Before revision: The Oswald 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.|} For conventional fixed-wing aircraft with moderate aspect ratio and sweep, Oswald efficiency number with wing flaps retracted is typically between 0.7 and 0.85.
After revision: , sometimes referred to as the span efficiency number,The Oswald 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.|} For conventional fixed-wing aircraft with moderate aspect ratio and sweep, Oswald efficiency number with wing flaps retracted is typically between 0.7 and 0.85.