Lu,
I think you are ducking the issue slightly. I possibly wasn't totally clear though in that I don't dispute that you can experience pylon rock at cruise speed but it is certainly more prevelant in the regime I have concentrated on. I was trying to ask how your explanation about it being a function of loss of lift came into play with no fwd speed.
I suspect that what you are refering to is simply the normal '2 per 1' vertical vibration experienced with a 2 bladed a/c. As Nick says, depending onthe harmonics being propogated and the various resonant frequencies of componants, you can get all sorts of "bits" bouncing up and down, including the pilots. Are you suggesting that the vertical bounce experienced in a badly track and balanced S76 is related to loss of lift? Of course not, no more than it is in a 212/206/214.
PS Sorry about the tone of my last; I was rather tired and emotional after a busy day in the class room (5 blades on this one!)
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Another day in paradise