Static Longitudinal Stability Questions and Answers

Answer: C

Static stability guarantees only that the initial response opposes the disturbance. Whether the resulting oscillation grows, decays or persists depends on damping, which is an entirely separate property of the aeroplane.

Answer: D

Correct answer: It is the point through which the lift always acts

That is the false one. The neutral point is the aerodynamic centre of the whole aeroplane: the point about which the pitching moment does not change with incidence, which means it is the point through which the *increment* of lift acts. The total lift acts through the centre of pressure, which moves with incidence. The other three statements are all correct restatements of the definition.

Answer: A

Correct answer: No, it reduces the static margin

The static margin is the distance from the centre of gravity to the neutral point, \( (h_n - h) \), so moving the centre of gravity aft closes the gap and reduces the margin. Stability falls, and at the neutral point itself it vanishes. The tail arm shortening as the centre of gravity moves aft works in the same direction, so it reinforces the answer rather than opposing it.

Answer: B

Correct answer: Increasing the wing area

The wing area sits in the denominator: \[ \bar{V}_H = \frac{S_t\,l_t}{S\,\bar{c}}. \] Enlarging the wing therefore reduces the tail volume ratio, which is why a wing growth on an existing airframe usually forces a tail growth with it. Reducing the mean chord, enlarging the tailplane and lengthening the arm all increase the ratio.

Answer: C

Excessive margin makes the aeroplane heavy on the controls, increases trim drag and restricts the forward loading limit. The design aim is a margin adequate for the handling required rather than the largest achievable.

Answer: D

Correct answer: Insufficient static margin

An insufficient static margin is what sets the *aft* limit, not the forward one - forward of the neutral point the margin only grows. The forward limit is set by control power: enough elevator to rotate on take-off, to flare at touchdown, and to trim at approach speed, all with the nose heavy and the tail carrying its largest download.

Answer: A

Correct answer: No, it usually carries a download

With the centre of gravity ahead of the wing’s centre of lift the wing produces a nose-down moment, and the tail must push down to balance it. The wing then carries the weight plus that download, which is the origin of trim drag. Only with the centre of gravity well aft does the load approach zero or reverse, and that is the condition the aft limit exists to prevent.

Answer: B

The neutral point is a property of the aeroplane's aerodynamics, not of how it happens to be loaded. Loading changes the static margin by moving the centre of gravity, while leaving the neutral point exactly where it was.

Answer: C

Correct answer: False, its restoring moment comes only from reflex or washout

A flying wing has no surface on a long moment arm, so its restoring moment must be generated by the wing itself - a reflexed trailing edge, or washout combined with sweep. Both are weak, so the neutral point lies close to the centre of gravity and the usable range of centre of gravity travel is small. Sweep helps, but it does not turn a small margin into a generous one.

Answer: D

Correct answer: An increase in the tail's stabilising contribution

Increased downwash reduces the tail’s share of the aeroplane’s change in incidence, since \( (1-\mathrm{d}\varepsilon/\mathrm{d}\alpha) \) falls. The tail therefore contributes less to stability, and the neutral point moves forward, shrinking the static margin for a given centre of gravity. The first three options are all consequences of that; only the last reverses it.