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Flying Handbook Menu > Transition to Turbopropeller Powered Airplanes > Turboprop Engine Types > Fixed Shaft
FIXED SHAFT
One type of turboprop engine is the fixed shaft constant speed
type such as the Garrett TPE331. [figure14-2] In this type
engine, ambient air is directed to the compressor section through
the engine inlet. An acceleration/diffusion process in the twostage
compressor increases air pressure and directs it rearward to
a combustor. The combustor is made up of a combustion chamber,
a transition liner, and a turbine plenum. Atomized fuel is added
to the air in the combustion chamber. Air also surrounds the
combustion chamber to provide for cooling and insulation of
the combustor.
The gas mixture is initially ignited by high-energy
igniter plugs, and the expanding combustion gases flow to the
turbine. The energy of the hot, high velocity gases is converted
to torque on the main shaft 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.
Powerplant (engine and propeller) control is
achieved by means of a power lever and a condition lever for
each engine. [figure14-3] 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.
figure14-3. Powerplant controls—fixed
shaft turboprop engine.
Powerplant instrumentation in a fixed shaft
turboprop engine typically consists of the following basic indicator.
[figure14-4]
figure14-4. Powerplant instrumentation—fixed
shaft turboprop engine.
• 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 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.
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