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14-5
affected propeller feathered. For this reason, the fixed
shaft turboprop engine is equipped with negative
torque sensing (NTS).
Negative torque sensing is a condition wherein
propeller torque drives the engine and the propeller is
automatically driven to high pitch to reduce drag. The
function of the negative torque sensing system is to
limit the torque the engine can extract from the
propeller during windmilling and thereby prevent large
drag forces on the airplane. The NTS system causes a
movement of the propeller blades automatically
toward their feathered position should the engine
suddenly lose power while in flight. The NTS system
is an emergency backup system in the event of sudden
engine failure. It is not a substitution for the feathering
device controlled by the condition lever.
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. [Figure 14-5] 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.
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.
Reduction
Gearbox
Propeller
Drive Shaft Fr ee (Power)
Tu r bine Compressor
Tu r bine
(Gas Producer)
Three Stage
Axial Flow
Compressor
Exhaust Outlet
Air Inlet
Centrifugal
Compressor
Igniter
Fuel Nozzle
Igniter
Fuel Nozzle
Accessory
Gearbox
Figure 14-5. Split shaft/free turbine engine.
Ch 14.qxd 5/7/04 10:09 AM Page 14-5
14-6
Powerplant (engine and propeller) operation is
achieved by three sets of controls for each engine: the
power lever, propeller lever, and condition lever.
[Figure 14-6] 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.
Engine instruments in a split shaft/free turbine engine
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