Lateral Stability Questions and Answers

Answer: A

Correct answer: Both Statement I and Statement II are correct

Both are true and the second is the reason for the first. An outboard aileron acts at the flexible wing tip, and the load it applies twists the wing leading-edge-down, reducing the wing's own angle of attack.

Above the reversal speed that twist more than cancels the aileron's lift change and the aeroplane rolls the wrong way. Locking the outboard surfaces out at high speed avoids the condition entirely, roll being provided instead by inboard ailerons near the stiff root and by spoilers, which produce no such twisting moment.

Answer: B

Correct answer: Both Statement I and Statement II are correct

Both are true and the second explains the first. Lowering an aileron raises the local angle of attack on a wing already near the stall, so the down-going aileron can stall its own wing tip and produce a roll in the wrong direction - or a wing drop.

Rudder and spoilers avoid the problem: neither raises local incidence anywhere. Washout, stall strips and boundary layer fences are the complementary design measures, arranging for the root to stall before the tip so that aileron authority survives to the stall.

Answer: C

Correct answer: 1, 2, 3, 4, 5

The chain is causal and the order is what makes lateral stability indirect. The gust banks the aeroplane and the lift vector tilts, so it no longer balances weight vertically.

The aeroplane therefore begins to slip towards the lower wing. The dihedral effect acts on that slip, raising the incidence and hence the lift on the lower wing. The lift difference is a restoring rolling moment, and the wings return towards level. Note that step 2 must precede step 3: without the sideslip there is nothing for the dihedral effect to act upon.

Answer: D

The first four add together and the last subtracts, which is exactly why anhedral appears on high mounted swept wings. Each contribution is estimated separately and the total is what determines the aeroplane's lateral behaviour.

Answer: A

Correct answer: 1, 2, 3, 4, 5

Each step follows from the one before. The pilot's input raises the right aileron and lowers the left, so the left wing gains lift - and with it induced drag.

The drag imbalance produces a yawing moment to the left, and the nose swings away from the intended right turn. That is adverse yaw, and the pilot's remedy is right rudder to coordinate. Note the order of steps 2 and 3: the drag difference is the cause and the yawing moment the effect, not the reverse.

Answer: B

The whole chain depends on the wing's torsional stiffness and on the dynamic pressure. Locking out the outboard ailerons above a certain speed breaks the sequence at its first step and leaves roll to the inboard surfaces and spoilers.

Answer: C

Correct answer: 1, 2, 3, 4

The ordering runs from favourable yaw to the largest adverse yaw. Spoilers are at one extreme, adding drag on the descending wing and so yawing the nose into the turn.

Frise ailerons come next, the projecting nose adding profile drag on the up-going side to offset the induced drag on the other. Differential ailerons do less, limiting the down travel rather than adding drag. Plain ailerons with equal travel do nothing at all about it and produce the most adverse yaw - which is why they need the most rudder.

Answer: D

Correct answer: Only three

The first three are correct. Geometric dihedral raises the lower wing's incidence in a sideslip; sweep contributes a stabilising effect that grows with lift coefficient; and a high wing gains from fuselage interference.

The fourth is wrong. Anhedral is negative dihedral and subtracts from the dihedral effect - which is precisely why it is fitted, on swept high-wing designs whose effective dihedral would otherwise be excessive and whose Dutch roll would be poorly damped.

Answer: A

Correct answer: Only three

Three of the pairs hold. Too much dihedral effect gives a poorly damped Dutch roll; too little gives spiral instability; and strong directional stability with weak dihedral gives spiral instability too - the same imbalance stated from the other side.

The fourth contradicts the first. Excess dihedral effect produces Dutch roll, not spiral instability; the two failures lie at opposite ends of the same balance. Where a list pairs one cause with two opposite consequences, one of the two must be the false entry.

Answer: B

Correct answer: Only three

The first three describe their devices correctly: differential ailerons travel further up than down, Frise ailerons project their nose into the flow on the up-going side, and spoilers add drag on the inside of the turn.

The fourth is wrong. Plain ailerons with equal travel do nothing about the drag imbalance - they are the case the other three exist to improve on, and they produce the most adverse yaw of any roll control. Equal travel is the problem, not the remedy.