Comparison With Span Efficiency Factor
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. But this assumes that the profile drag coefficient is independent of, which is certainly not true in general. Assuming that the profile drag itself has a constant+quadratic dependence on, an alternative drag coefficient breakdown can be given by
where
-
is the constant part of the profile drag coefficient, is the quadratic part of the profile drag coefficient, is the wing span efficiency factor from inviscid theory, such as Lifting-line theory
Equating the two expressions gives the relation between the Oswald efficiency number e0 and the lifting-line span efficiency e.
For the typical situation, we have .
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