Longitudinal Control and Trim Questions and Answers

Answer: C

The thrust acts along a line offset from the centre of gravity, so it generates a couple. Underslung engines therefore pitch the nose up as power is applied, which is a familiar effect on many propeller and jet aeroplanes alike.

Answer: D

Correct answer: Changing conditions at the tailplane

A propeller accelerates the air behind it, so the tailplane sits in a slipstream of higher dynamic pressure and altered downwash. Both change the tail load, and hence the trim, when power is changed.

That is why a single-engine aeroplane needs retrimming after every power change, and why the effect is most pronounced at low speed where the slipstream velocity is large compared with the free stream. The centre of gravity does not move, and the static margin is set by the aerodynamic configuration rather than by throttle position.

Answer: A

The aerodynamic centre moves aft from about a quarter chord towards mid chord, increasing the nose down moment. A Mach trim system senses the condition and applies compensating trim automatically to prevent the tendency developing.

Answer: B

It counters the nose down moment produced by the rearward shift of the aerodynamic centre. Without it the speed stability of the aeroplane would reverse, so that a small increase in speed would produce a tendency to accelerate further.

Answer: C

With a hydraulic actuator between the column and the surface, no aerodynamic load reaches the pilot's hand. A feel system must therefore generate a force that varies sensibly with airspeed and with the acceleration being commanded.

Answer: D

Sensing dynamic pressure reproduces the way real aerodynamic hinge moments behave, so the aeroplane feels heavier at high speed. Without such variation the pilot would have no natural cue to the speed at which manoeuvres are being made.

Answer: A

Correct answer: It stalls before the main wing

A canard is designed so the foreplane reaches its stalling angle first. When it does, its lift falls, the nose drops, and the angle of attack reduces - a self-limiting recovery that occurs before the main wing can stall.

The main wing therefore never departs, and the aeroplane resists the classic wing drop. The arrangement is not free of drag or of trimming demands, and the foreplane certainly can be stalled; it is precisely by stalling it first, deliberately, that the configuration protects itself.

Answer: B

A tailplane in disturbed low energy air produces far less lift change per degree of deflection. The problem is severe on T-tail aeroplanes at high incidence, where the deep stall condition can leave the elevator unable to recover the aircraft.

Answer: C

The aeroplane must be controllable with forces a pilot can sustain and with deflection still in hand at the limiting condition. Rotation, flare and the go around from an approach are the manoeuvres usually found to be critical.

Answer: D

The elevator is generally deflected at the trimmed condition, and it is the force that has been reduced to zero rather than the deflection. Confusing the two is a common misunderstanding of what trimming actually achieves.