The present invention provides a rotor blade provided at the top with a pairing (3) that is disposed below the rotating blades of the main rotor of a substantially vertical axis of the rotating rotor and covers a mechanism that allows the blades of the rotor blades of the main rotor to operate do. The streamlined shape of the pairing (3) is formed in the longitudinal direction as a truncated droplet at its rear portion. The truncated portion 6 of the water droplet side structure of the pairing 3 forms the rear wall of the pairing 3 and is provided with a hump 7 intermediate in the transverse direction. The hump 7 is provided with protrusions in the relief 9 arranged like waves extending continuously in the longitudinal direction along the sidewalls of the pairing 3, And is disposed on an increasingly inclined plane.
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