Persistent Aerodynamic Contrails From Propellor Tips

Trailspotter

Senior Member
Thanks for that. I read the interesting thread. The Bombardier Dash 8 is one of the most common planes here that don't transmit ADSB so I don't normally see them on SHCC. I will have to try to add MLAT which I can get from the HTML port 8080 output.
Most cloud was below 8km. today.
http://www.metoffice.gov.uk/public/lidarnet/lcbr-network.html
 
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Seems like it's a combination. More than just propellors at least, as there's contrail coming from most of the wing.
I suspect that the temperature is too high for any engine contrail, regardless of any differences of efficiency. The wing contrail indicates that conditions were right for aerodynamic contrails (just saturated, in this case, as the trail did not spread). The pressure drop across the prop would be much higher (I think) than the wing, and so contrails could be produced at a lower altitude than turbofan planes.
I can see a regular helix in the trail, in the last shot, but I might just be seeing what I want to see..
 
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Trailspotter

Senior Member
I can see a regular helix in the trail, in the last shot, but I might just be seeing what I want to see..
This reminded me a series of photos that I took in Lake District on August 10, 2016 at around 6:50 UTC. It features a Q400 contrail at 23,000 ft that went "from the horizon to the horizon". A close-up of the plane appears to show the contrail being triggered by the prop tips:
IMG_6321.JPG IMG_6322.JPG IMG_6325.JPG Screen Shot 2016-08-15 at 13.18.50.png

I also found a close up video of this plane and its contrail and posted it on youtube:

Source: https://youtu.be/JaKBZLEBgK4



In contrast, the same type aircraft passing over at 25,000 ft a few minutes later was contrail-less as long as I could follow it:
IMG_6326.JPG Screen Shot 2016-08-15 at 13.17.33.png
 
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Trailspotter

Senior Member
Very clear in photo #2 blown up. :) What is puzzling me is that they look like a 'single start thread' not a six start...
It looks like a triple helix to me (when it first appears behind the tail). It appears that the Q400 propeller blade angle is not fixed but controlled. Perhaps it could also vary between the blades, so only three blade tips were making contrails in this case:
Screen Shot 2016-11-21 at 11.41.43.png
A crop of a video frame (see the youtube link in my previous post).
 
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Trailspotter

Senior Member
Agreed. I am still trying to figure out what a screw thread looks like when is rotating and translating, and then frozen in time.. Should still be a six start thread, I think.
Sorry, but I've revised my previous post before your reply. It would have to be a six start thread, but only if all blades were at the same angle. But, as appears, their angles are controlled and possibly can vary between the blades.
 
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