To compare various profiles, it is useful to introduce
coefficients without dimensions to describe the forces acting on a
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:
(Q being the slope in degrees or tan(q)la slope in %)