Lateral Stability Questions and Answers

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 why the first is acceptable. A divergent spiral takes tens of seconds to double its bank angle, and a pilot corrects it with a glance and a small aileron input.

The alternative is worse. Strengthening the dihedral effect enough to stabilise the spiral degrades the Dutch roll, which oscillates in seconds and is tiring and sometimes difficult to arrest. Given a choice between a slow divergence and a poorly damped oscillation, the designer takes the divergence.

Answer: B

Correct answer: 1 and 2 only

The first two are correct. Lateral stability is measured by the rolling moment sideslip produces, and the stable case is a negative \( C_{l\beta} \) - a right sideslip giving a left rolling moment.

The third is false, and it is the point the whole subject turns on. Bank alone produces no restoring moment; it produces a sideslip, and the sideslip produces the moment. That is why the recovery is indirect and why the derivative is defined against \( \beta \).

Answer: C

Correct answer: 1 and 2 only

The first two are correct. Sweep adds to the dihedral effect through the lift difference between the forward and rear wings in a sideslip, and a high wing adds through fuselage interference.

The third reverses anhedral's role. Anhedral is negative dihedral and subtracts from the effect, which is why it is fitted - on designs whose sweep and wing position together give more than is wanted, and whose Dutch roll would otherwise be poorly damped.

Answer: D

Correct answer: 1 and 2 only

The first two are correct. Adverse yaw arises from the difference in induced drag between the wings when one aileron is lowered and the other raised, and both differential rigging and the Frise design reduce it - the first by limiting the down travel, the second by adding profile drag on the up-going side.

The third has the sign wrong. Spoilers add drag on the descending wing, inside the turn, so they produce favourable yaw. They are the one roll control that needs no rudder coordination.

Answer: A

Correct answer: 1 and 2 only

The first two are correct. Aileron reversal arises from wing twist cancelling the aileron's own lift change, and outboard ailerons are locked out at high speed for that reason.

The third reverses the comparison. Inboard ailerons twist the wing less, because the structure is deeper and torsionally stiffer near the root and the moment arm to the torsion box is shorter. That is precisely why roll control is moved inboard at high speed, and why the statement inverts the design logic.

Answer: B

Correct answer: A-1, B-2, C-3, D-4

Each contribution works by a different mechanism. Geometric dihedral raises the lower wing's incidence in a sideslip. Sweep gives the leading wing a lower effective sweep and so more lift, an effect that grows with lift coefficient. A high wing gains from the flow the fuselage deflects around it.

Anhedral is the odd one out, and the only one that subtracts: it exists to reduce a total the other three have made excessive. Reading for the negative contribution first settles that pairing at once.

Answer: C

Correct answer: A-1, B-2, C-3, D-4

The four form a ladder from worst to best. Plain ailerons with equal travel produce pronounced adverse yaw, doing nothing about the drag imbalance.

Differential ailerons reduce it by deflecting further up than down, limiting the induced drag added on the descending side. Frise ailerons reduce it by projecting the nose of the up-going surface into the flow, adding profile drag there. Spoilers reverse the sign altogether by adding drag inside the turn. Each step treats the same imbalance more aggressively.

Answer: D

The two lateral modes sit at opposite ends of the same balance, and improving one usually worsens the other. The third row is what designers aim for, accepting the slow spiral in exchange for a well behaved Dutch roll.

Answer: A

Correct answer: A-1, B-2, C-3, D-4

Each term has its own definition. The dihedral effect is the rolling moment produced by sideslip; the helix angle is \( pb/2V \), the non-dimensional roll rate; aileron reversal is wing twist overcoming the aileron's own lift change; and adverse yaw is the nose moving away from the intended turn during a roll.

Two of the four are quantities and two are named phenomena, which halves the matching at once. The pairing is then a matter of noting which quantity concerns sideslip and which roll rate.

Answer: B

Correct answer: Anhedral

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

That is exactly why it is used - on swept, high-mounted wings whose sweep and wing position have already given more effective dihedral than is wanted, and whose Dutch roll would otherwise be poorly damped. The other three all add: geometric dihedral directly, sweep through the lift difference in sideslip, and a high wing through fuselage interference.