Can the Span Efficiency Factor Exceed 1 in the Lifting-Line Theory?

[truffaldino]

Hello Everybody,

Span efficiency factor appears in the lifting-line theory of Prandtl describing Lift and vortex drag of a finite wing. According to this theory, the most efficient wing is an elliptical one and, roughly speaking, the span efficiency factor defines an efficiency of a given wing planform relatively to elliptic wing.

In the framework of the lifting line theory the span efficiency cannot exceed 1.

My question is: can this efficiency factor be more than 1, when going beyound lifting-line approximation ? (sorry for abusing notion of efficiency which supposed to be less <= 1).

In other words: does it exist such a wing whose induced drag is less than that of estimate for elliptic wing in the lifting-line theory (provided aspect ratios and lift coefficients are the same and viscous effects, such as skin friction and boundary layer, are neglected).

Thanks
Truffaldino

 

[Ryoko]

Short answer is yes, you can have a >1 efficiency factor. But you have to cheat. It's accomplished by using tandem wings or boxed wings. Winglets also allow for a >1 factor.