Supersonic airflow?

Question

I'm a little confused as to why an engine only works with subsonic flow? Also what effect does a shockwave have if it goes into the engine?

Answer 1

Supersonic and subsonic airflows are totally different in their behavior. Have a converging pipe with subsonic flow and the speed increases while pressure drops; but converging with supersonic airflow slows it down.

One can design an engine that would work with supersonic airflow, actually; but the point is that it would only work with supersonic airflow, and aircraft have to be able to operate at subsonic speed as well, if only so as to be able to takeoff.

Therefore, engines are designed as subsonic airflow machines (a lot easier that supersonic combustion, since supersonic would require careful design that may be adapted to a very narrow range of speeds--a compressor first stage that actually meets supersonic airflow would incur shocks within its blades--I believe only one engine ever had that capability, the Iroquois intended for the ill-fated late 1950's Arrow CF-105 interceptor) and if supersonic speed operation is required, air intake are designed with oblique shock compression ramps (that slows down air intake speed to subsonic), and tailpipe with convergent-divergent nozzle that brings exhaust gasses back to supersonic speed again.

Answer 2

When an aircraft hits the sound barrier the air cannot move out of the way fast enough, this causes the sonic boom - take a look at the bow of a ship the water begins to move away ahead of the ship. So effectively with an aircraft flying supersonic the air cannot be pushed away from the airframe and engine intakes fast enough and the air is therefore compressed to a very high density thereby causing a flame-out (just like Noch Einmal said).

If you go to Yahoo click on images then search sound barrier you will see some fantastic photos of aircraft as they break the sound barrier. If you are very technically minded you will find more in-depth explanations including the strange effects supersonic flight has on aircraft controls.

I have tried to explain this in simple terms and hope it helps.

Answer 3

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Answer 4

The shockwave and supersonic airflow will cause engine "flame-out".

To stop this happening, aircraft that are capable of supersonic flight, have intake ramps fitted to reduce the airflow speed into the engines.

Answer 5

Blow at a match.  The flame will probably go out.  At the combustion point in an engine, the air flow is about 25 fps.  

If a shockwave is formed in an engine intake, it will stall and push itself through the front of the engine.  This is because it is harder to travel through the engine than to go back out the way it came.