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Main rotor hub, transmission and engine of the American UH-60 Black Hawk helicopter.

Its called a Bifilar damper and its job is to reduce the vibrations from the main rotor before they are transferred into the transmission and airframe.

The UH-60 has an aft center of gravity to allow a range for loading and unloading the cabin. So Sikorsky canted the tail rotor 20° to lift the tail and correct the hover attitude in an aft CG.

But because the tail rotor provides lift now, as the pedals are pushed to change the tail rotor thrust and yaw the helicopter the nose will pitch up and down as well. So the UH-60 has a mixing unit in the controls that is governed by the AFCS (Automatic Flight Control System) which automatically applies a bit of fore and aft cyclic input to keep the nose level during a pedal turn.

The stabilizer on the tail also contributes. Because the UH-60 required such a large one, it was converted to a moving stabilator to aid with pitch control in forward flight.

At a hover and low speeds the stabilator has a high angle of incidence to be more in line with the downwash of the main rotor. And then as the helicopter adds power and airspeed the AFCS changes the stabilator angle to help maintain a level flight attitude. Without this then the stabilizer would cause a significant nose up tendency any time the helicopter tried to accelerate from a hover.

Wisteria garden.