Rotor hub and mast assembly comprising a constant velocity universal joint for connecting the rotor hub of two-blade, three blade or multi-blade helicopters with the drive shaft (mast) of the motion and torque, said rotor hub tilting with respect to the mast with two degrees of freedom, around the flapping axis and the feathering axis of the blades of the helicopter, said blades being fixed to the rotor hub, said constant velocity universal joint comprising: - a couple of universal concentric rings (5A, 5B) a first ring or internal universal ring coupled in a revolute way to the carrier, integral with the mast, by means of two rotating pins and a second ring or external universal ring coupled in a revolute way to the rotor hub by means of two swivel pins, said two universal rings being connected among each other by two connection pins in order to allow the relative rotation of the external universal ring with respect to the internal universal ring, substantially in quadrature with the pins of the mast and with the pins of the rotor hub; at least one connection pin having an extremity protruding towards the outside of said joint, coaxial to the connection pin; characterised in that the constant velocity universal joint further comprises - at least one bisector device comprising a first element and a second element respectively connected to the mast and the rotor hub by means of the relevant connecting rod, where said two elements (61, 62) are coupled in a prismatic manner among each other and each of the said elements (61, 62) is provided with at least one hole able to consent the passing of said connection pins protruding extremity so that each of said two elements (61, 62) is coupled in a sliding and swivelling manner on that connection pins protruding extremity, said device being configured in a way to maintain the axis of said pin of connection on plane that bisects the angle between that hub and the mast in order to guarantee the constant transmission of the rotating motion among the mast and the rotor hub even when said hub is tilted with respect to the mast of an angle not equal to zero. - a couple of universal concentric rings (5A, 5B) a first ring or internal universal ring coupled in a revolute way to the carrier, integral with the mast, by means of two rotating pins and a second ring or external universal ring coupled in a revolute way to the rotor hub by means of two swivel pins, said two universal rings being connected among each other by two connection pins in order to allow the relative rotation of the external universal ring with respect to the internal universal ring, substantially in quadrature with the pins of the mast and with the pins of the rotor hub; at least one connection pin having an extremity protruding towards the outside of said joint, coaxial to the connection pin; characterised in that the constant velocity universal joint further comprises - at least one bisector device comprising a first element and a second element respectively connected to the mast and the rotor hub by means of the relevant connecting rod, where said two elements (61, 62) are coupled in a prismatic manner among each other and each of the said elements (61, 62) is provided with at least one hole able to consent the passing of said connection pins protruding extremity so that each of said two elements (61, 62) is coupled in a sliding and swivelling manner on that connection pins protruding extremity, said device being configured in a way to maintain the axis of said pin of connection on plane that bisects the angle between that hub and the mast in order to guarantee the constant transmission of the rotating motion among the mast and the rotor hub even when said hub is tilted with respect to the mast of an angle not equal to zero.