Landing Gear Design Questions and Answers

Answer: A

Correct answer: Aft of the centre of gravity

The mains must sit behind the centre of gravity, and everything characteristic of the tricycle layout follows from it.

The weight ahead of them presses the nose wheel down, which is what makes the arrangement directionally stable on the ground: a yaw disturbance leaves the centre of gravity trailing behind the mains, and the resulting moment straightens the aircraft out. A tailwheel aircraft has the mains ahead of the centre of gravity, which reverses that moment and is why it ground-loops.

Answer: B

Correct answer: Six to twenty per cent

The nose gear carries a modest share - conventionally 6 to 20 per cent of the static weight, and the range is bounded at both ends.

Too little and the nose wheel lacks the traction to steer, and braking on the mains can lift it. Too much and the nose gear becomes heavy, the mains lose braking traction, and rotation on takeoff requires more elevator than the tail can supply. The fraction shifts with loading, so both extremes of the centre of gravity envelope must be checked.

Answer: C

Correct answer: The vertical at the main wheels and the line to the CG

The tip-back angle is measured at the main wheel contact point, between the vertical and the line running up to the centre of gravity.

It states how far the aircraft can rotate nose-up before the centre of gravity passes behind the wheels and the aircraft sits on its tail. The requirement is that it exceed the attitude needed at rotation, with a margin - typically 15 degrees or more - and it is checked at the most aft centre of gravity, where the angle is smallest.

Answer: D

Correct answer: Rolling over in a turn on the ground

The overturn angle measures resistance to tipping sideways during a turn on the ground - a fast taxi turn, or a swerve after a burst tyre.

It is taken between the vertical through the centre of gravity and the line from the centre of gravity to the tipping axis joining a main wheel and the nose wheel. A smaller angle means better resistance, and the usual limit is around 63 degrees. Nose-over under braking and sitting on the tail are governed by the tip-back angle instead.

Answer: A

Correct answer: Reduces the overturn angle

Widening the track moves the tipping axis outboard, so the line from the centre of gravity to that axis lies closer to the vertical and the angle between them is smaller.

A smaller overturn angle is the desirable outcome, which is the point most often got the wrong way round: the aircraft must lean further before its weight vector crosses the tipping axis. The tip-back angle is unaffected, being a matter of the longitudinal position of the mains rather than their lateral spacing.

Answer: B

Correct answer: Forcing oil through an orifice

The energy is dissipated by throttling: oil is forced through a metering orifice as the strut compresses, and the work done against that resistance turns into heat carried away by the fluid.

The gas spring stores energy rather than absorbing it - it returns what it took in, and its job is to support the static weight and extend the strut afterwards. That division is why an oleo reaches around 80 per cent efficiency where a pure spring reaches far less: only the oil damping removes energy from the system.

Answer: C

Correct answer: Lower efficiency and a tendency to bounce

A spring stores energy elastically and gives it back, so very little is removed from the aircraft - efficiency is around 50 per cent against an oleo's 80.

The energy returned is what produces the bounce, the leg pushing the aircraft back into the air after touchdown. The compensations are simplicity, low cost and almost no maintenance, which is why spring legs are common on light aircraft; they are not weightless, and they certainly do store energy, which is precisely the difficulty.

Answer: D

Correct answer: Under a tenth of the total

The tyre deflects at touchdown and absorbs some energy, but it is a small share - typically under 10 per cent - with the shock strut taking the great majority.

The tyre's stroke is short and much of what it takes in is returned elastically, so it is a spring rather than a damper. It contributes usefully to smoothing taxi over rough surfaces, where the strut's stroke is barely used, but the landing energy calculation is done on the strut and the tyre's help is a margin rather than a design allowance.

Answer: A

It is a legacy measure carried over from the days of actual cotton plies, and it now simply expresses strength. Speed is rated separately and both limits must be satisfied, since a tyre strong enough for the load may still be unrated for the takeoff speed.

Answer: B

Correct answer: Reduce the contact area

A tyre carries its load through the pressure acting over the contact patch, so to a good approximation

\[ A \approx \frac{W}{p} \]

With the load fixed, raising the pressure shrinks the area. That has a direct consequence for the pavement: the same load is delivered over less ground, so the stress rises and the aircraft becomes more demanding of the surfaces it can use. Load capacity in fact rises with pressure, which is why the first option is wrong.