Swept-Wing Flow Instability
Wave crisis on the aircraft lifting surfaces is the main obstacle to the increase of the flight speed. A lifting surface of a swept shape allows to postpone the beginning of the wave crisis up to the speeds corresponding to Mach numbers 0.8-0.95 due to three-dimensional effects appearing in case of the flow over a swept wing.
In case of the flow over a swept wing, the unperturbed flow is split into two flows: the one flowing along the normal to the leading edge, and the one flowing along the span. The latter has a constant velocity along the span and does not affect pressure distribution over the wing; it causes only skin friction. The flow along the normal to the leading edge has velocity that changes due to stagnation and acceleration of the flow over the airfoil and therefore affects both skin friction and pressure distribution over the wing cross section. Since this velocity is always smaller than the free stream velocity, the wave drag appears at larger free stream velocities in case of a swept wing than in case of a straight wing.
However, specificity of three-dimensional flow over a swept wing at large transonic flight speeds, the corresponding spreading of the boundary layer along the span towards the wing tip cross sections and the flow separation at the wing tips cause the increase of drag (and therefore the decrease of lift-to-drag ratio), as well as have an adverse effect on the aircraft stability and controllability.
Geometric and aerodynamic twist is used in order to reduce these adverse effects on swept wings.