A transport aircraft has a parabolic drag polar $C_D = 0.02 + 0.045 C_L^2$. What is the maximum lift-to-drag ratio, $(L/D)_{\max}$, achievable in steady level flight?

Answer Key:

16.67

### 5-Stage Derivation & Analytical Solution: 1. **Stage 1: Drag Polar Parameters:** $C_{D0} = 0.02$, $K = 0.045$. 2. **Stage 2: Optimal Lift Coefficient:** $C_{L,opt} = \sqrt{C_{D0} / K} = \sqrt{0.02 / 0.045} = \sqrt{0.4444} = 0.6667$. 3. **Stage 3: Minimum Drag:** $C_D = 2 C_{D0} = 0.04$. 4. **Stage 4: Aerodynamic Efficiency:** $(L/D)_{\max} = \frac{0.6667}{0.04} = 16.667 \approx 16.67$. 5. **Stage 5: Tolerance:** 16.5 to 16.8.

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