A rotor carrying a plurality of lift assemblies, each having a retention and mobility member. An abutment mechanism of a lift assembly includes an abutment track arranged on the retention and mobility member and a single cylindrical abutment that is movable in pivoting about a movement axis, said abutment extending over a height in elevation and also over a length and over a width. The length is greater than said width, and said height is greater than said length. A fly-weight is secured to pivot with said abutment, and a return spring exerts a force on said fly-weight.