thumb|upright=1.14| [[MiG-21|MiG-21MF inlet cone]]

Inlet cones (sometimes called shock cones or inlet centerbodies) are a component of some supersonic aircraft and missiles. They are primarily used on ramjets, 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.

Purpose

An inlet cone, as part of an Oswatitsch-type inlet used on a supersonic aircraft or missile, is the 3D-surface on which supersonic ram compression for a gas turbine engine or ramjet combustor takes place through oblique shock waves. Slowing the air to low supersonic speeds using oblique shocks generated by a cone minimizes loss in total pressure (increases pressure recovery). Also, the cone, together with the inlet cowl lip, determine the area which regulates the flow entering the inlet. If the flow is more than that required by the engine then shock position instability (buzz) can occur. If less than that required then the pressure recovery is lower which reduces engine thrust.

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.

Shape

The cone angle is chosen such that, at the design condition for the inlet (Mach 1.7 for the English Electric Lightning inlet), the shock wave that forms on its apex coincides with the cowl lip. The inlet passes its maximum airflow and achieves its maximum pressure recovery. A higher design speed may require two oblique shocks focussed on the lip to maintain an acceptable pressure recovery and pass maximum airflow. In this case a biconic cone is required with two angles ( the Bristol Thor ramjet has 24 and 31 degrees for a design speed of Mach 2.5). For higher speeds a more smoothly contoured transition between cone angles may be used in what is known as an isentropic spike (Marquardt RJ43 ramjet).

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. 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 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 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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File:English Electric Lightning F1 - Flickr - p a h.jpg|Lightning fixed cone

File:2H8A8730 (11020981025).jpg|F-111C quarter-cone

File:Su-22M-3 HuAF 2.jpg|Su-22 translating cone

File:Royal Military Museum Brussels 2007 279.JPG|Mirage 5 translating half-cone

File:SR71J58.png|SR-71 translating cone positions

File:BOMARC A Surface-to-Air Missile.jpg|Marquardt RJ43 ramjets with isentropic spike attached to Bomarc missiles

File:3M55 Yakhont Onyx SS-N-26 Armia 2018.jpg|P-800 Oniks

File:Old military hardware at Museum of Vladivostok Fortress 27.jpg|P-500 Bazalt anti-ship missile

</gallery>

See also

  • Index of aviation articles
  • Aerospike engine
  • Drag-reducing aerospike
  • Nose cone design
  • Splitter plate

References

  • Grundlagen der Gasdynamik; Klaus OSWATITSCH. Springer, 1976.
  • Benson, T. (2004). High Speed Aerodynamics Index. Retrieved Nov. 19, 2004.
  • Eden, P. & Moeng, S. (2002). Modern Military Aircraft Anatomy. Aerospace Publishing Ltd. .