24
In the flight or alpha range, the propeller is governed to a constant 100% RPM, and thrust
changes are made by changing fuel flow and blade angle rather than engine speed. Advancing
the throttle in the flight range causes an increase in fuel flow which results in an increase in
turbine inlet temperature and energy available to the turbine. The turbine extracts more energy
from the gas flow and transmits it the propeller in the form of additional torque. The propeller,
in order to maintain governing speed, increases blade angle to absorb the increased torque.
Operating the engine at constant RPM improves throttle response, as there is no “spool up” time,
and thrust increases can be made by changing blade angle.
Normally, the propellers are governed electronically through the synchrophaser. Electronic
governing provides:
Speed stabilization (rate feedback)
Throttle anticipation
Synchrophasing
There is a backup, purely mechanical governing mode. The MECH mode is selected via four
guarded switches on the copilot’s side shelf:
At the extreme end of the flight range is the “feather” position where the blade pitch is changed
so that the blades have their leading edge pointing into the direction of flight, offering minimum
resistance to the airflow. This mode allows the propeller rotation to be stopped, without adding
excessive drag to the aircraft. Feather is used to improve the performance of the aircraft after the
failure of an engine. In the C-130, there is no auto-feather feature and the feather condition must
be commanded by the aircrew, through either the condition lever or fire handle.
The propellers on the C-130 are not counterweighted, and the blade angle will flatten from
centrifugal force if hydraulic pressure is not maintained on the blade change mechanism. The
propellers must be driven towards feather via valve housing hydraulic pressure. An electrically
powered auxiliary feather motor is used to feather the propeller. When the propeller is signaled
to feather, either via the condition lever or fire handle, the auxiliary feather motor operates.
Operation of the auxiliary feather motor is indicated to the aircrew via solenoid-actuated feather
buttons. When the auxiliary feather motor is operating, the feather override button is pulled down
and a light illuminates. The crew can pull the feather motor override button out to shut off the
feather motor, or push the button in to complete the feather cycle.