Supercritical airfoil
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A supercritical airfoil is an airfoil designed, primarily, to delay the onset of wave drag in the transonic speed range. Supercritical airfoils are characterized by their flattened upper surface, highly cambered (curved) aft section, and greater leading edge radius as compared to traditional airfoil shapes. The supercritical airfoil was created in the 1960s, by then NASA scientist Richard Whitcomb, and was first tested on the TF-8A Crusader. While the design was initially developed as part of the supersonic transport (SST) project at NASA, it has since been mainly applied to increase the fuel efficiency of many subsonic aircraft. The supercritical airfoil shape is incorporated into the design of a supercritical wing.
Research aircraft of the 1950s and 60s found it difficult to break the sound barrier, or even reach Mach 0.9, with conventional airfoils. Supersonic airflow over the upper surface of the traditional airfoil induced excessive wave drag and a form of stability loss called mach tuck. Due to the airfoil shape used, supercritical wings experience these problems less severely and at much higher speeds, thus allowing the wing to maintain high performance at speeds closer to mach 1. The supercritical wing is still the number one choice for high-speed subsonic and transonic aircraft from the Airbus A380 to the Boeing F-15 fighter.
Supercritical airfoils have four main benefits: they have a higher critical mach number, they develop shock waves further aft than traditional airfoils, they greatly reduce shock-induced boundary layer separation, and their geometry allows for more efficient wing design (e.g., a thicker wing and/or reduced wing sweep, both of which may allow for a lighter wing). The aircraft airspeed at which air flowing over the top of the foil reaches the speed of sound is referred to as the critical Mach. When the airflow over a foil reaches this velocity a shockwave develops and airflow begins to separate from the surface of the foil aft of the shockwave. Both the shockwave itself and the resulting airflow separation have negative effects on the airfoil's performance.
In figure 1, the critical mach is denoted as Cp-crit, the shock is evidenced by a sharp drop in the pressure coefficient, and the position of the shock relative to the chord is indicated by the shock's position along the x-axis. The position of this shockwave is determined by the geometry of the airfoil; a supercritical foil is more efficient because the shockwave is minimized and is created as far aft as possible thus reducing drag.
In addition to improved transonic performance, a supercritical wing's enlarged leading edge gives it excellent high-lift characteristics. As a result, aircraft utilizing a supercritical wing have superior takeoff and landing performance. This makes the supercritical wing a favorite for designers of cargo transport aircraft. A notable example of one such heavy-lift aircraft that uses a supercritical wing is the C-17 Globemaster III.
