Comments on: Why do some planes leave long trails, but others don’t?

By: captfitch

captfitch — Tue, 24 Jan 2012 20:14:55 +0000

I can answer the engines question. All four engines should be operated as close as possible to each other at all times except possibly for taxi. The way to accomplish this is to vary the amount of fuel each engine recieves slightly as some engines run very slightly different than others. Now- on older engine the difference may be higher but if it’s hung on a 747 they should all be newer turbofan engines. But after the exhaust leaves the engines the aerodynamics have more affect on the trails than what the engines did.

By: vic.vega

vic.vega — Tue, 24 Jan 2012 15:54:17 +0000

Thanks for the quick reply and for the explanation Uncinus.That’s really interesting!:-)
Btw..from the superposition of frames it seems quite clear that what I thought at first (innermost persisting, outermost not) was not correct: in fact it’s more the other way round, although, as you pointed out, at a certain point it becomes really difficult to distinguish a trail from the near ones.
Thanks again!

By: Uncinus

Uncinus — Tue, 24 Jan 2012 15:30:43 +0000

Here I have stitched together some stills from the video, so you can see better what is going on:

I think what is happening is all the contrails essentially mix together. The two regions that persist longer are from the wingtip vortices. The low pressure in the vortex means the air is cooler faster, hence the ice crystals have longer to form, and end up slightly larger, and last longer.

This apparent four-into-two is not at all new. Something similar is found in the 1972 book Clouds of The World, section 11.2.4

https://picasaweb.google.com/Uncinus/CloudsOfTheWorld1972?feat=embedwebsite#5363662403468056450

By: vic.vega

vic.vega — Tue, 24 Jan 2012 14:16:32 +0000

Dear all, it would be nice to add some informations about the (not-so-common, perhaps) case in which one plane leaves, at the same time, contrails with different characteristics.
E.g., I’ve recently been pointed to an amateur video where there is a four-engine plane who leaves four contrails, one for every engine. However, the innermost two are “persistent”, while the outermost vanish after few seconds. Here it is:
http://www.youtube.com/watch?v=MFmvTOGGjyw

Of course, this is presented as a “proof” that there is something wrong going on, because, obviously, all the four engines are at the same altitude and the external conditions are the same.
As a scientist, I immediately thought about two possible explanations:
-) In a four-engine plane is not obvious that all the engines are driven at the same regime and some asymmetries (included possibly this one) could be related to this. To all the pilots who read this blog: is this assumption correct?It is possible/common to drive different engines (of the same plane) in different ways?
-) The outermost trails are influenced much more by the vortexes developed at the end of the wings, compared with the innermost ones.

I got interested in the question, so googled briefly but I couldn’t find that much about this kind of situations. It would be really nice to have some feedback here or to be pointed somewhere else (every link would be appreciated)
Cheers!!:-)

By: MikeC

MikeC — Sun, 22 Jan 2012 22:27:31 +0000

And then just browsing around I found this – RAF Typhoons making contrails – http://www.flickr.com/photos/fineclicks/6650336003/lightbox/