Lift coefficient

The section lift coefficient cl refers to the dynamic lift characteristics of a two-dimensional foil section, with the reference area replaced by the foil chord.

In aerodynamics and thin airfoil theory, the second axis is commonly in the chordwise direction: resulting in a coefficient: While in marine dynamics and for thick airfoils, the second axis is sometimes taken in the thickness direction: resulting in a different coefficient: The ratio between these two coefficients is the thickness ratio: The lift coefficient can be approximated using the lifting-line theory,[4] numerically calculated or measured in a wind tunnel test of a complete aircraft configuration.

The section lift coefficient is based on two-dimensional flow over a wing of infinite span and non-varying cross-section so the lift is independent of spanwise effects and is defined in terms of

Note this is directly analogous to the drag coefficient since the chord can be interpreted as the "area per unit span".

For a given angle of attack, cl can be calculated approximately using the thin airfoil theory,[6] calculated numerically or determined from wind tunnel tests on a finite-length test piece, with end-plates designed to ameliorate the three-dimensional effects.

Plots of cl versus angle of attack show the same general shape for all airfoils, but the particular numbers will vary.

At higher angles a maximum point is reached, after which the lift coefficient reduces.

The stall angle for a given profile is also increasing with increasing values of the Reynolds number, at higher speeds indeed the flow tends to stay attached to the profile for longer delaying the stall condition.

[7][8] For this reason sometimes wind tunnel testing performed at lower Reynolds numbers than the simulated real life condition can sometimes give conservative feedback overestimating the profiles stall.

Symmetric airfoils necessarily have plots of cl versus angle of attack symmetric about the cl axis, but for any airfoil with positive camber, i.e. asymmetrical, convex from above, there is still a small but positive lift coefficient with angles of attack less than zero.