At an airshow I attended last year the humidity was pretty high. A friend of mine came and we saw the same vortex trails from an AT-6 prop. He also mistook them for the spiral airflow caused by the prop. A debate ensued. It was pretty easy to disprove because the vorticies spirall in the opposite direction of rotation if the prop. It is not too hard to visualize the reason why they spiral in the opposite direction. Just imagine the position of the moving prop with respect to the rearward airflow. For any disbelievers take another good look at that photo of the Corsair.

Those were the days. I used to spend a lot of time at the observation area at the airport before they closed it. It was right at the end of the runway off to the side and there was a trail that took you to the very end of the runway. It was fun to stand there as a plane landed. It is amazing how powerful the wake of the plane seems when standing in it.

I was watching the documentary the other day on the Wright flyer that was recreated for the anniversary of flight. The image that stands out is the propeller being tested in the wind tunnel. They had a smoke trail running through the prop. I know that the prop was not turning all that fast, but there was no noticeable change in the flow after the prop. The twist in the airflow was not visible and it didn't look like the flow behind the prop was all that turbulent. Just an observation.