Image Number 5 for United States Patent #6676074.
A drag damper for use on a rotary wing aircraft rotor comprises a body defining two variable volume chambers linked by a piston. The chambers are filled with fluid and are connected by a restriction port (35) and by a channel (32) of larger cross-section than the restriction port. A secondary piston slidable and pressure-tight piston is fitted in the channel and loaded by an elastic bias (34). The equivalent mass of this secondary piston (33) and of the fluid which it displaces, and the stiffness of the elastic bias (34) are such that the secondary piston is resonant in the channel (32) at the rotation frequency of the rotor, to filter the dynamic component at this frequency of stresses applied to the damper (20). Furthermore at the natural drag frequency of the corresponding blade, the elastic bias substantially blocks the secondary piston (33) in its channel (32), and the flow of fluid between the chambers (29, 30) of the damper (20) takes place mainly via the restriction port (35) calibrated to provide substantial damping at this frequency.