Lateral Stability Questions and Answers

Answer: C

Correct answer: They add drag inside the turn

A spoiler is raised on the wing that is to descend - the inside of the turn - and it works by destroying lift, which necessarily adds drag at the same place.

Drag on the inside yaws the nose into the turn, the opposite of the aileron case, where the extra induced drag falls on the rising outer wing. Note that spoilers reduce lift on the descending wing rather than adding it, and they involve the rudder not at all; the reversal of sign follows entirely from which wing carries the extra drag.

Answer: D

Correct answer: Wing twist that would reverse their effect

The outboard ailerons act at the flexible wing tip, where the load they apply twists the wing leading-edge-down and reduces its incidence. Above the reversal speed that twist more than cancels the aileron's own lift change.

Locking them out at high speed avoids the condition entirely, and roll is provided by inboard ailerons near the stiff root and by spoilers, neither of which twists the wing appreciably. The measure costs roll rate, which is why the outboard surfaces are brought back at the low speeds where they are safe.

Answer: A

Correct answer: Aileron held per degree of steady sideslip

Lateral stability is defined by \( C_{l\beta} \), the rolling moment produced by sideslip, so the test that measures it directly is the steady sideslip: the aileron deflection needed to hold the wings level against a given slip is a measure of the rolling moment the slip is producing.

The other three measure related but different things - roll control power, the balance between the lateral and directional axes, and the spiral mode. Each depends on \( C_{l\beta} \) among other quantities, but none isolates it.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the second is precisely why the first must be so. An aeroplane at a bank angle experiences no rolling moment from the bank itself; it will hold that attitude indefinitely as far as roll is concerned.

What the bank does is tilt the lift vector so that it no longer balances weight vertically, and the aeroplane slips towards the lower wing. It is that sideslip the dihedral effect acts on. So the derivative that defines lateral stability must be taken with respect to \( \beta \), there being nothing to take it with respect to in bank.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the second explains the first. Sweep contributes a dihedral effect that grows with lift coefficient, and a high wing adds a further contribution through fuselage interference in sideslip.

Together they can give more effective dihedral than is wanted, and the symptom is a poorly damped Dutch roll. Anhedral subtracts from the total and restores the balance. The C-17 and the An-124 show the reasoning plainly - high, swept wings drooped noticeably at the tips.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the second is the mechanism. Sweep's dihedral effect works through the lift difference between the forward and rear wings in a sideslip, and that difference scales with the lift coefficient at which the wing is working.

On the approach the lift coefficient is high, so the contribution is at its largest and the aeroplane feels stiffer in roll. In cruise it is much reduced. That variation across the envelope is why a swept-wing aeroplane's Dutch roll is worst at low speed and why yaw dampers matter most there.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the second is the reason for the first. In a sideslip the fuselage deflects the flow, and the sense of that deflection depends on where the wing is mounted: a high wing gains, a low wing loses.

A low wing design must therefore supply more of the effective dihedral geometrically, and the angle shows on the drawing. High wing types frequently need almost none, and some carry slight anhedral. The two arrive at similar handling by different routes, which is what the visible difference conceals.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives both of the attractions. Spoilers add drag on the descending wing, inside the turn, so the yaw they produce is favourable rather than adverse and no coordinating rudder is needed.

And they generate a rolling moment by destroying lift rather than by loading the section aft, so they impose no twisting moment on the wing and are immune to the reversal that limits outboard ailerons. Both properties matter most on a large flexible wing, which is exactly where they are used.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the second is the reason for the first. Lowering an aileron raises the local angle of attack, and on a wing already near the stall that section may stall - producing a roll opposite to the one commanded, or dropping the wing further.

Rudder and spoilers avoid the difficulty because neither raises incidence anywhere. The complementary design measures are washout, stall strips and fences, arranging for the root to stall before the tip so that some aileron authority survives to the stall.

Answer: D

Correct answer: A is false but R is true

The reason is correct: anhedral does reduce the rolling moment that sideslip produces. But that is a reduction in lateral stability, not an increase, so the assertion states the opposite of what follows.

The reduction is nonetheless deliberate. Sweep and a high wing position can give more effective dihedral than is wanted, and the symptom is a poorly damped Dutch roll. Anhedral is fitted to trim the total back - sacrificing some lateral stability for better damping, which is a trade rather than an improvement in stability.