RAF CFS taught me that the vortex ring state affects the rotor induced flow by increasing it (the air above the disc has already been through the disc before as it is now recirculating outwards, upwards, over and down!). From the standard velocities diagram, this has the effect of decreasing the angle of attack of the blades whilst increasing the rotor drag.
The clearest description I have come across is from R. W. Prouty in his book "Helicopter Aerodynamics".
He shows, in a series of diagrams, the airflow in vortex ring. This is shown as a "doughnut with a hole" shaped (my interpretation - SC) vortex encompassing the entire rotor disc tip path (similar to a huge smoke ring as blown by some smokers, but upside down here, as the airflow direction is downwards at the centre) with the blade tips in the centre of the "dough / smoke" itself. The downwards / induced flow within the vortex is inboard from the blade tips and the corresponding "up draught" is outside of the tip path plane. He goes on to describe how underneath the "doughnut" is a higher pressure air bubble, fed from the downdraught flow within the "doughnut". The bubble cannot be sustained and randomly bursts below the aircraft and then quickly re-forms, causing the random rolling and pitching moments associated with this phenomena.
Prouty states that "from flight and wind tunnel tests, unsteadiness starts at about one-quarter, peaks at three quarters,and disappears at 1 and a quarter times the hover induced velocity". (Beyond this it is theoretically possible for the disc to go into autorotation but don't try this at home as you will probably hit the ground first - SC).
Easier seen with the aid of the diagrams than described though - I hope this can be understood!
Acknowledged to R.W. Prouty - Thanks!
[This message has been edited by Skycop (edited 20 January 2001).]