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vorticey,
When the tip path plane is not parallel to the mast plane, there will be a 2 per revolution speed variation between the two planes. This is because of the Hookes Joint Effect. This may be envisioned by considering that the tip path plane is attached to one end of the Hookes joint and the mast plane is attached to the other end. In addition, profile drag will vary as the rotor turns. The mast absorbs this rotational oscillation in most helicopters with teetering rotors.
There is no strong reason for lead-lag hinges in a 2-blade rotor because the two blades accelerate and decelerate together.
An exception to the above is the Kaman helicopter. It has two mechanically interconnected teetering rotors, for a total of four blades. Because of this interconnection, the helicopter has lead-lag and delta-3.
You are correct in regard to the weight shift and therefor the need for undersling.
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