Supercritical airfoils are designed to delay and reduce the transonic drag rise, due to both strong normal shock and shock-induced boundary layer separation.
As compared with a conventional airfoil a supercritical airfoil has reduced amount of camber, an increased leading edge radius, small surface curvature on the suction side, and a concavity in the rear part of the pressure side. Among these characteristics, there is the aft loading and the relatively high pitching moment. The first supercritical airfoils where designed and analyzed by Whitcomb at NASA Langley in the 1950s.
On the suction side the steep supersonic acceleration of the convetional airfoil is eliminated. The flow reaches supercritical speed, that is maintained over a large part of the airfoil. Then there is a small pressure plateau behind the shock and a pressure recovery in the trailing edge region.
The reduction in local Mach number also reduced the strength of the shock when the flow decelerates below critical speed.
On the pressure side the flow is maintained at subcritical speeds, with a flat distribution til a Stratford-like pressure recovery at the trailing edge.
Figure 1: Conventional vs supercritical airfoil
Design of supercritical airfoils has been made easier in recent years, due to the progress in theoretical methods for inverse design at transonic speeds.
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