Aspect ratio (foil)

The aspect ratio of a foil, rudder, or fin is AR = b^2 / S, span squared over planform area, which reduces to span over chord for a rectangular plan. It governs how steeply lift grows with incidence and how heavy the induced drag is: the 3D lift-curve slope is roughly dC_L/dalpha = 2 pi AR / (AR + 2), and the induced drag is C_Di = C_L^2 / (pi AR e) with e the span efficiency, so a high-AR surface lifts harder per degree and pays less induced drag. Ship rudders run low geometric AR, typically 1 to 2, because draft limits span; mounting the rudder against the skeg with an end plate roughly doubles the effective AR.