Empennage Sizing Questions and Answers

Answer: C

Correct answer: More lift than the aircraft's weight

A conventional tail trims a nose-down wing pitching moment by pushing down, and the wing must then carry both the weight and that download:

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

So a 500 kN aircraft with a 25 kN download needs 525 kN of wing lift. The extra lift costs extra induced drag, which is the trim drag penalty of a conventional tail - and the reason a canard, which trims with an upload, is attractive in principle, though it brings difficulties of its own.

Answer: D

A lifting surface ahead of the centre of gravity adds a destabilising moment, so stability must come from a suitably forward centre of gravity instead. The compensation is that the foreplane lifts rather than pushing down.

Answer: A

Correct answer: Keep it clear of the wing wake and jet efflux

Mounting the tailplane at the fin tip puts it above the wing's downwash and wake, and clear of the efflux of rear-fuselage engines, so it works in cleaner air and is more effective per unit area.

It also raises the fin's effective aspect ratio, the tailplane acting as an end plate. The costs are real: the fin must carry the tailplane's loads down to the fuselage and so is heavier, and the high tail can be blanketed at extreme angle of attack, which is the deep stall hazard.

Answer: B

Correct answer: T-tail with rear-mounted engines

Deep stall needs two things together. At extreme angle of attack the wing's separated wake rises and can blanket a high-mounted tailplane, so the elevator loses effectiveness at the very moment it is needed to lower the nose.

Rear-fuselage engines compound it: they place mass aft, and at high incidence their own intakes are disturbed. The BAC One-Eleven accident of 1963 established the phenomenon, and the standard remedies are a stick shaker and stick pusher to prevent entry, since recovery from a developed deep stall may be impossible.

Answer: C

Moving them together gives pitch and differentially gives yaw, so one pair of surfaces does both jobs. The name is simply the two functions run together, and the mixing linkage it demands is the layout's chief practical drawback.

Answer: D

Correct answer: Reduced wetted area and interference drag

A V-tail replaces three surfaces with two, so the total wetted area is smaller and there are two junctions with the fuselage rather than three - less skin friction and less interference drag.

The saving is smaller in practice than in theory, because the surfaces must be enlarged to give the same stability in both axes. And the drawback is real: pitch and yaw control are coupled, since a ruddervator deflection affects both, which demands a mixer in the control run and gives unusual handling. The Beechcraft Bonanza is the best-known example.

Answer: A

Correct answer: Conventional low tail on the rear fuselage

Most large jet transports carry a conventional tail, with the tailplane on the rear fuselage below the fin - the Boeing 737, 747, 777 and 787 and the Airbus A320, A330 and A350 among them.

The configuration is lighter than a T-tail, since the fin need not carry the tailplane's loads, and it is free of the deep stall hazard. T-tails appear where rear-fuselage engines demand it, as on the DC-9 family and the regional jets, and the arrangement follows from the engine position rather than being chosen for its own sake.

Answer: B

Correct answer: Crosswind landing and spin recovery

A single-engine aircraft has no asymmetric thrust case, so the sizing conditions that govern a twin's fin do not apply.

What remains are the two demands the question names: yawing the aircraft straight from its crab angle at the lowest speed of the approach, and generating the yawing moment needed to stop rotation in a spin, which requires rudder area that is not blanketed by the tailplane. Engine-out control governs a multi-engine aircraft, and cruise directional stability is met comfortably by a fin sized for either of these.

Answer: C

Correct answer: Delay fin stall at large sideslip angles

A dorsal fin is a low aspect ratio fillet extending the fin forward along the fuselage. Its swept leading edge sheds a stable vortex that keeps the flow attached to the fin well beyond its natural stall angle.

That preserves directional stability in a large sideslip and prevents rudder lock, where a stalled fin reverses the rudder's hinge moment and the surface floats hard over. It contributes little at small sideslip, where the fin is unstalled and working normally, so it is not a way of adding area for cruise stability.

Answer: D

Correct answer: Increases its lift-curve slope

Lift-curve slope rises with aspect ratio, because a longer, narrower surface loses less lift to the trailing vortices at its tips. For a straight wing the finite-wing relation is

\[ a = \frac{a_0}{1 + a_0/(\pi e A\!R)} \]

which approaches the two-dimensional value as \( A\!R \) grows. A higher slope means more tail force per degree of angle of attack, so a slender tail is more effective per unit area - but it is also structurally heavier for the same area, which is what limits the aspect ratio in practice.