What you’ve described is Retreating Blade Stall
Just as the stall of an aeroplane wing limits its low speed, the stall of a rotor blade limits the high-speed potential of a helicopter.
The airspeed of the retreating blade slows down as forward airspeed increases. It has to produce an equal amount of lift to that of the advancing blade so, as the airspeed of the retreating blade is decreased with forward airspeed, the angle of attack of that blade must be increased to equalise lift throughout the rotor disc area.
If the angle is increased further and further to equalise the lift, the retreating blade will eventually stall - at some high forward airspeed.
First indication is often a vibration - the equivalent of buffet in a fixed-wing.
The nose lifts and there's a tendency for the helicopter to roll in the direction of the stalled retreating blade.
If the pilot didn’t reduce the pitch of the blades, the stall would become worse, vibration would increases and control might eventually be lost.
Is it controllable?
Yes, in the same way that you don't allow a fixed-wing to stall - by recognising the warning signs and acting accordingly.
Just as in a fixed-wing you take action at the buffet stage to prevent an incipient stall developing into a full stall, so you take action in a helicopter.
In both cases, you reduce pitch.
High forward speed is only one of the circumstances in which RBS can occur; there are other causes.
NB: There are hundreds of expert pilots on this forum - I'm not one of them.
This is just a 'broad brush' answer until one of them explains it more accurately.
FL
Last edited by Flying Lawyer; 4th Jul 2006 at 23:38.