Longitudinal Control and Trim Questions and Answers

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the second is the hazard the pusher guards against. At very high incidence the wing's separated wake rises and can blanket a fin-mounted tailplane, leaving the elevator without the authority to lower the nose.

Because a developed deep stall may be unrecoverable, certification for such configurations rests on prevention: a stick shaker to warn and a stick pusher to force the nose down automatically before the critical angle is reached, rather than on a recovery technique.

Answer: B

Correct answer: 1 and 2 only

The first two are correct. Trimming zeroes the stick force, and the elevator generally remains deflected while trimmed, carrying the download that balances the wing's pitching moment.

The third confuses trim with stability. The static margin is the distance from the centre of gravity to the neutral point, both set by the mass distribution and the aerodynamic configuration. A trim tab changes neither; it alters the hinge moment the pilot feels, not the moment the aeroplane generates about its centre of gravity.

Answer: C

Correct answer: 1 and 2 only

The first two are correct. Proximity to the ground reduces the downwash reaching the tailplane, so the tail carries less download and a nose-down moment results, which must be countered with up elevator.

The third is the opposite of the truth. The flare is flown at the lowest speed of the approach, so the elevator's moment per degree is at its least. That coincidence of largest demand with least authority is exactly what makes the flare the sizing case for elevator power.

Answer: D

Correct answer: 1 and 2 only

The first two are correct. Shock formation moves the lift distribution rearward, taking the aerodynamic centre aft, and the same lift then acts further behind the centre of gravity to give a nose-down moment.

The third mistakes what a Mach trim system does. It applies compensating trim so that the stick force still builds conventionally with speed; it does not limit the speed. The limiting Mach number is enforced by the pilot and by overspeed warnings, not by the trim system.

Answer: A

Correct answer: 1 and 2 only

The first two are correct: a stabilator uses the whole surface as the control, and an anti-servo tab increases the stick force.

The third reverses the servo tab's function. A servo tab deflects opposite to the surface, driving it aerodynamically and reducing the force the pilot must apply - it is fitted where a surface is too heavy. The two tabs are named for what they do, and confusing them inverts the design intention in both cases.

Answer: B

Correct answer: A-1, B-2, C-3, D-4

The four are distinguished by how much of the surface moves and what it is asked to do. A conventional elevator is a hinged surface on a fixed tailplane. A variable incidence tailplane splits the work: the whole tailplane trims slowly while an elevator handles manoeuvring.

A stabilator has no fixed part at all, the whole surface acting as the control. Elevons combine pitch and roll on a tailless aeroplane, having no tailplane to work from. Reading for how much is fixed settles three of the four at once.

Answer: C

Correct answer: A-1, B-2, C-3, D-4

Each trim change has its own mechanism. Power on underslung engines acts through the thrust line below the centre of gravity, producing a couple. Ground effect in the flare acts through reduced downwash at the tailplane.

Mach tuck acts through the rearward shift of the aerodynamic centre. Flap selection acts through the altered lift distribution and the downwash change that follows. Two of the four work at the tail and two on the wing or the thrust line - a useful first division.

Answer: D

Correct answer: A-1, B-2, C-3, D-4

The two tabs are opposites and the trap is to interchange them. An anti-servo tab deflects with the surface, opposing its motion and increasing stick force on an all-moving tailplane. A servo tab deflects against the surface, driving it aerodynamically and reducing the force on a heavy control.

The q-feel unit generates artificial force varying with dynamic pressure, and the Mach trim system applies automatic nose-up trim to counter the tuck. Each is named for what it does, which is the reliable route through the matching.

Answer: A

Correct answer: A-1, B-2, C-3, D-4

Each condition is demanding for its own reason. The flare at forward centre of gravity combines the largest nose-up demand with the least authority. High incidence on a T-tail risks the tailplane entering the wing wake.

High Mach cruise brings the nose-down moment from the rearward aerodynamic centre shift. Power application on a propeller aeroplane changes trim through both the thrust line and the slipstream. Note that the first two are control-authority problems and the last two are trim-change problems.

Answer: B

Correct answer: A differentially deflected aileron

Ailerons deflected differentially produce a rolling moment, in the lateral axis, not a pitching one.

The other three are all longitudinal controls: the elevator hinged to a fixed tailplane, the stabilator moving as a whole, and elevons moving together. The qualification on the elevons matters and the question supplies it - the same surfaces roll the aeroplane when deflected differentially, so it is the mode of deflection rather than the surface that decides which axis is being controlled.