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by the turbine rotors. The reduction gear converts the
high r.p.m.—low torque of the main shaft to low
r.p.m.—high torque to drive the accessories and the
propeller. The spent gases leaving the turbine are
directed to the atmosphere by the exhaust pipe.
Only about 10 percent of the air which passes through
the engine is actually used in the combustion process.
Up to approximately 20 percent of the compressed air
may be bled off for the purpose of heating, cooling,
cabin pressurization, and pneumatic systems. Over
half the engine power is devoted to driving the
compressor, and it is the compressor which can
potentially produce very high drag in the case of a
failed, windmilling engine.
In the fixed shaft constant-speed engine, the engine
r.p.m. may be varied within a narrow range of 96
percent to 100 percent. During ground operation, the
r.p.m. may be reduced to 70 percent. In flight, the
engine operates at a constant speed, which is
maintained by the governing section of the propeller.
Power changes are made by increasing fuel flow and
propeller blade angle rather than engine speed. An
increase in fuel flow causes an increase in temperature
and a corresponding increase in energy available to the
turbine. The turbine absorbs more energy and
transmits it to the propeller in the form of torque. The
increased torque forces the propeller blade angle to be
increased to maintain the constant speed. Turbine
temperature is a very important factor to be considered
in power production. It is directly related to fuel flow
and thus to the power produced. It must be limited
because of strength and durability of the material in the
combustion and turbine section. The control system
schedules fuel flow to produce specific temperatures
and to limit those temperatures so that the temperature
tolerances of the combustion and turbine sections are
not exceeded. The engine is designed to operate for its
entire life at 100 percent. All of its components, such
as compressors and turbines, are most efficient when
operated at or near the r.p.m. design point.
Planetary
Reduction Gears
Air
Inlet
First-Stage
Centrifugal
Compressor
Second-Stage
Centrifugal
Compressor
Reverse-Flow
Annular Combustion
Chamber
Three-Stage
Axial Turbine
Fuel
Nozzle
Igniter
Exhaust
Outlet
Figure 14-2. Fixed shaft turboprop engine.
Ch 14.qxd 5/7/04 10:08 AM Page 14-3
14-4
Powerplant (engine and propeller) control is achieved
by means of a power lever and a condition lever for
each engine. [Figure 14-3] There is no mixture control
and/or r.p.m. lever as found on piston engine airplanes.
On the fixed shaft constant-speed turboprop engine,
the power lever is advanced or retarded to increase or
decrease forward thrust. The power lever is also used
to provide reverse thrust. The condition lever sets the
desired engine r.p.m. within a narrow range between
that appropriate for ground operations and flight.
Powerplant instrumentation in a fixed shaft turboprop
engine typically consists of the following basic
indicator. [Figure 14-4]
• Torque or horsepower.
• ITT – interturbine temperature.
• Fuel flow.
• RPM.
Torque developed by the turbine section is measured
by a torque sensor. The torque is then reflected on a
cockpit horsepower gauge calibrated in horsepower
times 100. Interturbine temperature (ITT) is a
measurement of the combustion gas temperature
between the first and second stages of the turbine
section. The gauge is calibrated in degrees Celsius.
Propeller r.p.m. is reflected on a cockpit tachometer as
a percentage of maximum r.p.m. Normally, a vernier
indicator on the gauge dial indicates r.p.m. in 1 percent
graduations as well. The fuel flow indicator indicates
fuel flow rate in pounds per hour.
Propeller feathering in a fixed shaft constant-speed
turboprop engine is normally accomplished with the
condition lever. An engine failure in this type engine,
however, will result in a serious drag condition due to
the large power requirements of the compressor being
absorbed by the propeller. This could create a serious
airplane control problem in twin-engine airplanes
unless the failure is recognized immediately and the
Power Levers
Condition Levers
Figure 14-3. Powerplant controls—fixed shaft turboprop engine.
Figure 14-4. Powerplant instrumentation—fixed shaft
turboprop engine.
Ch 14.qxd 5/7/04 10:09 AM Page 14-4
　
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