OK there is some formal info - I will go one step further - in the test -I also found that the string could actually be held lower than the tip of the blade (about 1/4") and was undisturbed.
This said to me (remember, I am a country boy ) " goll--y thet prop is not producing at the tips .
So to borrow from the blue sky guys - it looked like the faster air actually sucked the flow inward -from the tips - into the vortex.
back in 84 at the Reno Nats, we got a chance to look over and peek into a prototype pusher ,composite high speed personal transport.
the blade configuration said that something different was going on ,
they had a wild twist to them. The plane was a bit of a flop I understand.
I know that the Rutan planes work very well but I assumed the pusher position was a concession to the canard layout
I would not want a P39 type drive shaft passing between my legs --