Lift coefficient (CL)

The lift coefficient C_L is the non-dimensional lift of a foil, rudder, or fin, defined by L = 0.5 rho V^2 S C_L, with S the planform (or appendage) reference area and V the inflow speed. For a thin 2D section, lift grows linearly with angle of attack at the theoretical slope dC_L/dalpha = 2 pi per radian; a finite-aspect-ratio surface has a shallower slope, roughly 2 pi AR / (AR + 2), because downwash cuts the effective incidence. C_L rises with incidence until stall, where flow separates and lift collapses. In rudder and keel design C_L at the operating angle sets the side force.