TM 55-1510-219-10 AUTO FEATHER, located on the overhead circuit breaker panel (fig.  2-26). 2-45.  PROPELLER GOVERNORS. Two  governors  (a  constant  speed  governor,  and an   overspeed   governor)   control   propeller   RPM.      The constant speed governor, mounted on top of the propeller reduction housing, controls the propeller through its entire range.  The propeller control lever operates the propeller by   means   of   this   governor.      If   the   constant   speed governor should malfunction and request more than 2000 RPM,  the  overspeed  governor  cuts  in  at  2080  RPM  and dumps  oil  from  the  propeller  to  limit  RPM.    A  solenoid, actuated by the PROP GOV TEST switch located on the overhead  control  panel  (fig.    2-18),  is  provided  to  reset the  overspeed  governor  to  approximately  1830  to  1910 RPM  for  test  purposes.    If  the  propeller  sticks  or  moves too  slowly  to  prevent  an  overspeed  condition,  the  power turbine   governor,   contained   within   the   constant   speed governor housing, acts as a fuel topping governor.  When the    propeller    reaches    2120    RPM,    the    fuel    topping governor limits the fuel flow to the gas generator, thereby reducing    the    power    driving    the    propeller.        During operation    in    the    reverse    range,    the    power    turbine governor  is  reset  to  approximately  95%  propeller  RPM before the propeller reaches a negative pitch angle.  This insures  that  the  engine  power  is  limited  to  maintain  a propeller RPM of somewhat less than that of the constant speed  governor  setting.    The  constant  speed  governor will, therefore, always sense an underspeed condition and direct  oil  pressure  to  the  propeller  servo  piston  to  permit propeller operation in beta and reverse range. 2-46.  PROPELLER TEST SWITCHES. Two    two-position    switches    on    the    overhead control   panel   (fig.      2-18),   are   provided   for   operational testing  of  the  propeller  systems.    Placarding  above  the switches reads PROP GOV TEST.  Each switch controls test  circuits  for  the  corresponding  propeller.    In  the  test position, the switches are used to test the function of the corresponding  overspeed  governor.    Refer  to  Chapter  8, for test procedure.  Propeller test circuits are protected by one   5-ampere   circuit   breaker   placarded   PROP   GOV. located on the overhead circuit breaker panel (fig.  2-26). 2-47.  PROPELLER SYNCHROPHASER. a. Operation. The propeller synchrophaser automatically   matches   the   RPM   of   the   right   propeller (slave   propeller)   to   that   of   the   left   propeller   (master propeller) and maintains the blades of one propeller at a predetermined   relative   position   with   the   blades   of   the other propeller.  To prevent the right propeller from losing excessive RPM if the left propeller is feathered while the synchrophaser   is   on,   the   synchrophaser   has   a   limited range    of    control    from    the    manual    governor    setting. Normal     governor     operation     is     unchanged     but     the synchrophaser   will   continuously   monitor   propeller   RPM and  reset  the  governor  as  required.    A  magnetic  pickup mounted    in    each    propeller    overspeed    governor    and adjacent  to  each  propeller  deice  brush  block  transmits electric   pulses   to   a   transistorized   control   box   installed forward  of  the  pedestal.    The  right  propeller  RPM  and phase will automatically be adjusted to correspond to the left.  To change RPM, adjust both propeller controls at the same time.  This will keep the right governor setting within the     limiting     range     of     the     left     propeller.          If     the synchrophaser  is  on  but  is  unable  to  adjust  to  the  right propeller  to  match  the  left,  the  actuator  has  reached  the end   of   its   travel.      To   recenter,   turn   the   switch   off, synchronize  the  propellers  manually,  and  turn  the  switch back on. b. Control Box.  The control box converts any pulse rate   differences   into   correction   commands,   which   are transmitted to a stepping type actuator motor mounted on the  right  engine  cowl  forward  support  ring.    The  motor then  trims  the  right  propeller  governor  through  a  flexible shaft   and   trimmer   assembly   to   exactly   match   the   left propeller.    The  trimmer,  installed  between  the  governor control  arm  and  the  control  cable,  screws  in  or  out  to adjust the governor while leaving the control lever setting constant.      A   toggle   switch   installed   adjacent   to   the synchrophaser turns the system on.  With the switch off, the  actuator  automatically  runs  to  the  center  of  its  range of travel before stopping to assure normal function when used   again.      To   operate   the   system,   synchronize   the propeller in the normal manner and turn the synchrophaser  on.    The  system  is  designed  for  in-flight operations  and  is  placarded  to  be  off  for  take-off  and landing.    Therefore,  with  the  system  on  and  the  landing gear  extended,  the  master  caution  lights  will  illuminate and   a   yellow   light   on   the   caution/advisory   annunciator panel, PROP SYNC ON, will illuminate. c. Synchroscope.        The    propeller    synchroscope, provides an indication of synchronization of the propellers.    If  the  right  propeller  is  operating  at  a  higher RPM than the left, the face of the synchroscope, a black and   white   cross   pattern,   spins   in   a   clockwise   rotation. Left, or counterclockwise, rotation indicates a higher RPM of  the  left  propeller.    This  instrument  aids  the  pilot  in obtaining   complete   synchronization   of   propellers.      The system    is    protected    by    a    5-ampere    circuit    breaker placarded  PROP  SYNC,  located  on  the  overhead  circuit breaker panel (fig.  2-26). 2-40