Longitudinal Control and Trim Questions and Answers

Answer: B

A trimmed aircraft has zero pitching moment about its centre of gravity.

Trim means the aircraft flies in steady equilibrium with no tendency to pitch up or down. Rotation in pitch takes place about the centre of gravity, so the condition for trim is that the total pitching moment about the c.g. is zero, \(C_{m,cg} = 0\), with the tail or elevator setting chosen to achieve it.

The aerodynamic centre is the point about which the moment is independent of angle of attack; the moment there is generally not zero (it equals \(C_{m,ac}\)). The quarter-chord point is only an approximation to the aerodynamic centre of an airfoil, and 50% of the root chord has no special significance for trim.

Answer: C

Elevator effectiveness determines the forward-most location of the centre of gravity.

Moving the centre of gravity forward increases static stability, and with it the nose-down moment the elevator must overcome to trim at high lift coefficients, especially at landing with flaps down and in ground effect. The forward c.g. limit is the position at which full up-elevator can only just trim the aircraft at the required \(C_{L}\), so it is set by the elevator's power.

The aft-most limit is set by stability (the neutral point and the required static margin), not by elevator power. Turn radius and rate of climb are performance quantities set by load factor, thrust and drag.