Abstract: The invention relates to improvements to helicopters with coaxial rotors, of convertible type in particular, consisting of at least two contra rotative rotors carrying blades for lifting and propelling, the fuselage having an axis parallel to the axes of the rotors and wherein each rotor consists of a nave carrying arms radially arranged, the arms carrying a coating with a propeller blade section, movable in incidence through whirling on to said arms. The naves are axially displaced along and rotatably supported about a shaft coaxial with the fuselage axis and comprising a structural support member of the fuselage.