Also known as: SST
Definition: Aircraft that can fly faster than the speed of sound.
Significance: The speed of sound (Mach 1) is a milestone in the range of aircraft speeds, and aircraft that can fly faster than this speed are different in many ways from those that are not designed to exceed the speed of sound.
The first fifty years of aviation was characterized by ever-increasing aircraft speeds, as aerodynamic research found ways to reduce airplane drag and as engine and fuel improvements made possible continual improvements in thrust and power. By the 1940′s, highly streamlined airplanes with modern wing designs and powerful piston engines were capable of approaching the speed of sound in a full-power dive. When this occurred, as it did scores of times in some fighter aircraft of World War II, unexpected things began to happen which made the aircraft difficult, if not impossible, for the pilot to control, and sometimes caused the airplane to fail structurally. These problems aroused investigations in aerodynamic theory and led to experiments that verified high-speed flight experience. All combined to reveal huge increases in drag as the speed of sound. or Mach 1, was approached, leading to the definition of this speed as the “sound barrier.”
As a high-speed aircraft flies through the atmosphere, the surrounding air must accelerate as it moves around the plane. It is this acceleration that creates lift on the wing. The wing is shaped to cause more flow acceleration over its upper surface than its lower surface, and this results in lower pressures on top of the wing and, hence, lift. As aircraft speeds of 70 to 80 percent of the speed of sound are reached, portions of the flow over the wing will have accelerated to speeds greater than the speed of sound. Contrary to popular belief, it is not the existence of supersonic flow over the wing which causes the drag to rise. It is, rather, the normal inability of that supersonic flow to gradually slow back to subsonic speed that causes the problem.
When a supersonic flow over a wing tries to slow to subsonic speed it will usually do so in a very sudden speed drop, with an accompanying jump to a higher pressure. This sudden change in speed and pressure is known as a shock wave, and the pressure jump often causes the flow to separate or break away from the surface of the wing, resulting in a large, drag-producing wake. When this occurs, often at speeds between 70 and 80 percent of the speed of sound, the drag on the wing and, hence, the drag on the airplane begins to increase. The drag rises sharply as speed is increased beyond that point. At the same time, the new flow patterns over the wing change the way lift is produced on its surface and alter the way the aircraft tends to pitch nose up or down. Similar patterns of flow and separation over the stabilizer surfaces may result in an inability to use these to control the airplane, especially in view of the changed pitching moments. These changes, which can occur rather suddenly at the point of shock wave formation (the “critical” Mach number), can easily lead to loss of airplane control, along with increases in drag which quickly exceed the thrust of the engine. A plane not designed to handle these loading changes can suffer structural failure and even lose a wing or tail surface.
Breaking the Sound Barrier
The combination of drag increase and stability changes that occurred on a wing as the speed of sound was approached created a very real fear of the sound barrier among pilots of aircraft capable of reaching these speeds in high-speed dives. Fortunately for many of these pilots, the speed of sound is a function of air temperature and, hence, of altitude. As the diving plane descended to lower altitudes, the speed of sound increased, the plane’s Mach number (its speed divided by the speed of sound) decreased, and the shock waves disappeared, if the aircraft could hold together that long.
Aerodynamicists and others who studied the flows around wings at speeds near that of sound learned to design wings and control surfaces that could handle the changes in this transonic speed range and, in the 1940′s, they began to look at airplane designs which would allow flight beyond the sound barrier to supersonic speeds. The development of new jet and rocket engines also offered the hope of producing enough thrust to overcome the large increase in drag that occurred as Mach 1 was approached. One such design was the Bell X-1.
Despite the belief that drag became infinite at the speed of sound, a misinterpretation of aerodynamic theory, engineers at the National Advisory Committee for Aeronautics (NACA) knew that bullets and artillery shells flew at supersonic speeds. They realized that if an airplane could be supplied with enough thrust and if wings and stabilizers could be designed for transonic operation and control, it would be possible to break the so-called sound barrier. To help ensure their success, they modeled the shape of their new experimental airplane on an artillery shell.
The X-1 was equipped with a rocket engine and designed for air launch, in which it was dropped from beneath the wing of a B-29. Its rocket engine ignited for longer periods of time in subsequent flights, gradually pushing its speed toward Mach 1. Finally, on October 14, 1947, with Air Force Captain Charles E. “Chuck” Yeager at the controls, the X-1 reached and exceeded the speed of sound. At 700 miles per hour, Yeager had reached Mach 1.06 before cutting off the plane’s rocket engine. This X-1, which Yeager nicknamed “Glamorous Glennis,” after his wife, now hangs in the National Air and Space Museum in Washington, D.C.
The X-1 and other experimental planes explored ever-higher supersonic speeds and evaluated aerodynamic and control theories related to transonic and supersonic flight. The Bell X-5, a small aircraft with variable sweep wings, tested the theory that increasing the wing’s sweep would reduce the transonic drag rise. The Douglas Skyrocket reached Mach 2 in November, 1953, with Scott Cross field as pilot. The Bell X-2 reached Mach 3 on September 27, 1956, but then tumbled out of control, crashing and killing the Air Force pilot, Captain Milburn Apt.
Supersonic Design
The theory that sweeping the wings would decrease the drag rise that occurs as Mach 1 is approached was developed independently in Germany and the United States well before the flight of the X-1, but it was confirmed by the X-5. Sweeping the wing lowers the effective Mach number of the flow perpendicular to the wing, and it is this component of the flow that influences the increase in drag at transonic speeds. Tests confirmed that as the wing is swept aft, the transonic drag rise is both delayed and reduced. This allows an aircraft with a swept wing to fly faster before experiencing the transonic increase in drag and allows it to fly through the speed of sound with less thrust. Nothing is free, however, and the reduction in drag is accompanied by a reduction in the lifting capability of the wing and requires more wing area for a given amount of lift. One way to accomplish this is by the use of a triangular or delta-shaped wing, a wing shape used on many supersonic aircraft.
One of the first supersonic designs for a U.S. fighter aircraft using a delta wing was the Convair F-102. In early tests in 1953, however, it was determined that this aircraft could not reach the supersonic speeds for which it had been designed. This led to a redesign of the aircraft based on the area rule concept developed by NACA engineer Richard Whitcomb. Whitcomb realized that when the air moving around an aircraft had reached the speed of sound, it had been compressed or squeezed together as much as it could be compressed and that the only way the flow could move around the airplane was to push the surrounding air out of the way, creating more drag. His theory said that this drag could be decreased if the airplane body or fuselage could be squeezed in enough to make room for the flow that had to go around the wing. The resulting fuselage shape became known as the “Coke-bottle” or “wasp-waist” design and its use in the redesign of the F-102 proved his theory.
When Whitcomb’s area rule is applied to modern high speed aircraft, the dramatic necking-in of the fuselage is not as evident as on the F-102. Designers have learned to blend wing and fuselage to give the required ideal variation of cross-sectional area or volume in more subtle ways.
Richard Whitcomb went on to develop other improvements in transonic and supersonic aircraft design, such as his supercritical airfoil of the 1960′s. The supercritical airfoil is shaped in such a way as to produce a weaker shock wave than older wing designs as it accelerates toward Mach 1 and places that shock closer to the airfoil’s trailing edge, thus reducing the transonic drag rise. This development is used on almost all high-speed subsonic and supersonic aircraft designed since the 1970′s and allows subsonic aircraft to fly closer to the speed of sound with less thrust and fuel consumption than was possible with older airfoil shapes.
The SST
Many supersonic aircraft have been designed and flown since the flight of the X-1, and supersonic flight is now commonplace. The Concorde, developed jointly by British Aerospace and Aerospatiale of France in the late 1960′s, flew in prototype form in 1969 and went into passenger service in 1976, allowing anyone who can afford its premium-priced ticket to cross the Atlantic Ocean at Mach 2. The United States and the Soviet Union also had programs to develop supersonic transports, with Boeing selected as the firm to build the American SST. Boeing later canceled the project, believing that there was insufficient demand to allow either an airline or a manufacturer to make a profit on such a plane. The Tu-144 resulted from the Soviet SST project, but only a few were built and the aircraft was withdrawn from service after several crashes. The Concorde was also temporarily withdrawn from service after almost twenty-five years of continual service after a crash on a takeoff from Paris on July 25, 2000, which was caused by debris on the runway piercing its fuel tanks.
Supersonic airliner flight has been limited by international agreement to travel over the oceans, limiting their usefulness to transatlantic and Pacific routes, and the Concorde does not have the range required for nonstop flights across the Pacific. At supersonic speeds, the twin shock waves coming from the leading and trailing edge of an SST’s wings can result in loud and destructive sonic booms at ground level due to the sudden pressure change across the shock. As a result, flight of SST’s over land has been forbidden and military supersonic flight is restricted to defined training areas.
NASA and companies such as Boeing and Airbus have continued to explore designs for supersonic passenger planes of the future but, as of 2001, none is on the way to production. There have also been explorations by several companies of the possibility of building a commercially successful supersonic business jet, and such a plane could be built by 2010.
