ORIGINAL: fledermaus
FWIW: [link=http://www.nrc-cnrc.gc.ca/eng/multimedia/cavitation-tunnel.html]http://www.nrc-cnrc.gc.ca/eng/multimedia/cavitation-tunnel.html[/link] showing the spiral cavitation pattern caused by a ship propeller in water, in the National Research Council of Canada's experimental water tunnel.
[link=http://www.flightglobal.com/airspace/media/mokkel/propeller-vortices-2-7104.aspx]In this picture of a C-130 taking off[/link] one can see condensation vortices behind the props which are similar to the cavitation in water. The pressure changes around the prop blades cause water vapour to condense along the path of the blade.
One thought I have, however, is that these helical patterns do not necessarily mirror the airflow. The axial velocity of the air in the propwash is much greater than the tangential velocity (if it wasn't, it would be a pretty crappy propeller) and there is not that much momentum transferred tangentially as a result. This suggests that the prop wash effects might not be that substantial, but I'm not sure about that conclusion.
Maybe I'm just blowing smoke...
The sprial smoke trail doesn't prove there is spiral flow. If the air is moving straight back you will get a sprial smoke trail simply because the source is at a fixed point and rotating. Not saying there is none, there is, but it is not a strong force. Certainly not as strong as the smoke trail would seem to indicate.