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RPM varies with flight attitude in Continental or Lycoming aircraft engine
1. Oil transfer collar leak or malfunction. Once speed control and power
settings are made and established, the governor control valve will be
opened an amount to provide a pressure which balances the centrifugal twisting
moment and spring load, and an oil flow which is determined the leakage
through the crankshaft transfer bearing at that pressure. This will maintain
the blade angle such that set power may be absorbed at the desired speed.
If the attitude of the aircraft is altered and the airspeed changed, the
blade angle will be required to change in order to absorb the power at
this airspeed. This change in blade angle will cause the forces to vary,
and consequently will change the control pressure and the transfer bearing
leakage rate. Since the governor has been adjusted by the speed control
for the first set of conditions, the effect will be to change the effective
equilibrium speed of the propeller with no change in governor setting.
The amount of change varies with engine transfer bearing clearance. This
speed is generally a negligible amount and is not cause for concern since
it is a normal reaction of the control system. If the leakage out the bearing
transfer collar is great, then changes in engine speed may be excessive.
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