Before revision: The Oswald efficiency number, sometimes referred to as the span efficiency number , is a constant that represents the lifting efficiency of a three dimensional wing compared with another wing having the same aspect ratio and an elliptical lift distribution.Raymer, Daniel P., Aircraft Design: A Conceptual Approach, Section 12.6 (Fourth edition) The coefficient of induced drag on a wing with an elliptical lift distribution is: For a wing with a lift distribution that is not elliptical, the coefficient of induced drag is: where {| border="0" |- | || is the coefficient of induced drag , |- | || is the aircraft lift coefficient, |- | || is the circumference-to-diameter ratio of a circle, |- | || is the Oswald efficiency number |- | || is the aspect ratio .
After revision: The Oswald efficiency , sometimes (not quite correctly) referred to as the span efficiency number , is a correction factor that represents the rise in drag with lift of a three dimensional wing or airplane, as compared with an ideal wing having the same aspect ratio and an elliptical lift distribution.Raymer, Daniel P., Aircraft Design: A Conceptual Approach, Section 12.6 (Fourth edition) The coefficient of induced drag of the wing or airplane has a constant+quadratic dependence on the aircraft lift coefficient For a wing with a lift distribution that is not elliptical, the coefficient of induced drag is: where {| border="0" |- | || is the coefficient of induced drag , |- | || is the aircraft lift coefficient, |- | || is the circumference-to-diameter ratio of a circle, |- | || is the Oswald efficiency |- | || is the aspect ratio Oswald efficiency is defined only for the cases where the overalloverall Drag coefficient,|-| || is the zero-lift drag