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  #1  
Old 08-22-2009, 08:35 AM
Louise Hose's Avatar
Louise Hose Louise Hose is offline
 
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Question Observing an RV con trail

I taxied Mikey (RV6) down the runway yesterday morning to start a dawn patrol. The air was nearly saturated with moisture but the sun was about to rise and there was no forecast of fog. As I did my run up, I noticed a misty spray of moisture coming back along the left side of the cowl and from the prop. Confused, I slowed the RPMs back down and tried to determine where the moisture was coming from. The spray stopped. There was no water on the plane (or ground) and no rain in the immediate area. I run it up again and the spray re-started. I ran through any possibility I could think of...fuel, engine oil, prop oil? Slowing the RPM down again, I reached out to see if there was any oily film on the canopy or fuse. Nope. It was apparently just moisture. I didn't understand the physics but decided that the higher RPM was forcing condensation of the air vapor into tiny droplets. So, we headed out for a nice flight.

After I returned, Paul suggested that is was the equivalent to a con trail but he hadn't seen it occuring behind a prop before. Anyone else had this sort of observation? (I might add that the muted sunlight was coming from directly behind the mist, which might explain why I only saw it on the left side.)
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  #2  
Old 08-22-2009, 08:42 AM
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Default

Con trails can come from exhaust moisture or from pressure drops and do occur with propeller driven airplanes and from wing tips of any airplane. My father has spoken of con trails from B-17s that he crewed on. All of us have seen con trails from wing tips of fighters, particularly when pulling Gs.

I would think it your case it was just from the pressure drop of the accelerated air from the propeller.
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  #3  
Old 08-22-2009, 09:11 AM
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John Clark John Clark is offline
 
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Default Prop tips and moisture

Hi Louise,
I have seen what you described a couple of times in my '8' and many times in turboprops. Here is a photo off the web of an extreme case:



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  #4  
Old 08-22-2009, 09:16 AM
rv8ch rv8ch is online now
 
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Default cool picture!

I really like that picture - very cool! I have seen a similar phenomenon on a car I used to drive that had a radio antenna that would leave a "contrail" some mornings, when the conditions were just right. It was quite surprising.
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  #5  
Old 08-22-2009, 09:17 AM
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Happens a lot in seaplanes where we (obviously) work in very humid environments. Running the power up on takeoff you can really see the moisture coming off the prop.
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  #6  
Old 08-22-2009, 12:51 PM
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Default Yes, Louise....

....the bigger the engine and prop, the more visible it becomes. On very humid mornings, my Air Tractor with its 700 H.P. and 110" prop make quite a trail, screws around the airplane .

Yep,
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  #7  
Old 08-22-2009, 01:53 PM
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Multiple warbird departures early in the AM at OSH generated prop contrails that dissappeared about mid fuselage.
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Old 08-22-2009, 02:15 PM
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Last edited by N395V : 08-22-2009 at 02:21 PM.
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  #9  
Old 08-23-2009, 09:49 AM
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Default Not contrails, low-pressure areas...

Louise,

How many photos have you seen of the Blue Angels or USAF Thunderbirds making tight manuevers at speed showing the "cloud" of vapor above the wings? Same thing. I have seen exactly what you did on mornings when there is high-humidity and cool air when I am doing a pre-flight engine runup.

Here is a photo showing the phenomenon even at "low" speed due to the low-pressure area above the lifting surfaces of the strakes on this F-18.


Here is another good example above the wings showing a "cloud" in the low-pressure area providing lift for the airplane. The second phenomenon in this photo is the heat and carbon emissions seen in the jet exhaust path. At high-altitude where the air is cold and air-pressure is lower, the dark path behind this jet would be white water vapor forming the traditional contrail. The temperature and air density in the "exhaust" trail causes the dispersal of the light rays from the trees in the background to create the "out-of-focus" condition you see in this photo.


One of the combat zone maneuvers for the C-130 is rocket assisted takeoff from short fields. The heavy moisture in the air creates the visible arcs from propeller tip vortices behind the propellers. I noticed the same thing from my prop when I departed for LOE5 that morning as the ground fog was burning off.
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Last edited by n2prise : 08-23-2009 at 10:07 AM.
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  #10  
Old 08-23-2009, 10:20 AM
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The first time I saw it was 11-years ago on the first trip that my RV made to Oshkosh. I had spent the night at Boscobel airport (KOVS) and needed to wait till the fog from the river cleared before heading to OSH. The humidity was HIGH and we had the vapor coming off the prop during runup when we cycled the prop. I remember Brian saying that he could sit there all day watching the vapor coming off the prop.

Have seen it since then when HUMIDITY is very high and the prop is cycled on runup.

The photos before my post are good examples of this.
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