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
What is the polytrophic index n?
Approximately 1.4. (p*Vn=constant)
- In a steel spring, what is the relation between the reaction and the compression?
- What is the efficiency of the steel spring?
- The reaction is directly proportional to compression
- It has an efficiency of 0.5.
How can you obtain higher efficiency values on a steel spring?
Higher values are easily obtained by adding a preload, so that the spring starts to compress only when the reaction force exceeds it.
When should the shock absorber start to compress?
Before the maximum tire compression is reached.
Which amount of energy can be absorbed by an AC tire?
Typically 10% of the total AC weight.
What are the typical shock absorber efficiency values for:
- Steel spring
- Steel spring, with preload
- Air spring
- Rubber bungee
- Oleopneumatic, fixed orifice
- Oleopneumatic, metered orifice
- Tires
- 0.5
- ~0.6
- 0.45
- 0.58
- 0.65~0.80
- 0.75~0.90
- 0.47
When designers lay out the landing gear geometry, what are the first requirements to be consideres?
The first requirements that they consider are the ground pitch and roll angles that may occur in operation.
For propeller AC, how much is the ground clearance fo each propeller during taxiing, which airworthiness regulations specify?
A minimum ground clearance of 18cm for each propeller during taxiing is specified.
For transport AC, which lift value can be assumed?
The lift my be assumed to be equal to the AC weight.
What does the stroke of the landing gear depend on?
Mainly on the efficiency of the shock absorber (not on the AC mass).
For large transport AC at maximum ramp weight, the static compression can attain higher values. What values can be attained?
Up to 80% of the stroke and more.
How long is the total length of the oleo strut?
It is at leat 2.5 times the stroke.
- How is the diameter of the oleo strut determined?
- How is the static load of the main leg calculated?
- Simply by the load that it must carry.
- The static load on the main leg for a classic tricycle gear is half of the AC ramp weight minus the load fraction carried by the nose leg (10% assumed for the initial value).
How much larger is the external diameter than the piston diameter?
30% larger.
What is the disadvantage of retractable landing gear?
It imposes a weight penalty because of the large cutouts (Sicherungen) in the main structural elements.
Why does the nose wheel have a negative leg rake angle?
To prevent static and dynamic instability.
What is done to reduce steering forces of the nosewheel?
The rake angle is positive (between 7° and 15°) and the trail is less than 0.2 times the wheel radius.
What kind of system helps to detect if a wheel is locking and automatically reduce the hydraulic pressure in the corresponding brake unit?
Most modern AC are equipped with the anti-skid systems. (Antigleitschutz)
Anti-skid systems are inactive at low rolling speeds (typically below 20kt).
What kind of system did Airbus install to sense the pilots input electrically and elaborate by a computer that controls the brake hydraulic pressure?
"Brake-by-wire"
What is the typical lifetime of a wheel on a modern airliner?
~300~400 landings
How is tire burst prevented?
Modern AC wheels are equipped with a fusible (brennbar) plug to prevent tire burst. The plugs are filled with a low melting point material and allow air pressure to escape in case of overheating.
What are the main components of a tire?
- the tire interface with the wheel rim (Felgen), or bead (Reifenwulst).
- the main structure of the tire, or carcass. It is made of layers of rubberised nylon fabric (plies).
- The portion of the tire that comes into conract with the ground, or tread.
What is the most important thing for a tire?
It is of primary importance that the tires are kept at the correct inflation pressure.
What is true about Froude's propeller theory?
What is true about the pressure of the actuator?
Why is the flow of the actuator incompressible?
Because the density rho is constant.
How is the propulsive efficiency defined?
It is defined as the fraction of the mechanical power imparted to the airstream by the actuator disc that is converted in propulsive power.
On a propeller blade, where is most of the thrust generated?
Most of the thrust is generated in the outer part of the blade, where the rotational velocity component is higher.
What is the disdavantage of a large propeller?
A large propeller will have higher tip speeds.
What kind of propeller and efficiency fitts best for an AC with short take off distance and a low cruise speed?
A short take off aircraft with a low cruise speed will certainly benefit from a large propeller providing high efficiency and high thrust at low speeds.
What is true about the propeller diameter for a fast AC?
What is the difference between the effective pitch angle (teta) and the blade angle (beta)?
This difference corresponds to the angle of attack of the propeller blade section.
What are the principle factors affecting the efficiency of an airfoil section?
- The angle of attack
- Advance ratio J
- Blade angle beta
What are the peak efficiencies of modern variable propellers?
0.85-0.9
What is feathering?
In the event of an engine failure, propeller pitch must be set to a value that reduces drag to a minimum. This is called feathering.
What can happen if the propeller blades are set to small positive values or negative values?
With power applied large drag can be produced or even reverse thrust.
- What are integral tanks?
- What are the advantages?
- Integral tanks are part of the wing or tailplane structure.
- Weight advantages, the location is close to the aircraft centre of gravity and the wing root shear and bending loads are reduced when the tanks are full.
- What is a flexible tank?
- What is a rigid tank?
- A bladder (Blase) made of rubberised fabric or nylon is located in a specially built bay.
- There is no interference between tank and structure, except for the anchoring points (typical solution for fuselage tanks).
What is the task of the transfer pump?
The 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.
What are booster pumps?
Booster pumps (sometimes called engine feed pumps) are used to boost the fuel pressure in the engine feed lines. The reason for this is to prevent aeration (the presence of air bubbles in the fuel lines that could cause an engine flame-out).
