Concept

Canard — where it appears

A small lifting surface ahead of a wing, carrying part of the aircraft's weight while trimming it in pitch. Because it lifts upward where a tail usually pushes down, the question of what it saves is a question of how the lift is shared between the two surfaces.

Named by 3 essays across one field — each of them below, with the objects they name alongside it.

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

One bowl, and a line across it for every way of flying. The least induced drag of a wing, a canard and a tail together, as a function of the canard's and the tail's shares of the lift, drawn as rings of equal drag about the bowl's foot, where each carries 1.9 per cent. A static margin of a tenth of a chord and a wing pitching moment draw a straight line of trimmed splits across the bowl. The tail aircraft trims where its line crosses the axis of zero canard load; the three-surface aircraft slides along the same line to the point nearest the foot.

A third surface is worth a square

A canard and a tail each leave an aircraft one free share of its lift, and the static margin spends it. Give an aircraft both and one share stays free after trim, so the split can slide along a line to the point of least induced drag. What that slide is worth turns out to be a square: half the bowl's curvature times the square of how far the tail aircraft already trims from twice the bowl's foot. For a cruising wing the saving is under one per cent and less than the canard's own skin friction; it pays only for a wing whose pitching moment is as large as a flapped section's.

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.

Induced drag Model limit Neutral point The Trefftz plane Downwash Munk stagger Skin friction Static stability Trim Horseshoe vortex Optimisation Pitching moment

All concepts