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Supercritical Airfoils

NASA_F-8A_Crusader_Supercritical_Wing_Aircraft

Supercritical Airfoils

For any aircraft, while designing a wing one must select the airfoil with the higher value of lift and that of the drag as low as possible. It is observed from the several researches done by scientists of NASA that fuel consumption is efficient for the greater value of the cruise speed.

But as speed increases, the compressibility of air increases. What this means in practical terms is that the density of air increases at high speeds resulting in a greater drag on an airfoil.

What a supercritical airfoil is designed to do is delay the speed at which the compressibility effect becomes significant so that drag will be reduced. A supercritical airfoil is the one designed, primarily, to delay the onset of wave drag especially in the transonic speed range (i.e. from Mach no. 0.7 or 0.8 to 1.2 or 1.3).

Supercritical airfoils are mainly characterized by their flattened upper surface, highly cambered aft section, and larger leading edge radius compared with other airfoils such as NACA 6-series laminar airfoil shapes.

The very first supercritical airfoils were suggested in Germany in 1940 by K.A. Kawalki at Deutsche Berlin-Adlershof whose airfoils are characterized by elliptical leading edges, maximum thickness located downstream up to 50 percent chord and a flat upper surface.

In the U.S., supercritical airfoils were studied in the 1960s, by then NASA engineer Richard Whitcomb, and were first tested on a modified North American T-2C Buckeye.

The F-8 Crusader used as the SCW test aircraft was built by LTV Aerospace, Dallas, Tex., for the U.S. Navy. F-8s were the first carrier-based aircraft with speeds over 1,000 mph. A total of 1,261 F-8s were built between 1955 and 1965.

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. The supercritical airfoil shape is incorporated into the design of a supercritical wing in modern aircrafts.

Subsonic_and_trans-sonic_airfoils

Aircraft with a conventional wing nears a speed of sound (i.e. Mach 1), air flowing across the top of the wing moves faster and becomes supersonic. Due to this a shock wave is created on the wing’s upper surface, even though the aircraft as a whole, has not exceeded Mach 1.

The aircraft, at this point, is flying at what is called the critical speed. The Shockwave causes the smooth flow of air hugging the wing’s upper surface (on the boundary layer) to separate from the wing and create turbulence.

Separated boundary layers are like wakes behind a boat i.e. the air is unsteady and churning, and drag increases.

This increases fuel consumption, and it can also lead to a decrease in speed and cause vibrations. In rare cases, aircraft have also become uncontrollable due to boundary layer separation.

Supercritical wings have a flat-on-top “upside down” look. As air moves across the top of a SCW it does not speed up nearly as much as over a curved upper surface.

This delays the onset of the shock wave and also reduces aerodynamic drag associated with boundary layer separation. Lift that is lost with less curvature on the upper surface of the wing is regained by adding more curvature to the upper trailing edge.

Now the aircraft can cruise at a higher subsonic speed and easily fly up into the supercritical range. And with less drag, the aircraft is using less fuel than it would otherwise consume.

For a given aircraft, supercritical airfoils provide four main benefits; a higher drag divergence Mach number, developing shock waves further aft than traditional airfoils, greatly reducing shock-induced boundary layer separation, and allowing for more efficient wing design (e.g., a thicker wing and/or reduced wing sweep, each of which may allow for a lighter wing.

Higher subsonic cruise speeds and less drag translates into airliners and business jets getting to their destinations faster on less fuel, and they can fly farther. These are the factors that help keep the cost of passenger tickets and air freight down.

NASA’s two-year SCW flight test program between 1971 and 1973 substantiated predictions of better flying efficiency and reduced operating costs at a time when soaring fuel prices hit the aviation industry hard. Test data showed that the SCW on the F-8 test aircraft increased its efficiency by as much as 15%.

To give some idea of the practical benefit of the supercritical airfoil, Air Force and NASA researchers under the auspices of the Transonic Aircraft Technology (TACT) program modified a basic F-111 bomber and replaced the existing NACA 64-210 wing airfoil with a supercritical shape of equal thickness.

In so doing, they managed to increase the drag divergence Mach number by 16%, from Mach 0.76 to Mach 0.88. This example illustrates how significant the use of supercritical airfoils can be in improving the performance of aircraft cruising in the transonic regime.

Nowadays, Supercritical airfoil and wing technology is incorporated into the designs of commercial, business as well as in military aircraft all around the world. They include the 777 jetliner, C-17 Air Force transport, and AV-8B Harrier built by Boeing.

Similarly, others using SCW technology carry the corporate names of Bombardier, Lear, Challenger, Galaxy, Raytheon, Gulfstream, Cessna, Falcon, Airbus Industries, Dassault-Brequet-Dornier, and Israel Aircraft Industries. Boeing’s 757 and 767 jetliners, and the new generation of 737 aircraft, also have wings designed with some form of applied supercritical technology.

More importantly, several military aircraft in testing and development stages are being built with SCW technology. Among them are the Lockheed-Martin F-22 advanced technology fighter, and the two aircraft that will be considered for the U.S. military Joint Strike Fighter production contract, the Boeing X-32 and the Lockheed-Martin X-35.


Article Credit: Manish Sakhakarmy (Undergraduate Student Mechanical 4th Year)
Tribhuvan University, I.O.E
Image Credit: NASA

 

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