To: Nick Lappos
We are still talking about two different things. Pylon rock and the two per rev phenomenon. Although pylon rock can be excited by the vertical movement of the transmission and the interreaction of the lift link, I don’t think the comparison of Galloping Gertie and the oceans tides quite answers the question about what causes the vertical movement of the transmission and rotor system to a certain degree on some Bell Helicopters and especially those helicopters equipped with Nodamatic suspension on the transmission. You use the term dynamic response and I ask in response to what? I believe that it is caused by variations in lift as the rotor disc rotates now you can tell me what you think can exert the necessary energy to get a thousand pounds moving up and down at 500 times a minute. As indicated by Bell, the Nodamatic system was installed on the 206L, the 222 and the 214 in order to eliminate (Isolate) the vibratory energy introduced into the fuselage by the inherent 2 per rev that is common to two blade helicopters. In the process of isolating the vertical beat the Nodamatic system allows the transmission to move in relation to the fuselage. In the process of moving the Nodamatic system displaces heavy weights which absorb the energy. This is similar to the Bifilar system used by Sikorsky and the Pendular weights used by other helicopter manufacturers to eliminate vibratory forces from being transmitted to the rotorhead from the blades.
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The Cat