Torsional Oscillations due to flap
I agree that delta-three by flap hinge geometry will reduce (but not remove) torsional response.
However my earlier remarks were regarding the main rotor. The torsional response of the TR on most helicopters would be insignificant compared to the MR. Though it should not be ignored.
I'm sorry if that was ambiguous.
The main rotor will exhibit a RPM Response to flapping which will induce appriciable torsional loads. It is to this that I refer. Determining the exact magnitude of this oscillating torque is complicated and will be significantly effected by the size and rate of cyclic control and coning angle amongst other things. The belts help remove transient peaks from these loads by flexing and sliping.
Cheers
Cran