Aquaplane (rudder)

An aquaplane rudder carries a horizontal lifting surface (an end plate or fin) across the rudder to cut induced drag and raise side-force efficiency by increasing the rudder’s effective aspect ratio. A free-tipped rudder sheds a strong tip vortex that induces downwash and induced drag scaling as C_Di = C_y^2 / (pi AR_eff); an end plate or a horizontal foil at the tip blocks the cross-flow that feeds that vortex, so the surface behaves as if it had a larger span and develops the same side force for less drag. The principle is the one behind rudder end plates and high-aspect spade rudders: raise effective AR, suppress the tip vortex, recover the lift the finite span was throwing away.