Control Surface Hinge Moments Questions and Answers

Answer: C

Manual control forces grow with the square of speed, so at some point they reach the certification limits and the aeroplane cannot be manoeuvred as required, which is a genuine limitation on the maximum operating speed. Flutter is also a limit, and a harder one, but the question asks what stick forces contribute. Stall speed sets the bottom of the envelope. Tab range limits trimming rather than manoeuvring.

Answer: D

It is an anti-balance tab, fitted when the force or the force per g would otherwise be too low, or to restore a positive gradient on a surface that is close to overbalance. A balance tab moves the opposite way and lightens the control. A servo tab is the pilot's only direct connection, flying the surface aerodynamically. A trim tab sets the zero-force position without altering the gradient.

Answer: A

The whole art of manual control design rests on the fact that they are separable: aerodynamic balance changes the moment about the hinge while leaving the moment about the aeroplane's centre of gravity nearly untouched. If they were strictly proportional, no such device could work. Reducing area reduces both, but that is one particular change rather than a general rule. And it is hinge moment, not effectiveness, that depends strongly on hinge position. The practical test is to ask which quantity a change would move: the force, the gradient, the flutter margin or the trim.

Answer: B

A fighter in combat and a transport on approach make very different demands, so the acceptable band is set by class and by flight phase, exactly as the modal requirements are. Hinge moment coefficients are properties of a surface's geometry, not of a class. Heavier aeroplanes tend towards heavier controls unless powered, so the third option is backwards. And the specification structure follows from the engineering rather than the reverse.

Answer: C

Air leaking through the gap relieves the pressure difference across the surface, cutting effectiveness and altering the hinge moment, which is why gaps are sealed on surfaces where either matters, and why the internal balance arrangement makes deliberate use of a sealed chamber. Gaps have nothing to do with flutter prevention, and they certainly have an aerodynamic effect. Mass balance is a matter of weight distribution.

Answer: D

The moment is the product of a coefficient that grows with deflection and a dynamic pressure that grows with the square of speed, so the worst case is a large deflection at high speed - typically a recovery from an upset or a high-speed pull-up, and the case that sizes the actuator. Low speed at neutral is close to the minimum. The stall is at low dynamic pressure. And trimmed cruise has the surface near its zero-force position by definition. A design review checks each of these separately, since a fix for one can quietly worsen another.

Answer: A

An irreversible system is one in which the actuator holds the surface against the air load and no part of that load is fed back to the pilot's controls, which is why artificial feel becomes necessary. Trimming remains perfectly possible, through the feel unit's datum or a trim actuator. The surface returns to commanded position normally. And the absence of feel is the consequence rather than the definition.

Answer: B

A zero gradient means the pilot feels no force change as the speed departs from trim, so nothing cues a correction - the aeroplane will happily sit at any speed, which is tiring and is why a positive gradient is required. It is a handling characteristic rather than a failure. Trimming is still possible; indeed the aeroplane is effectively trimmed everywhere. And spiral stability is a lateral matter with no connection to the elevator force gradient.

Answer: C

Aft loading reduces the static margin and with it the change of elevator angle and force with speed, so the gradient goes shallow and a downspring restores it by adding a nearly constant nose-down moment. A forward-loaded aeroplane has the opposite problem, heavy but well-graded controls. A powered aeroplane gets its gradient from the feel unit. Altitude changes dynamic pressure but is not what drives the need.

Answer: D

Rigging concerns the mechanical system: free play, cable tensions, and the travel achieved at the stops, all of which affect flutter margin, control authority and feel. Static margin is a loading and design property, checked by weight and balance rather than by rigging. The lift curve slope and the Dutch roll damping are aerodynamic and dynamic properties that no ground inspection can measure.