Stick Fixed and Stick Free Stability Questions and Answers

Answer: C

In a steady pull-up the aeroplane is also pitching, and the resulting damping adds a restoring moment. Stick force per g therefore survives a little beyond the centre of gravity at which trim stability has already vanished.

Answer: D

Moving part of the surface area ahead of the hinge line lets the airload help rather than oppose the pilot. Overbalancing must be avoided, since a surface that assists too much produces control forces that reverse and becomes unsafe.

Answer: A

The horn projects forward of the hinge at the outboard end, so the airload acting on it opposes the hinge moment from the rest of the surface. It is simple and effective but adds a discontinuity that can affect the flow at high speed.

Answer: B

The airload on the area ahead of the hinge produces a moment opposing that from the area behind it. The degree of balance is set by how far back the hinge is placed, and too much produces overbalance and force reversal.

Answer: C

Correct answer: Deflecting to give zero stick force

A trim tab is a small surface at the trailing edge of the elevator, deflected in the sense opposite to the elevator itself. Its own aerodynamic moment about the hinge cancels the elevator's, so the net hinge moment is zero and the pilot can release the column.

Note what it does not do: it does not lock the surface, reduce its area or make it more effective. The elevator remains deflected at whatever angle the condition requires - trimming zeroes the force, not the deflection.

Answer: D

The pilot moves the tab rather than the surface, and the tab's aerodynamic moment then deflects the surface. It reduces the effort greatly but leaves little feel at low airspeed, where the tab itself has little authority.

Answer: A

A spring in the linkage lets the tab move progressively as the load rises, so it balances most where the forces are greatest. At low speed the spring barely deflects and the pilot retains conventional feel and authority.

Answer: B

The mass responds to the aeroplane's acceleration rather than to airspeed, so it increases stick force per g specifically. It is a simple way of meeting the manoeuvring force requirements without altering the aerodynamic balance.

Answer: C

Correct answer: The stick force can reverse

Overbalance means so much area lies ahead of the hinge line that the airload assists the deflection rather than resisting it. The surface then tends to deflect further of its own accord, and the pilot must push back to stop it.

A control whose force reverses gives the pilot no usable feedback about how much control is being applied, and the danger is inadvertent gross deflection. Balance is therefore a matter of degree: enough to make the force manageable, not so much that the sign of the gradient changes.

Answer: D

Forces that are too heavy tire the pilot and make manoeuvring imprecise, while forces that are too light invite inadvertent overstress. The permitted centre of gravity range is often set by keeping this parameter inside its bounds.