Lateral Stability Questions and Answers

Answer: C

Correct answer: Only three

The first three are right. The dihedral effect is the rolling moment produced by sideslip; the helix angle is \( pb/2V \); and aileron reversal is wing twist overcoming the aileron's own lift change.

The fourth misstates the dihedral effect as a moment produced by bank angle. Bank produces no rolling moment directly - that is why an aeroplane has no immediate tendency to level its wings. It produces a sideslip, and the sideslip produces the moment, which is the whole point of the first pair.

Answer: D

Correct answer: Only three

The first three are right. Locking out the outboard ailerons avoids reversal; inboard ailerons apply their load near the stiff root where little twist results; and spoilers give roll without any twisting moment at all.

The fourth is wrong on the purpose. Locking out the outboard ailerons reduces the roll rate available, since the surfaces with the longest moment arm are taken out of use. It is accepted as the price of avoiding reversal, which is why spoilers are brought in to make up the authority.

Answer: A

Correct answer: Anhedral

Anhedral is negative dihedral: the wings droop rather than rise, so in a sideslip the lower wing's incidence is reduced rather than raised and the dihedral effect is diminished.

That is exactly why it is fitted - on swept, high-mounted wings whose sweep and wing position between them give more effective dihedral than is wanted. The other three all add to the effect: geometric dihedral directly, sweep through the lift difference in sideslip, and a high wing through fuselage interference.

Answer: B

Correct answer: Spoilers

A spoiler is raised on the wing that is to descend, and it adds drag there - on the inside of the turn. Drag on the inside yaws the nose into the turn, which is favourable yaw.

The three aileron types all produce adverse yaw to some degree, differing only in how much they mitigate it: Frise and differential designs reduce it, and plain ailerons with equal travel produce the most. Freedom from adverse yaw is one of the reasons spoilers are used for roll on large transports.

Answer: C

Correct answer: Spiral instability

Spiral instability is the consequence of too little dihedral effect relative to directional stability, not too much: the fin weathercocks the nose into a developing turn faster than the dihedral can level the wings.

The other three do follow from excess. A poorly damped Dutch roll is the classic symptom; a strong rolling response to gusts follows from a large \( C_{l\beta} \); and holding a steady sideslip needs large aileron deflection precisely because the dihedral effect is fighting it.

Answer: D

Correct answer: Increasing the outboard aileron's span

That would make reversal worse. A larger outboard aileron applies a bigger load at the flexible wing tip, which is exactly where the twisting moment is greatest, so the reversal speed falls.

The three genuine measures all move the load away from the flexible tip or remove the twisting mechanism: locking the outboard ailerons out at high speed, using inboard ailerons near the stiff root, and using spoilers, which produce a rolling moment by destroying lift rather than by applying an aft load that twists the section.

Answer: A

Correct answer: The tailplane's area

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

The other three all do. Geometric dihedral raises the lower wing's incidence; sweep produces a lift difference between the forward and rear wings; and the fin's side force acts above the rolling axis, so the yawing surface contributes a rolling moment too - a reminder that the lateral and directional axes are coupled through the same sideslip.

Answer: B

Sweep and a high wing both add dihedral effect, and together they can overwhelm the directional stability. Drooping the wing tips removes part of the excess and restores an acceptable balance between the two lateral modes.

Answer: C

Correct answer: Adverse yaw from unequal induced drag

The symptom is diagnostic. The down-going aileron raises lift and induced drag on its wing, so the aeroplane carries more drag on the side that is rising, and the nose yaws away from the intended turn before the bank establishes the turn itself.

Aileron reversal would roll the aeroplane the wrong way, not yaw it, and would appear at high speed rather than low. A stalled fin would show as a directional divergence, and a misrigged rudder would give a persistent yaw rather than one tied to aileron input.

Answer: D

The aileron's load twists the outer wing leading edge downward, reducing local incidence. Above a certain speed that loss exceeds the aileron's own contribution, which is why outboard surfaces are locked out and inboard ones take over.