Empennage Sizing Questions and Answers

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the mechanism. A conventional aft tail trims the wing's nose-down pitching moment by pushing down, so the wing must carry the weight plus that download:

\[ L_{wing} = W + L_{tail,\,down} \]

The extra lift brings extra induced drag, and that is trim drag. It is why transports move fuel aft in cruise, reducing the download by shifting the centre of gravity towards the aft limit, and why a canard trimming with an upload is attractive in principle.

Answer: B

Correct answer: 1 and 2 only

The first two give the definitions correctly: the horizontal coefficient normalises on wing area and mean aerodynamic chord, the vertical on wing area and span.

The third is false, and follows from the first two. Because the span is much larger than the mean chord, the vertical coefficient's denominator is much bigger, and typical values differ by an order of magnitude - around 1.0 against 0.08. A fin is not undersized for having a small coefficient; the two numbers simply measure against different references.

Answer: C

The first two are correct. Static margin is a longitudinal quantity fixed by the wing and the horizontal tail, and the fin has no part in it; the two axes are sized against quite different critical cases and share no reference length.

Answer: D

Correct answer: 1 and 2 only

The first two are correct. A T-tail sits above the wing's downwash and wake and clear of rear-fuselage engine efflux, so it works in cleaner air; and its fin must carry the tailplane's loads down its full height, so it is structurally heavier.

The third is the opposite of the truth. The T-tail is the configuration most at risk of deep stall, precisely because a high tailplane can be blanketed by the wing wake at extreme angle of attack - which is why such aircraft carry stick pushers.

Answer: A

Correct answer: 1 and 2 only

The first two are correct. A conventional tailplane carries a download in trimmed cruise, balancing the wing's nose-down moment; and increasing its area strengthens its stabilising contribution, moving the neutral point aft.

The third contradicts the second. The aft centre of gravity limit is the neutral point less the required static margin, so anything that moves the neutral point aft moves the limit aft with it. The horizontal tail is therefore the principal determinant of the aft limit, not a bystander to it.

Answer: B

Correct answer: A-2, B-4, C-1, D-3

Each term has a distinct definition. The tail volume coefficient is tail area times arm normalised on wing reference quantities; the static margin is the distance from centre of gravity to neutral point, expressed in chords.

The tail arm is the physical distance between the wing and tail aerodynamic centres, and the neutral point is the centre of gravity position at which stability is neutral. Two of the four are dimensionless and two are positions or lengths, which is the quickest way to halve the matching.

Answer: C

Correct answer: A-2, B-1, C-4, D-3

The four are distinguished by where the horizontal surface sits. A T-tail puts it at the fin tip; a cruciform tail part way up the fin; a canard ahead of the wing entirely.

The V-tail is the odd one out in kind rather than position: it has no separate horizontal and vertical surfaces at all, two dihedral surfaces performing both pitch and yaw through ruddervators. Reading the list for the outlier first - the one defined by function rather than location - settles that pairing immediately.

Answer: D

Correct answer: A-2, B-1, C-3, D-4

Each case governs a different surface or a different aspect of one. Engine failure at low speed sizes the vertical tail and rudder against asymmetric thrust. Trim at the forward centre of gravity limit sizes the horizontal tail and elevator, that being the condition demanding the largest download.

Spin recovery is a rudder case but a distinct one, requiring authority at high angle of attack where the tailplane may blanket part of the rudder. Cruise trim drag concerns the horizontal tail's download and the induced drag it forces on the wing.

Answer: A

Correct answer: A-2, B-1, C-4, D-3

Each change acts in its own plane. More tailplane area strengthens the longitudinal contribution and moves the neutral point aft; more fin area strengthens the directional contribution and raises directional stability.

Lengthening the rear fuselage increases both tail arms at once, so it raises both tail volume coefficients - the only change here affecting both planes. Moving the centre of gravity aft changes no aerodynamic surface at all; it brings the centre of gravity closer to the neutral point and so reduces the static margin.

Answer: B

Correct answer: Balancing the wing's pitching moment in trim

Balancing the wing's pitching moment is the horizontal tail's work, done by carrying a download in the pitching plane. The fin generates side force and produces yawing moments; it plays no part in longitudinal trim and carries no load at all in symmetric flight.

The other three are exactly what the vertical tail exists for: yaw control in a crosswind, directional stability through the restoring moment in sideslip, and balancing asymmetric thrust after an engine failure - usually the case that sizes it.