Supersonic Airfoils

What makes a supersonic airfoil different than a typical one? Well that would be the shape and how it works. A typical subsonic airfoil, one that is used below the speed of sound Mach < 1, has a blunt leading edge and is meant to create a pressure difference using the velocity difference created from the shape. Subsonic airfoils typically do not include compressibility effects from the air, which is a safe assumption when the percent error at around a Mach number of 0.3 is around 5%, though some more recent subsonic airfoils have taken that into consideration. Below is a picture of how airfoils have evolved over time starting with the Wright's in 1908.

The problem with these airfoils is that when they travel at supersonic speed of Mach > 1 they start to exhibit compressibility effects such as shock waves. With a blunt nose a detached normal shock wave will form out in front of the leading edge which causes incredible drag and high temperatures and pressures. Richard Whitcomb came up with the supercritical airfoil design which improves aircraft performance in near sonic conditions as seen below. The supersonic airfoil looks very different because it has a sharp point in both the leading and trailing edges. These sharp points produce an attached oblique shock wave which has much less drag than a detached normal shock wave. The air compresses across this oblique shock wave then it goes over the top corner and into an expansion wave where the pressure drops and then the trailing edge also has an oblique shock trailing behind. 

Here below is a diagram taken from my compressible flow textbook. Area 1 is the free stream flow that is greater than the speed of sound, which varies with temperature and substance. Area 2 is behind the oblique shock wave with the high pressure. Area 3 is behind the expansion wave and has a lower pressure. Since this is a symmetric diamond airfoil at an angle of attack of zero degrees there will be no lift but there will still be drag from the pressure difference from areas 2 and 3. Supersonic airfoils do not have to be symmetric or a perfect diamond, different side lengths and angles will produce asymmetry that can cause lift, also an increase angle of attack will create more lift. 

Unfortunately no one wants to spill the beans on the specifications of the supersonic airfoils in use today for aircraft but they all must deal with the shock waves that form. If you ever look at the SR-71 you'll notice that the leading edges are all pointed and sharp, now you know why.