Supercritical airfoil
Overview
 
A supercritical airfoil is an airfoil
Airfoil
An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....

 designed, primarily, to delay the onset of wave drag
Wave drag
In aeronautics, wave drag is a component of the drag on aircraft, blade tips and projectiles moving at transonic and supersonic speeds, due to the presence of shock waves. Wave drag is independent of viscous effects.- Overview :...

 in the transonic
Transonic
Transonic speed is an aeronautics term referring to the condition of flight in which a range of velocities of airflow exist surrounding and flowing past an air vehicle or an airfoil that are concurrently below, at, and above the speed of sound in the range of Mach 0.8 to 1.2, i.e. 600–900 mph...

 speed range. Supercritical airfoils are characterized by their flattened upper surface, highly cambered
Camber (aerodynamics)
Camber, in aeronautics and aeronautical engineering, is the asymmetry between the top and the bottom surfaces of an aerofoil. An aerofoil that is not cambered is called a symmetric aerofoil...

 (curved) aft section, and greater leading edge
Leading edge
The leading edge is the part of the wing that first contacts the air; alternatively it is the foremost edge of an airfoil section. The first is an aerodynamic definition, the second a structural one....

 radius compared with traditional airfoil shapes. The supercritical airfoils were designed in the 1960s, by then NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 engineer Richard Whitcomb
Richard Whitcomb
Richard T. Whitcomb , was an American aeronautical engineer noted for his significant contributions to the science of aerodynamics....

, and were first tested on a modified North American T-2C Buckeye.
Encyclopedia
A supercritical airfoil is an airfoil
Airfoil
An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....

 designed, primarily, to delay the onset of wave drag
Wave drag
In aeronautics, wave drag is a component of the drag on aircraft, blade tips and projectiles moving at transonic and supersonic speeds, due to the presence of shock waves. Wave drag is independent of viscous effects.- Overview :...

 in the transonic
Transonic
Transonic speed is an aeronautics term referring to the condition of flight in which a range of velocities of airflow exist surrounding and flowing past an air vehicle or an airfoil that are concurrently below, at, and above the speed of sound in the range of Mach 0.8 to 1.2, i.e. 600–900 mph...

 speed range. Supercritical airfoils are characterized by their flattened upper surface, highly cambered
Camber (aerodynamics)
Camber, in aeronautics and aeronautical engineering, is the asymmetry between the top and the bottom surfaces of an aerofoil. An aerofoil that is not cambered is called a symmetric aerofoil...

 (curved) aft section, and greater leading edge
Leading edge
The leading edge is the part of the wing that first contacts the air; alternatively it is the foremost edge of an airfoil section. The first is an aerodynamic definition, the second a structural one....

 radius compared with traditional airfoil shapes. The supercritical airfoils were designed in the 1960s, by then NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 engineer Richard Whitcomb
Richard Whitcomb
Richard T. Whitcomb , was an American aeronautical engineer noted for his significant contributions to the science of aerodynamics....

, 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
F-8 Crusader
The Vought F-8 Crusader was a single-engine, supersonic, carrier-based air superiority jet aircraft built by Vought for the United States Navy and the U.S. Marine Corps, replacing the Vought F7U Cutlass...

. While the design was initially developed as part of the supersonic transport
Supersonic transport
A supersonic transport is a civilian supersonic aircraft designed to transport passengers at speeds greater than the speed of sound. The only SSTs to see regular service to date have been Concorde and the Tupolev Tu-144. The last passenger flight of the Tu-144 was in June 1978 with its last ever...

 (SST) project at NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

, 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
German Aerospace Center
The German Aerospace Center is the national centre for aerospace, energy and transportation research of the Federal Republic of Germany. It has multiple locations throughout Germany. Its headquarters are located in Cologne. It is engaged in a wide range of research and development projects in...

'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
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...

 airflow over the upper surface of the traditional airfoil induced excessive wave drag
Wave drag
In aeronautics, wave drag is a component of the drag on aircraft, blade tips and projectiles moving at transonic and supersonic speeds, due to the presence of shock waves. Wave drag is independent of viscous effects.- Overview :...

 and a form of stability loss called Mach tuck
Mach tuck
Mach tuck is an aerodynamic effect, whereby the nose of an aircraft tends to pitch downwards as the airflow around the wing reaches supersonic speeds...

. 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 Boeing 777
Boeing 777
The Boeing 777 is a long-range, wide-body twin-engine jet airliner manufactured by Boeing Commercial Airplanes. It is the world's largest twinjet and is commonly referred to as the "Triple Seven". The aircraft has seating for over 300 passengers and has a range from , depending on model...

 to the McDonnell Douglas AV-8B Harrier II.
Supercritical airfoils feature four main benefits: they have a higher drag divergence Mach number
Drag divergence Mach number
The drag divergence Mach number is the Mach number at which the aerodynamic drag on an airfoil or airframe begins to increase rapidly as the Mach number continues to increase...

, they develop shock waves
Shock Waves
Shock Waves, , is a 1977 horror movie written and directed by Ken Wiederhorn...

 farther aft than traditional airfoils, they greatly reduce shock-induced boundary layer
Boundary layer
In physics and fluid mechanics, a boundary layer is that layer of fluid in the immediate vicinity of a bounding surface where effects of viscosity of the fluid are considered in detail. In the Earth's atmosphere, the planetary boundary layer is the air layer near the ground affected by diurnal...

 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
Drag divergence Mach number
The drag divergence Mach number is the Mach number at which the aerodynamic drag on an airfoil or airframe begins to increase rapidly as the Mach number continues to increase...

, 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
Pressure coefficient
The pressure coefficient is a dimensionless number which describes the relative pressures throughout a flow field in fluid dynamics. The pressure coefficient is used in aerodynamics and hydrodynamics...

 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
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

. 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
C-17 Globemaster III
The Boeing C-17 Globemaster III is a large military transport aircraft. Developed for the United States Air Force from the 1980s to the early 1990s by McDonnell Douglas, the C-17 is used for rapid strategic airlift of troops and cargo to main operating bases or forward operating bases throughout...

.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK