Excellent stuff on Nick's pages, never seen that detail before and it was a timely reminder why I never became a test pilot
Hilico,
The best "pilot's perspective" I was given about how to visualise vortex ring was this:
Imagine a doughnut of air (annular / ring vortex) encompassing the outer part of the rotor disk, with the rotor mast in the hole in the middle of the doughnut. The air travels downwards through the disk, then out and upwards over the outer edge of the disk then back in over the tips and recirculating down again.
From this it might be easier to see why forward airspeed prevents the vortex ring building around the disk.
I had it demonstrated very briefly to me once in military training and some years later, very spectacularly at night by the relatively inexperienced pilot in the RHS. I was a QHI on type but acting as co-pilot on the flight. At the end of a long night he was attempting to achieve a hover at around 8,000 ft (military observation job). At very low IAS he allowed the ROD to build up, which I pointed out, but he took no recovery action and he eventually lost the plot. The aircraft suddenly began random pitching and rolling and the VSI went hard to the bottom stop so I yelled "airspeed" a couple of times more before I intervened and pushed the cyclic well forward for him. We flew out a few seconds later, having lost well over 3,500ft.