Abstract: A wind turbine for converting wind forces into usable energy has a main shaft rotatably mounted in the axis of rotation for the wind turbine, and a pair of coacting complementary longitudinally extending vane members are connected to each other by a plurality of support and transmission assemblies in the form of articulated members and to the main or driven shaft for driving engagement thereof. The coacting vane members may either have straight side edges or preferably have spiralled or helically shaped side edges which in the closed position are aligned and in abutment with each other to define and form a right circular cylinder in side elevation. The vanes are movable between a normally open starting position and a closed position to vary the total vane surface available for contact by the wind forces acting at any given time when the wind turbine is in operation.