If you're trying to fix MS Flight Sim's helicopter model - good luck. It has quite a few holes in it due to the basic background of a fixed wing model.
For example, try an autorotation at 60 KIAS, and then slow the airspeed to zero. At something below about 5 KIAS, the rotor slows to zero RPM.... (Not like the real thing, I can assure you).
There are a few models that have been made that try to get around the shortcomings - dodosims has done a pretty good job by all accounts. But mostly the helicopter model is quite a compromise.