# Find the angle of attack for this airfoil, if the lift per unit span produced is \(1400\,N\).

A NACA 0012 airfoil having a chord of \(2.3\,m\) is in an airstream velocity of \(70\,m/s\) at standard sea level conditions. Find the angle of attack for this airfoil, if the lift per unit span produced is \(1400\,N\).

Span is the wingspan of the airplane. It is the distance from one wing tip to other. It is measured in a straight line and is independent of the wing shape and sweep. Lift per unit span produced by a wing is given as \({L}’ = \frac{1}{2} \rho _{\infty}V_{\infty}^{2} c\left ( 1 \right )c_{l}\). Here, \(\frac{1}{2} \rho_{\infty}V_{\infty}^{2} \) is called the dynamic pressure , \(q_{\infty} \). Coffeicent of lift, \(c_{l}\), for this airfoil will be \[c_{l} = \frac{{L}’}{q_{\infty}c\left ( 1 \right )} \]\[\Rightarrow c_{l} = \frac{1400}{\left (\frac{1}{2} \times 1.225 \times 70^{2} \right )\left ( 2.3 \right )\left ( 1 \right )}\]\[\Rightarrow c_{l} = 0.203\]Therefore, coefficient of lift for this airfoil is \(0.203\). For, NACA \(0012\) airfoil from the lift curve -slope for this airfoil, the angle of attack is \( 2^{\circ} \).