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Inlet cones (sometimes called shock cones or inlet centerbodies NASA Dryden Centerbody inlet for F-15) are a component of some Sound barrier aircraft and missiles. They are primarily used on , such as the D-21 Tagboard and Lockheed X-7. Some turbojet aircraft including the Su-7, MiG-21, English Electric Lightning, and SR-71 also use an inlet cone. The inlet cone for circular/axisymmetric inlets has its equivalent in the intake ramp for 2-D/rectangular inlets.
An inlet with cone may be used to supply high pressure air for ramjet equipment which would normally be shaft-driven on a turbine engine, eg to drive turbopumps for the fuel pump on the Bristol Thor ramjet and hydraulic power on the Bristol Bloodhound missile.
The conical body may be a complete cone centerbody in a round inlet (MiG-21), a half cone in a side-fuselage inlet (Lockheed F-104 Starfighter) or a quarter cone in a side-fuselage/underwing inlet (General Dynamics F-111 Aardvark).
The rear of the cone beyond its maximum diameter, rear-facing and unseen inside the duct, is shaped for a similar reason to the protruding front part. The visible cone is a supersonic diffuser with a requirement for low loss in total pressure, and the rear, streamlined part, together with the internal surface profile of the duct, forms the subsonic diffuser, also with a requirement for low loss in total pressure as the air slows to the compressor entry Mach number.
For Mach numbers below about 2.2 all the shock compression is done externally. For higher Mach numbers part of the supersonic diffusion has to take place inside the duct, known as external/internal or mixed compression. In this case the rear part of the forward-facing conical surface, together with the internal surface profile of the duct, continues the supersonic diffusion with reflected oblique shocks until the final normal shock. In the case of the Lockheed SR-71 Blackbird with part of the supersonic compression taking place inside the ducting the spike and internal cowl surfaces were curved for gradual isentropic compression."Supersonic Inlet For Jet Engines' David H.Campbell, Lockheed Aircraft Corporation, United States Patent Office 3,477,455 The inlet cone also has different axial positions to control how the capture area varies with the duct internal throat area. For best intake operation this required area ratio gets bigger with increasing flight Mach number, hence the large inlet cone movement on the SR-71 which had to perform well from low speeds to Mach 3.2. On the SR-71 the cone moves back at higher speeds.
For higher flight speeds a moving cone becomes necessary to allow the supersonic compression to occur more efficiently over a wider range of speeds. With increasing flight speed, in typical Oswatitsch-type supersonic moving-cone inlet - the cone is moved forward (MiG-21), and if it is non-Oswatitsch-type cone inlet (SR-71) it is moved to the rear, or into the intake. In both cases, due to the shape of the cone surface and the internal duct surface the internal flow area gets less as required to continue compressing the air supersonically. The compression occurring in this path is called "internal compression" (as opposed to the "external compression" on the cone). At the minimum flow area, or throat, a normal or plane shock occurs. The flow area then increases for subsonic compression, or diffusion, up to the engine face.
The position of the cone within the intake is usually controlled automatically to keep the plane shock wave correctly located just downstream of the throat. Certain circumstances such as an engine surge can cause the shock wave to be expelled from the intake. This is known as an unstart.
Other supersonic aircraft such as the Concorde, Tu-144, F-15 Eagle, MiG-25 Foxbat, and the A-5 Vigilante use so-called 2D inlets, where the nacelle is rectangular and a flat intake ramp replaces the dual cones. The ramp is similarly adjusted in flight to ensure that the oblique shocks are properly positioned for efficient pressure recovery; such designs can also have supersonic compression be either all external, or mixed external/internal with the XB-70 Valkyrie being an example of the latter.
Some other supersonic aircraft (Eurofighter Typhoon) use a variable lower cowl liphttp://data3.primeportal.net/hangar/luc_colin3/eurofighter_typhoon_ehlw/images/eurofighter_typhoon_ehlw_58_of_59.jpg for high angle of attack operation and a bleed system (porous wall) incorporated on the intake ramp to facilitate stabilization of the shock system at supersonic Mach numbers. For the improvement of the intake flow (reduced distortion), air is dumped via an intake bleed slot on the ramp side downstream of the intake. The ramp, which is separated from the fuselage by a diverter, produces an oblique shock in order to decelerate the flow. The leading edge of the splitter plate separating the two intakes is located downstream of this oblique shock.
Many supersonic aircraft (F-16 Fighting Falcon, F/A-18 Hornet) dispense with the conical centrebody or complex variable ramps and employ a simple fixed-geometry Pitot tube intake, which is structurally lighter and more durable; a detached, strong normal shock appears directly in front of the inlet at supersonic flight speeds, which leads to poorer pressure recovery especially at higher Mach numbers. This was considered an acceptable tradeoff for aircraft that mainly operate in subsonic and transonic airspeeds with only transient supersonic dashes.
In newer aircraft, advances in aerodynamics have enabled fixed-geometry inlet designs to match the performance of variable inlet cones or ramps through careful shaping of the inlet geometry and using downstream pressure to control shock position. Examples include swept caret inlet ramps and cowls (F-22 Raptor, F/A-18 Super Hornet), which has a pair of fixed oblique ramps and a downstream bleed system to control and avoid shocks. Another is the diverterless supersonic inlet (F-35 Lightning II), which has a 3-D (non-axisymmetric) compression bump that acts similarly as a fixed half-cone to avoid shocks while also diverting the forebody boundary layer.
NASA has tested an alternative to the external/internal, or mixed compression inlet, needed for speeds above about Mach 2.2 (below that speed inlets with all-external compression are used). The mixed-compression inlet is susceptible to unstarts or expulsion of the internal shock to in front of the inlet. The NASA inlet, which they call a Parametric Inlet, does all the supersonic compression externally so there is no shock inside the ducting in a potentially unstable location.
== Different types of inlet cone ==
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