Lateral Stability Questions and Answers

Answer: C

Correct answer: False, the restoring moment comes from the resulting sideslip

An aeroplane at a bank angle feels no rolling moment from the bank itself. What follows is that the tilted lift vector no longer balances weight vertically, so the aeroplane slips towards the lower wing.

The dihedral effect acts on that sideslip, raising the lower wing's incidence and generating the restoring moment. So a restoring moment does appear - the second option is too strong - but by way of the slip, and dihedral acts on sideslip rather than on bank whatever the sweep.

Answer: D

Correct answer: They produce adverse yaw like ailerons

Spoilers produce favourable yaw, not adverse. A spoiler is raised on the wing that is to descend, inside the turn, and the drag it adds there yaws the nose into the turn rather than away from it.

The other three statements are true and are the reasons spoilers are used on large transports: they roll the aeroplane by reducing lift on the descending wing, they add drag inside the turn, and because they impose no aft load on the section they cause none of the wing twist that limits outboard ailerons.

Answer: A

Correct answer: No, it causes a poorly damped Dutch roll

The two lateral-directional failures lie at opposite ends of the same balance. Excessive dihedral effect relative to directional stability gives a poorly damped Dutch roll; excessive directional stability relative to the dihedral effect gives spiral instability.

So strong dihedral in fact stabilises the spiral, by levelling the wings faster than the fin can tighten a developing turn. Longitudinal instability belongs to the pitching plane and is unaffected by either. Fin size matters, but by shifting the balance rather than by changing which failure follows which excess.

Answer: B

Correct answer: Increasing the downward aileron travel

That makes matters worse. Adverse yaw comes from the induced drag added by the down-going aileron, so allowing it to travel further increases exactly the drag that causes the problem.

The genuine remedies work the other way. Differential rigging limits the down travel while increasing the up travel; a Frise aileron adds profile drag on the up-going side to offset the induced drag on the other; and spoilers add drag inside the turn, reversing the sign of the yaw altogether.

Answer: C

The aileron's load twists the flexible outer wing so that its incidence falls. Above a certain speed that loss exceeds the aileron's own contribution and the aeroplane rolls the wrong way with the rigging entirely correct.

Answer: D

Correct answer: Inboard surfaces give a higher roll rate at low speed

Inboard ailerons give a lower roll rate, because their moment arm about the roll axis is shorter. That is why outboard surfaces are used at low speed and brought back in for the approach - roll authority is greatest there.

The three genuine reasons all concern stiffness and twist: the wing is deeper and torsionally stiffer inboard, the arm through which the aileron load twists the torsion box is shorter, and the outboard surfaces risk reversal at high dynamic pressure. Roll rate is what is sacrificed, not what is gained.

Answer: A

Correct answer: No, the contribution grows with lift coefficient

Sweep's dihedral effect arises from the lift difference between the forward and rear wings in a sideslip, and that difference is proportional to the lift coefficient at which the wing is operating.

So the contribution is large at low speed and high \( C_L \), and much smaller in cruise - a variation across the envelope that a fixed geometric quantity such as the sweep angle cannot describe. It does not disappear above the critical Mach number, though compressibility alters it, and wing flexibility is not what drives the variation.

Answer: B

Correct answer: The tailplane's area

The tailplane acts in the pitching plane, contributing to longitudinal stability and to the neutral point's position. Nothing about its area produces a rolling moment in a sideslip.

The other three all do. Geometric dihedral raises the lower wing's incidence; sweep gives the leading wing more lift; and the fin's side force acts above the rolling axis, so the surface that provides directional stability contributes a rolling moment too. That last is a useful reminder that the lateral and directional axes are coupled through the same sideslip.

Answer: C

Correct answer: False, the lowered aileron may stall that section and worsen the drop

Lowering an aileron raises the local angle of attack on that section. On a wing already near the stall, that can take the section past the stall, so lift falls rather than rises and the wing drops further - the opposite of the intended correction.

The correct technique is to reduce the angle of attack first and to pick the wing up with rudder, or with spoilers where they are fitted. Note that the difficulty is not that ailerons are ineffective at low speed but that they act in the wrong direction once the section stalls.

Answer: D

Correct answer: Improved lateral stability

Anhedral reduces lateral stability by definition: it subtracts from the dihedral effect, so the rolling moment a sideslip produces is smaller.

The other three do follow from fitting it. The dihedral effect is reduced, which is the direct consequence; the Dutch roll damps better, since the imbalance causing the poor damping is removed; and the spiral mode becomes more divergent, that being the other end of the same trade. Anhedral buys damping at the cost of stability, which is precisely the exchange the designer intends.