Sweeping a wing backwards by an angle $\Lambda$ at transonic speeds delays the drag divergence Mach number primarily because:

(A) The effective flow velocity normal to the wing leading edge is reduced to $U_\infty \cos\Lambda$

(B) The Oswald efficiency factor $e$ increases above 1.5

(C) Induced drag is completely eliminated across the span

(D) The boundary layer becomes fully relaminarized

Answer Key:

A

### Solution: Simple sweep theory shows that aerodynamic pressure distribution is governed predominantly by the velocity component perpendicular to the swept line, $M_n = M_\infty \cos\Lambda < M_\infty$. Correct: A.

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