Dual piston drag damper for rotary-wing aircraft rotor

A drag damper for a rotary-wing aircraft rotor comprising a hub and at least two blades each of which is connected to said hub by a corresponding connecting device, the drag damper being designed to be fitted between two elements, one of which is a flapping mass comprising one of the blades and said corresponding connection device, and the other element is one of the hub and an adjacent flapping mass on the hub, so as to damp relative angular drag movements between said two elements, each flapping mass having by design a natural frequency in drag (ωδ) different from a nominal rotation frequency (Ω) at which the rotor is designed to be driven, the drag damper comprising:

• a tubular damper body closed by two end faces,
• a main piston, slidable axially, fitted in the body and delimiting in said body two opposing variable volume working chambers,
• a rod moving integrally with the main piston and running approximately axially through at least one end face of the body,
• said body and said rod each comprising a connector for connecting respectively to one of the two components between which said drag damper is fitted,
• a fluid filling at least the two working chambers in the body, and
• at least one restriction port made in the drag damper and capable of restricting fluid passing from one working chamber to the other when the main piston is moved in the body,
• wherein the drag damper further comprises at least one channel communicating with the two working chambers, the at least one channel being fitted with a slidable and substantially pressure-tight secondary piston loaded by an elastic bias tending to return said secondary piston to a neutral position in said channel, with the at least one channel having a greater cross-sectional area than the restriction port, and with the secondary piston and the elastic bias being dimensioned in such a way that the secondary piston has a resonance frequency in the channel substantially equal to the rotation frequency (Ω) of the rotor so as to filter a dynamic component of stresses applied to the damper at the rotational frequency of the rotor;
• and wherein the elastic bias substantially blocks the secondary piston in said channel at the natural drag frequency of the flapping mass;
• and wherein said at least one restriction port is calibrated so as to dampen substantially relative movements of the rod-main piston assembly and of the damper body at a frequency which is substantially equal to the natural drag frequency of the flapping mass.