Longitudinal Control and Trim Questions and Answers

Answer: A

Correct answer: Pitching moment per degree of deflection

Elevator power is the control derivative \( C_{m\delta e} \) - the pitching moment coefficient generated for each degree of elevator deflection.

It measures authority, not effort. What the pilot feels is the hinge moment and hence the stick force, which is a separate quantity and can be adjusted by aerodynamic balance or an artificial feel system without altering the power at all. Maximum deflection is a mechanical limit; the power says how much moment each degree of that travel buys.

Answer: B

Trimming at a lower speed means flying at a higher lift coefficient, which changes the balance of moments about the centre of gravity. The gradient of that requirement with speed is the flight test measure of stick fixed static stability.

Answer: C

A forward centre of gravity demands the largest nose up moment while low speed leaves the surface least effective. Adding flap deflection usually worsens the nose down moment further, which is why this combination sets the forward loading limit.

Answer: D

Correct answer: Deflection per unit normal acceleration

Elevator angle per g is the additional deflection needed to sustain each unit of normal acceleration in a steady manoeuvre, and it is the standard measure of manoeuvre stability.

It depends on the manoeuvre margin - the distance from the centre of gravity to the manoeuvre point - so it falls as the centre of gravity moves aft, and reaches zero at the manoeuvre point itself. Stick force per g is the related quantity the pilot feels, and certification sets minimum values for it to prevent inadvertent overstressing.

Answer: A

The ground constrains the wing's trailing vortices, reducing the downwash the tailplane sees and increasing its download. The resulting nose down moment must be countered with additional up elevator at exactly the moment least is available.

Answer: B

Deflecting the entire surface produces far more pitching moment than an elevator of the same total area could. The arrangement is common where control power is at a premium, and it usually carries an anti-servo tab to restore adequate feel.

Answer: C

Correct answer: The tailplane trims, the elevator manoeuvres

Splitting the two functions lets each be optimised. The tailplane's incidence is adjusted slowly by a screwjack to trim out the large, sustained moment changes of configuration and centre of gravity, and it holds that setting without any hydraulic power.

The elevator is then left with only the smaller, faster demands of manoeuvring, so it can be smaller and its actuators lighter. The arrangement does not remove the need for feel, and a trimming tailplane is heavier than a fixed one, not lighter - the gain is in what the elevator no longer has to do.

Answer: D

An all moving surface would otherwise be far too light and easy to overcontrol. The tab deliberately adds hinge moment so that the pilot feels a sensible force gradient and cannot inadvertently overstress the aeroplane.

Answer: A

Correct answer: Combine elevator and aileron functions

A tailless aeroplane has no tailplane to carry an elevator, so the wing trailing edge surfaces must do both jobs. Deflected together they pitch the aeroplane; deflected differentially they roll it, and a mixer combines the pilot's two inputs.

The compromise is real: the same surface cannot be optimised for both, and pitch trim on a tailless design costs lift, since an up-elevon reduces the wing's own lift. Ruddervators do the corresponding job on a V-tail, combining pitch and yaw instead.

Answer: B

The change in wing lift distribution and the extra downwash at the tail both alter the moment balance. Whether the aeroplane pitches nose up or nose down depends on the layout, and the trim change is a characteristic every pilot learns for the type.