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

Trim — the series

2 essays on one idea, from the one that introduces it to the one that assumes the rest.
  1. How the weight is shared between the two surfaces, against the centre of gravity. The wing's load and the tail's, for an aeroplane in level flight, against the static margin. The two must sum to the weight and their moments must cancel, and those two equations decide the split. At a forward centre of gravity the tail carries down and the wing carries more than the weight; the crossing is where the tail carries nothing.

    More lift than weight

    An aeroplane in level flight is drawn with one arrow up and one arrow down, equal and opposite. That equation collapses the whole configuration onto one number, and it is not true of any aeroplane with a tail behind it: there are two surfaces, two equations, and the second decides the split.

    part 1 · circulation
  2. No elevon acts through more than a quarter of the chord. The distance behind the quarter chord at which the load an elevon adds acts — its arm — against the elevon's share of the chord. A vanishing tab at the trailing edge has an arm of exactly a quarter chord; a tenth-chord elevon 0.217, a fifth 0.185, a third 0.145, half 0.0972, and a flap of the whole chord, which is simply a change of incidence, none. However far aft the hinge, the deflection loads the whole chord and most of the load sits near the leading edge.

    No elevon reaches past a quarter chord

    A wing with no tail trims itself with the back of its own section, and the back of a section is a short lever. Thin-aerofoil theory prices it exactly: the load an elevon adds acts at most a quarter of a chord behind the quarter-chord point, however small and far aft the elevon is. So every unit of trim moment costs at least four units of lift, where a tail three chords back costs a third, and a camber change shaped as a pure couple would cost nothing.

    part 2 · circulation

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