A way to check this might be to experiment with a water-screw.

It should be possible to observe the behaviour of suspensions in the water behind a prop. I'm thinking of a boat's prop, rotating in a glass of water, with some polystyrene beads floating in it. Or something.

If there exists a helical flow in water, then it is a fair bet that the same effect will be present in air.




Come to think of it, when I use a kitchen mixer, the fluid in the bowl does rotate. Mind you, someone did point out that, for a torque to exist at the prop-shaft, there must be a helical component to the airflow ... action-reaction.