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Why The CD nozzles of the fighter aircraft are always outside the fuselage, Why the whole engine is not buried in the fuselage??

If engine shroud also can be made variable area and varies with CD nozzle will it be possible to bury the whole engine inside fuselage???

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  • $\begingroup$ Can you think of any advantage to putting a shroud around the nozzle? $\endgroup$ – Hobbes Feb 18 '19 at 10:47
  • $\begingroup$ To reduce IR signature??? $\endgroup$ – Ashish Bhatt Feb 18 '19 at 11:42
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Some aircraft do what you're proposing, like the F-22 (photo below) and the F-117. So the technique has been known since the 1970s.

enter image description here

These are exceptions though. To effectively reduce the IR signature, you don't just need a shroud, you need to cool the exhaust stream. This means the design of the aft fuselage gets much more complicated, and it gets heavier (and if you use hydraulics to shape the shroud around the exhaust, that adds more weight).

In all designs, there are tradeoffs to be made. If a design feature isn't common, that usually indicates there are drawbacks, in this case the improvement in IR signature doesn't outweigh increased weight and complexity.

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  • $\begingroup$ The nozzle in the F-22 is still at the exit. It’s a variable area 2D converging-diverging nozzle. The reason you do not put the nozzle inside the fuselage, is because then you need a duct after that. Within that duct, the static pressure isn’t equal to ambient pressure, because of the constraining effect of the duct. So, the gas isn’t expanding to ambient static pressure through the nozzle, and hence the nozzle’s performance is significantly reduced. The exit at the end of the duct will still act like a nozzle. If that’s a fixed area, it will not work well over a high speed range $\endgroup$ – Penguin Feb 19 '19 at 9:08
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With an afterburning engine you can't bury the engine forward and use a long duct to the exhaust nozzle because the length of the afterburner chamber is limited, and the afterburner nozzle has to be the last thing the supersonic air passes through. If there is no afterburner, the exhaust duct can be fairly long with a simple convergent nozzle at the end (like an F-86, which has its engine midships with a long exhaust duct).

The nozzle on an afterburning engine does double duty. Burner off, it's a normal taipipe with simple convergent nozzle tip. Burner on, the nozzle morphs into a venturi shape (a convergent/divergent duct, like a rocket exhaust) to manage the supersonic flow created by the afterburner. So the nozzle has to be at the back, and the engine has to be close to the nozzle, how close determined by the length of the afterburner chamber.

You could put an afterburning engine amidships, but you'd then have to have a high tail to clear the exhaust nozzle which now has to be ahead of the tail (like an F-101 Voodoo). But if you want to also use the nozzles for steering, you want them as aft as possible to get the most pitch authority. So the engines on the F-22 end up at the tail and the only airplanes you see with fully buried engines and longish shrouded exhaust ducts (F-117 and B-2) are subsonic a/c without afterburners.

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  • $\begingroup$ Did you mean F-117 in the last sentence? F-111 Aardvark is a swing-wing low-penetration light bomber that has afterburner and is supersonic. F-117 Nighhawk, on the other hand, is the ugly edgy first stealth and is subsonic. $\endgroup$ – Jan Hudec Feb 19 '19 at 6:54
  • $\begingroup$ Yes. Thanks Jan. $\endgroup$ – John K Feb 19 '19 at 13:55

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