Supercritical airfoil

During 1940, K. A. Kawalki at Deutsche Versuchsanstalt für Luftfahrt Berlin-Adlershof designed a number of airfoils characterised by elliptical leading edges, maximal thickness located downstream up to 50% chord and a flat upper surface.

[2] The aviation authors Ernst Heinrich Hirschel, Horst Prem, and Gero Madelung have referred to the supercritical airfoil as being of equal importance, in terms of aerodynamics, as the innovation of the swept wing to high speed aircraft.

[3] During the 1950s and 1960s, a number of different high speed research aircraft equipped with conventional airfoils repeatedly encountered difficulties in breaking the sound barrier, or even reaching Mach 0.9.

Supersonic airflow over the upper surface of the traditional airfoil induced excessive wave drag, as well as a form of stability loss known as Mach tuck.

Aerodynamicists determined that, by appropriately shaping the airfoil used, the severity of these problems could be greatly reduced, allowing the aircraft to attain much higher speeds; this is the basis of the supercritical wing.

Hawker Siddeley's research subsequently served as the basis for the supercritical wing of the Airbus A300, a multinational wide-body airliner which first flew during 1972.

A specially modified North American T-2C Buckeye functioned as an early aerial testbed for the supercritical wing, performing numerous evaluation flights during this period in support of the research effort.

[15] According to Hirschel, Prem and Madelung, the supercritical wing has been regarded as being an essential element of modern jetliners, pointing towards its use on Airbus' product range.

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.

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).

[24] 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.