In some embodiments, a rotorcraft flight control actuator includes a driving member configured to receive mechanical energy from a power source, a driven member, a magnetorheological (MR) fluid disposed between the driving member and the driven member and configured to transmit a variable amount of mechanical energy from the driving member to the driven member, an output member configured to be coupled between the driven member and a flight control device of a rotor system, and a magnetic circuit configured to deliver a magnetic field towards the MR fluid, the magnetic circuit configured to vary the strength of the magnetic field in response to inputs.