Abstract: A rotor is intended to rotate in a fluid flowing in a flow direction and comprises a plurality of rotor blades, at the most four blades, which are arranged to rotate about an axis of rotation. Each blade forms a single-curved surface, the generatrix of which extends in a plane normal to the axis of rotation, and an imaginary plane interconnecting the leading edge and trailing edge of the blade forms an acute angle with the axis of rotation. In addition, each blade is pivotally mounted for pivotal movement about a pivot shaft in order to adjust the acute angle, the pivot shaft extends in a plane that is normal to the axis of rotation and is parallel to and spaced a distance from an imaginary radius radiating from the axis of rotation. The rotor is very silent and provides efficient energy conversion, for example extraction of wind power.