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 high subsonic aircraft. Research in 1940 by DVL'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. [1] 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. Techniques learned from studies of the original supercritical airfoil sections are used to design airfoils for high-speed subsonic and transonic aircraft from the Boeing 777 to the McDonnell Douglas AV-8B Harrier II. The upper surface is half a de Laval nozzle and so naturally accelerates the flow above it to supersonic speeds, but does not de-accelerate it. The supersonic flow-layer de-accelerates and increases pressure on narrowing and the subsonic flow-layer far above the wing accelerates and increases pressure on widening. As the pressure in the layer directly above the wing is low, the subsonic flow-layer has higher pressure and narrows the supersonic layer, de-accelerating it to subsonic speeds. In contrast to an inlet cone this is automatically stabilized. The supercritical airfoil tries to not to interfere with the deaccelaration by postponing the divergent part to the subsonic aft. The mechanical stiffness is achieved by the belly-bottom. Lift is generated in the aft.
Supercritical airfoils have four main benefits: they have a higher drag divergence Mach number,[2] they develop shock waves farther aft than traditional airfoils,[3] 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 slow 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 induces 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.
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.
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[edit] Notes
- ^ Hans-Ulrich Meier, Die Pfeilflügelentwicklung in Deutschland bis 1945, ISBN 3763761306 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.
