Question 5.4: Calculate the local skin friction at a point 0.5 m from the ......

Refer to Table 1.2 to obtain the static properties of undisturbed air at 6 \mathrm{~km} :

\begin{aligned} \rho_{\infty} & =0.6601 \mathrm{~kg} / \mathrm{m}^3 \\ \mu_{\infty} & =1.5949 \times 10^{-5} \mathrm{~kg} / \mathrm{s} \cdot \mathrm{m} \end{aligned}

Thus, using equation (5.26),

\operatorname{Re}_x=\frac{\rho_{\infty} U_{\infty} x}{\mu_{\infty}}     (5.26)

\begin{aligned} \operatorname{Re}_x & =\frac{\left(0.6601 \mathrm{~kg} / \mathrm{m}^3\right)(60 \mathrm{~m} / \mathrm{s})(0.5 \mathrm{~m})}{1.5949 \times 10^{-5} \mathrm{~kg} / \mathrm{s} \cdot \mathrm{m}} \\ & =1.242 \times 10^6 \end{aligned}

If the boundary layer is laminar,

\begin{aligned} C_f & =\frac{0.664}{\left(\operatorname{Re}_x\right)^{0.5}}=5.959 \times 10^{-4} \\ \tau & =C_f\left(\frac{1}{2} \rho_{\infty} U_{\infty}^2\right)=0.708 \mathrm{~N} / \mathrm{m}^2 \end{aligned}

If the boundary layer is turbulent,

\begin{aligned} C_f & =\frac{0.0583}{\left(\operatorname{Re}_x\right)^{0.2}}=3.522 \times 10^{-3} \\ \tau & =C_f\left(\frac{1}{2} \rho_{\infty} U_{\infty}^2\right)=4.185 \mathrm{~N} / \mathrm{m}^2 \end{aligned}