A site on Aerodynamics

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Dispositifs pour augmenter la portance
The Cx and the Cz
Polaire d'un profil

To compare various profiles, it is useful to introduce coefficients without dimensions to describe the forces acting on a wing.

The dimension of a force is a pressure multiplied by a surface. For a wing, the surface of reference is the projection of the wing on a horizontal surface. The pressure of reference is the pressure dynamique~: Pd=(1/2) * r*V2.

One compares the fictitious force F = Pd*S with the bearing pressure and the trail,

from where portance~: Fx = (1/2)*r*S*V2 * Cz and traînée~: Fx = (1/2)*r*S*V2 * Cx

The Cz coefficients and Cx strongly depend on the form of the profile (but also of the surface quality and the incidence). One defines the smoothness (noted F) as being: f=Portance/Traînée=Cz/Cx=1/tan(q)

(Q being the slope in degrees or tan(q)la slope in %)

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