Inlet Cone - Alternative Shapes

Alternative Shapes

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 . Also there are plans to measure the air in front of the inlet to detect turbulence and adjust the inlet in real-time.