Calculate the lift, drag and moment coefficient about quarter chord per unit span of an airfoil which is at an angle of attack of \({7\, ^ \circ }\) with a flow velocity of \(40\, m/s\) at standard sea level conditions. The airfoil has a chord length of \(1\, m\).
[For this airfoil; \({c_l} = 0.85\,,{c_{{m_{c/4}}}} =\, – 0.028,\,{c_d} = 0.075\)]
The dynamic pressure for the airfoil is
\[{q_\infty } = \frac{1}{2}{\rho _\infty }V_\infty ^2 = \frac{1}{2} \times 1.225 \times {\left( {40} \right)^2} = 980N/{m^2}\]
Area per unit span, \(S = 1 \times (c) = 1 \times 1 = 1\)
Therefore, lift per unit span,
\[L = {q_\infty }S{c_l} = 980 \times 1 \times 0.85 = 833\,N\]
Drag per unit span,
\[D = {q_\infty }S{c_d} = 982 \times 1 \times 0.075 = 73.5\,N\]
Moment about the quarter chord per unit span,
\[{M_{c/4}} = {q_\infty }Sc{c_{{m_{c/4}}}} = 980 \times 1 \times 1 \times \left( { – 0.028} \right) = – 27.44Nm\]
