As has been mentioned by MP andF1, there are a multitude of effects that determine the stick position, some aerodynamic, and others mechanical.
When looking at a `new` helo to determine the `apparent` longitudinal static stability ( ie does stick movement follow convention ie stick fwd-go faster ,etc) one would start at a mid-Cof G,at low speed and fixed collective, and open the envelope by 5-10 kt, over maybe +- 30 kts. Obviously, the a/c will descend as the power is fixed, at the higher speeds, so now one moves up the range and starts at about 40 kts, repeating the speed changes up to Vne, and then do it all again throughout the C of G range.This will then give one a total stick-plot- one would ,with a fully instrumented a/c, also have the lateral stick and yaw positions,TQ, engine parameters, attitudes etc,etc.
If during these tests it becomes apparent that for an increase in speed, the stick is/has stabilised in a reversed position, ie moving in the opposite direction(back) then it is time to quit and have a few beers , get the designer chappie in,and ask for an explanation !!
Of course, all this work cannot be done on a couple of sorties, and with modern simulators, this would have been caught early in the programme, but this was the way we did it when old Nick L, was in short pants, and couldn`t spell hicopleter...
It is also the way to do a quick-assessment, if one does not have access to an instrumented helo and limited time available.