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Output power of the power turbine (a function of the
speed and torque) is restricted by limiting the maximum
fuel flow to the gas producer. Maximum gas producer
rotor speed is set by the ENG COND (engine condition)
levers in the cockpit. The ENG COND levers electromechanically
positions the gas producer lever, which controls
the fuel control fuel shutoff valve and operating level
of the gas producer. During flight, the ENG COND levers
are left at FLT and the output shaft speed is regulated by
the power turbine speed (N2) governor.
The power turbine lever is electromechanically positioned
by the ENGINE BEEP TRIM switches, thrust control,
and EMERG ENG TRIM (emergency engine trim)
712 switches. Output shaft torques are limited by the
fuel flow limiter, which limits the maximum fuel flow. The
position of the main metering valve is determined by the
gas producer speed governor, power turbine speed governor,
the acceleration-deceleration control, or the fuel
flow limiter, depending on engine requirements at that
time. The governor or the control unit demanding the
least fuel flow overrides the other in regulating the metering
valve.
2-3-7. Speed Governing.
The power turbine speed governor senses the speed of
the power turbine and regulates the amount of fuel which
is supplied to the gas producer. This slows down or
speeds up the gas producer rotor so that power turbine
and rotor system speed remains nearly constant as loads
vary.
At minimum rotor blade pitch, the amount of power required
is at minimum. As pitch is increased, power turbine
speed (N2) starts to decrease since more power is
required from the engine to maintain a constant rotor
speed. The power turbine speed governor senses the
decrease of N2 RPM and increases the flow of fuel to the
gas producer. Decreasing pitch causes N2 to increase.
TM 1-1520-240-10
2-3-2
The power turbine governor senses the increases and
reduces the flow of fuel to the gas producer, thus decreasing
the engine output power.
The power turbine speed governor allows the power turbine
output speed to decrease (droop) approximately 10
percent when the power loading varies from minimum to
full load. This is minimized by a droop eliminator linked
to the thrust control rod. The droop eliminator automatically
changes the power turbine lever to compensate for
droop as pitch is increased or decreased. Another type
of droop, which is only transient, occurs as a result of the
time required for the engine to respond to changing loads
due to system lag.
2-3-8. ENG COND Levers 712
Two ENG COND (engine condition) levers, one for each
engine are on the ENG COND panel (fig. 2-3-1) of the
overhead switch panel. Each lever has three positions
labeled STOP, GND, and FLT. They are used to select
appropriate fuel flow rates for GND, FLT, and STOP (engine
shutdown). Power is supplied by the DC essential
buses through the ENGINE NO. 1 and NO. 2 COND
CONT circuit breakers on the No. 1 and No. 2 PDP.
Each ENG COND lever is spring-loaded outboard and is
inhibited by lock gates. They allow the pilot to proportionally
control acceleration of the gas producer from STOP
to FLT. Two engine control caution capsules are on the
master caution panel (fig 2-14-5). They are labeled NO.
1 ENG N1 CONT and NO. 2 ENG N1 CONT. The capsules
normally illuminate when the ENG COND levers or
the N1 actuators are at an intermediate position between
STOP, GND, or FLT. They extinguish when the ENG
COND lever and N1 actuator positions agree. However,
they remain illuminated if a component of the system
(actuator, control box, or condition panel) has failed in
other than a detent position. Power is supplied by the DC
essential bus through the LIGHTING CAUTION PNL circuit
breaker on the No. 1 PDP.
Figure 2-3-1. Engine Condition Panel 712
CAUTION
When the ENG COND lever is placed to
GND during start sequence, the N1 actuator
could inadvertently go beyond the
ground position. The respective ENG N1
COND caution capsule will illuminate.
However, ignition will still occur if the start
switch is moved to START, thus resulting
in a possible engine runaway.
CAUTION
When adjusting controls or switches on
the overhead switch panel, make sure
gloves or sleeves do not catch and inadvertently
move the ENG COND levers.
The ENG COND lever must be at GND before the engine
will start. When an ENG COND lever is advanced from
STOP to GND, power is then supplied to the electromechanical
actuator which establishes an appropriate fuel
　
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