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typically consist of the following basic indicators.
[Figure 14-7]
• ITT (interstage turbine temperature) indicator.
• Torquemeter.
• Propeller tachometer.
• N1 (gas generator) tachometer.
• Fuel flow indicator.
• Oil temperature/pressure indicator.
Figure 14-6. Powerplant controls—split shaft/free turbine
engine.
Figure 14-7. Engine instruments—split shaft/free turbine
engine.
Ch 14.qxd 5/7/04 10:09 AM Page 14-6
14-7
The ITT indicator gives an instantaneous reading of
engine gas temperature between the compressor
turbine and the power turbines. The torquemeter
responds to power lever movement and gives an
indication, in foot-pounds (ft/lb), of the torque being
applied to the propeller. Because in the free turbine
engine, the propeller is not attached physically to the
shaft of the gas turbine engine, two tachometers are
justified—one for the propeller and one for the gas
generator. The propeller tachometer is read directly in
revolutions per minute. The N1 or gas generator is read
in percent of r.p.m. In the Pratt & Whitney PT-6
engine, it is based on a figure of 37,000 r.p.m. at 100
percent. Maximum continuous gas generator is limited
to 38,100 r.p.m. or 101.5 percent N1.
The ITT indicator and torquemeter are used to set
takeoff power. Climb and cruise power are established
with the torquemeter and propeller tachometer while
observing ITT limits. Gas generator (N1) operation is
monitored by the gas generator tachometer. Proper
observation and interpretation of these instruments
provide an indication of engine performance
and condition.
REVERSE THRUST AND BETA RANGE
OPERATIONS
The thrust that a propeller provides is a function of the
angle of attack at which the air strikes the blades, and
the speed at which this occurs. The angle of attack
varies with the pitch angle of the propeller.
So called “flat pitch” is the blade position offering
minimum resistance to rotation and no net thrust for
moving the airplane. Forward pitch produces forward
thrust—higher pitch angles being required at higher
airplane speeds.
The “feathered” position is the highest pitch angle
obtainable. [Figure 14-8] The feathered position
produces no forward thrust. The propeller is generally
placed in feather only in case of in-flight engine failure
to minimize drag and prevent the air from using the
propeller as a turbine.
In the “reverse” pitch position, the engine/propeller
turns in the same direction as in the normal (forward)
pitch position, but the propeller blade angle is
positioned to the other side of flat pitch. [Figure 14-8]
In reverse pitch, air is pushed away from the airplane
rather than being drawn over it. Reverse pitch results
in braking action, rather than forward thrust of the airplane.
It is used for backing away from obstacles when
taxiing, controlling taxi speed, or to aid in bringing the
airplane to a stop during the landing roll. Reverse pitch
does not mean reverse rotation of the engine. The
engine delivers power just the same, no matter which
side of flat pitch the propeller blades are positioned.
With a turboprop engine, in order to obtain enough
power for flight, the power lever is placed somewhere
between flight idle (in some engines referred to as
“high idle”) and maximum. The power lever directs
signals to a fuel control unit to manually select fuel.
The propeller governor selects the propeller pitch
needed to keep the propeller/engine on speed. This is
referred to as the propeller governing or “alpha” mode
of operation. When positioned aft of flight idle, however,
the power lever directly controls propeller blade
angle. This is known as the “beta” range of operation.
The beta range of operation consists of power lever
positions from flight idle to maximum reverse.
Power Prop Condition
Reverse
Beta
Idle
Feather
Normal
"Forward" Pitch
Feather
"Maximum Forward
Pitch"
Flat Pitch
Reverse Pitch
Fuel
Cut
Off
Reverse
Fuel
Cut
Off
Reverse Feather
Fuel
Cut
Off
Feather
Fuel
Cut
Off
Low
Idle
Flt
Idle
Low
Idle
Flt
Idle
Low
Idle
Flt
Idle
Low
Idle
Flt
Idle
Pull
Up
Figure 14-8. Propeller pitch angle characteristics.
Ch 14.qxd 5/7/04 10:09 AM Page 14-7
　
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