What is the swept-wing effect on the critical Mach number?

• A. Wing sweep increases the effective critical Mach number, delaying the onset of shock waves by reducing the normal component of velocity to the leading edge.
• B. Wing sweep decreases the effective critical Mach number.
• C. Wing sweep has no effect on the critical Mach number.
• D. Wing sweep only affects lift, not Mach.

Answer: (A)

Sweeping the wing changes the speed the air sees normal to the leading edge. The onset of strong compressibility effects like shock waves depends on that normal velocity component, not the total free-stream speed. If the wing is swept back by an angle, the normal component of flow to the leading edge is approximately M cos(Lambda). So the flow must reach a higher freestream Mach before the normal component reaches the sonic condition, effectively raising the critical Mach number. In other words, sweep delays shock formation by reducing the normal velocity component, which is why the critical Mach number increases with sweep. While sweep also influences lift characteristics and drag, the primary effect described here is the increase in the effective critical Mach number due to the reduced normal velocity to the leading edge.