I stumbled into this thread a bit late, but have to chime in:
The blades all lift all the way around the mast. Anyone who believes otherwise please email me, I have bridge in Brooklyn that is for sale. I hope nobody has become convinced of this "fact" as it is wrong, and it is not a necessary piece of explaining the pylon rock issue.
Pylon rock is a natural oscillation common to the Bell two bladed systems, where the natural rotor inplane frequency is matched by the fuselage and transmission. All rotors have natural frequencies based on the rotational speed (RPM) and the number of blades, and the natural damping (energy absorbing) of the system. In early C model hueys (and the Cobra, which shares the same rotor design) there existed a match between the natural transmission rocking mode (the transmission is suspended on rubber mounts for vibration absorbtion) and the rotor's natural frequencies. This is quite similar in concept to a wing flutter condition in an airplane.
At some flight conditions, especially high speed and increased load factor) the whole rotor transmission system could go to a large limit cycle amplitude (get big and not die away). The Cobra had (has?) electronic motion transducers on the transmission feet to sense when the transmission starts rocking relative to the fuselage. This signal is sent to the SCAS (stability control augmentation system)which sends out control movements to oppose the transmission rocking, and damp them out.
Finding and fixing such "Pylon Rock" is a part of all helicopter testing, and keeping these dynamic frequencies under control is a big part of developing helicopters and tilt rotors.
Often, the back-up structure is stiffened or loostened to move the rocking mode away from the rotor mode, or damping is increased in the main rotor (no luck here for a teetering "semi-rigid" system).
The V-22 flight controls have several strong "notch filters" in them to keep the flight controls from sensing and amplifying similar rotor/transmission/fuselage modes. They work quite well, as evidenced by the lack of headlines about such problems. These rotor/airframe modes caused the crashes of the early tilt rotors, and stopped their development until better stiffer structures and faster flight controls could be developed (along with the smarter design programs needed to understand it all).
The other Bell rotors with 4 blades don't share the pylon rock problem because their natural frequencies are much higher, not anywhere near the transmission mode, and so there is no amplification of the lower transmission frequencies. A natural part of the design of a nodal transmission mount system is to keep these frequencies separated so that pylon rock does not happen.
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