| Inlet cone | |
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| Inlet cone of a MiG-21MF |
Inlet cones (sometimes called shock cones or inlet centerbodies[1]) are a component of some supersonic aircraft. They are primarily used on ramjets, such as the turboramjets of the SR-71 or the pure ramjets of the D-21 Tagboard and Lockheed X-7. Some conventional turbojet aircraft including the Su-7 Fitter and the MiG-21 Fishbed also use an inlet cone.
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The main purpose of an inlet cone is to slow the flow of air from supersonic flight speed to a subsonic speed before it enters the engine. Except for scramjet engines, all airbreathing jet engines need subsonic airflow to operate properly, and require a diffuser to prevent supersonic airflow inside the engine. At supersonic flight speeds a conical shock wave, sloping rearwards, forms at the apex of the cone. Air passing through the conical shock wave (and subsequent reflections) slows to a low supersonic speed. The air then passes through a strong normal shock wave, within the diffuser passage, and exits at a subsonic velocity. The resulting intake system is more efficient (in terms of pressure recovery) than the much simpler pitot intake.
The inlet cone is shaped so that the shock wave that forms on its apex is directed to the lip of the intake; this allows the engine to operate properly in supersonic flight. As speed increases, the shock wave becomes increasingly more oblique (the cone gets narrower). As a result, some inlet cones (for example, on the SR-71) are designed to move axially to maintain the shock-on-lip and allow efficient operation over a wider range of flight speeds. On MiG-21 this movement also makes the opening between the intake (diffusor) lip and the cone smaller, preventing the inside of the intake section from bursting open from dynamic air overpressure at the aircraft's highest speeds above Mach 2.
At subsonic flight speeds, the conical inlet operates much like pitot intake, it sucks air. However, as the vehicle goes supersonic a conical shock wave appears, emanating from the cone apex. Conical (and oblique) shock waves are akin to the bow wave on a ship. As the flight Mach number increases, the conical shock wave becomes more oblique and eventually impinges on the intake lip.
With increasing mach number the cone is not moved out of the inlet as it would make sense for the external compression, but inwards, because the cone sits in the cowl like a plug. By moving it inwards the path between the cone and the inlet narrows as needed for higher speed. The compression occurring in this path is called "internal compression" (opposed to the "external compression" on the cone) and can be subject to stall.
Care must be taken to prevent the normal shock wave, which lies behind the throat of the diffuser, coming forward through the throat, replacing the oblique shocks and reducing stagnation pressure and leading to excessive inlet temperatures burning the compressor.
To restore the engine after a surge the computer has to change inlet, turbine and nozzle geometry, fuel injection and counteract the yaw on a two engine airplane, by means of control surfaces and throttling back the second engine.
The boundary layer on the cone is stretched as it moves up the cone preventing flow separation, but for the internal compression and the subsonic compression the boundary layer still tends to separate and usually is sucked through tiny holes in the wall. As a side note on the aerospike engine the boundary layer gets thicker towards the end of the cone as needed for the greater speed difference between the air molecules just on the surface of the cone and the fully accelerated stream of air.
Some air inlets feature a biconic centrebody to form two conic shock waves, both focused on the lip of the intake. This improves pressure recovery. Some aircraft (F-35 Lightning II, Mirage III) use a semi-conic centrebody.
Concorde, F-15 Eagle, MiG-25 Foxbat, and the A-5 Vigilante use so-called 2D inlets, where the nacelle is rectangular and a flat inlet ramp replaces the dual cones just described. These allow good pressure recovery without spillover through the full range of speeds by matching their angle to the mach angle, but have problems with seals at high speed that is high pressure and temperature (like in a diesel engine). Inlet ramps allow for swept inlet cowls (F-22 Raptor, F-35 Lightning II) to avoid shocks. They are sealed by viton or a metal sheet bend in direction of the higher pressure. At least one supersonic and one subsonic ramp is used, but for improved seal multiple supersonic ramps can be used. The boundary layer (something which the subsonic pitote inlet avoids by external compression) tends to separate and the smaller boundary layer of the ramp inlet is an advantage compared to the inlet cone. To avoid separation vortex generators are used, which mix the boundary layer with the free flow (or the boundary layer is sucked away through a porous surface, leading to drag). After the fan the hot slow intermixed air is passed by the engine, while the fast cold air is delivered to the engine.
After the engine the comparatively cold bypass air is used as an isolation between the engine exhaust and the walls. Again two ramps can be used to form a variable supersonic nozzle. Often a mirror-symmetric set-up is used with ramps on top and on the bottom.
There is one possibility for a stable, shockless supersonic to subsonic transition. This is used in transonic wings and would ultimately mean to send the air into a loop, forming a vortex. Then the final shock to subsonic speed is oblique with the subsonic region moving from the outside of the vortex to the inside.
Many supersonic aircraft (Eurofighter Typhoon, F-16 Fighting Falcon) dispense with the conical centrebody and employ a simple pitot intake. A detached, strong normal shock appears directly in front of the inlet at supersonic flight speeds, which leads to poor pressure recovery.
Also NASA adds a gap through the whole compressor. Supersonic flow jumps over it by means of ramps, while subsonic flow is able to turn and exit through the gap. In this way a stall is easier to remove [1]. Also there are plans to measure the air in front of the inlet to detect turbulence and adjust the inlet in real-time.
As the compression of the inlet raises with speed the compression of the first compressor stage is reduced accordingly. The afterburner behind a turbine runs with a stoichiometric mixture like a ramjet but at higher pressure and thus more efficiency than a pure ramjet. It is claimed that an inlet at Mach 3.5 produces the same compression (44:1 [2]) as the whole compressor of a jet engine at zero speed, so the turbine should be bypassed then.
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