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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 ("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 will reach Mach 1 and shockwaves will 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, maximum thickness located downstream up to 50 per cent 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 '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. 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 for more efficient wing design (e.g., a thicker wing and/or reduced wing sweep, each of which may allow for 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.
- Harris, Charles (March 1990). "NASA Supercritical Airfoils: A Matrix of Family-Related Airfoils" (PDF). NASA Technical paper. 2969. Archived from the original (PDF) on 2011-10-18.
- Ernst Hirschel, Horst Prem, Gero Madelung, "Aeronautical Research in Germany: From Lilienthal until Today", Springer Science & Business Media, 2012. pp. 184-185.
- Bill Gunston, "Airbus, the Complete Story", 2nd ed., Haynes Publishing, 2009. p. 28, p. 51.
- Palmer, Willam E. and Donald W. Elliott, "Summary of T-2C Supercritical Wing Program", NASA SP-301 Supercritical Wing Technology: A Progress Report on Flight Evaluations, February 1972. pp. 13-34.
- Andrews, William H., "Status of the F-8 Supercritical Wing Program", NASA SP-301 Supercritical Wing Technology: A Progress Report on Flight Evaluations. NASA, February 1972. pp. 49-58.
- Hans-Ulrich Meier, Die Pfeilflügelentwicklung in Deutschland bis 1945, ISBN 3-7637-6130-6 Einspruch (1984) gegen US-Patentschrift NASA über »superkritische Profile«, basierend auf den Berechnungsmethoden von K. H. Kawalki (1940) p. 107. German
- Anderson, J: Fundamentals of Aerodynamics, p. 622. McGraw-Hill, 2001.
- ibid.: p. 623.