An upstanding bladed rotor is journalled from support structure and a windshield assembly is also journalled from the support structure for angular displacement about an upstanding axis about the rotor. The windshield assembly includes a wind-shielding portion which extends generally about one half the periphery of the rotor and further includes a wind vane operative to maintain the assembly in predetermined position relative to wind incident upon the rotor with the wind-shielding portion in position to shield generally one lateral half of the rotor from the incident wind. The windshield assembly additionally includes a combined wind booster and excess wind controlling vane supported therefrom and disposed outward of the other lateral half of the rotor into the sweep area thereof responsive to high velocity wind incident thereon so as to apply rotary torque to the windshield assembly partially overcoming the directional control thereon by the wind vane and swinging the wind-shielding portion into position across the side of the rotor upon which the wind is incident so as to reduce the amount of high velocity wind available to drive the rotor and thus prevent the latter from overspeeding.