A supersonic aircraft is an aircraft able to fly faster than the speed of sound (Mach number 1). Supersonic aircraft were developed in the second half of the twentieth century and have been used almost entirely for research and military purposes. Only two, Concorde and theTupolev Tu-144, were ever designed for civil use as airliners. Fighter jets are the most common example of supersonic aircraft, although they don't always travel at supersonic speed.
The aerodynamics of supersonic flight is called compressible flow because of the compression (physics) associated with the shock wavesor "sonic boom" created by any object travelling faster than sound. Aircraft flying at speeds above Mach 5 are often referred to as hypersonic aircraft. Supersonic flight brings with it substantial technical challenges, as the aerodynamics of supersonic flight are dramatically different from those of subsonic flight (i.e., flight at speeds slower than that of sound). In particular, aerodynamic drag rises sharply as the aircraft passes the transonic regime, requiring much greater engine power and more streamlined airframes. Wings[edit]To keep drag low, wing span must be limited, which also reduces the aerodynamic efficiency when flying slowly. Since a supersonic aircraft must take off and land at a relatively slow speed, its aerodynamic design must be a compromise between the requirements for both ends of the speed range. One approach to resolving this compromise is the use of a variable-geometry wing, commonly known as the "swing-wing," which spreads wide for low-speed flight and then sweeps sharply, usually backwards, for supersonic flight. However, swinging affects the longitudinal trim of the aircraft and the swinging mechanism adds weight and cost, so it is not often used. Another technique that has been used is a delta-wing design, such as used on Concorde. This has the advantage that it can attain a high angle of attack at low speeds, which generates a vortex on the upper surface which greatly increases lift and gives a lower landing speed. Other kinds of wings are the Short thin wing, Sweep back wing and the swept forward wing. Heating[edit]Main article: Aerodynamic heating Another problem is the heat generated by friction as the air flows over the aircraft. Most supersonic designs use aluminium alloys such as Duralumin, which are cheap and easy to work but lose their strength quickly at high temperatures. This limits maximum speed to around Mach 2.2. Most supersonic aircraft, including many military fighter aircraft, are designed to spend most of their flight at subsonic speeds, and only to exceed the speed of sound for short periods such as when intercepting an enemy aircraft or dropping a bomb on a ground target. A smaller number, such as the Lockheed SR-71 Blackbird military reconnaissance aircraft and the Concorde supersonic civilian transport, are designed to cruise continuously at speeds above the speed of sound, and with these designs the problems of supersonic flight are more severe. Engines[edit]Many early supersonic aircraft, including the very first, relied on rocket power to provide the necessary thrust, although rockets burn a lot of fuel and so flight times were short. Early turbojets were more fuel-efficient but did not have enough thrust and some experimental aircraft were fitted with both a turbojet for low-speed flight and a rocket engine for supersonic flight. The invention of the afterburner, in which extra fuel is burned in the jet exhaust, made these mixed powerplant types obsolete and none entered production. The turbofan engine passes additional cold air around the engine core, further increasing its fuel efficiency, and most supersonic aircraft have been powered by turbofans fitted with afterburners. Supersonic aircraft usually use low bypass turbofans as they give good efficiency below the speed of sound as well as above; or if extended supercruise is needed turbojetengines are desirable as they give less nacelle drag at supersonic speeds. Another high-speed powerplant is the ramjet. This needs to be flying fairly fast before it will work at all. The Pratt & Whitney J58 engines of the Lockheed SR-71 Blackbirdoperated in mixed modes, taking off and landing as turbojets, using the afterburner to accelerate to higher speeds when the inner jet core was shut down and all the air was fed round the bypass duct to the afterburner, so that the engine now operated as a ramjet. This allowed the Blackbird to fly at well over Mach 3, faster than any other production aircraft. The heating effect of friction at these speeds meant that a special fuel had to be developed which did not break down in the heat and clog the fuel pipes on its way to the burner. Transonic flight[edit]Main article: Transonic flight Airflow can speed up or slow down locally at different points over an aircraft. In the region around Mach 1, some areas may experience supersonic flow while others are subsonic. This regime is called transonic flight. As the aircraft speed changes, pressure waves will form or move around. This can affect the trim, stability and controllability of the aircraft, and the designer needs to ensure that these effects are taken into account at all speeds. Hypersonic flight[edit]Main article: Hypersonic flight Flight at speeds above about Mach 5 is often referred to as hypersonic. In this region the problems of drag and heating are even more acute. It is difficult to make materials which can stand the forces and temperatures generated by air resistance at these speeds, and hypersonic flight for any significant length of time has not yet been achieved.
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