Control Surface Hinge Moments Questions and Answers

Answer: C

Roll control is the one: aileron hinge moments and aeroelastic reversal both bite hardest in roll, so spoilers take over some or all of the function at high speed on many transports, with the outboard ailerons locked out. Pitch and yaw keep their conventional surfaces, powered where necessary. Trim is a function rather than an axis. The reason roll suffers first is partly that two surfaces must be moved at once and partly that the wing twists under aileron load.

Answer: D

It causes overbalance and a reversal of the stick force gradient, so beyond some deflection the surface drives itself further from neutral and the column pulls out of the pilot's hand. It is an aerodynamic condition and has nothing to do with flutter, which is a mass and stiffness problem. Trim authority depends on the tab. And effectiveness about the aeroplane's centre of gravity is unchanged, which is what makes the condition so insidious: the control still works, but the feel has inverted.

Answer: A

Trimming means driving the hinge moment to zero at the deflection the aeroplane needs, so the pilot can release the controls and the surface stays put. Reaching a stop is a travel limit, not a trim condition. Stick force per g is a manoeuvre characteristic that trimming does not target. And putting the centre of gravity on the hinge is mass balancing, an entirely separate operation performed on the ground with weights.

Answer: B

An unshielded horn projects clear of the fixed surface, so it sees the free stream directly: more balance per unit area, but its behaviour changes more with incidence and sideslip, which makes overbalance in a corner of the envelope easier to stumble into. A shielded horn is smoother and weaker. Neither addresses flutter, which needs mass. And both work throughout the speed range, with the usual transonic complications.

Answer: C

The hinge moment is aerodynamic and depends on the surface, its deflection, the local flow and the dynamic pressure - none of which the linkage can change. Everything else on the list is scaled by gearing: the force at the column, the travel the pilot must make, and the force per g. That separation is why gearing is the cheapest adjustment available and why it cannot solve an overbalance problem, which lives on the aerodynamic side.

Answer: D

A fixed trim tab is a small metal tab bent by a mechanic on the ground to remove a standing out-of-trim condition, common on light aeroplanes and on ailerons that will not sit level. A bobweight is a fixed installation sized at design. A servo tab is flown by the pilot in the air. An artificial feel unit is an active system. Only the first is set by hand between flights, which is exactly what ground-adjustable means.

Answer: A

The coefficient is a function of the flow over the surface, so deflection and local incidence dominate, with Mach number and any tab position as further arguments. Weight and altitude affect the flight condition and therefore the dynamic pressure, which multiplies the coefficient rather than setting it. Fuel state is a mass matter. The number of hinges is a structural detail with no aerodynamic significance at all.

Answer: B

In an irreversible system the actuator carries the entire hinge moment and none of it reaches the pilot, which is why an artificial feel system must be provided. Shared loading describes a power-assisted or boosted control, which is a different arrangement with its own feel characteristics. Threshold-speed assistance is not how these systems work. And tab-driven surfaces are servo tab systems, the pre-hydraulic answer to the same problem.

Answer: C

Rigging and cable tension are the first suspects, along with friction in the run and any misassembled balance weight or tab linkage, because they are the things maintenance can have changed. Static margin depends on loading and would show as a change in trim behaviour rather than in friction-like heaviness. Dutch roll damping and the lift curve slope are aerodynamic properties that maintenance does not touch. Reasoning from what was recently disturbed is the quickest route to the cause.

Answer: D

The hinge moment is the one the pilot fights, about the surface's own hinge; the pitching moment is what the surface does to the aeroplane, about its centre of gravity. Control effectiveness is the derivative that quantifies the second. The manoeuvre margin is a stability property. Confusing the two moments is the single most common error on this topic, and the whole design art of balance depends on keeping them separate, since balance changes one and not the other.