Longitudinal Control and Trim Questions and Answers

Answer: A

Powered controls isolate the pilot from real hinge moments, so the feel must be generated artificially. Losing the dynamic pressure input removes the cue that tells the pilot how fast the aeroplane is going through the feel of the column.

Answer: B

A thrust line below the centre of gravity produces a couple, and the slipstream changes the dynamic pressure and downwash the tail sees. Both act together on a propeller aeroplane and both must be trimmed out as power changes.

Answer: C

Correct answer: A deep stall

Loss of pitching moment from the elevator at very high angle of attack on a T-tail is the signature of deep stall. The wing's separated wake rises and blankets the fin-mounted tailplane, so the surface works in low-energy air and produces almost no lift change per degree.

The condition is self-sustaining, since the aeroplane descends at high incidence with the wake still over the tail, and recovery may be impossible. Mach tuck belongs to high speed, and overbalance concerns hinge moments rather than the moment about the centre of gravity.

Answer: D

Correct answer: power

Elevator power is the control derivative \( C_{m\delta e} \), the pitching moment coefficient produced per degree of deflection. It is the measure of the surface's authority.

The other terms describe different things. Balance concerns the hinge moment and hence the force the pilot feels; travel is the mechanical range available; and float is a free surface trailing with the airflow. Power and stick force are independent: an artificial feel system can change what the pilot feels without altering the moment the surface produces at all.

Answer: A

Correct answer: stick force

Trimming adjusts a tab or the tailplane incidence until the aerodynamic hinge moment is balanced and the pilot need hold no force. The elevator itself generally remains deflected.

That is the point of the definition: at most trimmed conditions a download is needed at the tail to balance the wing's pitching moment, so the surface sits several degrees from neutral. Airspeed and angle of attack are the flight condition being trimmed at, not what trimming reduces to zero.

Answer: B

Correct answer: Mach

Mach tuck is the nose-down tendency arising as shock waves form at high subsonic speed and the aerodynamic centre moves aft.

With the aerodynamic centre further behind the centre of gravity, the same lift produces a larger nose-down moment. The danger is that it is self-reinforcing: the nose drops, the aeroplane accelerates, the Mach number rises and the moment grows. A Mach trim system senses Mach and applies compensating movement automatically, which is why the condition is manageable on transports.

Answer: C

Correct answer: anti-servo

An anti-servo tab is linked so that it deflects in the same direction as the surface, opposing its motion aerodynamically and so increasing the hinge moment the pilot must overcome.

It is fitted to all-moving tailplanes because their own hinge moments are small while their control power is large, giving a stick force per g too low to meet certification. A servo tab does the reverse, deflecting oppositely to reduce the force on a heavy surface, and a trim tab is set to hold a condition at zero force.

Answer: D

Correct answer: q

A q-feel system senses dynamic pressure - conventionally written \( q \) - and generates stick force in proportion to it, so the control stiffens with the square of airspeed.

It is needed because a fully powered control isolates the pilot from the real hinge moments entirely, and feel that did not vary with speed would invite gross overcontrol at high speed. Springs and bob weights are the other elements of an artificial feel system, providing centring force and force per g respectively.

Answer: A

Only four degrees remain in hand at the flare, which is the tightest case in the table. Certification requires margin to remain at the critical condition, and it is that requirement which fixes how far forward the aeroplane may be loaded.

Answer: B

Correct answer: Propeller slipstream and ground effect

Both act by changing the conditions the tailplane works in. The slipstream raises the dynamic pressure at the tail and alters the downwash reaching it; ground effect reduces the downwash, raising the tail's effective angle of attack so it carries less download.

The other two act elsewhere. Flap lowering changes the wing's own lift distribution and pitching moment, the downwash change being a secondary consequence; and an underslung thrust line produces a couple about the centre of gravity directly, with no involvement of the tail at all.