Empennage Sizing Questions and Answers

Answer: C

Correct answer: Only three

The first three are right. More tail area and a longer tail arm both increase the tail's stabilising contribution, moving the neutral point aft; a canard sits ahead of the centre of gravity and moves it forward.

The fourth confuses the axes. The fin generates side force and governs directional stability - the yawing plane - and has no effect on the neutral point, which is a longitudinal quantity concerning pitch. The directional counterpart is the yawing moment derivative, which fin area certainly does affect.

Answer: D

Correct answer: Only three

Three pairs are correct. Engine failure at low speed and crosswind landing both size the vertical tail, and trim at the forward centre of gravity limit sizes the horizontal tail - that being the condition demanding the largest download.

The fourth is wrong on the surface named. Cruise trim drag is a horizontal tail matter, arising from the download the tailplane carries and the extra wing lift it forces; the fin carries no trim load in symmetric flight and contributes nothing to it.

Answer: A

Correct answer: Trim drag in cruise

Trim drag arises from the horizontal tail. Its download forces the wing to carry more than the weight, and the extra lift brings extra induced drag - a longitudinal matter entirely.

The fin carries no load in symmetric flight, so it contributes nothing to trim drag. The other three are exactly what the vertical tail is sized for: balancing asymmetric thrust after an engine failure, de-crabbing in a crosswind, and providing the restoring yawing moment that makes the aircraft directionally stable.

Answer: B

Correct answer: Adding a canard foreplane

A canard sits ahead of the centre of gravity, so its lift in a disturbance increases the angle of attack rather than reducing it. Its contribution to the pitching moment derivative is destabilising, and it moves the neutral point forward.

The other three all move it aft by increasing the tail's stabilising contribution: more area, a longer arm, and a higher aspect ratio giving a steeper lift-curve slope and so more force per degree. This is why a canard aircraft needs a forward centre of gravity range, and why its foreplane is often deliberately stalled first.

Answer: C

Correct answer: Dorsal fin

A dorsal fin is a fixed fillet extending the fin forward along the fuselage. It has no movable element and no actuator, and the pilot cannot deflect it - it changes the moment produced by sideslip, not the moment produced by an input.

The other three are control surfaces: the elevator for pitch, the rudder for yaw, and the ruddervator, which on a V-tail performs both functions through a mixer in the control run. Keeping stability devices and control surfaces distinct matters because they are sized by quite different criteria.

Answer: D

Correct answer: Increasing the wing area

Wing area sits in the denominator of both coefficients:

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

so increasing \( S \) lowers the tail volume for an unchanged tail. That is a real effect and not merely arithmetic: a bigger wing generates larger moments the tail must balance, so the same tail is relatively less effective.

The other three raise it. Tail area and tail arm are both in the numerator, and lengthening the rear fuselage is how the arm is increased.

Answer: A

Correct answer: Elevon

An elevon is a control surface, not a tail arrangement. It combines the elevator and aileron functions in one surface at the wing trailing edge, and it is used on tailless and delta aircraft, which have no tailplane to carry a separate elevator.

The other three are genuine empennage configurations: the T-tail with the tailplane at the fin tip, the cruciform with it part way up, and the V-tail replacing both surfaces with two. The elevon belongs to the same family as the ruddervator - a combined-function surface - but it is on the wing.

Answer: B

Tail volume depends on area and arm, and the arm has grown with the rear plug. The extra volume is often what makes a stretch possible without a new tail, though an aft shift of the centre of gravity can spend the surplus again.

Answer: C

Correct answer: Increase the horizontal tail area to move the neutral point aft

Marginal stability at the aft limit means the centre of gravity is too close to the neutral point there. The direct remedy is to move the neutral point aft, and enlarging the horizontal tail does exactly that by increasing its stabilising contribution.

Increasing the wing area moves the neutral point the wrong way, since it raises the wing's share of the total moment. Reducing the elevator chord affects control power, not stability. Fin area works in the yawing plane and has no bearing on longitudinal stability at all.

Answer: D

Correct answer: The fin and rudder may need enlarging for the engine-out case

Asymmetric yawing moment is thrust times the engine's distance from the centreline. The stations are unchanged but the thrust is greater, so the moment to be balanced after a failure grows in proportion.

Since the fin and rudder were sized against the original figure, the minimum control speed rises unless they are enlarged - which may be unacceptable against the take-off performance the new engines are meant to buy. The tailplane is unaffected, being sized by trim and stability rather than by thrust, and the tail arm is fixed by the fuselage.