Control Surface Hinge Moments Questions and Answers

Answer: C

The division is clean: aerodynamic balance addresses the force the pilot feels, and mass balance addresses a dynamic instability. They are often fitted to the same surface and are sometimes physically combined in one horn, which is why they are confused; but a perfectly mass-balanced surface can still be far too heavy to move, and a beautifully light one can flutter. The last option simply swaps them, which is the error the question exists to catch.

Answer: D

It is carried by hydraulic actuators, with the pilot's inputs commanding them and an artificial feel system supplying the force gradient. Cables direct to the surface would give impossible forces at cruise dynamic pressure. Servo tabs were used on some large aeroplanes of an earlier generation and remain on a few types, but they are not the modern arrangement. Aerodynamic balance alone cannot reduce forces by the factor required at these speeds.

Answer: A

The first two are correct: hinge moment carries dynamic pressure, and its coefficient is a function of both deflection and incidence through the two derivatives. The third is wrong. Static margin is fixed by the aeroplane's geometry and centre of gravity, not by the hinge; hinge moments determine the difference between the stick-fixed and stick-free margins, which is a related but distinct statement, and the confusion between them is worth guarding against.

Answer: B

Reducing \( C_{h_\delta} \) by aerodynamic balance lightens the control while leaving the moment about the aeroplane's centre of gravity essentially untouched - which is precisely the trick. Reducing area or moving the surface forward would both cut effectiveness along with the force. Changing the gearing lightens the force too, but at the cost of column travel or of control authority at the stops, so it is a compromise rather than a free gain.

Answer: C

It is the area of the movable surface aft of the hinge line, paired with that surface's own mean chord aft of the hinge. Using the wing area would give a coefficient two orders of magnitude smaller and quite incomparable with published values. The whole tailplane includes the fixed portion, which carries no hinge moment. Frontal area is never used here. Reference geometry is the first thing to check when a hinge moment figure looks implausible.

Answer: D

An anti-balance tab is geared to move with the surface, adding hinge moment and so raising the stick force. Designers fit one when the force per g would otherwise be below the certification minimum, or to cure an over-balanced surface. The other three all reduce the hinge moment: a balance tab by moving opposite to the surface, and the set-back hinge and horn by placing area ahead of the hinge line.

Answer: A

In a steady manoeuvre the elevator is displaced from its trim position and the aeroplane has a pitch rate, so both the deflection and the rate contribute to the hinge moment the pilot must hold, which is why stick force per g depends on the manoeuvre margin rather than the static margin. Weight enters through the load factor demanded. The trim tab fixes the zero-force point rather than the gradient. Thrust plays no part in the hinge balance.

Answer: B

Self-centring requires \( C_{h_\delta} < 0 \), so the moment always opposes the displacement and the surface returns to neutral when released. A positive value is overbalance. Zero gives a surface with no preference for any position, which would make the control feel dead and would leave the surface at the mercy of the local flow. The incidence derivative decides floating rather than centring, and its sign is a separate question.

Answer: C

Servo tabs were the answer: the pilot moved only a small tab, whose aerodynamic moment flew the main surface. It gave acceptable forces on very large aeroplanes with purely mechanical runs. Fly-by-wire and side-sticks are decades later. Fully powered irreversible actuators are the modern solution but needed hydraulic systems of a reliability that did not yet exist, which is exactly the gap servo tabs filled.

Answer: D

A centre of gravity aft of the hinge leaves the surface's inertia free to drive it out of phase with structural motion, so the surface is flutter-prone and needs mass balance. Aerodynamic balance is about area ahead of the hinge, not mass. Self-trimming is not a recognised behaviour. Overbalance refers to the sign of the deflection derivative, an aerodynamic condition that has nothing to do with where the mass sits.