TM 1-1510-223-10 2-46            2-43.   PROPELLER GOVERNORS. Two governors, a constant speed governor, and an overspeed    governor,    control    propeller    RPM.    The constant speed governor, mounted on top of the reduc- tion  housing,  controls  the  propeller  through  its  entire range. The propeller controller lever controls the propel- ler by means of this governor. If the constant speed gov- ernor should malfunction and request more than 1700 RPM, the overspeed governor cuts in at 1802 RPM and dumps oil from the propeller to keep the RPM from ex- ceeding approximately 1802 RPM. A solenoid, actuated by the GOVERNOR TEST switch located in the over- head control panel (fig. 2-13), is provided for resetting the overspeed governor to approximately 1540 to 1580 RPM for  test  purposes.  If  the  propeller  sticks  or  moves  too slowly during a transient condition causing the propeller governor to act too slowly to prevent an overspeed condi- tion,  the  power  turbine  governor,  contained  within  the constant speed governor housing, acts as a fuel topping governor. When the propeller reaches 106% of selected N₂ RPM, the power turbine governor limits the fuel flow to the gas generator, reducing N₁ RPM, which in turn pre- vents the propeller from exceeding approximately 1802 RPM. During operation in the reverse range, the power turbine governor is reset to approximately 95% of propel- ler RPM before the propeller reaches a negative pitch angle. This ensures that engine power is limited to main- tain a propeller RPM of somewhat less than that of the constant  speed  governor  setting.  The  constant  speed governor  therefore,  will  always  sense  an  underspeed condition and direct oil pressure to the propeller servo piston to permit propeller operation in beta and reverse ranges. 2-44.   LOW PITCH STOP. Low pitch propeller position is determined by a me- chanically-monitored hydraulic low pitch stop. The pro- peller servo piston is connected by four spring-loaded sliding rods to the beta collar, mounted behind the propel- ler. A carbon brush block riding in the beta collar transfers the movement of the collar through the propeller revers- ing lever to the beta valve of the governor. The initial for- ward motion of the beta valve from its rigged position blocks off the flow of oil to the propeller. Further motion dumps the oil from the propeller into the reduction gear box sump. A mechanical stop limits the forward motion of the beta valve. Rearward movement of the beta valve from its rigged position does not affect normal propeller control. When the propeller is rotating at a speed lower than that selected on the governor, the governor pump provides  oil  pressure  to  the  servo  piston,  decreasing pitch of the propeller blades until the feedback of motion from the beta collar pulls the beta valve into a position blocking the supply of oil to the propeller, thus preventing further pitch changes. 2-45.   GROUND FINE. CAUTION Propeller speeds below 1000 RPM are not au- thorized, unless the propeller is feathered. Lifting the POWER levers and moving them aft past the flight idle stop will place the POWER levers into the ground fine position. Approximately half way back to the ground fine gate, a mechanical linkage at the propeller governor will begin to bleed P_y air from the fuel control unit,  provided  the  PROP  levers  are  positioned  to  the minimum  RPM  position.  This  results  in  a  decrease  in both  engine  N₁,  torque,  and  propeller  RPM.  With  the POWER  levers at the ground fine gate, engine N₁ should be within the range of 62% to 67%, and propeller RPM should not be less than 1000 RPM. 2-46.   PROPELLER SYNCHROPHASER. a.   Description. The propeller synchrophaser matches left and right propeller RPM as well as propeller phase relationship. This phase relationship is designed to decrease cabin noise, and is not adjustable in flight. A toggle   switch,   placarded   PROP   SYN   -   ON   -   OFF, installed adjacent to the synchroscope on the pilot’s in- strument panel (fig. 2-16), turns the system on/off. Signal  pulses  occurring  once  per  revolution  of  the propeller are obtained from magnetic pickups (located in the  front  of  the  engine  on  the  deice  brush  mounting bracket) when the target (mounted on the aft side of the spinner bulkhead) passes the magnetic pickup. The sig- nal pulses are sent to a control box installed forward of the  pedestal.  The  control  box  received  these  signal pulses and compares them for pulse rate and relative position. Differences in pulse rate and/or propeller posi- tion cause the control box to vary the voltage in the prima- ry governor coil, which in turn increases propeller speed until the correct speed and phasing are obtained. A governor coil increases the speed set by the propel- ler control lever, but never decreases the speed set by the control lever. The maximum synchrophaser range is approximately 20 RPM. This limited range prevents ei- ther propeller from losing more than a limited RPM if the other propeller is feathered with the synchrophaser ON. There is no master or slave engine in this type sys- tem. There is a limited range for synchronizing, called the “holding range”. There is a maximum RPM differential (capture   range),   at   which   the   synchrophaser,   when turned on, will begin to synchronize the propellers. For this reason the propellers should be manually synchro- nized before turning the synchrophaser on.