= Skin friction coefficient
{title2=$c_f$}
The local <skin friction coefficient> normalizes <wall shear stress> by the outer <dynamic pressure>:
$$
c_f(x)=\frac{\tau_w(x)}{\tfrac12\rho U^2}\simeq\frac{0.664115}{\sqrt{\mathrm{Re}_x}}.
$$
For the ideal <Blasius boundary layer> on one face of length $L$, averaging the local <skin friction coefficient> gives $\overline c_f\simeq1.328230/\sqrt{\mathrm{Re}_L}$. The factor of two follows from integrating $x^{-1/2}$, not from counting both plate faces. <Drag> on two equally exposed faces doubles when the reference area remains that of one face.
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