Induced drag (foil)

Induced drag is the drag penalty a finite-span lifting surface pays for producing lift, arising because the trailing vortex sheet shed at the tips induces a downwash that tilts the local lift vector backward. For an elliptically loaded foil the induced drag coefficient is C_Di = C_L^2 / (pi AR), so it scales with the square of lift and inversely with aspect ratio AR = b^2 / S. Low-aspect-ratio rudders and control foils therefore carry heavy induced drag at angle, which is why a deeper, higher-AR rudder or an end plate that raises effective AR cuts the penalty. Induced drag is distinct from the viscous profile drag of the section and vanishes only at zero lift.