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

Centre of pressure — where it appears

The point on a wing or body through which the resultant aerodynamic force acts, so that the force there has no moment. For a cambered aerofoil it moves with incidence, which is why the fixed aerodynamic centre is the more useful reference.

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

Finding the aerodynamic centre by sweeping the chord. How fast the pitching moment changes with incidence, plotted against where along the chord the moment is taken. The curve crosses zero once, and that crossing is the aerodynamic centre — the single point about which pitching the wing does not change the moment.

Where the lift acts

Lift gets drawn as an arrow, and an arrow has to start somewhere. The pressure is spread over the whole surface, so the choice of where to put the arrow is free — except that exactly one point on the chord has the property of not changing its answer when the wing is pitched.

circulation · Moment 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.

circulation · Trim

Named alongside it

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

Aerodynamic centrePitching momentThin-aerofoil theoryCamberLift coefficientLift curve slopeModel limitStabilityStatic marginTrim

All concepts