What is the common unit of the "aircraft lift curve slope" [CLα]?
The shear and bending loads are independent of speed.
What happens to a beam (fixed on both ends) when its heated/cooled?
It will expand (when heated) or shrink (when cooled).
- Expandation will create a compression load
- Shrinking will create a tension load
When can thermal induced loads become significant?
At supersonic speed.
What are the minimum load factor limitation of commuter and normal aircrafts?
Positive loads: 2.9 to 3.8
Negative loads: -1.2 to -1.6
How do you calculate the ultimate loads?
limit loads multiplied by a safety factor of 1.5
What must the aircraft structure withstand while testing it?
The aircraft structure can withstand the ultimate load for at least 3 seconds without failure (permanent deformations are allowed).
What do fast aircraft that are designed to fly at low altitude in heavy turbulence have, to reduce the load factor increments due to gust?
a relatively high wing loading and a short wing span
(-> the lighter the aircraft, the higher the load factors due to gusts)
What is Aeroelasticity?
the subject that describes the interaction of aerodynamic, inertia and elastic forces for a flexible structure and the phenomena that can result
Torsional divergence: What happens to the angle of attack and the lift distribution for a straight wing?
- the angle of attack of the wing sections towards the tip will increase more than close to the root
- spanwise redistribution of lift: c.p. will move outward and the bending moment at the wing root will increase.
What is the formula for the total energy absorbed by the landing gear?
E = 0.5 * m * v2
What happens in the case of a combined bending and twisting oscillation when maximum twist occurs at zero bending and vice versa?
During the wing upward motion, at the position of zero bending the twisting of the wing causes a positive geometric incidence and therefore an aerodynamic lift force in the same direction as the wing motion. A similar but reversed situation occurs when the wing is moving downwards.
What is Vf?
What happens above the speed Vf?
the oscillation will diverge and destroy the wing structure
What is "Aileron Buzz"?
- a form of flutter associated with shock waves oscillating back and forth on the upper and lower wing surfaces and interacting with the boundary layer
- The sharp changes in pressure distribution cause a high frequency oscillation of the aileron
What ist "Buffeting"?
the most common form of buffeting is that of the tail, which can experience severe vibrations when it is invested by the turbulent wake of the stalled wing
What is "Dynamic response"?
an elastic aircraft in an accelerated flight condition may experience loads which are much greater than those predicted assuming a perfectly rigid structure. External loads, such as gust and landing loads, excite oscillations of the structure. The additional inertia forces associated with these oscillations can produce dynamic load overshoots.
During landing, the maximum vertical velocity at touchdown is defined by requirements and specifications, depending on the class of the aircraft. What is the max. vertical velocity for civil aircrafts?
3.05 m/s (10 ft/s) at maximum landing weight (MLW)
Why is the landing gear load factor important for the landing gear sizing?
the higher its value, the shorter the landing gear compression required to absorb the vertical kinetic energy at landing
What are the advantages and disadvantages of the old taildragger arrangement landing gears?
- + lower weight (nose gear is much heavier)
- + less drag (if the gear is not retractable)
- + more space available in the aircraft nose (particularly if the gear is retractable)
- + better suited for operation on rough fields
- - laterally unstable when rolling
- - heavy braking may bring the danger of tipping the aircraft over on its nose
What are the advantages of the tricycle landing gear?
- the aircraft does not need to be aligned with the runway (a crab landing is possible)
- it is much easier for the pilot to keep it on the centreline, even with strong side gusts
- offers better visibility for the pilots during taxiing
- easier loading and unloading of the aircraft (and more comfort for the passengers) thanks to the nearly horizontal floor
- low drag during take off acceleration
- is more forgiving when landing with insufficient or no flare (the aircraft tends to reduce the angle of attack after touchdown, and is less prone to bouncing back in the air)
What are the advantages and disadvantages of the bicycle gear?
- + lower weight
- + less drag (when gear is extended)
- + used when not enough space for langing gear
- - difficult take-off and landing
- - In crosswind conditions, care must be taken not to land on one of the outrigger gear legs, which are not designed to take substantial landing loads.
What is the maximum take-off mass for a single wheel per main gear leg?
23,000 kg (although for safety reasons it would be better to fit twin wheels)
For which take-off mass are there usually two wheels per main leg?
23,000 - 68,000 kg (This configuration is sometimes found also on heavier aircraft with a take-off mass of up to 100,000 kg)
For which take-off mass is the four-wheel bogie common?
90,000 - 180,000 kg
What types of shock absorbers exist?
- spring type
- bungee type
- oleopneumatic shock absorbers (oleo struts)
Explain the simple spring type shock absorber.
- gear legs are elastically deformed under load
- typical of small general aviation aircraft
- The vertical kinetic energy is transmitted gradually to the structure
- practically no damping effect, except for the energy dissipated by lateral friction of the tires (when the gear deflects, the wheels move sideways)
- aircraft tends to bounce back in a hard landing
Explain the simple bungee type shock absorber.
- the main gear legs are hinged at the side of the fuselage and connected by rubber bungees (bundles of rubber
strands packed in a cloth tube) that stretch as the gear deflects
- common in early light aircraft
- In addition to the lateral friction of the tires, some energy is dissipated by rubber internal friction and hysteresis