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 ("downward-curved") aft section, and larger leading-edge radius compared with NACA 6-series laminar airfoil shapes. Standard wing shapes are designed to create lower pressure over the top of the wing. The camber of the wing determines how much the air accelerates around the wing. As the speed of the aircraft approaches the speed of sound, the air accelerating around the wing reaches Mach 1 and shockwaves begin to form. The formation of these shockwaves causes wave drag. Supercritical airfoils are designed to minimize this effect by flattening the upper surface of the wing.
The supercritical airfoils were suggested first in Germany in 1940, when K. A. Kawalki at Deutsche Versuchsanstalt für Luftfahrt Berlin-Adlershof designed airfoils characterised by elliptical leading edges, maximal thickness located downstream up to 50% chord and a flat upper surface. Testing of these airfoils was reported by B. Göthert and K. A. Kawalki in 1944. Kawalki's airfoil shapes were identical to Richard Whitcomb's. Hawker-Siddeley in Hatfield, England, designed in 1959–1965 improved airfoil profiles known as rooftop rear-loaded airfoils, which were the basis of the Airbus A300 supercritical wing, which first flew in 1972.
In the U.S., supercritical airfoils were studied in the 1960s, by then NASA engineer Richard Whitcomb, and were first tested on a modified North American T-2C Buckeye. After this first test, the airfoils were tested at higher speeds 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 high-subsonic aircraft. The supercritical airfoil shape is incorporated into the design of a supercritical wing.
Kawalki's research was the basis for the objection in 1984 against the US-patent specification for the supercritical airfoil.
Research aircraft of the 1950s and 1960s 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. Techniques learned from studies of the original supercritical airfoil sections are used in designing airfoils for high-speed subsonic and transonic aircraft from the Airbus A300 and Boeing 777 to the McDonnell Douglas AV-8B Harrier II.
Supercritical airfoils feature four main benefits: they have a higher drag-divergence Mach number, they develop shock waves further aft than traditional airfoils, they greatly reduce shock-induced boundary layer separation, and their geometry allows more efficient wing design (e.g., a thicker wing and/or reduced wing sweep, each of which may allow a lighter wing). At a particular speed for a given airfoil section, the critical Mach number, flow over the upper surface of an airfoil can become locally supersonic, but slows down to match the pressure at the trailing edge of the lower surface without a shock. However, at a certain higher speed, the drag-divergence Mach number, a shock is required to recover enough pressure to match the pressures at the trailing edge. This shock causes transonic wave drag and can induce flow separation behind it; both have negative effects on the airfoil's performance.
At a certain point along the airfoil, a shock is generated, which increases the pressure coefficient to the critical value Cp-crit, where the local flow velocity will be Mach 1. 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. Compared to a typical airfoil section, the supercritical airfoil creates more of its lift at the aft end, due to its more even pressure distribution over the upper surface.
In addition to improved transonic performance, a supercritical wing's enlarged leading edge gives it excellent high-lift characteristics. Consequently, 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.
The stall behavior of supercritical profile is unlike that of low-speed airfoils. The boundary layer along the leading edge of a supercritical wing begins thin and laminar at cruise angles. As angle of attack (AOA) increases, this laminar layer detaches in a narrow region and forms a short bubble. The airflow, now turbulent, reattaches to the surface aft of the bubble; the increase in drag is not extreme in this condition. However, if AOA is increased to the stalling point, an adverse pressure gradient builds, and a shockwave can form within the thin boundary layer ahead of the bubble, even at relatively low speed. At the critical angle, the bubble rapidly expands ("bursts"), causing airflow to suddenly detach from the entire surface (from leading to trailing edge). The abrupt loss of lift is exacerbated by the lack of traditional stall "warning" or buffet as a low-speed contour would provide. Due to this lack of buffet warning, aircraft using supercritical wings are routinely equipped with stick-shaker alert and stick-pusher recovery systems, to meet certification requirements.