Longitudinal Control and Trim Questions and Answers

Answer: C

Correct answer: Only three

The first three are right. An anti-servo tab increases stick force on an all-moving surface, a Mach trim system counters the nose-down moment at high Mach, and a q-feel system varies artificial force with dynamic pressure.

The fourth contradicts the first. A tab that reduces stick force is a servo tab, used where the surface is heavy; the anti-servo tab does the opposite, and is fitted precisely because a stabilator's own hinge moments are too small.

Answer: D

Correct answer: Only three

Three of the pairs give the right reason. Forward centre of gravity at low speed combines the largest nose-up demand with the least authority; the landing flare adds a nose-down moment through ground effect; and a T-tail at high angle of attack risks the tailplane entering the wing wake.

The fourth is wrong. Aft centre of gravity reduces the elevator deflection needed, so control power is at its most comfortable there. What aft loading threatens is stability - a shrinking static margin - which is a different limit.

Answer: A

Correct answer: A differentially deflected aileron pair

Ailerons deflected differentially produce a rolling moment, not a pitching one - that is roll control, in the lateral axis.

The other three all control pitch: the conventional elevator hinged to a fixed tailplane, the all-moving stabilator, and elevons, which pitch when deflected together on a tailless aeroplane. Elevons are the case worth noting, since they do both jobs; deflected together they are a longitudinal control, and only their differential component rolls.

Answer: B

Correct answer: The aeroplane is at its highest speed

The flare is flown at the lowest speed of the approach, not the highest. That is exactly why it is demanding: the elevator's moment per degree goes with dynamic pressure, so authority is at its minimum.

The other three are the genuine reasons. Ground effect reduces downwash at the tail and adds a nose-down moment; the centre of gravity may be at its forward limit, demanding the largest download; and low speed makes the surface least effective. Maximum demand coincides with minimum authority.

Answer: C

Correct answer: A change in the aeroplane's registration

Registration is an administrative matter and changes nothing about the aerodynamics or the mass distribution, so it can produce no trim change.

The other three all do. Lowering flap alters the lift distribution and the downwash at the tail; applying power with an offset thrust line produces a couple about the centre of gravity; and entering ground effect reduces downwash at the tailplane. Fuel burn and passenger movement belong to the same list, acting through the centre of gravity instead.

Answer: D

Correct answer: A trim tab on the elevator

A trim tab is an aerodynamic device: it deflects into the airstream to balance the real hinge moment on a manually controlled surface. On a fully powered control there is no hinge moment reaching the pilot for it to trim, so it has no place in the feel system.

The other three are the standard components: a q-feel unit stiffening the stick with dynamic pressure, springs providing centring force, and a bob weight adding force in proportion to normal acceleration - which is what gives stick force per g.

Answer: A

Correct answer: Fitting an all moving tailplane

An all-moving tailplane increases effectiveness. Deflecting the whole surface changes its angle of attack directly rather than merely cambering it, giving substantially more pitching moment per degree than an elevator of the same area.

The other three all reduce it. Low airspeed reduces the dynamic pressure the surface works in; very high angle of attack and the tailplane entering the wing wake both reduce the local dynamic pressure and the lift change per degree - the deep stall mechanism in its severe form.

Answer: B

A forward centre of gravity gives ample stability but demands the largest nose up moment at the lowest speed. Certification requires travel to remain in hand, which is precisely how the forward limit is fixed during development.

Answer: C

Shock formation moves the aerodynamic centre aft and produces a nose down moment that reverses the speed stability. A Mach trim system senses the condition and applies compensating trim before the tendency reaches the pilot.

Answer: D

Correct answer: Fit an anti-servo tab to increase the stick force

The symptom is a control that is too powerful for the force it demands: a stabilator generates a large moment per degree while its own hinge moments are small, so the stick force per g is low and the aeroplane is easy to overstress.

An anti-servo tab deflects with the surface, opposing its motion and raising the hinge moment the pilot must overcome. A servo tab would do the opposite and make matters worse; enlarging the tailplane increases the power that is already excessive; and moving the centre of gravity aft lowers the stick force per g further still.