When I posted the earlier comments on why a VRS intensifies, I wanted to add the following comments but chose to wait to see if the comments would be anywhere near correct.
A vortex also has the property of being "semi-static". By this I mean the rotational energy and velocity of a vortex causes it to stablize its position in the airmass that it was created in. However it will move with the prevailing winds and air currents that move the airmass it exists in. That's why it's "semi-static", it remains positionally static within the surrounding airmass, but moves when the surrounding airmass moves.
I think a very good example of visualizing a vortex ring occurs when a prop driven aircraft is taking off in high humidity air. In these conditions you can "see" the prop tip vortexes being generated by the prop tip, by seeing the moisture inside those vortexes. I think this happens because the rotational energy of the vortex causes a low pressure area inside the vortex (like in a tornado or hurricane) to develop, and the low pressure area causes the inside air's "dew point" to be reached and the inside air "fogs". You also notice that the visible moisture soon dissipates as the vortexes dissipates.
Being able to "see" these circular "ring" shaped vortexes on the prop tips makes it easier to understand how vortex ring state develops in a helicopter. If you've ever seen this, you know that the visible circular vortexes move towards the rear of the aircraft in a spiral corkscrew shape as the aircraft accelerates down the runway. On occasion you can even see this corkscrew pattern when a prop driven aircraft has its engines under power but is stationary with its brakes applied.
The corkscrew pattern is being developed because the vortexes are stationary within an airmass that is being moved towards the rear of the aircraft by the force of the propellers. Now use your imagination and suppose that a rather hefty tug were put on the nosewheel of the aircraft. Imagine that a pilot in the aircraft pushed the throttles forward on a very humid day (so you could see the vortexes) and the tug operator began to push the aircraft backwards until the speed of the aircraft moving backwards began to equal the speed of the airmass being driven backwards by the propellers. The ring vortexes on the prop tips would no longer be stretched out in a corkscrew pattern, but all of the vortexes being generated would now accumulate in one spot around the arc of the propeller, within the propeller driven airmass that is now stationary with respect to the rearward moving airframe. I hope you can visualize this.
In a helicopter "settling with power", this is exactly what you have. The rotor is driving down an airmass around the helicopter's airframe, while the airframe itself is falling down at a rate that is at or near the airspeed of the decending air column generated by the rotors. The rotor tip vortexes are no longer being spread out in a corkscrew pattern, but are being allowed to accumulate at the rotor. This causes the effect of "vortex ring state" to intensify as the rotor tip vortexes have nowhere to go, and they just keep adding energy to the vortex ring that already exists.
I agree completely with the methods of escape described in the other posts on this topic. Since the vortex ring is "semi-static" within the descending air column, then the best method of escape is to cut collective and/or power and to push the cyclic forward, leaving the vortex ring behind. Side stepping the vortex ring with side cyclic could also work, which the Chinook pilots seem to find helpful. For a tiltrotor, forward cyclic would seem to work best as this would seem to remove both rotors from the vortex rings at the same time.
Please correct me if any of this sounds wrong.
------------------
Safe flying to you...
[This message has been edited by Flight Safety (edited 25 January 2001).]