waketurbulence
Posts: 1264
Joined: Sun Apr 18, 2004 12:33 pm

Engines Fogging On Throttle Up

Sun Sep 03, 2006 12:19 pm

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
 
RichardPrice
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Joined: Sat Apr 23, 2005 5:12 am

RE: Engines Fogging On Throttle Up

Sun Sep 03, 2006 7:29 pm

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.
 
BoeingOnFinal
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Joined: Fri Apr 14, 2006 6:47 am

RE: Engines Fogging On Throttle Up

Sun Sep 03, 2006 8:32 pm

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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Starlionblue
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RE: Engines Fogging On Throttle Up

Sun Sep 03, 2006 8:57 pm

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." - John Ringo
 
Smalbany
Posts: 255
Joined: Fri Aug 06, 2004 2:29 am

RE: Engines Fogging On Throttle Up

Sun Sep 03, 2006 9:28 pm

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.
 
EMBQA
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RE: Engines Fogging On Throttle Up

Mon Sep 04, 2006 12:53 am

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"
 
zotan
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Joined: Thu Jan 13, 2005 7:42 am

RE: Engines Fogging On Throttle Up

Mon Sep 04, 2006 1:48 am

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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