Abstract: An air foil which is coupled to a wind energy conversion machine is provided with a flexible and resilent trailing spoiler that limits the rotational speed of the machine and prevents overspeeding. The trailing spoiler is a flexible and resilient sheet that is laminated to the upwind side of a rigid blade so that a portion of the sheet extends past the trailing edge of the blade. The portion of the sheet that extends past the trailing edge bends towards the wind during high wind conditions to prevent overspeeding of the machine.