Definition
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
-
is the overall drag coefficient, is the zero-lift drag coefficient, 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. At supersonic speeds, Oswald efficiency number decreases substantially. For example, at Mach 1.2 Oswald efficiency number is likely to be between 0.3 and 0.5.
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