1.

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»

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.



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