Constant speeding propeller:
The definition of a Constant Speeding propeller is 'A propeller, the pitch setting of which varies automatically to maintain a preselected constant rotational speed'.
The variable pitch propeller overcomes the disadvantages of the fixed and two pitch propellers by maintaining the best propeller speed to suit the engine power output. The propeller control unit (PCU) or constant speed unit (CSU) maintains the selected RPM through all changes in flight conditions without any assistance from the pilot, it will automatically select the optimum angle of attack for the propeller blades at all times.
Other facilities will be incorporated into the mechanism to enable 'feathering' and power on braking.
Figure Illustrates the principles of operation of a fixed cylinder constant speed system.