Fig. 5. Flow and force vectors relating to a wing element. U, velocity of wing element; U_r, relative velocity of air at a wing element; w₀, vertical component of induced downwash velocity; , geometric angle of attack; _r, effective angle of attack; , downwash angle; F_h' and F_v', orthogonal horizontal and vertical forces; F_R', single resultant force; L' and D_pro', orthogonal lift and profile drag forces.