Longitudinal Control and Trim Questions and Answers

Answer: C

Correct answer: False, it removes the stick force instead

Trimming adjusts a tab or the tailplane incidence until the aerodynamic hinge moment is balanced, so the pilot can release the column. The elevator itself generally remains deflected.

At most trimmed conditions a download is needed at the tail to balance the wing's nose-down pitching moment, so the surface sits several degrees from neutral. Correct rigging does not change this - a correctly rigged trim tab zeroes the force at whatever deflection the condition requires.

Answer: D

Correct answer: It increases downwash at the tailplane

Ground effect reduces downwash at the tailplane. The ground constrains the wing's trailing vortices, so less downwash reaches the tail, its effective angle of attack rises, it carries less download and the nose drops.

The other three statements are true and follow from that one: the downwash is reduced, a nose-down moment results, and it arrives in the flare at the lowest speed of the approach where the elevator is least effective. That coincidence is why the flare sizes the surface.

Answer: A

Correct answer: No, it carries an anti-servo tab to increase the force

A stabilator's problem is the opposite of a heavy control. Its power is high, because deflecting the whole surface changes its angle of attack directly, while its hinge moment is small, because it is closely balanced about the pivot.

The resulting stick force per g is too low for certification, so an anti-servo tab is fitted to deflect with the surface and oppose its motion, raising the force. A servo tab would lighten a control that is already too light, and the surface is not left untabbed.

Answer: B

Correct answer: Moving the centre of gravity aft

Moving the centre of gravity aft reduces the elevator deflection needed to trim, since the wing's lift acts closer to the centre of gravity and the tail download required is smaller. That is why aft loading is comfortable for control and limited instead by stability.

The other three all increase the demand: forward loading needs a larger download, entering ground effect adds a nose-down moment to be countered, and a flap-induced nose-down pitch must be balanced with up elevator.

Answer: C

Correct answer: No, it applies compensating trim as Mach number rises

A Mach trim system senses Mach number and applies compensating nose-up trim, so that the stick force continues to build conventionally with speed instead of reversing.

It is a handling device, not a limiter: it does not restrict thrust and does not prevent the limiting Mach number being exceeded, which remains the pilot's responsibility supported by overspeed warnings. Nor is it without effect on trim - adjusting the trim is precisely what it does.

Answer: D

Correct answer: The elevator becomes overbalanced at high incidence

Overbalance is a hinge moment problem: the surface tends to deflect further of its own accord, and the pilot feels the force reverse. It concerns the force at the stick, not the pitching moment the surface generates.

The genuine reasons are the other three, and they are one mechanism stated three ways: at very high angles of attack the tailplane enters the wing wake, and the wake carries little energy, so the surface generates far less moment per degree. That is the deep stall mechanism.

Answer: A

Correct answer: No, it pitches the nose up

Underslung engines put the thrust line below the centre of gravity, so thrust acting forward at a distance below produces a nose-up couple. Adding power pitches the nose up and reducing it pitches down.

The effect is not universal, which is why the second option overstates it: an aeroplane with the thrust line above the centre of gravity behaves in the opposite sense. Flap position is irrelevant, and thrust offset from the centre of gravity certainly does produce a pitching moment.

Answer: B

Correct answer: The elevator trim tab

A trim tab is an aerodynamic device that balances the real hinge moment on a manually controlled surface. On a fully powered control no hinge moment reaches the pilot, so there is nothing for it to trim; the trim function is performed instead by moving the zero-force point of the feel system.

The other three are the standard feel components: a q-feel unit varying force with dynamic pressure, a centring spring giving force with displacement, and a bob weight giving force with normal acceleration.

Answer: C

Correct answer: False, it is set by the elevator power available

Forward loading increases stability, since the centre of gravity moves further from the neutral point and the static margin grows. Stability is what limits the aeroplane at the other end of the range.

The forward limit is a control limit: at forward centre of gravity the tail must carry its largest download, and in the flare that demand arrives at the lowest speed, where the elevator is least effective. The limit is placed where enough travel remains after trim, manoeuvre and gusts. The landing gear position is a ground-handling consideration, not this one.

Answer: D

Correct answer: Reducing the tailplane's moment arm

Control power depends on the tail's force and on the arm through which it acts, so shortening the arm reduces the moment each degree of deflection produces.

The other three all increase it: an all-moving tailplane changes the surface's angle of attack directly rather than merely cambering it, a larger area generates more force for a given angle, and a longer arm turns the same force into a larger moment. The last two are the same terms that appear in the tail volume coefficient, which is why control power and stability improve together.