Static Longitudinal Stability Questions and Answers

Answer: C

Correct answer: Only three

Moving the centre of gravity forward increases the margin and so the stick forces; lengthening the tail arm increases \( V_H \); and greater downwash reduces the tail\'s effective angle of attack change and with it its contribution. The fourth pair is wrong: moving the centre of gravity aft reduces the static margin and makes the aeroplane less stable, which is why an aft limit exists at all.

Answer: D

Correct answer: Only three

The first three are right: a conventional tail carries a download in trim, a canard foreplane lifts upward, and a tailless aeroplane obtains its restoring moment from a reflexed trailing edge or from washout on a swept wing. The fourth is wrong. A canard foreplane sits ahead of the centre of gravity and is destabilising in itself, exactly as a wing with an aft centre of gravity is; the stability of a canard layout comes from the main wing behind, and the foreplane is sized to stall first so that the nose drops.

Answer: A

Correct answer: The tailplane

The tailplane is the stabilising component: sitting well aft, an increase in incidence raises its lift and produces a nose-down moment. The fuselage is destabilising, a wing with the centre of gravity aft of its aerodynamic centre is destabilising, and a canard foreplane, being ahead of the centre of gravity, is destabilising in itself. Everything forward of the centre of gravity tends to destabilise; the aft surface is what restores.

Answer: B

Correct answer: Moving the centre of gravity aft

Moving the centre of gravity aft reduces the static margin \( h_n - h \) and so reduces stability - taken far enough it reaches the neutral point and the aeroplane becomes unstable. The other three all increase stability: moving the centre of gravity forward increases the margin directly, and enlarging the tail or lengthening its arm both raise \( V_H \), which moves the neutral point aft.

Answer: C

Correct answer: The point through which the lift always acts

That describes the centre of pressure, which travels with incidence and is not fixed. The neutral point is the centre of gravity position at which the moment slope becomes zero, and so equivalently the point at which the static margin vanishes and the aft limit of static stability lies. The three correct descriptions are three ways of saying the same thing; the odd one out belongs to a different concept altogether.

Answer: D

Correct answer: Light stick forces in manoeuvre

A large static margin gives heavy stick forces, because the force per g rises with the margin. It also increases the trim download and hence the trim drag, and it pushes the centre of gravity forward towards the limit at which elevator power runs out. Light stick forces are the symptom of a small margin, that is of an aft centre of gravity.

Answer: A

Correct answer: The wing’s aspect ratio

The tail volume ratio is

which contains the tail area, the tail arm, the wing area and the wing mean chord, and nothing else. Aspect ratio does not appear. It does influence stability by another route, through the lift curve slopes and the downwash gradient, but it leaves \( V_H \) itself untouched.

Answer: B

Moving the centre of gravity aft reduces the distance to the neutral point and with it the restoring moment. Stick forces fall in proportion, and near the neutral point the aeroplane holds no attitude of its own accord.

Answer: C

Correct answer: The forward centre of gravity limit, set by elevator power

Loading forward gives a large static margin, which is why the cruise felt solid, but it also increases the nose-down moment the elevator must overcome. On the approach the speed is low, so the dynamic pressure at the tail and the moment per degree of elevator are at their smallest. The two effects meet, and full aft elevator barely produces enough moment to flare. This is precisely the condition the forward limit exists to prevent, and it shows that the limit is about control authority, not about stability.

Answer: D

Volume ratio scales directly with the moment arm, so the tail's stabilising power rises in proportion. Lengthening the fuselage is often preferred to enlarging the tail, since area brings weight and drag as well as stability.