I was just peering at a Virgin Atlantic 747 through my binoculars no doubt as it was descending across London towards Heathrow and I noticed that now and then a sudden load of condensation would appear behind the wings. We all know that the engines will produce condensation at high altitudes known as contrails but has anyone seen any of this mysterious condensation appear right along the entire wing very suddenly and then disappear just as suddenly? It would be great if someone could come up with a possible explanation why this would happen to this particular boeing and not any of the other large planes floating about up there at the moment. Maybe something to do with the fact that it was raining a few hours ago and now the sun has come out?
If he's on the landing approach, it could be the wing flaps being operated leading to a change in pressure and what you describe happening.
http://en.wikipedia.org/wiki/Prandtl-Glauert_singularity It forms if you fly at near-supersonic speeds I think.
Hmm I don't think Virgin planes are quite that good. But yeah, in fact it did look a lot like the kind of condensation you see when planes fly at super sonic speeds. The difference is that the cloud is shaped flat behind the back edge of the wing rather than in a big cone behind the plane. It probably did have something to do with the flaps being adjusted. I can't say that I've ever seen it or seen pictures or videos or heard of it before though so I wondered if anyone else had?
A 747 on landing approach will not be flying fast enough to produce a shock collar. The Prandtl-Glauert singularity normally occurs near the speed of sound under normal atmospheric conditions. 747s have a cruising speed of about 567 mph, well below the speed of sound. I think you're just seeing some normal contrails caused by the difference in pressure as air flows over the wing. Edit: Here it is: condensation from wing tip pressure. According to Wikipedia, it can occur in jet liners during take off and landing. Perhaps you saw an ectoplasm? -monkey
The plane doesn't necessarily have to be going that fast to form vapour patterns as you describe - the levels of moisture in the air/humidity have a lot to do with it! Take a look at this video - F-22 Raptor demo at Point Mugu, California, in high humidity conditions. Not supersonic in this pass, but look at the vapour it pulls in turns... http://www.youtube.com/watch?v=Ba3-Sqgco7A
It must have been ectoplasm. When there weren't big clouds of vapour behind the plane there were actually trails of condensation from the wing tips. The big clouds must have formed when the flaps caused a drop in pressure in the air behind the wings causing a drop in temperature and thus condensing the moisture out of the air. Mystery solved. Edit: here's what I saw: http://www.airliners.net/open.file/856005/L/
Maybe as the air changes pressure (going over top surface of wing..?) it cools quickly, so the water condenses out of it. maybe this only works when the wings are at a certain angle to the airflow??
I see this all the time when flying (i tend to look out the window alot), usually on easyjet or ryanair 737s or airbus A300s.
Dah da-daa! (That's my entry music to science related threads btw). Ok, so actually I don't really know why this happens, but here's an educated reasoned guess: The air the aircraft was flying through was near dew-point, therefore a slight decrease in temperature (which can be caused by a drop in pressure) will cause the moisture to condense, forming a "cloud." However, the air was at such a temperature and moisture content that this wouldn't occur when the plane simply flew through the air, but only when it passed through turbulance would the pressure decrease enough for condensation to occur. Of course, after the plane can caused a local depressurisation leading to said condensation, the air would contract back to it's original pressure, and the moisture would evaporate.
Could be like woof said. Could also be something similar that happends when the plane is adjusting the altitude. (this will change the pressure over the wing)
It's more or less been covered, but I thought I'd wander in and try to tie everything up since there still seemed to be some questions. The condensation you saw is functionally the same as the vapor over the wings of the F-22 doing high-G turns. In both cases, the airflow over the wing is starting to separate from the wing surface, as the plane is flying at a much higher angle of attack than under normal cirucumstances. This lowers the air pressure over the wing, allowing moisture in the air to condense briefly in that area. The Prandtl-Glauert Singularity is the same phenomenon but is caused by pressure differences within the shock waves coming off the plane, rather than by flow separation. The vapor trails off of wingtips at high altitude are, again, the same physical occurrence but in this case they are caused by high-speed vortices created where high-pressure air from under the wing flows up into the low-pressure area above the wing. This movement up and around the wingtip creates a very fast-spinning cone of air behind the wingtip, and because it's moving so quickly it has lower pressure than the surroundings, once again allowing moisture to condense.
