Blade pitch change control system

A pitch change control system for use in a pitch change actuation system for adjusting the pitch of a blade of a variable pitch propeller, the blade rotatively mounted to a hub of the propeller, said pitch change actuation system having a pitch change actuator piston operatively connected to the blade and said pitch change control system operatively connected to the pitch change actuator piston for selectively pressuring the pitch change actuator piston to effectuate a desired change in the pitch of the blade, said pitch change control system comprising:

• a. a primary electrohydraulic valve operative during normal operation to receive a pitch change fluid at a supply pressure and to pass a first portion of the pitch change fluid therefrom through a first conduit as a coarse pitch fluid at a modulated pressure less than the supply pressure and to pass a second portion of the pitch change fluid therefrom through a second conduit as a fine pitch fluid at the supply pressure, said primary electrohydraulic valve having a valve chamber and a spool selectively positionable within the valve chamber, the modulated pressure at which the coarse pitch fluid is passed through said first conduit to the pitch change actuator being a function of the position of the spool within the valve chamber;
• b. an electronic control means operatively associated with said electrohydraulic valve for selectively positioning the spool thereof within the valve chamber thereof to provide blade pitch control, overspeed protection, low pitch stop limit protection and feathering during normal operation;
• c. a protection valve disposed between said primary electrohydraulic valve and the pitch change actuator and having a valve chamber communicating with said first and second conduits and a spool selectively positionable within the valve chamber, said protection valve operative to pass the coarse pitch fluid at a first pressure and the fine pitch fluid at a second pressure to the pitch change actuator piston, the first pressure at which the coarse pitch fluid is passed and the second at which the fine pitch fluid is passed to the pitch change actuator being a function of the position of the protection valve spool within the valve protection chamber;
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• d. a protection solenoid operatively associated with said electronic control means and said protection valve, said protection solenoid operative in the event of a failure of said electronic control means to selectively position the spool of said protection valve away from a first position at which said protection valve passes the coarse pitch fluid at the modulated pressure and the fine pitch fluid at the supply pressure as received from said primary electrohydraulic valve whereby authority for supply of the coarse and the fine pitch change fluid to the pitch change actuator piston is transferred from said primary electrohydraulic valve to said protection valve.