Question

The induced drag (Di’) in terms of lift per unit span (L’) for a finite wing is_____

(a) Di’=L’sin sinαi

(b) Di’=L’αi

(c) Di’=πL’αi

(d) Di’=L’cos cosαi

I have been asked this question in my homework.

This interesting question is from Prandtl’s Classical Lifting-Line Theory topic in section Flow over Airfoils over Finite Wing of Aerodynamics

Answer 1

Right choice is (a) Di’=L’sin sinαi

Easiest explanation: For a finite wing flat plate airfoil the induced drag is obtained by the component of lift in the direction of free-stream velocity i.e. Di’=L’sin sinαi. For a thin airfoil, we can make the small angle approximation in sine but not for all general airfoils.