INNflight From Austria, joined Apr 2004, 3589 posts, RR: 71 Posted (5 years 1 month 2 weeks 5 days 7 hours ago) and read 1136 times:
Hello!
I have seen quite a few images with condensation coming from props, how do they start? What conditions are needed? Can this happen to all kind of aircraft?
Fly2HMO From United States, joined Jan 2004, 3963 posts, RR: 4 Reply 1, posted (5 years 1 month 2 weeks 5 days 6 hours ago) and read 1105 times:
This is what I remember this from high school physics:
The props change the air pressure. When there's a change in pressure of a gas, its temperature also changes, in this case there is enough humidity and the temperature of the air going through the prop tips is cool enough to form condensation.
Hopefully, I am correct
Happiness is just an illusion caused by the temporary absence of reality
QantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 50 Reply 2, posted (5 years 1 month 2 weeks 5 days 5 hours ago) and read 999 times:
Fly2HMO is correct. Put simply, the drop in pressure "across"/over the propellor blades cools the air - depending on atmospheric conditions, this can also condense the air, leading to the effects sometimes seen.
Cheers,
QantasA332
(It's good to be back again! I just arrived home from visiting relatives in the US for a while...hence my past silence ).
FredT From United Kingdom, joined Feb 2002, 2101 posts, RR: 30 Reply 3, posted (5 years 1 month 2 weeks 5 days 4 hours ago) and read 957 times:
There is a pressure gradient over the propeller disc with a pressure drop in front of it. However, these condensation trails are not due to this gradient, other than indirectly. They are just the same as the condensation trails forming at wingtips and flaps during high lift coefficient flight in humid conditions.
You have a pressure difference between the front side and the rear side of the blade. Air leaking around the blade tip forms a vortex in the wake of the blade tip, just the same as a wing tip vortex. Centrifugal (centripetal bla bla bla) forces make sure that the pressure is lower inside this vortex. Lower pressure means lower temperature.
If the dewpoint is close to the air temperature, the temperature inside the vortex will drop below the dewpoint. The humidity in the air inside the vortex will condense and the condensation trails are formed.
Cheers,
Fred
(Hey QA332, welcome back! Was just about to mail you to ask what you were up to!)
I thought I was doing good trying to avoid those airport hotels... and look at me now.
QantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 50 Reply 5, posted (5 years 1 month 2 weeks 4 days 16 hours ago) and read 639 times:
Hey Fred, thanks for the extended explanation! Just as I was going to bed last night I figured I should have clarified the fact that it is condensed prop-tip vortices that you're actually seeing with that effect but hey, I reckon I did well enough considering I had just arrived home!
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