal power which is converted into propulsive power.
The cruise range of an AC is...
The AC range is...
Which assumptions are made to analyse the work of the Joule Cycle?
- the compression and expansion processes are isentropic (adiabatic and reversible);
- there are no pressure losses in the combustion chamber;
- the working fluid is a perfect gas;
- the gas mass flow is constant throughout the cycle.
How is a jet engine with no turbine and compressor called?
Ramjet -->Used for missiles, where auxiliary rockets are used to accelerate to the high Mach numbers needed to start the ramjet.
Which parameter is the main sizing parameter for engines?
The air mass flow rate m0. It determines the engine face diameter.
Which is the primary parameter that sizes the engine itself?
The TO gross weight of the AC.
What are the main functions of the AC fuel system?
- fuel storage
- fuel tank pressurisation and venting;
- engine feed
- fuel transfer
- refuelling and defuelling
- engine oil cooling
- fuel jettison
- in-flight refuelling
Whcih components form the fuel system?
- fuel tanks
- pumps
- valves
- fuel-oil heat exchangers
- fuel quantity sensors
- fuel management computers
What are integral tanks?
Integral tanks are part of the wing or tailplane structure.
What are the advantages of integral tanks?
The location is close to the AC centre of gravity and the wing root shear and bending loads are reduced when the tanks are full.
When are flexible or rigid tanks installed?
When the structure can't be used as a tank.
In a flexible tank a bladder made of rubberised fabric or nylon is located in a specially built bay (for helicopter tanks).
In a rigid tank is no interference between tank and structure, except for the anchoring points (typical solution for fuselage tanks).
What is installed in the wider tanks to mitigate the slosh problem?
Anti slosh baffles can be installed.
How are the rib lightening holes used to prevent liquid sloshing in high wing tanks?
They are often closed with flap check valves, allowing fuel to collect in the root area and prevent it to slosh towards the tip.
What is the total fuel capacity of an A-340?
140640 litres
What are transfer pumps?
Transfer pumps move fuel between tanks to maintain mass balance and to supply fuel to the collector tanks from where fuel is drawn to feed the engines.
How are booster pumps usually driven?
Electronically; for smaller AC, they are powered by the AC 28VDC system with delivery pressure in the range of 10 to 15 psi (0.7 to 1 bar) and flow rates up to 2.5 kg/s.
