Yep, that would show the flow pretty well. I don't think it would take that much of an extension either. I have an electric motor gearbox by Leisure with about a 3 inch extension. An inboard model boat drive shaft would work excellent also.

I just have to make sure I keep the wool tufts short enough that they can't wind around the shaft....

I'm not in any rush ( got quite enough on my plate just now) but this would be a fine bit of research for later this spring. I've got a test power supply that runs off a car battery so it would be easy to set this up. Just need twe wires across the airflow with some yarn fibers on it.

And with the lower speed of a gear drive electric one could even attach a thread on the tip of the prop and using a strobe light timed to the revs you'd see a standing prop with a moving thread (actually muliple images of the thread in different positions) so you could see how much of a tip vortex there is. Hmmmm.....