2 is especially valuable in applications wherein the magnitude of the vibratory load active in on the rotating system varies as a function of operating regime or operating speed in a rotary wing aircraft for example it is common to require the highest levels of vibration isolation in high speed forward flight i e where the rotor blades are experiencing the largest differential in aerodynamic loading from advancing to retreating sides of the rotor system consequently it may be expected that the drive system 30 produce the maximum load vector p 10 max such as illustrated in fig3 a in yet another example it is anticipated that the lowest levels of vibration isolation would occur in a loiter or hovering operating mode where the rotor blades are exposed to the generally equivalent aerodynamic and gyroscopic affects consequently it may be expected that the drive means 30 produce no or a minimum load vector p 10 min such as illustrated in fig3 c thus far the discussion herein has concentrated on the rotational speed and angular position of the masses m 1 m 2 to produce vibration isolation while this feature of the invention is a primary aspect of the invention the configuration of the inventive isolation system 10 produces counteracting load vectors p 10 which act though the rotational axis of the rotor shaft 4 that is the line of action of the load vector p 10 whether the masses m 1 m 2 are coincident or opposing.