A glider wing of rectangular planform has span $b = 15\text{ m}$, chord $c = 1.0\text{ m}$, and Oswald efficiency factor $e = 0.90$. The zero-lift drag coefficient is $C_{D0} = 0.012$. What is the minimum drag coefficient $C_D$ of the entire wing when operating at $C_L = 0.8$?

Answer Key:

0.0271

### Solution: Aspect ratio $AR = b / c = 15 / 1.0 = 15$. Induced drag $C_{Di} = \frac{C_L^2}{\pi e AR} = \frac{0.8^2}{\pi \times 0.90 \times 15} = \frac{0.64}{42.4115} \approx 0.01509$. Total drag $C_D = C_{D0} + C_{Di} = 0.0120 + 0.01509 = 0.02709 \approx 0.0271$. Correct: 0.0271 (range 0.0265 to 0.0278).

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