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Engines Fogging On Throttle Up  
User currently offlineWakeTurbulence From United States of America, joined Apr 2004, 1294 posts, RR: 17
Posted (7 years 11 months 2 days 4 hours ago) and read 2231 times:

I did a search on this topic, and I think it has been discussed before, but I couldn't find what I was looking for. Anyway, I was spotting at LAX the other day and noticed some very nice condensation forming in the engines during the takeoff roll. What causes this effect to occur? Also, why is it much thicker just after the throttle has been pushed up from idle? As the aircraft started to roll the condensation would come and go during the t/o run. Thanks for any help.
-Mat


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6 replies: All unread, jump to last
 
User currently offlineRichardPrice From , joined Dec 1969, posts, RR:
Reply 1, posted (7 years 11 months 1 day 21 hours ago) and read 2163 times:

Its because the pressure immediately infront of the fan drops drastically, causing the condensation held in the air to no longer be able to be supported, manifesting itself as mist.

Its pressure changes that cause it, so increasing intake of air into the engine will drop the pressure initially until air further out starts moving inward.


User currently offlineBoeingOnFinal From Norway, joined Apr 2006, 476 posts, RR: 0
Reply 2, posted (7 years 11 months 1 day 20 hours ago) and read 2146 times:

Is it so that the temperature and dewpoint has something to do with this? I know very little about this subject, but I would assume that this is a factor, since general condensation on wings and so on also is affected by dewpoint.


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User currently offlineStarlionblue From Greenland, joined Feb 2004, 17001 posts, RR: 67
Reply 3, posted (7 years 11 months 1 day 19 hours ago) and read 2134 times:

Quoting BoeingOnFinal (Reply 2):
Is it so that the temperature and dewpoint has something to do with this? I know very little about this subject, but I would assume that this is a factor, since general condensation on wings and so on also is affected by dewpoint.

Of course. These do affect condensation.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineSmAlbany From United States of America, joined Aug 2004, 285 posts, RR: 0
Reply 4, posted (7 years 11 months 1 day 19 hours ago) and read 2124 times:

Quoting BoeingOnFinal (Reply 2):
Is it so that the temperature and dewpoint has something to do with this? I know very little about this subject, but I would assume that this is a factor, since general condensation on wings and so on also is affected by dewpoint.

To elaborate on the last post, the closer the ambient temperature is to the dewpoint (ie the higher the relative humidity), the less the amount of pressure change required to form condensation.


User currently offlineEMBQA From United States of America, joined Oct 2003, 9364 posts, RR: 11
Reply 5, posted (7 years 11 months 1 day 15 hours ago) and read 2064 times:

As said above.. it comes from the drop in pressure at the inlet.. but will only happen if the weather conditions are right. My bet is you would also see it on the upper surface of the wings during rotation for the same reason...and might also see it in the cabin at the air vents.


"It's not the size of the dog in the fight, but the size of the fight in the dog"
User currently offlineZOTAN From United States of America, joined Jan 2005, 609 posts, RR: 0
Reply 6, posted (7 years 11 months 1 day 14 hours ago) and read 2045 times:

It's due to the drop in pressure.

The ideal gas law is PV=nRT

P = Pressure
V = Volume
n = mols
R = Universal Gas Constant
T = Temperature

When the pressure drops on the left side of the equation, something must also have to drop on the right to keep it equal. Therefore, the temperature drops. If it drops below the dew point, condensation will form.


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