the rotatable frame which is the rotatable frame shown in fig7 the upper and lower ones of each pair of blade support arms 11 of the rotatable frame 8 support between distal ends thereof a blade spin axle 13 of the respective blade the rotatable frame 8 has a radius r that is the horizontal distance between a center of the frame arbor 12 and a center of the blade spin axle 13 fig9 is a diagram of the rectangular blade 9 that carries four rollers 10 and that is the rectangular blade 9 shown in fig7 the rectangular blade 9 has upper and lower edges each having opposite ends each carrying a roller 10 the roller 10 is slidable within the recessed rail 7 of the rim like rack 6 for reducing resistance and noise depending upon applications the rectangular blade 9 can be made of various materials such as plate or a sail the rectangular blade 9 has a center line that in the embodiment illustrated coincides with the blade spin axle 13 the blade spin axle 13 has a height that is substantially equal to the thickness of the blade support arms 11 see fig8 plus thickness of rollers height of the rectangular blade 9 and a suitable amount of tolerance the parameter s of the curve tw r s is the distance between centers of left and right side rollers of the blade fig1 is a diagram of the rim like rack 6 which is the rim like rack 6 shown in fig6 the rim like rack 6 has a raised along edge wall having a surface that is recessed to form a recessed rail 7 for receiving the rollers 10 of the rectangular blades 9 to slide in the recessed channel of.