Originally Posted by
AMDEC
Do not think about it. It would be a perfect shaker.
The flapping motion of the blade induces some vertical force at the flapping hinge that directly varies with the flapping angle. If you sum the contributions of the different blades, it gives you a constant moment in the fixed frame... as soon as you have 3 blades or more.
With a two-bladed rotor, the vertical force on both blades is opposed (when one blade is up, the other is down). This makes a pure moment at the hub center, which varies cyclically, like the flapping. The only way to get rid of it is to have no flapping hinge offset. All 2-bladed rotors are see-saw rotors.
Finally a reasonable and satisfying explanation to the original question! Thank you, Sir (or M'am)!