Abstract: A windmill for generating electricity in which each blade of the air-screw is connected to the hub by means of at least two resilient strips which are oppositely inclined both in the plane of rotation of the blade and in a plane extending through the rotational axis of the hub. This arrangement results in automatic pitch increase of the blades with increase of air-screw rotation due to the increasing centrifugal force exerted on the blades. This regulates air-screw speed and prevents excessive speed, especially at low load under high-speed wind. This pitch-varying mechanism can also be applied to automotive cooling fans.