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supercritical airfoil
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supercritical airfoil

A Vought F-8A Crusader was selected by NASA as the test-bed aircraft to install an experimental supercritical wing in place of the conventional wing. The unique design of the supercritical wing reduces the effect of shock waves on the upper surface near Mach 1, which in turn reduces drag.

Sometimes, breakthroughs in technology are not used in the way that their inventors intend. They may lead to surprising applications. The supercritical airfoil, first developed by a NASA aerodynamicist in the 1960s, provides an excellent example.

An airfoil is the shape of a wing's cross-section. Slice a wing like you would slice salami and you can see the shape of the airfoil. This shape defines how much lift the wing generates at various speeds. By the 1950s, airfoil research in the United States and most other countries had reached a standstill. Aerodynamicists had become more interested in other research questions, such as how the air flowed over an aircraft's skin when it travelled faster than the speed of sound (Mach 1) and how much wings should be swept back (or angled back) from the fuselage of the plane. Although designers still developed new airfoils, they did so on a case-by-case basis for use on specific airplanes. Few undertook basic research on airfoil shapes for a whole class of aircraft.

That changed in the 1960s when NASA scientist Richard T. Whitcomb developed the supercritical airfoil. Whitcomb was already famous for developing the "Area Rule" of supersonic flight which won him the Collier Trophy and which resulted in some fighters of the mid 1950s having a pinch midway along their fuselage, like an hourglass. Probably no one understood how air flowed over an aircraft as it approached the speed of sound better than Whitcomb, and he applied that knowledge to a new problem associated with the compressibility of air over an aircraft wing as it approached the speed of sound.

At the time, commercial passenger jets like the Boeing 707 cruised at speeds of around Mach 0.7 to Mach 0.8 ("cruise speed" is the speed at which an airplane is most fuel efficient; commercial airplanes operate at this speed in order to be economical and not waste fuel). The people who ran the airlines wanted planes that could travel even faster, at Mach 0.9 or 0.95, and still be fuel efficient.

But as an aircraft approaches the speed of sound, it reaches a point where the air flowing over the wings reaches supersonic speeds though the plane itself is still moving slower than Mach 1, causing a dramatic increase in drag. The airspeed at which this occurs is called the critical Mach number for the wing. For example, if the air flowing over a wing reaches Mach 1 when the wing is only moving at Mach 0.8, the wing's critical Mach number is 0.8. The spot where this happens on the wing is usually about halfway between the leading edge and the trailing edge of the wing.

Cross-sections of three supercritical airfoils. These are considerably blunter and thicker than conventional transonic airfoils.

Designers deal with this dramatic increase in drag by angling the wings back from the fuselage, making them thinner, and using other features designed to reduce drag. But all of these solutions increase structural weight, decreasing range and fuel economy, and making them unattractive for commercial use. In addition, thinner wings cannot be used to store fuel, a common location for fuel tanks on passenger planes.

In the early 1960s, Whitcomb sought to develop a new airfoil shape that would allow the wing to reach a higher speed before the airflow over it reached the speed of sound. He proposed a new airfoil shape featuring a well-rounded leading edge, relatively flatter upper surface (not as curved or cambered as other wings) that pushed the critical Mach point farther back on the wings, and a sharply down-curving trailing edge that increased lift. He called this the "supercritical" airfoil. Whitcomb tested this wing in NASA's 8-foot transonic pressure tunnel at Langley, Virginia. These tests suggested that the supercritical wing might allow planes to travel up to 10 percent faster. Alternatively, a plane with the new wing could fly more efficiently at the same speed (for example, a plane that normally cruised at Mach 0.7 could be equipped with a supercritical wing and achieve better fuel economy).

Supercritical wing on the F-8 research airplane.

The wind tunnel tests, however, involved small models with low Reynolds numbers making tests of the supercritical wing unreliable. For full-scale tests, NASA engineers chose a Navy Vought F-8U fighter as a test aircraft. The F-8U was normally a fighter aircraft capable of supersonic flight, but NASA engineers wanted to use it to determine if an aircraft could cruise just below the speed of sound, a speed range known as the transonic region. NASA engineers equipped the plane with a slender, graceful supercritical wing and tested it in 1971. The F-8U flight tests proved that Whitcomb's wind tunnel results were correct: the supercritical airfoil would allow planes to cruise at higher speeds. Passenger jets could be equipped with wings that would allow them to fly at Mach 0.9 or 0.95 instead of Mach 0.7 or Mach 0.8, and still be relatively fuel efficient.

NASA presented the resulting wind tunnel and flight test data at a conference in 1972. Industry designers were intrigued by the data and started to evaluate it. They then came up with a surprising conclusion: instead of increasing cruise speed to around Mach 0.9, they would keep the speed around Mach 0.8, but use the supercritical shapes to increase fuel efficiency. A more efficient aircraft could travel farther on the same amount of fuel. The commercial airlines had told the airplane builders that this was what they wanted—planes that could fly farther more economically rather than planes that could fly faster.

By the mid-1970s, supercritical wings were being incorporated into a whole range of aircraft, from subsonic transports to business jets. In addition to being more fuel efficient, the blunt leading edge of a supercritical wing improved takeoff and landing performance, as well as manoeuvrability. As a result, the most enthusiastic users of supercritical airfoils are designers of cargo transport planes. The Air Force C-17 has a supercritical wing that gives it excellent performance for a plane of its massive size. The supercritical wing may also eventually see use for its original purpose: commercial aircraft designers have recently begun looking at the possibility of designing large passenger jets that can cruise just below the speed of sound.