Feathering
Feathering is achieved through a mechanical linkage that overrides the flyweights and speeder spring. When the cockpit control is moved to ³feather,² the governor lever and shaft are turned beyond normal low-RPM operating limits. As the threaded shaft backs out, the shaft lift rod engages the pilot valve spindle and lifts the pilot valve. This causes oil to flow out of the propeller, and it moves to feather pitch position. (Fig. 12)
Unless the airplane is equipped with the unfeathering accumulator option, the pilot can ³unfeather² the propeller by moving the propeller control to high RPM (low pitch) and engaging the engine starter. When the engine is turning over fast enough to develop sufficient oil pressure, the propeller blades will be forced out of feather.
The unfeathering accumulator option permits a feathered propeller to be unfeathered in flight for air-starting the engine. With this option, the governor is modified to provide an external high-pressure oil outlet through a check valve, as well as a device for unseating the check valve. The external outlet is connected to an accumulator. One side of the accumulator is filled with compressed nitrogen and the other side with oil. This allows the oil to be stored under high pressure, as it is during normal flight. (Fig. 13) When the propeller is feathered, the check valve maintains oil pressure in the accumulator. (Fig. 14) When the propeller control is moved from feather to low pitch, the check valve is unseated, permitting the high-pressure oil in the accumulator to flow to the governor pilot valve. With the governor control lever and shaft in low pitch, the speeder spring forces the pilot valve down so that the oil flows to the propeller and moves the blades to low pitch. (Fig. 15)
