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 cosαiI 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

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

1.	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 cosαiI 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» 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.

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