Control Surface Hinge Moments Questions and Answers

Answer: C

It is the column force per unit normal acceleration, and it is the quantity certification rules constrain from both sides: too light and the aeroplane can be overstressed inadvertently, too heavy and it cannot be manoeuvred. It depends on the manoeuvre margin, the hinge moments and the gearing together. Force from neutral in level flight is a breakout or friction matter. Trim change with speed is the speed stability gradient, a separate requirement.

Answer: D

A bobweight is a mass that responds to normal acceleration, so it adds a force proportional to g and raises the stick force per g without affecting the force at one g in level flight. It is the classic fix for an aeroplane whose manoeuvre forces are too light. It does nothing about the speed dependence of hinge moments. Flutter is prevented by mass balancing at the surface, which is a different weight in a different place. Artificial feel units are separate devices.

Answer: A

A downspring applies a nearly constant nose-down moment, so it steepens the variation of stick force with speed and can rescue an aeroplane whose speed stability gradient is too shallow, typically one loaded well aft. Because its force is constant it does not change the force per g, which is the bobweight's job. Balancing about the hinge is achieved by mass or aerodynamic balance. Deflection limits come from stops.

Answer: B

Hinge moment goes with dynamic pressure and therefore with the square of speed, so doubling the speed quadruples the force:

\[ F = 45 \times 2^{2} = 180 \ \mathrm{N} \]

which is at the edge of what a pilot can hold one-handed for any length of time, and precisely why manual controls run out of acceptability as speed rises. 90 N assumes a linear dependence on speed. 45 N assumes none at all. 22 N has the trend backwards.

Answer: C

Both bounds are imposed: a minimum so the aeroplane cannot be overstressed by an inadvertent pull, and a maximum so a pilot of average strength can achieve the required load factor. That two-sided constraint is why bobweights and balance devices are tuned rather than simply minimised. Limiting only the low side would permit a control so heavy the aeroplane could not be manoeuvred, and limiting only the high side would permit one dangerously light.

Answer: D

Moving the hinge aft puts part of the surface ahead of it, so the load on that forward portion produces a moment opposing the rest and reduces \( C_{h_\delta} \). It is the simplest and most common form of aerodynamic balance. Effectiveness about the aeroplane's centre of gravity is essentially unchanged, which is the point: lighter forces for the same control power. Flutter needs mass balance. Trim tabs remain necessary for trimming. Each of these is measured differently and adjusted by a different means, which is why they are specified separately.

Answer: A

A horn balance is area forward of the hinge, concentrated at the tip, where it is easy to arrange without a full-span set-back. Unshielded and shielded variants differ in whether the horn projects beyond the fixed surface. Mass ahead of the hinge is mass balance, aimed at flutter. Sealing the gap is the internal balance arrangement. A tab moving the other way is a balance tab, which is a third and separate method.

Answer: B

The surface's nose extends into a chamber sealed by a flexible diaphragm, and the pressure difference across that chamber acts ahead of the hinge to oppose the hinge moment. It gives effective balance with no external area and little drag penalty, which is why it is common on transports. No engine air is involved. A spring in the actuator would change the feel rather than the aerodynamic moment. Reducing the aft chord would reduce effectiveness as well as hinge moment.

Answer: C

A set-back hinge lightens the control, mass ahead of the hinge prevents flutter, and an anti-balance tab deliberately heaviens it, so A-Q, B-P, C-R. The third pairing surprises people: a designer sometimes needs more force, either to meet the minimum stick force per g or to stop an over-balanced surface running away. Confusing mass balance with aerodynamic balance is the most consequential of the possible errors, since one addresses a flutter mechanism and the other a force level.

Answer: D

A balance tab is geared to move opposite to the surface, so its load assists the deflection and reduces the force the pilot must apply. An anti-balance tab is geared the other way and increases the force, which is what the second option describes. A tab that returns to neutral independently of the surface is a trim tab, driven by the pilot or an actuator. Pilot-selected movement alone also describes a trim tab rather than a geared balance tab.