The line normal to the shaft axis is called either propeller reference line or
directrix. In the case of controllable pitch propeller the spindle axis is used as
synonymous with the reference line.
• Generator line: The line formed by intersection of the pitch helices and the
plane containing the shaft axis and propeller reference line.
• The aerofoil sections which together comprise the blade of a propeller are
defined on the surfaces of cylinders whose axes are concentric with the shaft
axis.