Static Longitudinal Stability Questions and Answers

Answer: C

Correct answer: More stable but heavier on the controls

Moving the centre of gravity forward increases the static margin \( h_n - h \), which makes the moment slope more negative and the aeroplane more resistant to a disturbance in pitch. The same margin sets the stick force per g, so the controls become heavier in the same proportion. Stability and control feel move together: the pilot pays for the added stability in effort, and beyond the forward limit the elevator can no longer produce the nose-up moment needed to flare.

Answer: D

Correct answer: The minimum acceptable static margin

As the centre of gravity moves aft the static margin shrinks, the moment slope flattens, and the stick forces lighten until the aeroplane wanders in pitch. The neutral point is where the margin reaches zero, and the certificated aft limit is placed some way forward of it to keep a positive margin at every loading. It is a stability limit, in contrast with the forward limit, which is a control-authority limit set by the elevator power available at low speed.

Answer: A

A very forward centre of gravity gives ample stability but demands more elevator than the surface can provide at approach speeds. Landing flare and rotation on take-off are the manoeuvres that bind at that end of the range.

Answer: B

Correct answer: Moves, and the static margin changes accordingly

Which way it moves depends on where the tanks sit relative to the centre of gravity. Burning fuel from tanks aft of it draws the centre of gravity forward and increases the margin; burning from tanks ahead of it does the opposite. The neutral point does not move, so every shift of the centre of gravity changes the margin directly, and the loading schedule must keep the whole burn sequence inside the limits, not merely the take-off and landing conditions.

Answer: C

A foreplane ahead of the centre of gravity is destabilising in the same way a forward wing is, so stability must be obtained by careful placement of the centre of gravity. The compensation is that the foreplane lifts upward in trim rather than downward.

Answer: D

Correct answer: A downward load, which the wing must offset with extra lift

On a conventional layout the centre of gravity is ahead of the wing\'s aerodynamic centre, so the wing lift produces a nose-down moment that the tail must balance with a download. The wing then has to carry the weight plus that download, which raises the induced drag - the trim drag penalty of the conventional arrangement. A canard reverses this: its foreplane lifts upward and shares the weight, which is the aerodynamic argument in its favour.

Answer: A

Without a tailplane the wing itself must provide the restoring moment, which is achieved by reflexing the trailing edge or by sweep with washout. The available margin is small, which is why such aeroplanes are demanding to balance.

Answer: B

The manoeuvre point plays the same role for pulling g that the neutral point plays for trim. Both the handling qualities and the certification requirements depend on that relationship being kept within sensible bounds.

Answer: C

Static stability guarantees only that the initial response opposes the disturbance. Whether the subsequent oscillation grows, decays or persists is a dynamic question that depends on damping as well as on the restoring moment.

Answer: D

Locating it near the quarter chord for a subsonic aerofoil simplifies the whole analysis, since the moment about that point can be treated as constant. The centre of pressure by contrast moves with angle of attack and is far less convenient.