Airfoil designed primarily to delay the onset of wave drag in the transonic speed range
Conventional (1) and supercritical (2) airfoils at identical free stream Mach number. Illustrated are: A– supersonic flow region, B– shock wave, C– area of separated flow. The supersonic flow over a supercritical airfoil terminates in a weaker shock, thereby postponing shock-induced boundary layer separation.
A supercritical aerofoil (supercritical airfoil in American English) 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.[1] Standard wing shapes are designed to create lower pressure over the top of the wing. Both the thickness distribution and the camber of the wing determine 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 reaches Mach 1 and shockwaves 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 origins of the supercritical airfoil can be traced back to the German aerodynamicist K.A. Kawalki, who designed a number of airfoils during the Second World War. Following the end of the conflict, multiple nations continued research into the field, including Germany, the United Kingdom, and the United States. In particular, Hawker Siddeley Aviation designed a number of advanced airfoils that were, amongst other programmes, incorporated into the Airbus A300. In America, the aerodynamicist Richard Whitcomb produced supercritical airfoils similar to Kawalki's earlier work; these were used to devise a supercritical wing that was, in turn, incorporated into both civil and military aircraft. Accordingly, techniques learned from studies of the original supercritical airfoil sections have been used to design airfoils for several high-speed subsonic and transonic aircraft, from the Airbus A310 and Boeing 777 airliners to the McDonnell Douglas AV-8B Harrier II jumpjet.
The supercritical airfoil was first suggested by aerodynamicists in Germany during the Second World War. 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. Testing of these airfoils was reported by B. Göthert and K.A. Kawalki in 1944. Kawalki's airfoil shapes were similar to those subsequently produced by the American aerodynamicist Richard Whitcomb.[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 and allow the aircraft to attain much higher speeds; this is the basis of the supercritical wing. Its design allows the wing to maintain high performance levels at speeds closer to Mach 1 than traditional counterparts.
In 1962 the Vickers VC-10 was rolled out which had wing super-critical characteristics, it also being the first airliner to have a wing section that was specifically designed for the plane ( rather than a standard shape). The design was worked on by Vickers and the UKs research institutes.[4][5] Between 1959 and 1968, the British aerospace manufacturer Hawker Siddeley Aviation, based in Hatfield, England, designed its own improved airfoil profiles, which were sometimes referred to as rooftop rear-loaded airfoils. 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.[6][7] In parallel, postwar Germany and the Netherlands also conducted their own research efforts into optimal transonic airfoil designs, intending for these efforts to support civil aviation programmes.[8] Up until the 1970s, there was considerable focus upon developing an airfoil that performed isentropic recompression, a shock-free return of the airflow to subsonic speeds.[9]
In the United States, the supercritical airfoil was an area of research during the 1960s; one of the leading American figures in the field was Richard Whitcomb. 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.[10] Following initial flight testing, the new airfoils were tested at increasingly higher speeds on another modified military aircraft, the TF-8A Crusader.[11]
While the supercritical airfoil had been initially worked on by NASA as part of the United States' National Supersonic Transport programme, the supersonic airliner that was being developed to harness it, the Boeing 2707, was ultimately cancelled due to a combination of technical challenges and relatively high costs.[12][13] Despite this, the work was one aspect of the programme that survived the cancellation of its principal intended recipient. The supercritical airfoil shape was incorporated into the design of the supercritical wing.
In such a manner, the technology has subsequently been successfully applied to several high-subsonic aircraft, noticeably increasing their fuel efficiency.[14] Early examples include the Boeing 757 and Boeing 767 airliners, both of which were developed during the 1970s.[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.[9]
During 1984, Kawalki's research was cited as the basis for a formal objection against the US patent specification that had been issued for the supercritical airfoil.[16] Around this time, Kawalki's work was reportedly playing an active role in the design of new airliners, such as the Airbus A310.[9] Additionally, some aircraft have been redesigned to incorporate supercritical wings; such as the Hawker Siddeley Harrier, popularly known as the Harrier jump jet, which had a second generation AV-8B Harrier II model that adopted a new one-piece supercritical wing to improve cruise performance by delaying the rise in drag and increasing lift-to-drag ratio.[17]
The adoption of the supercritical airfoil amongst modern jet aircraft has diminished the use of some other methods of decreasing wave drag. The anti-shock body was one such method, having also been derived from Richard Whitcomb's work as well as that of the German aerodynamicist Dietrich Küchemann.[18] Alternatively referred to as "Whitcomb bodies" or "Küchemann carrots", it is closely associated with the area rule, a recent innovation of the era to minimise wave drag by having a cross-sectional area which changes smoothly along the length of the aircraft.[19][20]
Description
Benefits
Supercritical airfoils feature four main benefits: they have a higher drag-divergence Mach number,[21] they develop shock waves farther aft than traditional airfoils,[22] they greatly reduce shock-induced boundary layer separation, and their geometry allows more efficient wing design (e.g., a thicker wing and/or reduced wing sweep, each of which may allow 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.
Supercritical airfoil Mach number/pressure coefficient diagram (y axis: Mach number, or pressure coefficient, negative up; x axis: position along chord, leading edge left). The sudden increase in pressure coefficient at midchord is due to the shock.
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 Boeing C-17 Globemaster III.[23]
Stall characteristics
The stall behavior of supercritical profile is unlike that of low-speed airfoils. The boundary layer along the leading edge of a supercritical wing begins thin and laminar at cruise angles. As angle of attack (AOA) increases, this laminar layer detaches in a narrow region and forms a short bubble. The airflow, now turbulent, reattaches to the surface aft of the bubble; the increase in drag is not extreme in this condition. 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). The abrupt loss of lift is exacerbated by the lack of traditional stall "warning" or buffet as a low-speed contour would provide.[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. Since wing fences "prevent the entire wing from stalling at once", they may also form an alternative means of providing recovery in this respect.[25]
Aerodynamics is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dynamics and its subfield of gas dynamics, and is an important domain of study in aeronautics. The term aerodynamics is often used synonymously with gas dynamics, the difference being that "gas dynamics" applies to the study of the motion of all gases, and is not limited to air. The formal study of aerodynamics began in the modern sense in the eighteenth century, although observations of fundamental concepts such as aerodynamic drag were recorded much earlier. Most of the early efforts in aerodynamics were directed toward achieving heavier-than-air flight, which was first demonstrated by Otto Lilienthal in 1891. Since then, the use of aerodynamics through mathematical analysis, empirical approximations, wind tunnel experimentation, and computer simulations has formed a rational basis for the development of heavier-than-air flight and a number of other technologies. Recent work in aerodynamics has focused on issues related to compressible flow, turbulence, and boundary layers and has become increasingly computational in nature.
The Whitcomb area rule, named after NACA engineer Richard Whitcomb and also called the transonic area rule, is a design procedure used to reduce an aircraft's drag at transonic speeds which occur between about Mach 0.75 and 1.2. For supersonic speeds a different procedure called the supersonic area rule, developed by NACA aerodynamicist Robert Jones, is used.
A delta wing is a wing shaped in the form of a triangle. It is named for its similarity in shape to the Greek uppercase letter delta (Δ).
The sound barrier or sonic barrier is the large increase in aerodynamic drag and other undesirable effects experienced by an aircraft or other object when it approaches the speed of sound. When aircraft first approached the speed of sound, these effects were seen as constituting a barrier, making faster speeds very difficult or impossible. The term sound barrier is still sometimes used today to refer to aircraft approaching supersonic flight in this high drag regime. Flying faster than sound produces a sonic boom.
A supersonic transport (SST) or a supersonic airliner is a civilian supersonic aircraft designed to transport passengers at speeds greater than the speed of sound. To date, the only SSTs to see regular service have been Concorde and the Tupolev Tu-144. The last passenger flight of the Tu-144 was in June 1978 and it was last flown in 1999 by NASA. Concorde's last commercial flight was in October 2003, with a November 26, 2003 ferry flight being its last airborne operation. Following the permanent cessation of flying by Concorde, there are no remaining SSTs in commercial service. Several companies have each proposed a supersonic business jet, which may bring supersonic transport back again.
A waverider is a hypersonic aircraft design that improves its supersonic lift-to-drag ratio by using the shock waves being generated by its own flight as a lifting surface, a phenomenon known as compression lift.
A swept wing is a wing that angles either backward or occasionally forward from its root rather than in a straight sideways direction.
Transonic flow is air flowing around an object at a speed that generates regions of both subsonic and supersonic airflow around that object. The exact range of speeds depends on the object's critical Mach number, but transonic flow is seen at flight speeds close to the speed of sound, typically between Mach 0.8 and 1.2.
In aeronautics, wave drag is a component of the aerodynamic drag on aircraft wings and fuselage, propeller blade tips and projectiles moving at transonic and supersonic speeds, due to the presence of shock waves. Wave drag is independent of viscous effects, and tends to present itself as a sudden and dramatic increase in drag as the vehicle increases speed to the critical Mach number. It is the sudden and dramatic rise of wave drag that leads to the concept of a sound barrier.
Richard Travis Whitcomb was an American aeronautical engineer who was noted for his contributions to the science of aerodynamics.
Wingtip devices are intended to improve the efficiency of fixed-wing aircraft by reducing drag. Although there are several types of wing tip devices which function in different manners, their intended effect is always to reduce an aircraft's drag. Wingtip devices can also improve aircraft handling characteristics and enhance safety for following aircraft. Such devices increase the effective aspect ratio of a wing without greatly increasing the wingspan. Extending the span would lower lift-induced drag, but would increase parasitic drag and would require boosting the strength and weight of the wing. At some point, there is no net benefit from further increased span. There may also be operational considerations that limit the allowable wingspan.
The Boeing YC-14 is a twinjet short take-off and landing (STOL) tactical military transport aircraft. It was Boeing's entrant into the United States Air Force's Advanced Medium STOL Transport (AMST) competition, which aimed to replace the Lockheed C-130 Hercules as the USAF's standard STOL tactical transport. Although both the YC-14 and the competing McDonnell Douglas YC-15 were successful, neither aircraft entered production. The AMST project was ended in 1979 and replaced by the C-X program.
A flap is a high-lift device used to reduce the stalling speed of an aircraft wing at a given weight. Flaps are usually mounted on the wing trailing edges of a fixed-wing aircraft. Flaps are used to reduce the take-off distance and the landing distance. Flaps also cause an increase in drag so they are retracted when not needed.
In aerodynamics, the critical Mach number of an aircraft is the lowest Mach number at which the airflow over some point of the aircraft reaches the speed of sound, but does not exceed it. At the lower critical Mach number, airflow around the entire aircraft is subsonic. Supersonic aircraft such as the Concorde and combat aircraft also have an upper critical Mach number at which the airflow around the entire aircraft is supersonic.
Mach tuck is an aerodynamic effect whereby the nose of an aircraft tends to pitch downward as the airflow around the wing reaches supersonic speeds. This diving tendency is also known as tuck under. The aircraft will first experience this effect at significantly below Mach 1.
The VFW VAK 191B was an experimental German vertical take-off and landing (VTOL) strike fighter of the early 1970s. VAK was the abbreviation for Vertikalstartendes Aufklärungs- und Kampfflugzeug. Designed and built by the Vereinigte Flugtechnische Werke (VFW), it was developed with the purpose of eventually serving as a replacement for the Italian Fiat G.91 then in service with the German Air Force. Operationally, it was intended to have been armed with nuclear weapons as a deterrent against aggression from the Soviet Union and, in the event of a major war breaking out, to survive the first wave of attacks by deploying to dispersed locations, rather than conventional airfields, and to retaliate against targets behind enemy lines.
Anti-shock body is the name given by Richard T. Whitcomb to a pod positioned on the upper surface of a wing. Its purpose is to reduce wave drag while travelling at transonic speeds, which includes the typical cruising range of conventional jet airliners. 'The Cambridge Aerospace Dictionary' defines shock body as a streamlined volume added to improve area rule distribution.
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. This increase can cause the drag coefficient to rise to more than ten times its low-speed value.
A supersonic airfoil is a cross-section geometry designed to generate lift efficiently at supersonic speeds. The need for such a design arises when an aircraft is required to operate consistently in the supersonic flight regime.
The crescent wing is a fixed-wing aircraft configuration in which a swept wing has a greater sweep angle on the inboard section than the outboard, giving the wing a crescent shape.
↑ 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, ISBN3-7637-6130-6. Einspruch (1984) gegen US-Patentschrift NASA über »superkritische Profile«, basierend auf den Berechnungsmethoden von K. H. Kawalki (1940) p. 107. (in German)
↑ Tanner, Clinton E., Bombardier Business Aircraft Senior Advisor, "The Effect of Wing Leading Edge Contamination on the Stall Characteristics of Aircraft" (reported in 24 December 2018 article in Aviation Week & Space TechnologyThin Margins in Wintry Takeoffs).
Hirschel, Ernst Heinrich; Prem, Horst; Madelung, Gero (2012). Aeronautical Research in Germany: From Lilienthal until Today. Heidelberg: Springer Berlin Heidelberg. ISBN978-3-642-18484-0.
Obert, Ed. Aerodynamic Design of Transport Aircraft IOS Press, 2009. ISBN1-5860-3970-9.
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