The Oswald efficiency, similar to the span efficiency, is a correction factor that represents the change 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.
[1] The Oswald efficiency is defined for the cases where the overall coefficient of drag of the wing or airplane has a constant+quadratic dependence on the aircraft lift coefficient where 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.
At supersonic speeds, Oswald efficiency number decreases substantially.
[1] It is frequently assumed that Oswald efficiency number is the same as the span efficiency factor which appears in lifting-line theory, and in fact the same symbol e is typically used for both.
, an alternative drag coefficient breakdown can be given by[citation needed] where Equating the two