Condensation Cloud Clarification

[cad2blender]

Ok I am not sure if this is the right place to post this but here it goes:

I know we've all seen those pictures where an airplane is caught "breaking the sound barrier" You know, that nice white cloud in front of the wing, as if the plane is "crossing" into the supersonic realm. As an Aerospace Major I really dislike when people say this because this is not entirely true, what are those clouds called? How are they made? My prof told me that it is because the air become so dense that it condenses and raises temperature, and you get a cloud. If this is true then what is up with those clouds where an airliner is taking off or a fighter is pulling high G's and thick clouds form on its wings sometimes covering the whole airplane like in this pictures?
http://i198.photobucket.com/albums/aa215/cad2blender/1197485.jpg
http://i198.photobucket.com/albums/aa215/cad2blender/0981791.jpg

[pmackenzie]

That is caused by the low pressure zones cooling the air causing the liquid to condense out of it.

High Cl results in lower pressure ( or visa versa depending on how you look at it ;) )

Pat MacKenzie

[nmasters]

Yep, compresion cooling, just like in your fridg

[vintage1]

No, expansion cooling, just like your fridge :D

Google 'adiabatic'

[HELModels]

This link is probably more useful and relevant, since you are looking at a micro meteorlogical event.

http://en.wikipedia.org/wiki/Dew_point

Dew point is more a meteorlogical term and the above link refers frequently to constant pressure, so read between the lines. Since air is a gas, pressure, volume, temperature functions apply. Dew point is sensitive to temperature and pressure and is visible when reached. As a wing produces lift it produces changes in temperature and pressure, which both pictures capture.

[Mike the Snake]

Killer pics

I'm not stopping until I go fast enough to form vapor trails off the wingtips of my Opus! LOL

[vintage1]

Killer pics

I'm not stopping until I go fast enough to form vapor trails off the wingtips of my Opus! LOL

Fly in near mist conditions, and you will;

I've even seen it coming off the back wing of an F1 car..

[Gary Warner]

When I worked at an FBO, we had to do pressure testing of pressurized airplanes. We would hook up a very large blower to pressurize the cabin. When the test was done the air pressure falls very fast inside. The inside of the cabin would instantly turn into a fog. Same principles involved as those in the pictures.

BTW, breaking the sound barrier is not a requirement for the phenomenon, but the pressure wave in front of the plane and the rapid falling of the pressure behind the wave does seem to be more often seen as planes approach the speed of sound. I saw this happen to an F18 at an air show this summer with the classic cone-shaped con-disk surrounding the plane. That was so cool.

[nmasters]

I saw this happen to an F18 at an air show this summer with the classic cone-shaped con-disk surrounding the plane. That was so cool.

That's called a prandtl glauert singularity (http://images.google.com/images?hl=en&q=prandtl+glauert+singularity&um=1&ie=UTF-8&sa=N&tab=wi). As the plane goes faster the cone moves forward. At Mach one the apex of the vapor cone stands just behind the nose. The OP's pictures are due to high CL the PGS is due to expansion behind a shock wave. Completely different phenomena that occur at opposite ends of the flight envelope but both result in local cooling

[N74463]

nmasters got it right for the planes that are flying close to the speed of sound. And vintage is right for the slow speed effect that is visible in the pictures in the original post. They are two different effects. I'm not enough of an aerodynamicist to explain the P-G singularity. The slow speed condensation is easier: low pressure over the top of the wing drops the temperature. If the air is humid enough, the temperature drop causes water vapor to condense into visible moisture.

Joe

[HELModels]

Killer pics

I'm not stopping until I go fast enough to form vapor trails off the wingtips of my Opus! LOL

Fly your Opus along the coast. The salt in the sea air is hygroscopic and "fog" will form easier. I wonder if these pictures might have been at seaside airports because the effect seems extreme.

[vintage1]

nmasters got it right for the planes that are flying close to the speed of sound. And vintage is right for the slow speed effect that is visible in the pictures in the original post. They are two different effects. I'm not enough of an aerodynamicist to explain the P-G singularity. The slow speed condensation is easier: low pressure over the top of the wing drops the temperature. If the air is humid enough, the temperature drop causes water vapor to condense into visible moisture.

Joe


I've seen it often enough on on airliner wings coming in to land in the tropics.

No great speed required..

I had assumed that the transonic stuff was also due to localised pressure drops, but the means of generating them was different..

[HELModels]

I agree with you here, Vantage, that it is a pressure and/or temperature drop that causes the condensation. If you have high pressure in one spot, there will be low pressure somewhere nearby and clouds maybe somewhere along the boundary. Look at the intakes on jet engines, condensation can form there too. Inside the intake is high pressure, but there is a large pressure drop needed somewhere like right in front of the blades and the cloud will form if the humidity and temperature and pressure meet the saturation point. Condensation nuclei need to be present too apparently. A marine environment really encourages it because obviously it is a moist environment with salt attracting water molecules. More condensation nuclei mean
denser clouds. With the supersonic example, it must be a humungous pressure drop as well as huge pressure rise.

[N74463]

I'm not sure if southern California qualifies as the tropics, but you can sometimes see the upper wing surface condensation on airliners approaching LA International (which is right on the coast) or when they rotate on their takeoff run.
A more common condensation effect at LAX is streamers of visible moisture at the core of the wingtip vortices. I'm sure it's the same as the others, pressure drop (at the vortex core) causes temperature drop results in visible moisture. On some airliners there will also be a streamer at the outboard end of the deployed flaps, where there's another vortex. Sorry, I don't have a picture of the effect, but it looks like they're dragging long gray ribbons from their wingtips. It's pretty cool.

Joe

[cad2blender]

Ok guys thanks for the clarification. Let me get this straight:

1. low pressure on top of wing, high pressure on the bottom, otherwise plane don't fly...
2. At high AOA, in the case of the F-22 Raptor, the pressure drops so much that the air is forced to go below under its dew point, therefore you see a cloud. I also believe that temperature should also drop right?

Well thanks for the help, here are some more pics... :)

http://i198.photobucket.com/albums/aa215/cad2blender/0326528.jpg
http://i198.photobucket.com/albums/aa215/cad2blender/1157718.jpg
http://i198.photobucket.com/albums/aa215/cad2blender/1278211.jpg

[N74463]

cad2blender,

2. The pressure drop causes the temperature to fall. And then, yes, the air is cooled below its dew point and the water vapor comes out of solution as visible droplets of moisture, just like a cloud.

The F/A-18 is dragging the visible vortex cores I described seeing on airliners. And the same thing is happening at each of the prop tips on the Herc. Cool pictures. That "tornado" going into the intake on the transport is pretty impressive.

Joe

[Aphorism's Dream]

I think the really impressive thing is the volume of span wise flow on the airliner wing!!
I wonder what the flow would look like with properly designed winglets?
Great images!

[Aphorism's Dream]

For a really neat image showing lift distribution as condensation see the files linked in this forum thread.
http://www.rcgroups.com/forums/showthread.php?t=794429

[seanreit]

Actually if you want to learn more about this, talk to your local HVAC Engineer. Because all of this phenomenon is due to psychrometrics. You can google that and learn how and when this will happen at any given temperature and dew point :)

[Aphorism's Dream]

Yep, exactly correct!

[HELModels]

Actually if you want to learn more about this, talk to your local HVAC Engineer. Because all of this phenomenon is due to psychrometrics. You can google that and learn how and when this will happen at any given temperature and dew point :)

Psychrometrics literally means the measurement of psychro, whatever that greek root implies, and I would bet it is greek and not latin. A phenomenon can not be due to taking a measurement unless youre in some particle accelerator or some other useless experiment. The phenomenon which is exhibited in those photos is the result of air(a fluid with some properties which psychrometry will quantify) interacting with a solid such that the air is visibly affected.

The 'fog' that is visible in those pictures should not be considered a representation of the entire lift distribution, even at maximum visibility. Although what is visible is the result of lift, the visible phenomenon ends way before the freestream. It is a fleeting effect which varies according to AOA and Cl, with some AOA yielding a big puff and others just a hint.

So, no, it is not exactly correct to say a phenomenon is due to a branch of science. To collect atmospheric data around a wing in flight which some psychrometric chart might explain would require alot more than the local HVAC specialist.

[seanreit]

A psychrometric chart will tell you what temperature/pressure water will come out of the air at a given humidity. A condensation trail is formed by a change in pressure (temperature) at a given humidity.

Interestingly, there is an RC Jet that will create a condensation trail, the Jet Model Products firebird in the right conditions will do it at the wingtips, and it lasts just half a second or so.

[Nigelp]

If you go back to basics.....Warm air holds more water vapour than cool air.

Example: If you force 'warm' air from the flatlands to rise over a mountain, as the 'warm' air rises it decompresses - that cools it down. Eventually the excess water vapour falls out of the now 'cool' air, in the form of rain (or snow !)

When the 'warm' air is cooled to the point where its ability to hold on to all of the water vapour is affected and the excess starts to 'fall out' of the air - that equals 100% (plus) relative humidity.

Nigel