Abstract: A reaction-type turbine provided with an output shaft coupled to a power generator, such as an electric generator, and having a plurality of variable-pitch flexible impeller blades, or sails, mounted between the periphery of the output shaft and a concentric tubular shroud rotatable in unison with the output shaft. The pitch of the flexible impeller blades or sails is varied by a centrifugally-actuated pitch controlling unit such that the output shaft has a constant angular velocity irrespective of variations in the velocity of the fluid flowing through the turbine. For actuation by wind, the turbine is mounted on a rotatable platform which is automatically oriented in the direction of the prevalent wind by a servo system controlled by a wind direction detecting device. The mass of air flowing through the turbine is automatically controlled as a function of the wind velocity by an appropriate throttling mechanism controlled by a wind velocity detector.