Why do supersonic planes have swept wings?

As an aircraft approaches the speed of sound, air over the thickest part of the wing accelerates past Mach 1 even though the aircraft itself is still subsonic. Shock waves form, the boundary layer separates behind them, and drag rises steeply: this is the transonic drag rise, and it appears from about Mach 0.75 on a straight, thick wing. Beyond Mach 1 a second penalty appears, wave drag, the energy carried away by the shock waves that the whole airframe drags along.

Sweep works because only the component of the airflow perpendicular to the leading edge matters for the pressure field over the wing section. The flow parallel to the leading edge simply slides along the span and does nothing aerodynamically. A wing swept 45 degrees at Mach 0.9 therefore behaves as though the airflow were about Mach 0.64, and its critical Mach number rises accordingly. This was understood in Germany in the 1930s, from work by Adolf Busemann and Albert Betz, and the captured research shaped the F-86, the B-47 and every jet airliner since.

At supersonic speed the same idea takes a different form. The nose generates a Mach cone whose half-angle shrinks as speed increases. If the wing's leading edge lies inside that cone, the air it meets is still subsonic in the relevant direction, the leading edge can be rounded, and wave drag stays modest. This is why Concorde's ogival delta was swept so sharply near the root, and why Mach 2 fighters use deltas or sweep angles above 50 degrees. Thin sections and a smooth cross-sectional area distribution, the area rule, reduce wave drag further.

A swept wing produces less lift at low speed, tends to stall at the tips first, which pitches the nose up, and is heavier and more flexible for its span. Landing speeds rise and low-speed handling worsens. Designers answer with leading-edge slats, complex flaps, or moving the sweep itself: the F-111, F-14, Tornado and B-1 pivot their wings forward for take-off and aft for high-speed flight. The trade is always the same: sweep buys speed and costs field performance.

Some supersonic types, notably the F-104 and the Bell X-1 before it, used very thin straight wings instead. A wing thin enough keeps wave drag acceptable without sweep, at the cost of poor low-speed lift and razor-like leading edges. Modern designs blend the two: moderate sweep, thin sections and careful area ruling, tuned to the speed range the aircraft will actually use.