Question

The figures shown in the options are schematics of airfoil shapes (not to scale). For a civilian transport aircraft designed for a cruise Mach number of \(0.8\), which among them is aerodynamically best suited as a wing section?

• A. A
• B. B
• C. C
• D. D

Hint:

For transonic cruise (\(M \approx 0.8\)), the supercritical airfoil is optimal: flat upper surface, aft camber, and blunt leading edge. It delays shock and minimizes drag rise.

Answer 1

The Correct Option is D

Step 1: Cruise condition at Mach 0.8.
Civilian transport aircraft generally cruise in the transonic regime (\(M \approx 0.8\)). Conventional airfoils (like A and B) produce strong shock waves on the upper surface, leading to wave drag rise. Thus, an airfoil that reduces or delays shock formation is required.

Step 2: Characteristics of supercritical airfoil.
- Flattened upper surface: reduces local flow acceleration, delaying shock to further aft.
- Aft camber on the lower surface: maintains lift while keeping upper-surface pressure distribution favorable.
- Blunt leading edge: improves low-speed handling.
- Thin trailing edge: aids smooth pressure recovery.

Step 3: Eliminate other options.
(A) Conventional airfoil: suffers early shock formation.
(B) Strongly cambered: even worse drag divergence at transonic speeds.
(C) Very thin symmetric: lacks sufficient lift at cruise \(C_L\).
(D) Matches supercritical features: best suited for Mach 0.8 cruise.

Final Answer:
\[ \boxed{\text{Option (D) is aerodynamically best suited.}} \]