Originally Posted by
The Sultan
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.
Indeed. Commonly referred to as biomechanical feedback. The first Bristow EC225 had an interesting experience on its initial ferry from Marignane, after stopping in Norwich.