the spiralling slipstream is largely a nonsense.
what happens is that the air flows straight back over the fuselage.
as the propellor blade scythes through the air there is a small sheet of vortices coming off the trailing edge. these are quite thin. these then move back in the slipstream and appear to spiral down the fuselage.
you can prove this for yourself with a piece of tape in the oil streaks on the underside of the fuselage. if you accidentally get it right you get a 2 per rev flap flap flap sound as the vortex sheet passes which is totally dependent on rpm.
look in the oil streaks after the flight and it will be apparent what is occurring.
a squared has given a pretty good accounting btw.