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Engine Air Intakes
editThe speed of air entering the compressor of a normal jet engine must not be supersonic, even for supersonic aircraft. While this does not affect subsonic aircraft, supersonic aircraft face a problem: current turbine engines are unable to handle the shock waves associated with supersonic airflow, and might be damaged if supersonic airflow passes through turbine blades.
Thus, on aircraft traveling above the speed of sound, the airflow must be slowed down to subsonic speeds before entering the engine. In modern jet aircraft, this is done in a variety of ways through design of the inlet. The complexity of these inlets increases with an increase in top speed. Planes with top speeds over Mach 2 require much more elaborate inlet designs. Thus, most modern aircraft today rarely exceed the Mach 2 speed [dubious – discuss], usually limiting their top speed to Mach 1.8-2.0.
To sustain the flow mechanics of the compressor and turbine blades, the air reaching the compressor of a normal jet engine must be traveling below the speed of sound, even for supersonic aircraft.
The Air intake (Inlet) — The standard reference frame for a jet engine is the aircraft itself. For subsonic aircraft, the air intake to a jet engine presents no special difficulties, and consists essentially of an opening which is designed to minimize drag, as with any other aircraft component. At supersonic flight speeds, shockwaves form in the intake system and reduce the recovered pressure at inlet to the compressor. So some supersonic intakes use devices, such as a cone or ramp, to increase pressure recovery, by making more efficient use of the shock wave system.
Compressor or Fan — The compressor is made up of stages. Each stage consists of vanes which rotate, and stators which remain stationary. As air is drawn deeper through the compressor, its heat and pressure increases. Energy is derived from the turbine (see below), passed along the shaft.
Development
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Used
editThe DSI bump functions as a compression surface and creates a pressure distribution that prevents the majority of the boundary layer air from entering the inlet at speeds up to Mach 2. In essence, the DSI does away with complex and heavy mechanical systems.
The Inlet Far ahead of the airplane, the speed of the air relative to the engine is equal to the airplane's flight speed. Our air volume enters the inlet and slows down considerably. A well-designed inlet will straighten out the flow, leaving it uniform and without much turbulence. This is important because compressors and fans need to be fed distortion-free air. Supersonic inlets, found on many military jets, are usually much more complex and use shock waves to slow down the air.[1]
The DSI concept was introduced into the JAST/JSF program as a trade study item in mid-1994. It was compared with a traditional "caret" style inlet. The trade studies involved additional CFD, testing, and weight and cost analyses. The new inlet earned its way into the JSF design after proving to be thirty percent lighter and showing lower production and maintenance costs over traditional inlets while still meeting all performance requirements.
The overall inlet design, called a diverterless supersonic inlet or DSI, moved from concept to reality when it was installed and flown on a Block 30 F-16 in a highly successful demonstration program.
The new inlet showed slightly better subsonic specific excess power than a production inlet and that verified the overall system benefits of eliminating the diverter. Test pilots remarked that military power settings and thrust characteristics were very similar to standard production F-16 aircraft with the same General Electric F110-GE-129 engine. Considering the overall goal of the flight test program was to demonstrate the viability of this advanced inlet technology, the results were excellent. Diverterless Inlet The F-35's diverterless inlet lightens the overall weight of the aircraft. Traditional aircraft inlets were comprised of many moving parts and are much heavier than newer diverterless inlets. The diverterless inlet also eliminates all moving parts.[2]