Static Longitudinal 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 reason gives the cause. A tailless aeroplane has no surface on a long arm, so its restoring moment must come from reflexed trailing edges or from washout on a swept wing - both small effects acting on a short arm. The neutral point is therefore close to the centre of gravity, and only a small movement of the latter uses up the whole margin.

Answer: B

Correct answer: 1 and 2 only

Statements 1 and 2 are the two conditions for static longitudinal stability: the slope \( \mathrm{d}C_m/\mathrm{d}\alpha \) must be negative so that a disturbance is opposed, and the curve must cross zero within the usable range so that a trimmed condition actually exists. Statement 3 is wrong - static stability says only that the initial tendency is to return; whether the resulting oscillation damps out or diverges is a separate, dynamic question.

Answer: C

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: the neutral point is the centre of gravity position at which the moment slope is zero, and it therefore sets the aft limit, with a margin held back from it. Statement 3 confuses the neutral point with a fixed centre of pressure - it is the point through which a *change* in lift acts, not the total lift, which is why the moment about it does not vary with incidence.

Answer: D

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: the fuselage produces a destabilising moment that grows with incidence, and the tailplane is the principal stabilising surface. Statement 3 is inverted - with the centre of gravity ahead of the wing’s aerodynamic centre the wing contribution is stabilising, and it is a centre of gravity aft of the aerodynamic centre that destabilises.

Answer: A

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: the aft limit is set by the least static margin considered acceptable, and the forward limit by the elevator power needed to rotate, flare and trim at low speed with the nose heavy. Statement 3 is false - fuel burn moves the centre of gravity, sometimes considerably on a long flight, which is precisely why fuel is transferred between tanks and why the loading schedule must keep the aeroplane inside the envelope throughout, not merely at take-off.

Answer: B

The first two describe the aeroplane as loaded and the last two its aerodynamics. Keeping the aerodynamic centre of a wing distinct from the neutral point of the whole aeroplane is essential to following any stability argument.

Answer: C

Only one of the four is stabilising, which is why the tail must be sized to overcome all the others. A canard achieves stability by placing the centre of gravity far enough forward rather than by any contribution from the foreplane.

Answer: D

Four different criteria bound the loading, and the operational limits sit inside the aerodynamic ones. The manoeuvre point lies aft of the neutral point, which is why stick force per g survives a little beyond neutral static stability.

Answer: A

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

Four arrangements of the same job. The conventional tail sits behind the wing and normally trims with a download. The canard puts the surface ahead of the wing, lifting upward. The tailless aeroplane has no separate surface and relies on reflex or washout. And the T-tail places the tailplane above the fin to keep it clear of the wing wake - at the cost of the deep stall risk, since at high incidence the wake can blanket it entirely.

Answer: B

Correct answer: The tailplane

The tailplane is the stabilising surface: an increase in incidence increases its lift, and because it acts well behind the centre of gravity that produces a nose-down restoring moment. The other three all work the other way - a wing whose aerodynamic centre is ahead of the centre of gravity, the fuselage, and a canard foreplane, which lifts ahead of the centre of gravity and so is destabilising by nature.