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I was reading a couple of threads on the site I learned that the air flow going into the intake needs to be at subsonic speeds. Which is why the F-16 intake is slightly seperated from the main body and why the F-15 and F-14 have movable flaps in front of their intakes. But why can't the air going into the intake be at supersonic speeds. I have my own theories but they dont completely make sense to me.

Thanks for the info guys

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The air entering the intake can be supersonic, but the flow must be reduced to subsonic speeds before it reaches the turbine inlet face. To explain why would require an understanding of compressible flow theory.

The intake is slightly separated from the fuselage to avoid ingesting the turbulent boundary layer flow near the fuselage. It has nothing to do with supersonic flow or speed. This is not unique to the F-16, it is a design feature found on virtually every modern jet aircraft.

Last edited by allenperos on Jul 09, 2005 - 05:21 PM; edited 1 time in total

Habu2 has it right, except I think he meant to say instead of air going into the turbine inlet, it is the fan inlet and variable inlet guide vanes first. To let supersonic air flow enter the powerplant would have negative consequences due to the fact that the airflow now becomes incompressible, reversing "Bernouli's" law. What this means is airflow will now come out the front instead of the rear, a compressor stall and it sounds like a big bang.

Rotor tip speed may be supersonic, and if this occurs, you can get compressor stall, every engine has N1 and N2 maximum values, unless it is a Rolls Royce engine, that has a third spool, N3. Refer to Irwin Treagor's "Jet Engine Technology".

Yes, I meant the "front face of the engine" Embarassed

The geometry of the F-16 inlet introduces a series of shock waves which slow the flow to subsonic before it reaches the engine. This design is supposedly good up to about M2.5, but other factors (unrelated to the inlet design) restrict the Viper's top speed to ~M2.05.

Others, like the F-14 and F-15, use variable ramps in the inlet to more accurately position and control the shock waves at higher speeds.

Thanks for the answers guys. I know I have a lack of understanding when it come to compressible flow theory and things of this nature. You guys helped alot and I think I have the basic understanding of it, so thanks for your replies.

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Air speed into the intake