Question

The skin-friction drag coefficient for turbulent flow on the bottom surface of an airfoil, with chord length being 1m is _____ (Reynolds number at the trailing edge Rec=32×10^5)

(a) 0.074

(b) 0.0037

(c) 0.027

(d) 0.0074

I have been asked this question in semester exam.

This intriguing question comes from Turbulent Flow in section Incompressible Flow over Airfoils of Aerodynamics

Answer 1

Correct choice is (b) 0.0037

To explain: The turbulent skin-friction coefficient, Cf=\(\frac {0.074}{\sqrt[5]{Re_c}}\). Putting the given values, we get the answer as 0.01328. This is the coefficient for any one side of the airfoil and is the same for both top and bottom surfaces.