Lateral Stability Questions and Answers

Answer: A

The aeroplane has no natural tendency to return to wings level from a bank alone, since banking produces no restoring moment by itself. Stability appears only through the sideslip that a bank eventually causes, which is why the two are linked.

Answer: B

Correct answer: Negative, so that a right sideslip rolls the aeroplane left

Lateral static stability requires a restoring rolling moment, so \( C_{l\beta} \) must be negative: a sideslip to the right must produce a rolling moment to the left, raising the wing the aeroplane is slipping towards.

The sign convention is what makes the derivative negative rather than positive, and the quantity is usually called the dihedral effect after its principal source. It is independent of \( C_{n\beta} \), the directional derivative, though the balance between the two decides whether the aeroplane's spiral and Dutch roll modes are well behaved.

Answer: C

Correct answer: The lower wing gains incidence

In a sideslip the relative wind has a spanwise component. On a wing with dihedral, that component meets the lower wing from slightly below, raising its effective angle of attack, and the upper wing from slightly above, reducing its own.

The lower wing therefore generates more lift, and the difference is a rolling moment that raises it back towards level. The effect depends on the sideslip, not on the bank itself, which is why an aeroplane has no direct tendency to level its wings until the bank has produced a slip.

Answer: D

In sideslip the forward wing meets the flow more nearly perpendicular and produces more lift than the trailing one. The effect can be so powerful on a swept wing aeroplane that anhedral must be added to keep it within bounds.

Answer: A

The contribution depends on the difference in lift between the two wings, which grows as the whole wing works harder. That is why a swept wing aeroplane feels most strongly stable in roll at low speed and high angle of attack.

Answer: B

The flow around the fuselage in sideslip is deflected upward on the windward side of a high wing and downward on the leeward side. The effect is equivalent to several degrees of geometric dihedral and is one reason high wing aeroplanes need little or none.

Answer: C

Correct answer: Fuselage interference works against it

In a sideslip the fuselage deflects the flow, and the direction of that deflection depends on where the wing is mounted. On a high wing the flow is pushed up under the lower wing and down over the upper one, adding to the dihedral effect. On a low wing the same mechanism works in reverse and subtracts from it.

So a low wing design must make up the deficit with geometric dihedral, which is why low wing light aircraft show a visible dihedral angle where high wing types often show almost none.

Answer: D

A side force generated above the aeroplane's centre of gravity produces a rolling moment as well as the yawing moment it is fitted for. The contribution is modest but it is one more link between the lateral and directional axes.

Answer: A

A high mounted swept wing can produce far more dihedral effect than is wanted, giving a badly damped Dutch roll. Drooping the wing tips downward removes part of it and restores an acceptable balance between the two lateral modes.

Answer: B

The strong rolling response to sideslip drives an oscillation in which yaw and roll couple together. Too little dihedral effect gives the opposite failing, a divergent spiral, so the two must be kept in a workable balance.