A symmetric thin airfoil has zero-lift angle of attack $\alpha_{L=0} = 0^\circ$. If the airfoil is operated at an angle of attack $\alpha = 4^\circ$, what is the section lift coefficient $C_l$? (Take $\pi \approx 3.1416$, round off to two decimal places).

Answer Key:

0.44

### Solution: Angle in radians: $\alpha = 4 \times \frac{\pi}{180} = \frac{4 \times 3.1416}{180} = 0.069813\text{ rad}$. $C_l = 2\pi \alpha = 2 \times 3.1416 \times 0.069813 = 6.2832 \times 0.069813 \approx 0.4386 \approx 0.44$. Correct: 0.44 (range 0.42 to 0.46).

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