Empennage Sizing Questions and Answers

Answer: C

Correct answer: Increasing the wing area

Wing area is in the denominator:

\[ V_H = \frac{S_H\,l_H}{S\,\bar{c}} \]

so enlarging the wing lowers the coefficient for an unchanged tail. The effect is physical as well as arithmetic: a bigger wing produces larger moments for the tail to balance, so the same tail is relatively less effective.

The other three raise it. Tail area is in the numerator, and both lengthening the rear fuselage and moving the tailplane aft increase the arm, which is also in the numerator.

Answer: D

Correct answer: It increases as the centre of gravity moves aft

The static margin is the distance from the centre of gravity to the neutral point, so moving the centre of gravity aft brings the two closer and the margin decreases.

The other three statements are true: it is expressed as a fraction of the mean aerodynamic chord, it is the standard measure of longitudinal static stability, and a negative value means the centre of gravity has passed the neutral point and the aircraft is unstable. That is exactly why an aft loading limit exists.

Answer: A

Correct answer: Reduced tailplane effectiveness in normal flight

In normal flight a T-tail is more effective, not less, since it sits above the wing's downwash and wake and clear of engine efflux, working in cleaner air with a higher local dynamic pressure.

The genuine drawbacks are the other three: a heavier fin, which must carry the tailplane's loads over its full height; the deep stall risk at extreme angle of attack, where the wake can finally blanket the surface; and the complexity and cost of the fin-to-tailplane joint, which must transfer those loads and stay flutter-free.

Answer: B

Correct answer: Wrong, since yawing moments are referred to the span

The reference length must match the moments the surface produces. The fin generates yawing and rolling moments, and both are non-dimensionalised on the span, just as pitching moments are on the mean aerodynamic chord.

Using the chord for the vertical coefficient would give a number that could not be compared with any published data and would carry no consistent meaning. The rule is general - it holds for every configuration and every wing planform - so the options qualifying it by tail type or sweep are wrong for that reason too.

Answer: C

Correct answer: A twin generally faces a larger asymmetric moment than a four-engine aircraft of similar thrust

Total thrust being equal, a twin's engines are individually larger and further outboard, so losing one removes a bigger share of the thrust at a longer arm. A four-engine aircraft loses a quarter of its thrust at the arm of an inboard or outboard engine, which gives a smaller yawing moment.

The other options invert the case: it sizes the vertical tail, not the horizontal, and it becomes more critical as speed falls, since the rudder's opposing moment goes with dynamic pressure while the thrust asymmetry does not.

Answer: D

A forward centre of gravity needs a larger download to balance the wing, so trim drag grows. The other three all reduce the download the tail must carry, and aft fuel transfer is used in service for exactly this reason.

Answer: A

A surface ahead of the centre of gravity is destabilising; stability has to be recovered by moving the centre of gravity forward. Designing the foreplane to stall first is a standard safety feature, since it drops the nose.

Answer: B

Correct answer: Halve the horizontal tail volume coefficient

The coefficient is linear in the arm:

\[ V_H = \frac{S_H\,l_H}{S\,\bar{c}} \]

so with \( S_H \) fixed, halving \( l_H \) halves \( V_H \). The distractor "quarter it" would follow if the coefficient went as the square of the arm, which it does not.

Restoring the original tail volume would then require doubling the tail area - the inverse trade seen from the other side, and the reason short-fuselage aircraft carry proportionately large tails.

Answer: C

Correct answer: The neutral point is further forward on long aircraft

That statement is not a reason, and it is not generally true: neutral point position depends on the balance of wing, fuselage and tail contributions, and a longer tail arm tends to move it aft rather than forward.

The other three are the genuine reasons, and all say the same thing from different sides. A longer aircraft has a longer tail arm; the arm is in the numerator of the tail volume coefficient, so the required volume is reached with less area; and each unit of tail load produces a larger moment.

Answer: D

The blanketed tailplane leaves no pitch authority to lower the nose, which is why prevention by stick pusher is the accepted answer rather than any recovery technique. It is a high angle of attack, low speed phenomenon of high-mounted tails.