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
Name the main loads acting on the empennage.
- aerodynamic loads due to angle of attack and sideslip, pitching, yawing and rolling;
- control loads due to rudder and elevator deflection;
- inertia forces and moments relative to the structure and equipment.
Which localisation methods are used to find the location of a constructive part, of a point or of a plane?
- Reference coordinate system;
- Station planes system;
- zoning system.
In a conventional AC structure, where are holes cut to reduce weight and to accommodate control cables, wiring and fuel or hydraulic ducts.
Holes are cut at positions where the local stress is low.
Describe a semi-monocoque construction.
The fuselage of modern transport aircraft is a reinforced shell commonly referred to as semi-monocoque construction.
A pure monocoque shell is a simple tube of thin skins, and as such it is structurally inefficient since unsupported thin skins become unstable under compression and shear loading.
To summarize: a structural system comprised of a thin-skinned shell stiffened by longitudinal stringers which, in turn, are supported by transverse frames forms a semi-monocoque structure. Semi-monocoque structures are very efficient, that is, they have a high strength to weight ratio, and are well suited for unusual load combinations and locations. They can withstand local failures without collapsing through load redistribution.
- What are the similarities between the structural components of a fuselage and a wing?
- What are the differences?
- The function of the stringers and skins of fuselages and wings are equivalent.
- Fuselage longerons and stringers and wing beam caps serve similar functions of carrying axial loads induced by bending.
- Fuselage frames are equivalent in function to wing ribs.
- Fuselage skins have a greater curvature and are more stable under compression and shear loads.
- External pressure loads are much lower on the fuselage than on the wing. The skin thickness on a fuselage is generally lower than on a wing.
- In the fuselage, transverse shear loads are carried by the skin while in the wing these loads are predominately resisted by the spar webs.
- The design of wing ribs is influenced by local airloads whereas the design of fuselage frames may be influenced by loads resulting from equipment mounted in the fuselage.
Classical differential construction method: Name some advantages of joining parts together with rivets (Nieten). (This type of construction was universally used until the 1950s)
- good repairability : damaged parts can be simply replaced
- good crack-stopping properties (Risse verhindern): cracks are effectively stopped at the next part and do not cause the whole structural component to fail;
- good elasticity
Name some disadvantages of constructions with rivets.
- relatively high weight: the many rivet holes between the different parts are stress concentration points that require a more generous sizing to avoid fatigue problems;
- high fabrication costs: assembly of many different parts with rivets is work intensive and can only in special cases be automatised;
- the joints can loosen, particularly because of vibrations, and can be sensitive to corrosion if special measures are not taken;
- the surface quality is not optimal because of the rivet heads and of ondulations (Wellenbildung) resulting from the riveting process.
What is integral construction?
Large structural components, such as a wing skin with its stringers are machined from a single piece of material.
What are the advantages of intergral construction?
- better structural efficiency, resulting in lower weight: extreme stress concentration points are avoided by design and the material can be loaded more uniformly;
- high precision of components produced on CNC (computised numerically controlled) machines;
- excellent surface quality;
- better resistance to vibration and corrosion, hence longer structural life.
What are the disadvantages of integral construction?
- the possibility of structural repairs is limited: in case of damage, the whole structural component must be replaced;
- higher material costs: in the production of a typical integral structural component, up to 90% of the material is scrapped as shavings (zu Späne verschrottet);
- relatively large investments in sophisticated CNC machines; the high rigidity of the structural components can be a disadvantage
What is the largest contribution to the fatigue loading of an AC?
The atmospheric turbulence.
How is fatigue strength determined?
Fatigue strength is determined by applying different levels of cyclic stress to individual test specimens and measuring the number of cycles to failure.
Why is it generally more economical to design some parts of the structure as fail-safe rather than safe life?
Because such components can be lighter.
How is the damage tolerance philosophy defined?
Under the damage-tolerant design concept, the structure must contain the growth of initial assumed damage for a specified period of service, maintaining a minimum level of residual static strength both during and at the end of this period. It is the objective of the Damage Tolerant requirements to ensure that cracks do not reach levels that could impair the safety of the aircraft during the expected lifetime of the aircraft.
Which definitions of inspections do exist? Explain.
- In-flight evident inspectable: If the nature and extent of damage occurring in flight will result directly in characteristics which make the flight crew immediately and unmistakably aware that significant damage has occurred and that the mission should not be continued.
- Ground evident inspectable: If the nature and extent of damage will be readily and unmistakably obvious to ground personnel without specifically inspecting the structure for damage.
- Walkaround inspectable: If the nature and extent of damage is unlikely to be overlooked by personnel conducting a visual inspection of the structure.
- Special visual inspectable: If the nature and extent of damage is unlikely to be overlooked by personnel conducting a detailed visual inspection of the aircraft for the purpose of finding damaged structure. The procedures may include removal of access panels and doors, and may permit simple visual aids such as mirrors and magnifying glasses.
- Depot or base level inspectable: If the nature and extent of damage will be detected utilizing one or more selected nondestructive inspection procedures.
- In-service non-inspectable structure: If either damage size or accessibility preclude detection during one or more of the above inspections.
What does the category Slow Crack Growth mean?
In the Slow Crack Growth category, structures are designed such that initial damage will grow at a stable, slow rate under service environment and not achieve a size large enough to cause rapid unstable propagation.
How many percentage of the maximum take-off weight does the landing gear represent?
Between 3% and 7%, depending of the AC category.
How high is the drag (in percent) of a fixed landing gear?
Up to 40% of the total zero-lift drag.
What are the basic functions and requirements of a landing gear system?
- to support the aircraft on ground;
- to allow aircraft control (steering and braking) during taxiing;
- to ensure that no other part of the aircraft touches the ground during taxiing, lift off and touchdown.
- to absorb the vertical kinetic energy component at landing without transmitting excessive loads to the airframe and the occupants;
- to withstand the side loads associated with crosswind take-off and landing;
- to dissipate the horizontal kinetic energy component during landing and in case of an
- aborted take-off;
- to smooth out the bumps caused by uneven terrain and provide for a smooth and comfortable rolling.
According to FAR Part 23 and EASA CS 23 regulations (general aviation and commuter AC), what is the value of lift to be assumed for design and testing?
Not more than 2/3 of the weight of the AC. Hence nz=ngear+2/3.
According to FAR Part 25 and EASA CS 25 regulations (transport AC), what is the value of lift to be assumed for design and testing?
It may be assumed to be equal to the AC weight. Hence nz=ngear+1.
The higher the landing gear load factor,....
What are the typical landing gear load factors for following AC?
- Commercial AC/Transport
- General aviation
- Ground based fighter
- Military trainer
- STOL (Short Take-off and Landing AC)
- Carrier based fighter
- 0.7~1.5
- 2~3
- 3~4
- 3.5~4.5
- 4~5
- 5~6
Why did the designer select a low landing gear load factor for transport AC?
For transport aircraft, whose structure is designed for normal load factors between 2.5 and 3.8 in flight, a low landing gear load factor value is selected, so that the main airframe structure does not need to be sized for the landing case.
What is the most common landing gear arrangement nowadays?
It is the tricycle, with two main gear legs and nose leg; each leg can have one or more wheels depending on the AC weight.
What are the advantages with a tricycle landing gear compared to a taildragger landing gear?
Better control at landing (keeping AC on centreline) and it is easier to land with tricycle landing gear because the pilot does not need to align the AC with the RWY.
As a pilot, you have better visibility during taxiing. It is easier to load and unload the AC, low drag during take off acceleration and it is more forgiving when landing with insufficient or no flare (no bouncing back in the air).
What is a bicycle gear? What is the advantage?
The bicycle gear, with two legs below the fuselage centerline (one in front and one aft of the aircraft center of gravity) and two small outrigger legs to keep the aircraft balanced when stationary or rolling at low speed, is not very common.
Advantage: The only advantages of this type of landing gear with respect to the others are a lower weight and less drag (when the gear is extended, of course).
What is the general rule for tire pressure and loads per tire when operating at an airport?
As a general rule, if the load per tire exceeds 45,000N and the tire pressure is higher than 4 Bar ( 60 psi), operation is possible only at airports which have high strength flexible (asphalt) pavements or, better, rigid (concrete) pavements
What is the allowable tire pressure on the best RWYs?
The allowable tire pressures are not much higher than 14 Bar (~200psi)
To keep the tire size within reasonable limits, what is the maximum load on a single wheel?
Approximately 150'000 N.
When is the four-wheel bogie used?
The four-wheel bogie is common for aircraft with a take-off mass of between 90,000 and 180,000 kg.
- In a simple spring type, the gear legs are....
- What are the disadvantages of a simple spring type landing gear?
- elastically deformed under load.
- The vertical kinetic energy is transmitted gradually to the structure, but there is practically no dumping effect, except for the energy dissipated by lateral friction of the tires (when the gear deflects, the wheels move sideways). An AC equipped with such a gear tends to bounce back in hard landing.
How is the spring effect on an oleo strut created?
Oleo struts combine a spring effect, created by a variable chamber filled with compressed air or nitrogen, with the damping effect of a piston which forces oil through small orifices (Düsen).
What are the three most common oleo strut configurations?
- telescopic gear: The main structure of the gear leg is formed by the oleo strut itself. It is quite heavy because the oleo strut must provide the full required amount of compression (+lateral and longitudinal loads)
- triangulated gear: Conceptually similar to the bungee type: when gear deflects, the oleo strut is compressed
- levered, or trailing link gear: like previous configuration
- What are the advantages of a triangulated gear?
- What are the disadvantages of a triangulated gear?
- The wheel lateral and longitudinal loads are carried by the solid gear legs, and a shorter and smaller oleo strut can be used. Maintenance is easier, since the oleo strut can be replaced or overhauled without removing the wheel assembly and the landing gear leg.
- This configuration is usually a little heavier than the telescopic one, and the gear deflections causes lateral friction on the tires, which shortens their life.
- What are the advantages of a trailing link gear?
- In a trailing link gear, the wheels travel slightly backwards when the oleo strut is compressed; this makes it easier for a wheel to overcome a bump. This type of construction is very robust, even if somewhat heavier than the telescopic leg type, and is well suited to aircraft that operate on aircraft carriers or to short take off and landing aircraft designed to take off and land from unprepared runways (such as the Pilatus PC-12).
What does cause the compression dumping in an oleo strut?
The flow pressure losses caused by the metered orifice.
What can be regulated by appropriate shaping of the metering pin?
With appropriate shaping of the metering pin, the shock absorber can be made softer when reacting to small bumps during taxiing and harder when reacting to high loads during landing.
What does the area below the reaction [N] vs. compression [m] curve indicate?
This area is the absorbed energy.
In an air spring is the elastic reaction force F is generated by the increase of gas pressure in an accumulator. How does the reaction vs. compression curve look like?
The curve reflects a polytrophic pressure vs. volume relationship, p*Vn=constant. Since the compression of a shock absorber at landing is quite fast, the process is adiabatic.
