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Video Question: Why Condensation?  
User currently offlineGlobalDreams From United States of America, joined Dec 2006, 43 posts, RR: 0
Posted (4 years 3 months 2 weeks 2 days 15 hours ago) and read 1465 times:

http://www.youtube.com/watch?v=54bz7fkPcgE

I was watching some aviation videos and noticed that, on some of them, some kind of condensation or water vapor forms in the engines once power is increased, and that more forms on top of the wing as the plane takes off? I know it's not anything dangerous, but why does it form only at certain times of the takeoff and flight?

5 replies: All unread, jump to last
 
User currently offlineB727fan From United States of America, joined Apr 2004, 298 posts, RR: 0
Reply 1, posted (4 years 3 months 2 weeks 2 days 15 hours ago) and read 1456 times:

I believe it has to do with air pressure, which causes the vapor (gas state) to condense.

User currently offlineEDICHC From , joined Dec 1969, posts, RR:
Reply 2, posted (4 years 3 months 2 weeks 2 days 14 hours ago) and read 1396 times:

It's all to do with the effect of motion on air pressure. As air moves more quickly it reduces in pressure. As air pressure reduces so does the dewpoint temperature. When the dewpoint temp becomes lower than the ambient air temp that's when you see condensation forming. The reason you see this in engine cowlings and on the upper surface of wings is because of the significant rise in the speed of the airflow at these points.

User currently offlineVikkyvik From United States of America, joined Jul 2003, 9413 posts, RR: 27
Reply 3, posted (4 years 3 months 2 weeks 2 days 13 hours ago) and read 1327 times:
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Quoting EDICHC (Reply 2):
It's all to do with the effect of motion on air pressure. As air moves more quickly it reduces in pressure. As air pressure reduces so does the dewpoint temperature. When the dewpoint temp becomes lower than the ambient air temp that's when you see condensation forming. The reason you see this in engine cowlings and on the upper surface of wings is because of the significant rise in the speed of the airflow at these points.

Some slight misunderstandings here.

First of all, the total pressure of the air does not change. When air is in motion, its static pressure is reduced - that energy goes into dynamic pressure.

As far as condensation is concerned, static pressure is what counts. When the static pressure drops, the temperature of the air also drops. If the temperature of the air drops below the dew point of the air, then condensation will form (this is equivalent to saying the relative humidity of the air has exceeded 100%).

EDICHC just got the dew point temp and the air temp mixed up. In his situation (with the dew point temp starting out above the air temp) condensation would already be there; when the dew point temp dropped below the air temp, condensation would cease.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineEDICHC From , joined Dec 1969, posts, RR:
Reply 4, posted (4 years 3 months 2 weeks 2 days 12 hours ago) and read 1282 times:



Quoting Vikkyvik (Reply 3):
EDICHC just got the dew point temp and the air temp mixed up.

Yup...never text a post when recovering from a hangover!  laughing   vomit 


User currently offlineDocLightning From United States of America, joined Nov 2005, 18713 posts, RR: 58
Reply 5, posted (4 years 3 months 2 weeks 2 days 11 hours ago) and read 1260 times:

B727fan is correct.

At high power settings, the fan is pulling so much air that it lowers the pressure of the air in the inlet. As air pressure drops, its temperature tends to drop too. This sudden drop in temperature causes the moisture present in the atmosphere to condense into droplets, causing visible fog. This is also why the fog forms over the wings on take-off. The pressure above the wing is far lower than atmospheric pressure because the wings are at a high angle of attack. Also, in wake vortices, you can see a cone of mist because the spinning air tends to fly outward, causing low pressure in the center of the vortex. That's why you see those trails of mist coming off wingtips or flap edges.

This phenomenon is only visible under situations where the ambient humidity is high enough. In dry environments you won't see it. But if the relative humidity is high, then it doesn't take much to cause condensation.


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