Originally Posted by
wrench1
Bell calls it collective bounce which I believe is the same term VH uses in its bulletin on the matter which goes into more detail. I just used the same terminology as the previous poster. I've always been instructed its main cause was pilot vs collective interaction that was magnified due to improper collective system friction which was addressed in the Bell OSN. There are also a number of other docs on the issue and it’s my understanding any 2-blade teetering system is susceptible to it regardless of blade type.
What you are describing is PIO (pilot induced oscillation) and it is not limited to two bladed helicopters. What I am alluding to is actual “slop” in the swashplate assembly due to worn components. If these are present the swashplate has a degree of freedom to pump the rotor independent of actual control position. This occurs when the sum of the pitch link loads oscillates through zero which is around the 40% torque range. If the pitch link loads between blades are equal this zone can entered without a perceived change in vibration. If the mean pitch link load between blades are not equal (generally due to sloppy track and balance) this imbalance pumps the rotor without crew input.