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Can some or all helicopters have contrails? If only some, why only those? If not, why can they not? Pictures would be really helpful. I do know that turboprops and piston props can leave contrails, but I haven't been able to find anything about contrails on helicopters.

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    $\begingroup$ They don't typically fly high enough for contrails. $\endgroup$ – Ron Beyer Mar 3 at 21:19
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    $\begingroup$ You don't have to be high to leave contrails. I once saw a Cessna 150 with a short contrail while doing circuits at 1600' ASL on a cold -38C day. $\endgroup$ – Mike Sowsun Mar 3 at 22:10
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    $\begingroup$ Contrails behind Russian airliner at takeoff: i.stack.imgur.com/AyLnu.gif $\endgroup$ – Mike Sowsun Mar 3 at 22:16
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    $\begingroup$ It all depends on how you define the term "contrail". Conventionally they are considered to be the condensation of water vapor from jet exhaust at high altitudes. However, I have seen people argue that condensation of atmospheric moisture even at sea level from wingtip vortices is considered a contrail. $\endgroup$ – Michael Hall Mar 3 at 22:31
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    $\begingroup$ @MichaelHall Given that "contrail" is just short for "condensation trail"; it seems fair to me that the second definition is accurate. That seems to be how Wikipedia treats it as well. Nothing about the term "contrail" makes it clear that it has to be from exhaust. If anything, that's just a bias because of how much more we observe those contrails compared to vortex induced contrails. $\endgroup$ – JMac Mar 4 at 19:54
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If the temperature and conditions are right, yes.

Any aircraft which has a heat engine that burns hydrocarbon fuel, which is pretty much any reciprocating or gas turbine engine used in aviation, will emit carbon dioxide and water vapor as byproducts of combustion. It’s just a byproduct of the chemical reaction to produce heat energy in the engine. And when the temperatures and humidity in the ambient air are low enough and high enough respectively, the water vapor in the exhaust stream will freeze into ice crystals that remain suspended in the atmosphere for a sustained period of time.

You typically won’t see this at your average airport or from the average helicopter flying over you as the temperatures simply are not low enough at the altitude the aircraft operates at for contrails to properly form. That being said, see picture below.

enter image description here
Image source: https://koreaaero.tistory.com/40 (in Korean)

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    $\begingroup$ Carlo, that picture appears to be vortices trailing from the rotor tips. (and the aircraft is flying above translational lift) $\endgroup$ – KorvinStarmast Mar 4 at 14:10
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    $\begingroup$ @KorvinStarmast I'm not sure how you can state that conclusively. Contrails don't visibly connect directly to the engines (for example: upload.wikimedia.org/wikipedia/commons/4/47/…) and the movement of the rotors may mean they don't point directly back to the engines as a result. $\endgroup$ – ceejayoz Mar 4 at 14:12
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    $\begingroup$ Wooooow low altitude rotating chemtrail dispenser ! Sweet ! $\endgroup$ – Quentin H Mar 4 at 16:07
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    $\begingroup$ I very much doubt that those 'contrails' have anything to do with engine exhaust. As I asserted in my answer, the engine exhaust is hugely diluted and dispersed by the downwash and cannot possible leave a trail. The trails seen above are almost certainly from atmospheric moisture. $\endgroup$ – Fiddlesticks Mar 4 at 17:25
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    $\begingroup$ @Fiddlesticks Contrails from fixed wing aircraft are ‘hugely diluted by wingtip vortices as well. A helo will produce vortices, rotor wash or no, if the conditions are right. $\endgroup$ – Carlo Felicione Mar 4 at 17:59
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Contrails off the engine exhausts is unlikely. But in the right conditions, ie damp air, you can get contrails off the "wingtip" vortices, ie off the rotor blade tips.

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    $\begingroup$ Rotor vertices are not contrails i.e. condensed water vapor from engine exhaust $\endgroup$ – Carlo Felicione Mar 4 at 14:05
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    $\begingroup$ @CarloFelicione I think you're over-constraining the definition of "contrails" when you shouldn't be. "Contrail" is just short for condensation trail. Although they are typically produced by the water vapour from engine exhaust, it's not critical to their definition AFAIK. At least Wikipedia doesn't seem to think so, the article spends quite a bit of time talking about contrails due to changes in air pressure; which is what the contrails due to wingtip vortices would produce. $\endgroup$ – JMac Mar 4 at 19:52
  • $\begingroup$ @JMac "contrails" was originally a contraction of those two words, but now has a clearly understood definition related to exhaust - that is why the phrase "wing tip votex" exists seperately. $\endgroup$ – Mike Brockington Mar 5 at 12:44
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    $\begingroup$ @MikeBrockington The phrase "wing tip vortex" exists separately. A wing tip vortex isn't the same thing as a contrail or a condensation trail. A wing tip vortex can trigger a condensation trail because of the low pressure zone; but a wing tip vortex and the condensation it leaves behind are two different things. $\endgroup$ – JMac Mar 5 at 13:11
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In a word: NO.

All existing helicopters have their engine exhausts somewhere beneath the main rotor, and that means that the exhaust gases are hugely diluted and dispersed by the enormous downwash from the main rotor. It simply is not possible for a contrail to form under such dilution and turbulence.

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    $\begingroup$ @KorvinStarmast Note in the first photo what appears to be denser condensation at the bottom of the trail, in line of the engines. The first photo also debunks the idea that the rotor downwash wipes out any visible condensation put forward in this answer. $\endgroup$ – ceejayoz Mar 4 at 14:07
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    $\begingroup$ I think that this answer would be better if the first line is: "generally, no" But that's my opinion. $\endgroup$ – KorvinStarmast Mar 4 at 14:15
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    $\begingroup$ @GalacticCowboy Links to copyrighted material don't violate copyright law. Copying it over to Imgur does. Links have a tendency to die; on StackOverflow a links-only answer gets removed. I'm not familiar enough with the rules here to know if the same is true here. $\endgroup$ – ceejayoz Mar 4 at 22:51
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    $\begingroup$ @ymb1 Read that answer again. "That means that the images you upload are covered under our 'attribution required' license just like anything else you contribute." I cannot license someone else's photos under that Creative Commons license. $\endgroup$ – ceejayoz Mar 5 at 13:27

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