an anemometer for measuring the component of air or of a similar fluid speed along its axis of rotation , in which the anemometer has a plurality of generally radially directed , helicoidal airfoils . the airfoil effect is produced by the helicoidal twisting which results in a vacuum being produced on one side thereof according to the direction of rotation . each of the airfoils has a plurality of helicoidal edges , each edge being formed by the intersections of two or more plane surfaces so as to form airfoils having parallelogram or substantially parallelogram cross - sections . each airfoil extends outwardly from a rotatable hub and is attached to the hub at its inner end but is spaced in part from the hub by a cutout area , such as a slot across the airfoil . the spaces formed along the surfaces of the hub cause each airfoil to effectively perform as a true airfoil with trailing vortices produced at the airfoil tip and next to the hub as a result of the aerodynamic lift forces on the blades , whereby the blades respond accurately to the component of wind along the axis of rotation .