show schematically the head of a four bladed helicopter main rotor substantially as described in ep 0 213 016 with reference to figures 4 and 5 in that document to which reference may advantageously be made for further information it will be recalled that this rotor head comprises a one piece tubular mast hub 1 the lower substantially cylindrical part of which forms a mast 2 with a base 4 designed to be connected in rotation to a main gearbox of the helicopter h shown schematically in phantom outline to drive the rotor in rotation about the axis a of the mast hub 1 the latter also comprises an upper part forming a hub 3 which is an extension of the mast 2 and has the general external shape substantially of a truncated cone hollowed out at the end opposite to the mast 2 as far as a reinforcement ring 5 which constitutes the upper free end thickened locally on its periphery of the hub 3 radial relative to axis a openings 6 identical and equal in number to the number of rotor blades are made in the portion of hub 3 which is directly adjacent the ring 5 and are regularly distributed in a circumferential direction over the periphery of this portion of hub 3 each opening 6 has a substantially half moon or greatly rounded bean shape the general concavity of which is facing the ring 5 which surrounds the opening in the upper end of the hub 3 this ring 5 the shape of which in plan see fig2 is that of a pseudo regular polygon with small sides 5 a equal rounded and convex.