At Mach 1, the speed of sound, wings develop shock waves that add drag and subtract lift. When the wings are swept, they look thinner to the passing air and the shock formation is delayed and occurs at a higher airspeed. It's the difference between slicing salami straight across or cutting it at an angle: A swept wing is like salami cut at an angle. Swept wings are nothing new, but one lesser known fact about them is that when it comes to making the wing appear thin to the air, sweeping the wings forward is as good as sweeping them aft. And forward sweep has advantages. At high speed, the air passing over conventional aft-swept wings tends to flow out to the wingtips, a phenomenon called spanwise flow.
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