PURPOSE: To increase the strength by constructing a core to be inserted in a wing-shaped vane in the form of a channel, installing ends of two legs at a hub, and by setting a core mounting hole in the hub so that it is oriented at the specified angle of the wing-shaped vane.

CONSTITUTION: Each core 3 in the form of a channel is inserted in mounting holes 4, which are arranged in pairs each having a distance from each other to suit the distance of two legs of the core 3 and which are oriented at the same angle as the specified angle of a wing-shaped vane 2, and the two members area welded fast from inside the peripheral cylindrical part 1a of the hub. Be cause the central part 3b of each core 3 is so located as to be stretching longitu dinal to the profile of the wing-shaped vane 2 and identical to its axis, it sustains surely with the stress generated in the vane by centrifugal force.