an installation for extracting useful work , such as driving an electric generator from a fluid stream , includes a platform rotatable about a central axis and supporting a plurality of blades or vanes , each of which is rotatable about its individual blade axis to vary the angle of attack between the blades and the fluid stream during each revolution of the platform . the blades are coupled together so that each blade executes a similar motion during each platform revolution and so that the angle of attack of the fluid stream relative to a given blade is substantially zero when the fluid stream direction is perpendicular to the plane including the central platform axis and the axis of the given blade , the amount of variation in blade angle of attack during each revolution is variable to compensate for variations in the fluid flow .