Question

Select the incorrect equation for the induced drag coefficient for a finite wing.

(a) CD,i=\(\frac {C_l^2}{\pi AR}\)(1+δ)

(b) CD,i=\(\frac {C_l^2}{\pi AR}\)

(c) CD,i=\(\frac {C_l^2s}{\pi eb^2 }\)

(d) CD,i=\(\frac {C_l^2}{\pi eAR}\)

I had been asked this question in a job interview.

I need to ask this question from Prandtl’s Classical Lifting-Line Theory in portion Flow over Airfoils over Finite Wing of Aerodynamics

Answer 1

Correct option is (b) CD,i=\(\frac {C_l^2}{\pi AR}\)

For explanation: The correct formula for induced drag coefficient for a general lift distribution is CD,i=\(\frac {C_l^2}{\pi eAR}\) where e=\(\frac {1}{1+\delta}\) and AR=\(\frac {b^2}{S’}\) where δ is related to the Fourier coefficients and e is the span efficiency factor. Thus, the option CD,i=\(\frac {C_l^2}{\pi AR}\) is wrong.