Windrose

a small windwheel, usually with many blades, placed behind or in front of a driving windwheel so that their planes of rotation are perpendicular to one another. The windrose automatically orients the head of the windmill relative to the air current. When the direction of the wind coincides with its plane of rotation, it is motionless; when the direction of the wind changes, lift force comes into play on its blades. As the windrose rotates, it turns the head until the plane of rotation of the driving windwheel is perpendicular to the air current. In this position the wind-mill will produce the most power. Windroses are most often used in high-speed windmills with at least 2 hp.