Aircraft Stability and Control Questions and Answers

Answer: A

Static stability concerns only the direction of the initial response, not whether the subsequent motion converges. An aeroplane can be statically stable and dynamically unstable, so the two properties must always be assessed separately.

Answer: B

A negative slope means an increase in angle of attack produces a nose down moment that opposes it. The curve must also cross zero at a useful positive angle of attack, or the aeroplane would only trim at a condition it cannot fly.

Answer: C

At that point the aeroplane will hold its attitude without any pitching control input. Stability concerns what happens when it is disturbed from there, which is a separate question from where the trim point itself lies.

Answer: D

Moving the centre of gravity aft to that point removes the stabilising slope entirely, leaving the aeroplane neutrally stable. Any further aft movement makes it statically unstable, which is why the neutral point defines the aft limit.

Answer: A

A larger static margin means a steeper restoring moment and a more stable aeroplane. It also means heavier stick forces and more trim drag, which is why designers aim for a margin that is adequate rather than as large as possible.

Answer: B

Correct answer: Statically unstable in pitch

The aerodynamic centre is the point about which the wing\'s pitching moment does not change with incidence, so all of the lift increment produced by a disturbance acts there. If the centre of gravity lies behind it, that increment sits ahead of the centre of gravity and pitches the nose further up, increasing the incidence that caused it. The moment slope is positive and the disturbance grows. Only with the centre of gravity ahead of the aerodynamic centre does a wing alone restore itself, which is why a conventional aeroplane needs a tail to move the neutral point aft of the centre of gravity.

Answer: C

Correct answer: A destabilising pitching moment

A slender body inclined to a flow develops a moment that acts to increase its incidence - the classical Munk result for a body of revolution. The fuselage therefore adds a positive term to \( \mathrm{d}C_m/\mathrm{d}C_L \) at every speed, typically of the order of \( +0.03 \) to \( +0.05 \). It is one of the two destabilising contributions the tailplane must overcome, and it is why a long forward fuselage tends to require a larger tail.

Answer: D

An increase in angle of attack raises the tail's lift, and its long moment arm turns that into a substantial nose down moment. The leverage matters as much as the tail's area, which is why the two are combined into a single volume ratio.

Answer: A

Combining area and moment arm into one non-dimensional group captures the tailplane's stabilising power in a single number. Two aeroplanes with the same volume ratio have similar tail contributions despite quite different proportions.

Answer: B

The tail sees the flow already deflected downward by the wing, so its incidence changes less than the aeroplane's does. The rate of change of downwash with angle of attack therefore appears directly in the stability expression.