Why do swept wings increase the critical Mach number (MCRIT)?

Swept wings increase the critical Mach number (MCRIT) primarily because they reduce the effective chord-wise velocity. When a wing is swept, the airflow over the wing is effectively redirected, which decreases the component of the airflow speed parallel to the leading edge of the wing. This reduction in the effective velocity at which air flows over the wing surface means that the wing can reach a higher overall airspeed before shock waves begin to form on the wing, which is what typically defines critical Mach number.

This aerodynamic principle allows aircraft with swept wings to operate efficiently at higher speeds without encountering compressibility effects that would lead to drag increases and stability issues associated with shockwave formation. Thus, the design of swept wings helps circumvent some of the limitations faced by wings with straight configurations at higher subsonic speeds, thereby raising the critical Mach number.