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WeekendFlyer
7th Jun 2014, 20:01
Quick question for any guru-level aerodynamicists or engine intake designers out there. If a subsonic jet engine intake for a high bypass turbofan engine (e.g. as used on most airliners) experiences freestream airflow above Mach 1, what happens? I realise a normal shock will form across the inlet or just inside it but will that be all, or will there be some nasty destructive shock patterns inside the inlet as well? I was wondering if the inlet profile could effectively be seen as a convergent-divergent nozzle by the airflow, thus resulting in supersonic flow entering the divergent duct.

Also, why are smaller pitot intakes (such as those seen on Hawk, Jaguar, etc) fine to use above Mach 1? Clearly they have much sharper lips to the intake and the normal shock sits across the whole inlet area, so presumably behind it the air is all subsonic and the rest of the intake behaves as it would at subsonic speeds.

Any references or diagrams would be much appreciated!

Owain Glyndwr
8th Jun 2014, 06:28
If the intake has been sized to match engine requirements in subsonic cruise then it will be too big for the engine at Mach 1~1.2 ish. That will mean that excess airflow has to be spilled out of the front and the intake will develop a strong normal shock which stands off the intake LE. Flow behind that shock would be subsonic everywhere so there would be no shocks inside the intake itself. In effect it becomes a subsonic intake operating behind a shock wave that transforms the oncoming supersonic flow into something the intake can handle, albeit with significant loss of total pressure and increased drag.

So far as I can see, the same is true of the pitot intakes used on early fighters, but because of the sharper leading edges the bow shock wave would be weaker.

underfire
8th Jun 2014, 11:22
Pretty sure the the smaller pitot type intakes have a spill door on the intake....this allows for subsonic flight.

With subsonic engines as you are talking about, look at the mechanism that creates the shockwave. When Mach increases, the spillage goes supersonic, creating oblique shockwaves around the exterior of the cowling.

Should a high bypass engine be designed to go to those Mach levels, the intake structure would be designed to be variable, so that as Mach increases, bypass flow is variable...maintaining correct pressures and flow.

As Owain stated, its all about maintaining the correct pressures.

John Farley
8th Jun 2014, 16:22
For what it is worth the standard production Sea Harrier FRS1 would reach M1.3 in a dive wth no aparent handling or engine abnormalities. But please don't ask me to explain why that should be so! The Pegasus turbofan concerned had a bypass ratio of some 1.3 and the only doors in the intake were typical boundary layer bleed doors. The row of suck in doors (used at low speed) were of course well and truly shut.

Mozella
9th Jun 2014, 06:32
HERE (http://naca.central.cranfield.ac.uk/reports/arc/rm/3565.pdf)is a link about intakes and shock waves you might find interesting.

It's one thing to drive a transonic aircraft using a high bypass fan engine to low supersonic mach numbers in a steady state dive. It's quite something else to then ask that engine to behave nicely at high angles of attack both in pitch and yaw or to expect to operate at even higher mach numbers.

To cite one example of supersonic fan engines, the F-14 intake was a marvel of efficiency back in the day, but it was VERY complicated. The TF-30 didn't take kindly to uneven pressures across the face of the compressor/fan. So, Grumman's answer was the fancy intake system discussed HERE (http://www.anft.net/f-14/f14-detail-airintake.htm). Part of the reason for a complicated intake is that unlike some high bypass afterburning engines, the fan air was not bypassed around the outside of the afterburner but was fed into the air exiting the turbine. In other words, the fan air fed the burner too and that tended to make the already fussy engine more unstable; hence the need for a sophisticated intake to insure good airflow under all conditions including short periods of flying backwards. I flew that aircraft and I still remember the "tail slide" which was part of my first familiarization flight; no big deal as it turns out. I also remember the engine failure I had on the first flight. :ugh: Again no big deal but something of a disappointment.

If there was any way to make this engine behave properly with a simple airliner-style intake, believe me they would have done so. I suspect the intake system and it's associated software cost more then the engine.

pattern_is_full
9th Jun 2014, 17:32
Concorde used a similar intake to the F-14 as described by Mozella.

It is an interesting side question that the inlets for both planes are boxy and rectangular. I don't know if this was simply to accomodate simpler flat inlet ramps and vents, or if there is some aerodynamic advantage at supersonic speeds to having rectangular knife-edge nacelles/inlets.

The XB-70 also had a box inlet - but with a splitter knife-edge extending forward between the intakes. This configuration produced compression lift, where the shock-compressed air pushed up on the wing from below - essentially, a bit of "ground effect" you could carry along with you.

A lot of early Mach+ planes (Mig-21, Electric Lightning, SR-71) had central inlet cones or spikes, in a round inlet, to slow intake air to subsonic speed - often movable fore and aft, to adjust for the shape of the shock wave at different speeds, and change the volume of the inlet. Some planes (Mirage III) with engines in the fuselage used semicircular side inlets with "half cones."

A recent development is the "diverterless inlet" for fuselage inlets, which uses a forward-swept inlet edge, in combination with a fuselage "bump" inside the inlet (analogous to the Mirage half-cone) to control the shock waves and intake air speed.

http://aviationintel.com/wp-content/uploads/2012/10/20120218110258_3.jpg

The F-35 uses this approach, as do several recent Chinese fighters.