Lateral (rolling) static stability
Just as the mainplane provides the main contribution to the lateral static stability of a fixed wing aircraft, so the main rotor provides the major contribution in a helicopter. Other contributions come from the tail rotor, fin, fuselage and horizontal stabilizer. Also, as with fixed wing aircraft, no direct restoring moment arises as a result of a disturbance in bank angle (it has zero stiffness about the roll axis). The lateral static stability is, in fact, provided by the sideslipping motion that occurs subsequent to a change in bank angle. Suppose a disturbance in bank angle occurs and is followed by a sideslip to starboard, a rolling moment to port is required to restore equilibrium: Lv must therefore be negative for stability.
Main rotor contribution to Lv
Sideslip induces the characteristic flapback effect with the rotor flapping away from the relative wind direction. This causes a tilt of the thrust vector that will produce a restoring moment about the CG. The magnitude of the rolling moment will be dependent on the size of the flapping angle and the height of the rotor hub above the CG. Offset hinges will also increase its magnitude.