Month in the Patent Office

A control system for a variable pitch propeller includes pitch lock means operable by the onset of a predetermined negative‐torque in the propeller drive transmission to lock the propeller against pitch fining movement. In the embodiment of Fig. 1, a double acting hydraulic pitch change motor 12 having a fixed brake stop 26 and a fixed feathering stop 27 has a fine pitch line 13 and a coarse pitch line 15. To coarsen the pitch oil is supplied under pressure via line 14, by‐pass 16 and non‐return valve 29 to the left hand side of the motor piston 10, oil on the other side exhausting through line 13. To reduce pitch oil under pressure is supplied to the line 13 from which it branches to act on the right hand side of the piston 10 and to move the pitch lock valve 17 against the action of its spring 22 so that oil from the left‐hand side of the piston can exhaust through said valve into line 14. The port 32 of the reduction gearing driving the propeller shaft 30 is connected to hydraulic piston and cylinder units 33 forming port of a torquemeter so that when negative‐torque is experienced, i.e. when the propeller windmills and drives the engine, the port 32 moves clockwise, thus tripping a lever 25 to raise a spring‐loaded spill valve 23 which connects the fine pitch line 13 with a low pressure line 24, thereby relieving pressure on the valve 17. The spring 22 then closes said valve, forming a hydraulic lock in the motor cylinder so that fining of the propeller pitch with consequent increase in windmilling drag, is prevented. In another embodiment the spill valve is replaced by a multi‐purpose increase pitch valve and the system includes main control, servo, and negative‐torque signal valves so arranged that with the onset of negative‐torque the lock valve 17 closes as in the first embodiment but the coarse pitch line is placed in direct communication with the delivery side of the governor pump. The propeller moves to a coarser pitch, the windmilling and the negative‐torque are reduced, and the propeller is returned to the control of the main control valve.