To mitigate that drawback, devices have been made that are arranged on the rotor head. In a first solution, devices are known that seek to concentrate an oscillating mass in the vicinity of the axis of rotation of the rotor, which devices are referred to as “concentrated-mass devices” for convenience.
According to document FR 2 416 838, a first concentrated-mass device is mounted on the top portion of the hub in order to reduce said vibration.
That first concentrated-mass device includes an oscillating mass that is held radially in a housing that is secured to the hub by resilient means. The mass is also fastened to the top end of a rod.
The rod is placed substantially on the axis of rotation of the rotor in a recess in the rotor mast of the rotor. The bottom end of the rod is then hinged at a point that is situated on the axis of rotation of the rotor.
Thus, with the resilient means tending to keep the oscillating mass in a rest position, the oscillating mass moves in a plane that is substantially parallel to the top portion of the lift rotor hub, i.e. in a plane that is substantially perpendicular to the vertical axis of rotation of the rotor. The oscillating mass then directly opposes the vibration generated by the rotor.
However, the forces that induce the vibration that is to be reduced and that is generated in a rotor head may be described by a force torque, sometimes referred to by the person skilled in the art as the “rotor head torque”. The rotor head torque is then made up of three moments together with three resultant forces relative to three mutually perpendicular axes, namely:                two axes referred to, for convenience below, as the “first and second resultant axes” defining a plane referred to below as the “first resultant plane” extending parallel to the top portion of the rotor hub and thus substantially perpendicular to the vertical axis of rotation of the rotor; and        an axis referred to below as the “third resultant axis” that is perpendicular to said first resultant plane.        
The first concentrated-mass device is then effective in filtering the resultant forces of the “rotor head torque” along the first and second resultant axes that are substantially parallel to the plane containing said oscillating mass, but is ineffective for the resultant force of the “rotor head torque” along the third resultant axis that is substantially parallel to the axis of rotation of the lift rotor and perpendicular to the plane containing the oscillating mass.
Furthermore, since the oscillating mass used is constant, the first device is particularly effective when the excitation frequency of the vibration that is to be reduced is close to the resonant frequency of said oscillating mass, which frequency is in fact constant.
The first concentrated-mass device is thus not really suitable for variable frequencies.
To remedy that, a second concentrated-mass device is disclosed in document FR 2 749 901.
Like the first concentrated-mass device, the second concentrated-mass device has a main oscillating mass held radially in a housing. In addition, it is provided with an adjuster oscillating mass suitable for sliding along the rod that is secured to the main oscillating mass and that is hinged at a point situated on the axis of rotation of the rotor.
By moving the adjuster oscillating mass, it then becomes possible to adapt the device so as to enable it to reduce vibration at varying frequencies.
Nevertheless, it continues not to filter the resultant forces of the “rotor head torque” along all three mutually perpendicular axes, in particular it does not filter vibration along the third resultant axis that is substantially parallel to the axis of rotation of the lift rotor.
Document FR 2 575 800 presents a vibration damper for a helicopter rotor hub, which damper is provided with a central oscillating mass suspended by mechanical and pneumatic springs inside an outer casing so as to be free to move along three mutually perpendicular directions.
Thus, a single mass is used for combating all three resultant forces of the “rotor head torque”.
Although effective, that damper is limited by the fact that the excitation along the third resultant axis differs from the excitation along the first and second resultant axes. As a result that device is difficult to tune.
In the same manner, Document EP 0 790 180 provides for suspending a central oscillating mass by a plurality of springs, the central oscillating mass also being arranged on a movement limiter extending along a vertical axis.
Finally, document U.S. Pat. No. 6,443,273 discloses the possibility of suspending a central mass in a casing by means of springs.
It should be observed that unlike the first above-mentioned solution, a second solution seeks to distribute the oscillating masses about the axis of rotation of the lift rotor by means of devices that are referred to as “distributed mass devices” for convenience.
Document WO 2005/079200 presents a first distributed mass device having two coaxial masses and control means, the control means being suitable for controlling the angular velocity of said masses and their relative angular position.
Like the first and second concentrated-mass devices, the first distributed mass device appears to be insufficient to counter the resultant forces of the “rotor head torque” along three mutually perpendicular axes, and more particularly to counter the resultant force along the third resultant axis that is substantially parallel to the axis of rotation of the lift rotor.
Similarly, Document FR 2 435 391 presents a second distributed mass device having two masses fastened to a shaft passing through the cuff connecting the blade to the hub of a lift rotor.
Document FR 2 808 256 presents a third distributed mass device provided with oscillating masses located between the blades.
Each oscillating mass is then connected by a pivoting connection to an arm of a support secured to the rotor hub in such a manner as to constitute a pendulum.
Under those conditions, the third distributed mass device would appear to be capable of countering the resultant forces of the “rotor head torque” along three mutually perpendicular axes.
However, it is particularly difficult to adjust the third distributed mass device, which device is effective either in countering the resultant forces of the “rotor head torque” along the first and second resultant axes substantially parallel to the plane containing the support of the masses, or else it is effective in countering the resultant force of the “rotor head torque” along the third resultant axis that is substantially parallel to the axis of rotation of the lift rotor.
Document FR 2 733 483 presents a fourth distributed mass device having at least one pendular body mounted on the rotor so as to be capable of oscillating about a pendular movement axis that is spaced apart from the center of inertia of the pendular body.
Consequently, the state of the art provides either devices that are suitable for filtering the resultant forces of the “rotor head torque” along one or two axes, or else devices that are suitable for filtering the resultant forces of the “rotor head torque” along three mutually perpendicular axes, but that are relatively difficult to adjust to obtain results that are of good performance along all three mutually perpendicular axes.