Why do aircraft have swept wings?

Modern planes fly farther and faster than most could have ever imagined when the first aircraft were designed in the early 20th century.  As late as 1935, the fastest aircraft only approached about 250 mph, and a large obstacle was the drag that aircraft experienced at higher speeds.  As more powerful engines were developed and the thrust capabilities became much higher, aviators began to notice that as aircraft approached about 3/4 of the speed of sound, the drag on the aircraft became exponentially higher.  The drag became such a barrier that few engines could provide enough thrust to increase speed even incrementally, and it became apparent that the wings were largely responsible for the impediment.



Increasingly swept wings from left to right. 

The problem is due to the aerodynamic properties of subsonic flight.  With any projectile, the craft can only travel by displacing air around is body, which at low speeds encourages aerodynamic designs that allow air to flow around its shape easily, such as rounded edges.  In cars, this leads to designs that look like bullets, but airplanes can only fly by displacing air downwards, requiring it to have a certain amount of aerodynamic inefficiency.  Wings are effective at creating lift, but approaching the speed of sound, this lift is negated by the drag.


How swept wings displace air versus straigh wings.

When wings displace air quickly, it creates momentary high pressure regions in front of the wing and low pressure regions behind the wing.  When these regions equilibrate, a shockwave is propagate on the wing and create drag and instability.  The combat this, airplane designers began to focus on making extremely thin wings, but the Germans instead relied upon trying a new wing design that was more aerodynamic while still creating lift:  swept wings.


The difference in lift given by swept and straight wing designs.

In the aftermath of WWII, the allied forces found these designs and immediately recognized their value.  Unlike straight wing configurations, swept wings experience much less drag at the same speed by allowing the air to flow over the wings without causing such drastic changes in air pressure.  Because of this, the efficiency of aircraft at high speeds is much greater with swept wing designs, leading to its proliferation in aircraft today.