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Observing an RV con trail

Louise Hose

Well Known Member
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.)
 
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.
 
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:

130contrail.jpg


John Clark
RV8 N18U "Sunshine"
KSBA
 
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.
 
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.
 
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,
 
Multiple warbird departures early in the AM at OSH generated prop contrails that dissappeared about mid fuselage.
 
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contrails_Medium.jpg
 
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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.
blue010.jpg


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.
blue017.jpg


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.
blue009.jpg
 
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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.
 
The old high humidity and cool temps in the morning make it happen.

We used to get that from our helicopters when I was in the Florida Guard, hovering in the tree tops waiting to pounce, and I never got a picture (kind of busy flying at the time) really cool looking!!

I'd love to see some pictures of that coming off of Mikey, or any other RV

cheers
 
Many years ago, when I was travelling a lot, I would be fascinated at the flap extension mechanics on the B-737 as we approached to land. I always liked to get a window seat where I could watch all the worm drives and linkages in the wing. On very humid days, you could see the "high to low" pressure regions downstream of the flaps marked by the moisture trails. Very neat!
I've never seen it on a small plane, so thanks, Louise, for letting us know an RV prop can generate them.
 
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.
blue010.jpg


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.
blue017.jpg


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.
blue009.jpg

The first photo is not at low speed. its the results of a 6-8g pull from low altitude. Seen it many times at airshows and believe me F18s dont get that close to the ground at low speed without gear and flaps.
 
A friend of mine was visiting the Maldives last year. He was on a trip with Malvidian Air Taxi and took this picture.

propcontrails.jpg
 
Low Pressure

I know you're gonna think "he's lying", but I have seen this off the tips of my 65hp Aeronca Champ on two occasions! One was in South Texas.
 
Helicopter Tip Vortices

The morning was cool and damp at Reklaw Fly In 2009. During arrival of an EC135 airmed ship, I noticed vortices coming from the rotor blade tips, similar to what we see coming from a prop.
img3565x.jpg
I also saw some of this coming from props of fixed wing aircraft (like Beautiful Doll).
 
The missing physics explanation

Yes, it does require high relative humidity, near 100%. It is the pressure drop, but not exactly. Air (any gas) cools when it expands. Check Boyle's law about that. It expands (it must) when it moves from where it is to relatively lower pressure. It is the cooling that causes the change in dew point which produces condensation. To observe this cooling phenomenon, put your hand in front of a stream of compressed air escaping to ambient temps (like from your compressor or from a tire).

That said, it's not the same as the contrails of jets at high altitude because those are ice crystals and not from a drop in pressure but from the added moisture from the jet's exhaust. The exhaust from burning hydrocarbons is water vapor and carbon dioxide. Looks the same, but not really the same cause nor the same "state".
 
We used to get that from our helicopters when I was in the Florida Guard, hovering in the tree tops waiting to pounce, and I never got a picture (kind of busy flying at the time) really cool looking!!

I was just reading along thinking about how the Jetstream's and Mu2's I used to fly would do it, always wanted to get a pic but was always "busy." ;)
 
I am with Larry. It is a function of compression and decompression of moist air. WE would get them on an F-16 right under the intake. A tornado looking affair sometimes touching the ground. Thus the name Viper, as it looked like a toungue flicking out.
Hack
 
Here's a couple of prop contrail pictures of me flying with Mike Seager:

DSC_8224cropped.JPG


DSC_8225Cropped.JPG


Skylor
 
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