Empennage Sizing Questions and Answers

Answer: A

Correct answer: Statement I is incorrect but Statement II is correct

The second statement is right: a dorsal fin's swept, low aspect ratio leading edge sheds a vortex that keeps the flow attached well beyond the fin's natural stall angle, preserving stability in a large sideslip and preventing rudder lock.

The first is wrong. At small sideslip the fin is unstalled and working normally, and the dorsal fillet adds very little side force there - its contribution is negligible until the angles become large. It is a fix for the extreme case, not a way of adding cruise directional stability.

Answer: B

Correct answer: Both Statement I and Statement II are correct

Both are true and the second is the cause of the first. At extreme angle of attack the wing's separated wake rises and can blanket a high-mounted tailplane, so the surface sits in low-energy air and the elevator loses the authority to lower the nose.

The stall then becomes self-sustaining, since the aircraft descends at high incidence with the wake still over the tail - recovery may be impossible. The remedies are preventive: stick shaker and stick pusher to stop entry, established after the BAC One-Eleven accident of 1963.

Answer: C

The coefficient comes first from precedent, then the arm from the layout already fixed, then the area falls out of the definition, and only then is the resulting tail checked for control authority. Computing an area before knowing the arm is meaningless.

Answer: D

The failure creates the asymmetry, the asymmetry creates the moment, the rudder answers it, and the slowest speed at which it can still do so is the certified minimum control speed. Each step depends on the one before it.

Answer: A

Correct answer: 4, 2, 1, 3

The ordering follows how much of the tailplane's load the fin must carry. A conventional low tail puts the tailplane on the fuselage, so the fin carries only its own side load and is lightest.

A cruciform tail mounts the tailplane part way up, so the fin carries it over part of its height. A T-tail carries it at the very top, over the full height, in bending and torsion. A T-tail with rear-fuselage engines is heavier still, the fin also being stiffened against the flutter and inertia effects of the aft mass. Each step adds load path, hence weight.

Answer: B

The wing sets the starting point, the tail moves it aft, the aft loading limit is then measured against it, and the margin is what that comparison yields. Reversing any pair leaves you reporting a number you have not yet computed.

Answer: C

Length buys arm, arm buys tail volume, and the surplus volume is what lets the original tail stand. It is the cheapest outcome a stretch can have, though the aft centre of gravity movement sometimes spends the surplus again.

Answer: D

Correct answer: Only three

The first three are correct: the horizontal tail volume is normalised on wing area and mean aerodynamic chord, the vertical on wing area and span, and the static margin is expressed as a fraction of \( \bar{c} \).

The fourth is wrong. The tail arm is a physical length in metres - the distance between the wing and tail aerodynamic centres - and is not normalised on anything. It appears in the numerator of both tail volume coefficients, which is where the normalising happens.

Answer: A

Correct answer: Only three

Three of the pairs hold. A T-tail brings deep stall risk and a heavier fin; a V-tail couples pitch and yaw control through its ruddervators; and a canard foreplane is destabilising, moving the neutral point forward and demanding a forward centre of gravity.

The fourth is wrong. Rudder authority in a crosswind is a matter of fin and rudder sizing, not of tail configuration, and a conventional tail is no worse placed than any other - if anything better, since its rudder is not blanketed by a tailplane above it.

Answer: B

The first three are right. A dorsal fin is not a control surface at all; it is a fixed fillet ahead of the fin that delays fin stall at large sideslip angles, and roll on a conventional aircraft belongs to the ailerons and spoilers.