A bow shock wave exists for free-stream Mach numbers above 1.0.
The Mach cone becomes increasingly swept back with increasing Mach numbers.
A delta wing has the advantage of a large sweep angle but also greater wing area than a simple swept wing to compensate for the loss of lift usually experienced in sweepback.
This figure shows qualitatively the drag advantage that a straight wing has over a swept or delta wing at higher Mach numbers.
This figure shows (L/D)max, a measure of aerodynamic efficiency, plotted against Mach number for an optimum straight-wing and swept-wing airplane.
Various swing-wing airplanes.
Lockheed SST configurations.
A bow shock wave will exist for free-stream Mach numbers above 1.0. In three dimensions, the bow shock is in reality a cone in shape (a Mach cone) as it extends back from the nose of the airplane. As long as the wing is swept back behind the Mach cone, there is subsonic flow over most of the wing and relatively low drag. A delta wing has the advantage of a large sweep angle but also greater wing area than a simple swept wing to compensate for the loss of lift usually experienced in sweepback. But, at still higher supersonic Mach numbers, the Mach cone may approach the leading edge of even a highly swept delta wing. This condition causes the total drag to increase rapidly and, in fact, a straight wing (no sweep) becomes preferable.
Sweepback has been used primarily in the interest of minimizing transonic and supersonic wave drag. At subsonic Mach numbers, however, the disadvantages dominate. They include high induced drag (due to small wing span or low aspect ratio), high angles of attack for maximum lift, and reduced effectiveness of trailing-edge flaps. The straight-wing airplane does not have these disadvantages. For an airplane that is designed to be multimission, for example, cruise at both subsonic and supersonic velocities, it would be advantageous to combine a straight wing and swept wing design. This is the logic for the variable sweep or swing-wing. Although not necessarily equal to the optimum configurations in their respective speed regimes, it is evident that an airplane with a swing-wing capability can, in a multimissioned role, over the total speed regime, be better than the other airplanes individually. One major drawback of the swing-wing airplane is the added weight and complexity of the sweep mechanisms. But technological advances are solving these problems also.
In addition to low-aspect-ratio wings at supersonic speeds, supersonic wave drag may also be minimized by using thin wings and area ruling. Also, long, slender, cambered fuselages minimize drag and improve the spanwise lift distribution.
On June 5, 1963, in a speech before the graduating class of the United States Air Force Academy, President John F. Kennedy committed the United States to "develop at the earliest practical date the prototype of a commercially successful supersonic transport superior to that being built in any other country in the world...." What lay ahead was years of development, competition, controversy, and ultimately rejection of the supersonic transport (SST) by the United States.
The National Aeronautics and Space Administration (NASA) did considerable work, starting in 1959, on basic configurations for the SST. There evolved four basic types of layout that were studied further by private industry. The aircraft manufacturer Lockheed chose to go with a fixed-wing delta design; whereas another aircraft company, Boeing, initially chose a swing-wing design.
One problem associated with the SST is the tendency of the nose to pitch down as it flies from subsonic to supersonic flight. The swing-wing can maintain the airplane balance and counteract the pitch-down motion. Lockheed needed to install canards (small wings placed toward the airplane nose to counteract pitch down). Eventually, the Lockheed design used a double-delta configuration and the canards were no longer needed. This design proved to have many exciting aerodynamic advantages. The forward delta begins to generate lift supersonically (negating pitch down). At low speeds the vortices trailing from the leading edge of the double delta increase lift. This means that many flaps and slats could be reduced or done away with entirely and a simpler wing design provided. In landing, the double delta experiences a ground-cushion effect that allows for lower landing speeds. This is important since three-quarters of airplane accidents occur in takeoff and landing. The British-French Concorde and the Russian Tupolev Tu-144 prototypes use a variation of the double delta wing called the ogee wing. It, too, uses the vortex-lift concept for improvement in low-speed subsonic flight.
Ultimately, Boeing with a swing-wing design was selected as the winner of the U.S. SST competition. The size of the Boeing SST design grew to meet airline payload requirements. Major design changes were incorporated into the Boeing 2707-100 design. The supersonic cruise lift-drag ratio increased from 6.75 to 8.2, and the engines were moved farther back to alleviate the exhaust impinging on the rear tail surfaces. Despite the advantages previously quoted for a swing-wing concept, technological advances in construction did not appear in time. Because of the swing-wing mechanisms and beefed-up structure due to engine placement, incurable problems in reduction of payload resulted. Boeing had no recourse but to adopt a fixed-wing conceptthe B2707-300. Political, economic, and environmental factors led the United States to cancel the project in 1972.
While the British-French Concorde and Russian Tu-144 fly, research is still continuing into advanced supersonic transports in the United States. Whereas, the Concorde and Tu-144 cruise at Mach = 2.2 to 2.4, and the Boeing design cruised at Mach = 2.7, configurations with a cruise speed of Mach = 3.2 have been being analyzed.
One of the more objectionable of the problems facing any supersonic transport is commonly referred to as the "sonic boom." To explain sonic boom, one must return to a description of the shock-wave formation about an airplane flying supersonically. A typical airplane generates two main shock waves, one at the nose (bow shock) and one off the tail (tail shock). Shock waves coming off the canopy, wing leading edges, engine nacelles, etc. tend to merge with the main shocks some distance from the airplane. The resulting pressure pulse changes appear to be N-shaped. To an observer on the ground, this pulse is felt as an abrupt compression above atmospheric pressure followed by a rapid decompression below atmospheric pressure and a final recompression to atmospheric pressure. The total change takes place in one-tenth of a second or less and is felt and heard as a double jolt or boom.
The sonic boom, or the overpressures that cause them, is controlled by factors such as airplane angle of attack, altitude, cross-sectional area, Mach number, atmospheric turbulence, atmospheric conditions, and terrain. The overpressures will increase with increasing airplane angle of attack and cross-sectional area, will decrease with increasing altitude, and first increase and then decrease with increasing Mach number.
Turbulence in the atmosphere may smooth the "N" wave profile and thus lessen the impact of the boom or, on the other hand, may in fact amplify the overpressures. Reflections of the overpressures by terrain and buildings may cause multiple booms or post-boom aftershocks. In a normal atmospheric profile, the speed of sound increases with decreasing altitude. The directions in which the overpressures travel are refracted in this normal case and they will at some point curve away from the Earth. The strongest sonic boom is felt directly beneath the airplane and decreases to nothing on either side of the flight path. It is interesting to note that a turning supersonic airplane may concentrate the set of shock waves locally where they intersect the ground and produce a superboom.
Perhaps the greatest concern expressed about the sonic boom is its effect on the public. The effects run from structural damage (cracked building plaster and broken windows) down to heightened tensions and annoyance of the citizenry. For this reason, the world's airlines have been forbidden to operate supersonically over the continental United States. This necessitates, for SST operation, that supersonic flight be limited to overwater operations. Research for ways in which to reduce the sonic boom continues.
Adapted from Talay, Theodore A. Introduction to the Aerodynamics of Flight. SP-367, Scientific and Technical Information Office, National Aeronautics and Space Administration, Washington, D.C. 1975. Available at http://history.nasa.gov/SP-367/cover367.htm
For Further Reading:
Anderson, Jr., John D. A History of Aerodynamics. Cambridge, England: Cambridge University Press, 1997.
Dwiggins, Don. The SST: Here It Comes Ready or Not. Garden City, New York: Doubleday, 1968.
Shurcliff, William. S/S/T and Sonic Boom Handbook. New York: Ballantine Books, 1970.
Wegener, Peter P. What Makes Airplanes Fly? New York: Springer-Verlag, 1991.
Lifting vortices of double delta wing. At low speeds, the vortices trailing from the leading edge of the double delta increase lift.
The British-French Concorde and the Russian TU-144 prototypes use a variation of the double delta wing called the ogee wing.
The evolution of the Boeing SST design was originally derived from one of the NASA designs.
An advanced SST configurations with a cruise speed of M = 3.2 was tested at the NASA Langley Research Center.
Sonic boom generation.
Factors affecting sonic-boom overpressures.
Refraction of shock waves. This figure shows that the directions in which the overpressures travel are refracted in this normal case and that they will at some point curve away from the Earth.