We all know engineers wouldn't adopt a global technique on fast aircraft just to make them look cool, so the real answer is that it lets them fly faster by reducing drag.
A swept wing just looks like it has less drag. Explaining why is more difficult - and the answer may surprise you. Sweeping the wings makes the wing feel like it's flying slower. That, in turn, delays the onset of supersonic airflow over the wing - which delays wave drag. But it's not all benefit - there's a hefty price which shows up at slow speeds.
Flying Almost Supersonic Means The Air's Going Supersonic
You know that the air accelerates as it travels over the top of a wing - it's a basic part of Bernoulli's lift. So, if you're flying near the speed of sound - say Mach .8, the air flowing over the wing could speed up to Mach 1. Now you have supersonic flow. Your critical Mach number is the speed where air flowing over the wing first reaches Mach 1.
What's the problem with that? The airflow doesn't stay supersonic forever - it speeds up, exceeds Mach, and then slows back down to a subsonic speed. The faster you fly, the more supersonic air travels over the wing.
However, when the air slows down below Mach 1, it creates a shock wave. As the air flows along the wing, it sends out pressure waves - which move at the speed of sound. That means that the pressure waves can't move forward through the supersonic air flow. Instead, they build up into a massive pressure, or shock wave.
That shock wave generates lots of drag. The air flowing over the wing crosses a massive pressure boundary, which sucks energy out of the airflow - causing drag. Plus, the air can lose so much energy that it separates from the wing, causing more drag. This drag is called wave drag.
Sweeping The Wing Back Delays Supersonic Flow
How does wing sweep help prevent wave drag? It delays the start of supersonic flow, by reducing the amount of acceleration over the wing.
On a straight wing airplane, all of the airflow over the wing travels parallel to the aircraft's chord line. But, on a swept wing, only some of the air flows parallel to the chord line. The other part flows perpendicular to the chord - this is called spanwise flow.
Only the component of airflow flowing parallel to the chord line accelerates. So, by reducing the amount of airflow flowing parallel to the chord line, you've reduced the amount of acceleration - and delayed your critical Mach number. Now you can fly at a higher Mach number before you start to create wave drag.
It's Not All Good News - Slow Speed Flight Suffers
When you reduce the amount of air flowing parallel to the chord line, you reduce the amount of lift the wing creates. At high speed, this isn't a problem - your high airspeed requires a small angle of attack to create lift. However, at slow speeds, you're at a high angle of attack, and sweeping the wing can force a very high angle of attack - nearing your stalling angle of attack.
To counter this, swept wing aircraft use extensive flap systems - like fowler flaps, and leading edge slats. Check out this 747 - lots of flaps.
Sweeping the wings also affects the stall pattern. The amount of spanwise flow compounds as you approach the wingtip, decreasing the wingtip's effective airspeed and thickening the boundary layer. This can cause the wingtip to stall before the wing root - meaning you lose aileron control at the onset of the stall. That could make for a wild ride.
To counter this, engineer's place flow fences on the wing to stop the spanwise flow from building up. Here's a great example on a Russian OKB-1 150 bomber.
The Forward Sweep
Forward swept wings provide the same benefits of aft swept wings - reducing the critical Mach number. And, they eliminate the wingtip stall problem. So, why don't we see them? NASA experimented with them on the X-29 - and while the design holds promise, there are many design problems - like flutter, wingtip stress problems and manufacturing complexity. But, with new composites and advanced production systems, we may see them become a reality. And how cool would that look on the ramp.
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