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Flying Handbook Menu > Transition to Turbopropeller Powered Airplanes > Turboprop Engine Types > Split Shaft / Free
Turbine Engine
In a free power-turbine engine, such as the
Pratt & Whitney PT-6 engine, the propeller is driven by
a separate turbine through reduction gearing. The propeller
is not on the same shaft as the basic engine turbine and compressor.
[figure14-5] Unlike the fixed shaft engine, in the split shaft
engine the propeller can be feathered in flight or on the ground
with the basic engine still running. The free power-turbine
design allows the pilot to select a desired propeller governing
r.p.m., regardless of basic engine r.p.m.
figure14-5. Split shaft/free turbine
engine.
A typical free power-turbine engine has two
independent counter-rotating turbines. One turbine drives the
compressor, while the other drives the propeller through a reduction
gearbox. The compressor in the basic engine consists of three
axial flow compressor stages combined with a single centrifugal
compressor stage. The axial and centrifugal stages are assembled
on the same shaft, and operate as a single unit.
Inlet air enters the engine via a circular
plenum near the rear of the engine, and flows forward through
the successive compressor stages. The flow is directed outward
by the centrifugal compressor stage through radial diffusers
before entering the combustion chamber, where the flow direction
is actually reversed. The gases produced by combustion are once
again reversed to expand forward through each turbine stage.
After leaving the turbines, the gases are collected in a peripheral
exhaust scroll, and are discharged to the atmosphere through
two exhaust ports near the front of the engine.
Apneumatic fuel control system schedules fuel
flow to maintain the power set by the gas generator power lever.
Except in the beta range, propeller speed within the governing
range remains constant at any selected propeller control lever
position through the action of a propeller governor.
The accessory drive at the aft end of the engine
provides power to drive fuel pumps, fuel control, oil pumps,
a starter/generator, and a tachometer transmitter. At this point,
the speed of the drive (N1) is the true speed of the compressor
side of the engine, approximately 37,500 r.p.m.
Powerplant (engine and propeller) operation
is achieved by three sets of controls for each engine: the power
lever, propeller lever, and condition lever. [figure14-6] The
power lever serves to control engine power in the range from
idle through takeoff power. Forward or aft motion of the power
lever increases or decreases gas generator r.p.m. (N1) and thereby
increases or decreases engine power. The propeller lever is
operated conventionally and controls the constant-speed propellers
through the primary governor. The propeller r.p.m. range is
normally from 1,500 to 1,900. The condition lever controls the
flow of fuel to the engine. Like the mixture lever in a piston-powered
airplane, the condition lever is located at the far right of
the power quadrant. But the condition lever on a turboprop engine
is really just an on/off valve for delivering fuel. There are
HIGH IDLE and LOW IDLE positions for ground operations, but
condition levers have no metering function. Leaning is not required
in turbine engines; this function is performed automatically
by a dedicated fuel control unit.
figure14-6. Powerplant controls—split
shaft/free turbine engine.
Engine instruments in a split shaft/free turbine
engine typically consist of the following basic indicators.
[figure14-7]
• ITT (interstage turbine temperature)
indicator.
• Torquemeter.
• Propeller tachometer.
• N1 (gas generator) tachometer.
• Fuel flow indicator.
• Oil temperature/pressure indicator.
figure14-7. Engine instruments—split
shaft/free turbine engine.
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 figureof 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.
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