Based on the Kutta-Joukowski theorem and the definition of the lift coefficient, the following relationship between the section circulation, [GAMMA]([theta]), and the lift coefficient, [C.sub.L]([theta]), is
In the proceeding of calculating the section lift coefficients and the induced drag, the magnitudes of local velocity are both used in the Kutta-Joukowski theorem and the definition of the lift coefficient.
Applying the Kutta-Joukowski theorem to a straight lifting surface, the force acting on a spanwise differential segment of the lifting surface related with the section circulation can be derived as