Control Surface Hinge Moments Questions and Answers

Answer: A

Gearing is purely mechanical, so it scales the force with no effect on the aerodynamic hinge moment and therefore no risk of a sign reversal; the cost is column travel or authority at the stops. The other three all work by increasing aerodynamic balance, and each brings the surface closer to overbalance at large deflection. That is why gearing is usually the first adjustment tried and balance the second.

Answer: B

Rolling requires the pilot to overcome both ailerons at once, and the wheel or stick offers less mechanical advantage laterally than the column does in pitch, so roll forces dominate on many manual aeroplanes - one reason spoilers and powered assistance appeared first in roll. Elevator area is often larger but only one surface is involved. The rudder's arm affects the aeroplane's yawing moment, not the hinge moment. And nothing makes the three equal.

Answer: C

Breakout force is the amount required to overcome friction, preload and any centring spring before the surface begins to move, and it is specified with limits: too little feels sloppy, too much masks the force gradient near neutral. Stick force per g is the manoeuvre measure. Hinge moment is the aerodynamic load at the surface. Gradient is the rate at which force builds with deflection or with g, which breakout sits on top of rather than replacing.

Answer: D

Mass balance is a weight fitted to the surface itself, ahead of its hinge line, and it works by moving the surface's centre of gravity onto the hinge so inertia cannot drive it out of phase with structural motion. The other three all live in the control run or the cockpit: a bobweight adds force with normal acceleration, a downspring adds a constant moment to steepen the speed gradient, and an artificial feel unit manufactures the whole force in a powered system. Where a device sits is a good clue to what it does.

Answer: A

Hinge moment is a moment and carries units of newton metres. Its coefficient is dimensionless, being the moment divided by dynamic pressure, area and chord. Stick force is a force in newtons, obtained from the moment through the gearing. The gearing ratio is itself dimensionless or in metres per newton metre depending on how it is written. Sorting the four by units is a quick way to check any expression that relates them.

Answer: B

Hinge moment grows with the square of the speed, because it carries dynamic pressure. The coefficient is nearly constant at a given Mach number, which is why it is the quantity tabulated. The elevator angle needed to trim actually decreases as speed rises, since the same lift needs less incidence. And the float angle is a ratio of two coefficients and so barely changes at all. Only one of the four carries the dynamic pressure, and it is the one the pilot feels.

Answer: C

A stop limits travel mechanically, protecting the structure and the actuator and defining the control authority available. It does nothing about the hinge moment, which is aerodynamic. Balancing is achieved by mass or by area ahead of the hinge. Sealing the gap is the internal-balance arrangement or a simple seal fitted to preserve effectiveness. Stops are also where the maximum hinge moment case usually occurs, which is why the actuator is sized against the load at full deflection.

Answer: D

It schedules against dynamic pressure, so the control feels progressively heavier as the aeroplane goes faster, reproducing the cue a manual control would have given and discouraging large, rapid inputs at high speed. Fuel state, temperature and cabin altitude have nothing to do with the force a pilot should feel on the column. Some systems add a further term in incidence to give a tactile stall warning, but the primary schedule is q.

Answer: A

Flutter is the symptom: an unbalanced surface lags the structure's motion and feeds energy into it, producing an oscillation that grows with speed. Heavy forces are an aerodynamic balance question, addressed with set-back, horns or tabs. A shallow speed gradient is a static stability or downspring matter. Poor trim authority means the tab is too small or its travel too limited. Each symptom points to one class of device, which is the practical value of keeping the four apart.

Answer: B

The surface floats to the angle where its total hinge moment is zero, which is set by the ratio of the incidence derivative to the deflection derivative. Static margin is a whole-aeroplane property that the float angle affects rather than the reverse. Gearing scales the force the pilot feels and cannot influence where a released surface settles. Mass matters in a manoeuvre and for flutter, but the steady float is purely aerodynamic.