How do square nozzles reduce IR?

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collimatrix

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Unread post23 Jan 2019, 10:07

I was curious if anyone knew, and is allowed to discuss, how exactly the square nozzles reduce the IR plume of the engines on the F-22. Do the big chevrons generate vortices that stir cool air into the plume, like on the LOAN? Or is it something else? I note that the flat nozzles on the F-117 and B-2 have no obvious vortex-generating features.
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hornetfinn

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Unread post23 Jan 2019, 14:25

AFAIK, they flatten the exhaust plume which make the plume wider and shorter. They also probably generate vortices that mix the cool air with the plume. I think big thing is that engine hot parts are really well shielded from most viewing angles.

Interesting patent: https://patents.google.com/patent/US6000635A/en
and interesting article relating to this: http://www.jmargolin.com/laser/ref7_AWS ... -IR-bw.pdf
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collimatrix

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Unread post24 Jan 2019, 13:28

Thanks!

I wonder if LOAN makes flat nozzles obsolete.
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charlielima223

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Unread post27 Jan 2019, 06:24

collimatrix wrote:Thanks!

I wonder if LOAN makes flat nozzles obsolete.


From my understanding the LOAN was the advantage of being easier to maintain and offers lighter structural weight over the flat serrated nozzles the F-22 uses. However this is a bit of an apples to oranges comparison in some cases. Obviously the LOAN isn't optimized for TVC unlike that of the F-22 which blended TVC along with radar and some IR low observable features. I would think that it is improvement over the conventional "turkey feathers"
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charlielima223

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Unread post27 Jan 2019, 06:42

I would think that LM and Pratt&Whitney engineers also added some active IR suppressing measures like introducing ambient air into the exhaust stream

Image

note the gap of separation in the early YF-119 engine test/prototype model
Image

I've also heard or read somewhere that the F-22 has a longer exhaust pipe possibly where cooler ambient air is introduced into the exhaust.
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collimatrix

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Unread post27 Jan 2019, 09:25

In theory, the larger a nozzle is the lower an IR signature it should have. The closer the nozzle can get to ideal isentropic expansion, the cooler the exhaust gas should be. Excess heat in the exhaust plume is, after all, energy that isn't being used to make thrust. I'm sure this runs into all sorts of practical limitations.
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SpudmanWP

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Unread post27 Jan 2019, 09:52

charlielima223 wrote:I would think that LM and Pratt&Whitney engineers also added some active IR suppressing measures like introducing ambient air into the exhaust stream
That's pretty much what a 3-stream engine is.
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