Following from our previous discussion on contrails, Helen on the Taking Flight fear of flying support forum asked about what looked like smoke in the front of a jet engine at takeoff, and posted an interesting video from Youtube.  Here’s the video, and my reply. (The “smoke” appears about 1:30 into the video)

[youtube]http://www.youtube.com/watch?v=Ii0nUcm24Jw[/youtube]

Inside the engine at the front is a big fan which is sucking in air. As this air is sucked in its pressure is lowered by the suck and because it is getting funnelled into a small hole, so its temperature decreases a little. With just the right humidity the reduction in pressure and temp causes condensation to occur, and that’s what you are seeing.

You don’t usually see this because the humidity needs to be “just right” for this effect to occur.

Interestingly, you can see condensation occurring over the entire length of the wing. The whole “Bernoulli” principle of how the wing works states that you get an area of low pressure over the top of the wing, because of it’s shape and the angle it makes through the air (air moving faster over the top of the wing causes low pressure)… it is one way of looking at how the airplane wing works, and the condensation we see in your video proves this to be true (low pressure = low temperature = condensation if high humidity).

Very cool demo!