Longitudinal Control and Trim Questions and Answers

Answer: A

Correct answer: Both Statement I and Statement II are correct

Both are true and the second gives the mechanism. 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 therefore pitches the nose up and reducing it pitches down - the trim change every pilot of such an aeroplane learns to anticipate, and a significant handling consideration during a go-around. An aeroplane with the thrust line above the centre of gravity, such as some flying boats, shows the opposite behaviour.

Answer: B

Correct answer: Both Statement I and Statement II are correct

Both are true and the second is how the first is achieved. The foreplane is sized and set so that it reaches its stalling angle before the main wing.

When it does, its lift falls, the nose drops, and the angle of attack reduces automatically - the aeroplane cannot be held beyond that point, whatever the pilot does with the stick. The main wing therefore never stalls, and the classic wing drop and departure are avoided. It is a designed-in limit rather than a warning system.

Answer: C

The additional demand arrives at the moment the surface is least effective, since the aeroplane is also at its lowest speed. That combination is why the flare so often turns out to be the critical case for elevator power.

Answer: D

Correct answer: 1, 2, 3, 4, 5

The sequence is the logic of the calculation. The critical trim case must be identified first - flare or rotation, at forward centre of gravity - since everything else is computed at that condition.

The deflection required to trim there follows, then the additional deflection for manoeuvre and gusts, and the total is compared with the travel available. The limit is finally placed where sufficient travel remains in hand. Note that step 3 must precede step 4: trim alone is not what the surface must deliver.

Answer: A

Each step involves more of the surface in producing the control moment. The most powerful arrangement is also the lightest to move, which is why it normally carries an anti-servo tab to restore a sensible force gradient.

Answer: B

Without the compensating trim the aeroplane would tend to accelerate further as it tucked, reversing its speed stability. Sensing Mach number and applying trim automatically prevents the sequence from ever reaching the pilot.

Answer: C

The trim change on flap selection is a characteristic of each type that a pilot learns during conversion. Trimming removes the force rather than the deflection, which remains whatever the new balance of moments requires.

Answer: D

Correct answer: Only three

The first three are correct: elevator power is the pitching moment per degree, elevator angle per g is the deflection sustaining a normal acceleration, and trim is holding a condition at zero stick force.

The fourth misdefines trim as zero elevator deflection. Trimming zeroes the force, not the deflection - at most trimmed conditions the elevator is deflected several degrees, carrying the download that balances the wing's pitching moment.

Answer: A

Correct answer: Only three

The first three describe their arrangements correctly. A stabilator is an all-moving surface; a variable incidence tailplane trims slowly by screwjack while the elevator handles rapid inputs; elevons combine pitch and roll on a tailless aeroplane.

The fourth contradicts the first. A fixed surface with a hinged control is a conventional tailplane and elevator, which is precisely what a stabilator is not - the whole point of the arrangement is that no part of it is fixed.

Answer: B

Correct answer: Only three

The first three pair effects with their true causes: the nose-up pitch from underslung engines comes from the thrust line below the centre of gravity, Mach tuck from the rearward shift of the aerodynamic centre, and the nose-down moment in the flare from reduced downwash in ground effect.

The fourth gives Mach tuck a second and wrong cause. Hinge moments concern the force the pilot feels, not the moment about the centre of gravity, and tuck would occur on a fully powered control that transmits no hinge moment at all.