My last post on subsonic lift got me thinking about the possible differences with supersonic lift. It has turned out to be an interesting area due to the difference in lift mechanisms and flow patterns.
In subsonic flight the wing generates lift by means of accelerating the flow around the top surface of the airfoil, this is achieved by altering the chord, upper & lower camber, curvature and the angle of attack in order to produce an area of lower pressure above the airfoil. In supersonic flight, lift is generated only by the angle of attack. Bernoulli’s Principle simply describes the pressure along a streamline as a function of the local velocity, assuming an inviscid, incompressible and irrotational flow. Supersonic flows have high viscous dissipation, the flow as a whole is no longer inviscid, and is also compressible. The lift, however, is still generated by the pressure differences but in the case of supersonic flow it is generated in a different way (cjl, 2012).
Pressure difference in supersonic flight is generated by shocks at the airfoil’s leading edge (Figure “oblique shock”), the shock going down (angle of attack dependant) is stronger than the shock going up, because of this the pressure jump across the shock going down from the leading edge is larger than the one across the shock going up. This causes a greater pressure under the airfoil and therefore creates lift (cjl, 2012). The presence of shock waves, along with the compressibility effects of high-velocity fluids, is the central difference between supersonic and subsonic aerodynamic flow. The figure below shows a supersonic airfoil with angle of attack (θ), as the angle of attack increases the lift generated increases, up until the stall point (separation point well forward of the trailing edge). The Oblique shock in supersonic flow moves from the centre of the airfoil in subsonic flow to the trailing edge in supersonic flow, this effect increases drag, the effect is known as wave drag and it can affect lift.
cjl. (2011). Supersonic. Retrieved 04 2012, from physicsforums.com: