What is the principal advantage of a sweepback design wing over a straight wing design?

The principal advantage of a sweepback wing design over a straight wing design is that the critical Mach number will increase significantly. This occurs because the sweepback angle reduces the effective aspect ratio of the wing, which allows the aircraft to achieve higher speeds before encountering compressibility effects.

As an aircraft approaches the speed of sound, pressure waves build up, leading to drag increments and potentially shockwave formation. A wing with a sweepback design delays these compressibility effects by allowing for a smoother airflow over the wing's surface, thus enabling higher speeds and improved performance in transonic flight regimes.

While aspects such as drag reduction and fuel efficiency may be related to the design, the critical Mach number increase is the fundamental advantage that allows aircraft to fly faster and more efficiently at high speeds.