Unique Aerodynamic Design
In variable-pitch fan as describe later the blades are turned in the hub about their longitudinal or pitch-change axis. Clearly the mechanism provided to produce this pitch change must be capable of exerting sufficient force to overccome any mechanical or aerodynamic opposing force set up by the blades themeselves. It will be useful then to see exactly what these force are.
The "Mechanical Force" involved is known as Centrifugal Twisting Moment (C.T.M) which as its name implies is closely allied to the normal centrifugal force acting on the blades when the fan is rotating about the shaft. It is turning couple brought about by the fact that the blade section are inclined at an angle to the fan of rotation and results in the natural tendency for any fan blade. when rotating turned about its longitudinal axis towards zero pitch so that the blades section are turned in to the fan blade rotation
The main factor that determines the force developed by an Aerofoil secction is the angle at which it is inclined to the relative airflow, i.e. the angle of attack in order, therefore, for the fan blade section to develop the requisite aerodynamic force for propulsion, each section along the blade must be inclined at the appropriate angle of attack to the relative airflow direction pertaining to the section hence knowing angle of attack is asimple matter to indicate the force developed from which it will be noted that the total blade angle of attack and the helix angle.