Lu
Thanks for the info.
As you said, it is assumed that the blades are tracking properly etc. My query relates to the aerodynamic source(s) of the low frequency vibration. You and others have provided quite a bit of interesting information about methods used to minimize or offset these vibrations.
Perhaps the sinusoidal wave of the blade, which you mentioned, is caused by a varying lift at different blade radiuses, azimuths and forward speeds.
Perhaps the source is the rotor's downwash striking the fuselage or tail.
Perhaps the source is ....?????
The illogical logic here is; if the source is eliminated then the remedy is eliminated.