A windmill converting wind forces into rotational energy that can be used to drive a generator or other apparatus requiring rotational energy to work. The windmill includes panels for capturing the wind arranged around a vertically rotating shaft. The panels are able to move from a vertical position when receiving the force of the wind to a horizontal position when rotated to the upwind side of the rotation, thus minimizing their aerodynamic resistance. The panels can also be moved toward or away from the vertical shaft and the area of the panels can be changed so as to maintain a constant speed of rotation of the vertical shaft.