Have you ever looked out your plane window and wondered why the wing angles backwards. It looks natural, like the wings have been pulled back by the shear speed of the aircraft, but planes weren't always designed this way. Before the close of World War 2, most planes were designed with a straight wing. It wasn't until the end of the war, mostly due to captured German data and designs, that the allies realised the advantages of the swept wing. So what do swept wings do? They allow the plane to fly faster. The Airbus A380 for example can fly up to 85% the speed of sound, something that was unimaginable not so long ago. Let’s look at how this fascinating technology works.
Just because the plane is going less than the speed of sound, does not mean that the air flowing over the aircraft cannot exceed the speed of sound. The wing is shaped in a way that will force the air to accelerate over the top and this is how the wing generates lift, due to Bernoulli’s principle, which states that an increase in speed in a fluid will cause a corresponding decrease in pressure. Let’s look at the airflow over a straight wing at transonic speeds.
The airfoil shape has forced the air to accelerate, thus causing localised areas of supersonic flow. When this occurs shockwaves form that will reduce the lift and increase the drag on the wing. As the speed of the plane increases larger volumes of air around the plane will begin to flow supersonically and this can cause some serious problems, as airflow can separate from the surface of the wing. Watch my video on this subject below to learn more:
In order to increase our critical mach number, this is the speed at which airflow somewhere on the plane reaches supersonic speeds, we need to decrease the acceleration of air over the wing. One way to do this is to angle the wings backwards. This converts some of the airflow over the wing into spanwise flow, which does not accelerate and thus does not affect the critical mach number. This reduces the airflow over the chord, this is the flow that generates lift, thus reducing the acceleration.
This increases the critical mach number, but it also reduces the lift generated by the wing, this is okay at higher speeds, but at lower speeds it can cause some problems. To combat this the wing has several control surfaces that can be used to alter the lift characteristics of the wing. If you have looked out your window while landing, I'm sure you have noticed the movements of the control surfaces on the wing. The plane can extend it's trailing edge flaps and leading edge flaps/slots in order to maintain adequate lift while landing