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 aft section,
and smaller leading edge radius compared with traditional airfoil shapes. The
supercritical airfoils were designed 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 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.
Research in 1940 by Deutsche Versuchsanstalt für Luftfahrt's K.A.
Kawalki led to subsonic profiles very similar to the supercritical profiles,
which was the basis for the objection in 1984 against the US-patent specification
for the supercritical airfoil. Description
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. 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.
See also Whitcomb area rule
NACA airfoil Notes
External links Supercritical Airfoils at Aerospaceweb
Supercritical airfoil - U.S. Centennial of Flight Commission
[1] The Supercritical Airfoil - NASA Dryden Flight Research Center