Abstract: A MagnetoRheological Fluid Elastic (MRFE) lag damper system for adaptive lead-lag damping of helicopter main rotors. Embodiments include snubber dampers especially for hingeless helicopter rotors, and concentric bearing dampers. The snubber lag dampers include a flexible snubber body defining a cavity, a flexible or rigid interior (e.g., center) wall subdividing the cavity, and a flow valve in the interior wall or external to the cavity. The flexible snubber body may comprise elastomeric materials and metal rings stacked together to create a sealed MR fluid cavity. The shear deformation of the snubber body induces MR fluid flow through the valve, controlled by a magnetic field in the valve. An MRFE concentric bearing damper is also disclosed, comprising a pair of concentric tubes with elastomeric material injected and cured in an annular gap between the two tubes, and an MR fluid reservoir with piston-mounted MR valve housed inside the innermost tube.