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enter image description hereno IR suppressor

enter image description here IR suppressor

All I know is that it blends the hot air before releasing it How does an IR suppressor help an aircraft avoid detection from a missile? Wouldn’t the entire helicopter show up on the IR?

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An IR sensor detects objects that are at a higher (or lower but that's not how missiles work) temperature than the surrounding environment, most of the helicopter won't be that much hotter or cooler than the air around it, so wouldn't distinguish it much unless you get very close. Jet engine exhaust is much hotter than the surrounding environment, so it shows up very clearly to an IR sensor from far away.

A suppressor diffuses hot exhaust into a larger air mass, lowering the temperature of the exhaust compared to the surrounding air before the sensor can see it. Lower temperature means less heat for a sensor to see. A suppressor also makes the infra-red picture fuzzier, i.e. less of a distinct point. The lower temperature and fuzzier image decreases the effective range for a sensor, meaning it would have to be closer to detect it. Suppressors often deflect exhaust, pointing it away from where a sensor would be expected to read, or shield the exhaust.

Suppressors don't prevent an IR detection in the same way that stealth doesn't prevent radar returns, but a suppressor means you need to get closer.

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    $\begingroup$ Larger volume of air I think you mean, not surface area? Or perhaps "mass" of air, because number of jiggling molecules is proportional to mass even at non-constant temperature. (And that's what matters for temperature of the mixture). Although volume around the chopper would be what matters for making it fuzzy, although from any given direction you're seeing the cross-sectional area of the parts that are "deep" enough to see. $\endgroup$
    – Peter Cordes
    Feb 13, 2020 at 14:10
  • $\begingroup$ That's a good point @PeterCordes, I've rephrased to say "air mass" instead as you suggest. $\endgroup$
    – GdD
    Feb 13, 2020 at 14:16
  • $\begingroup$ The key is in the spreading around bit, right? A can of paint is not going to provide the same intensity on a square mile as it would on a square foot, while there's still the same amount of paint. $\endgroup$
    – Mast
    Feb 13, 2020 at 19:28
  • $\begingroup$ That works as an analogy @Mast. Another way to say it is imagine you have 1 100 watt light bulb or 10 10 watt bulbs spread over a couple of square meters of space, which are you going to see from a distance? $\endgroup$
    – GdD
    Feb 13, 2020 at 19:33
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To add to GdD's fine answer:

The environment provides plenty of different heat sources for the missile to see. To get a target lock, the "signature" of the heatsource has to be of quite a specific kind. Anything simply hotter than the surroundings will not be sufficient. The specifics of the way a missile lock is aquired and maintained is of course classified, but as the example in the picture shows, a quite simple contraption quite effectively reduces the threat of IR missiles.

It mixes the exhaust with cooler air as you described, directs it away from the probable direction of the threat (surface to air) as GdD described and in addition to those, the rotor downwash blends the exhaust gasses even further to make the IR signature fuzzier.

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    $\begingroup$ The specifics are somewhat unknown, but I think it's safe to say the missile is looking for a bright beacon of heat compared to the background. Something warm or even somewhat hot is ignored - it needs to be very hot to get a lock, and any aircraft without countermeasures will provide that very hot exhaust. $\endgroup$
    – JPhi1618
    Feb 12, 2020 at 19:52
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    $\begingroup$ @JPhi1618 On older missiles (Sidewinder AIM-9 circa mid 70s comes to mind) I have seen the seeker track a person a few hundred feet away when fitted for a training mission (only the seeker section of the missile was loaded on the pylon). There was also an amusing event of a live firing that was confused by two flares and the missile went between them. $\endgroup$
    – Peter Smith
    Feb 13, 2020 at 11:11
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    $\begingroup$ Missiles may also expect the IR source to be steady. IR jammers like the AN/ALQ-144 contain a powerful IR source that is occluded by spinning vanes. $\endgroup$
    – StackOverthrow
    Feb 14, 2020 at 16:38

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