Fluid Flow & Aerodynamics
why things fly, and why the usual explanation is wrong
Concept

Static stability — where it appears

The tendency of an aircraft disturbed in pitch to return to its trimmed attitude without the pilot's help. It holds when the centre of gravity lies ahead of the neutral point, and the distance between them, the static margin, measures how strongly.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

The drag is set by the split, and stability sets the split. The least induced drag of a wing and a smaller surface together, as a multiple of the elliptic wing's alone, against the share of the lift the smaller surface carries. By Munk's stagger theorem the curve is the same whichever surface is in front. Its minimum, 0.9984, is at a share of 1.9 per cent. At a static margin of a tenth of a chord, a tail trims with 6.7 per cent of the weight on it, upwards, and pays 1.009; a canard trims with 19 per cent and pays 1.128.

A canard pays for its stability in induced drag

The argument for a canard is that both of its surfaces lift upwards, while a tail pushes down and makes the wing carry the difference. Munk's stagger theorem turns the question into arithmetic: two surfaces' least induced drag depends only on how the lift is split between them, not on which is in front. Static margin sets the split. At a margin of a tenth of a chord a tail carries a small upload and costs under one per cent; a canard must carry a fifth of the weight on a third of the span and costs twelve, and the more stable it is made, the more it pays.

circulation · Moment Holding the neutral point with a canard ahead takes a tail two and a half times the size. The tail area, as a fraction of the wing's, that puts the neutral point where the tail aircraft with a tail of 0.2 has it, against the tail's height above the wing's plane in spans: with no canard, with the canard, and with the canard but its wake at the tail removed. At the reference height the tail aircraft needs 0.2; with the canard it needs 0.496, of which 0.22 is for the canard's own lift ahead of the centre of gravity and 0.077 for its wake at the tail. A tail 0.25 span up needs 0.354 with the canard and 0.154 without.

The tail a canard needs costs more than the canard saves

A canard added to an aircraft as a second trimming surface saves induced drag only if the neutral point is held where it was, and holding it is not free. The canard's own lift ahead of the centre of gravity pulls the neutral point forward, its wake reaches the tail and weakens it, and the tail has to grow to put the neutral point back. For a canard of a tenth of the wing's area the tail grows two and a half times, and its skin friction is twenty to thirty counts against a saving of one to twenty. A T-tail escapes most of the wake and a third of the bill, and still only a flapped wing comes near to paying.

circulation · Moment

Named alongside it

The objects these essays reach for when they reach for this one.

CanardDownwashInduced dragModel limitNeutral pointThe Trefftz planeHorseshoe vortexMunk staggerSkin frictionStallTail volumeTrim

All concepts