How Things Work: Supersonic Inlets

How Things Work: Supersonic Inlets

The Lockheed SR-71. (Lockheed Martin)
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Many other currently operational fighters also have boundary layer diverters. Air that clings to the surface of an aircraft in flight is known as boundary layer air, and it tends to cause turbulence in the air flowing into the engine, especially when it interacts with shock waves. Inlet designers try to keep out as much boundary layer air as possible, frequently positioning the inlet several inches away from the surface of the fuselage and its boundary layer air and employing a duct system to whisk the undesirable air away. (The SR-71 inlet rids itself of boundary layer air by sucking it in through slots on the spike and passing it through ducts that exit the nacelle.)

The F-35 inlet, however, is positioned flush against the fuselage, and just in front of the inlet opening is a raised surface, or bump, that pushes much of the boundary layer air off to the sides and away from the inlet. The bump serves another purpose: During supersonic flight, it compresses and slows the air passing over it into an oblique shock wave. The air is still moving supersonically, however, and it is slowed down to subsonic speeds after passing through a normal shock wave that forms at the mouth of the inlet. The simplicity of the JSF design makes for an inlet that requires less maintenance, reduces aircraft weight by 300 pounds, and costs $500,000 less than a traditional fighter inlet.

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