15 MODULE : GAS TURBINE ENGINES
15 MODULE
GAS TURBINE ENGINE
AS OUTLINED IN PART-66, APPENDIX 1.
NOTES
SUB-MODULE 17 - TURBO-SHAFT ENGINES
Arrangements, drive systems, reduction gearing, couplings, control systems.
15 Sub Module : 17 Turbo Shaft Engines
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15 MODULE : GAS TURBINE ENGINES
TURBOSHAFT ENGINES
1. The turbine engines used in helicopters come in
various sizes from a range of 300 to 3000 or more
horsepower.
2. They are referred to as shaft turbine or turbo shafts.
This is because the power extracted from the engine is
used to turn a shaft. The shaft turns the transmission
for the main rotor and tail rotor of the helicopter.
3. The engine drives the main rotor through a
transmission and belt or centrifugal clutch system. The
anti-torque rotor is driven from the transmission.
4. The shaft turbine may produce some thrust but it is
primarily designed to produce shaft horse power.
Accomplished by using the same basic components
found in a turbojet with the addition of turbine wheel (wheels) to absorb the power of the escaping
gases of combustion.
5. Turbo shaft
engine may
also contain a
gear reduction
to reduce the
speed of the
turbine, which
exceeds
30,000 RPM
on some
engines.
6. CLASSIFICATION :
A) Direct shaft turbine
B) Free shaft turbine
7. Both types have been
successful, however the free
turbine is the most popular.
8. Turbine engines are ideally
suited for powering
helicopters, because they
operate most efficiently at
the constant RPM required
by a helicopter.
9. Most turbo-shaft engines drive their
output shaft with multistage free
turbine that extracts as must energy
as possible from the exhaust gases.
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
Fig, Turbo shaft engine design
Typical turboshaft with Axial flow compressor and free turbine
COMPONENTS
1. Basic components : compressor, diffuser, combustor,
turbine wheel or wheels, and exhaust.
2. The free turbine derives its power from a separate turbine not connected to the compressor (figs.)
3. The direct drive turbo shaft engine must run at a constant speed, with the compressor and
the power output attached
to the same shaft.
4. The free turbine, however,
can vary the speed of the
compressor as required to
maintain power
turbine output.
1. Engine inlet and
particle separator
head
2. Air Starter and
accessory box
3. Oil cooler fan
4. TV3-117 VM engine
Fig. Engine fitted on Mi-8 Russian Helicopter
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15 MODULE : GAS TURBINE ENGINES
5. VR – 14 Main Transmission of Main Gear Box
6. Tail rotor drive shaft
ENGINE OPERATION
COMPRESSOR
1. The airflow through the turbo shaft engine begins at the inlet where the air enters the compressor .
2. The most popular shaft turbine today is a combination of radial and centrifugal compressors.
3. Usually first stages of compression will be axial while the last stage will be centrifugal.
4. COMBINATION OF AXIAL and CENTRIFUGAL COMPRESSOR REASONS
a) By reducing the number stages of axial flow compressor, the size of the engine may be
reduced in length without an appreciable change in diameter and if the engine were built
with a centrifugal compressor, only the diameter of the engine would increase.
b) The likely hood of foreign object damage is greatly reduced by eliminating the smaller
stages of axial compression.
c) The most significant reason, the speed of the airflow through the engine is increased by
using this combination of axial and centrifugal compressor.
DIFFUSER
1. The air from the combined compressor is then directed to the
diffuser section where it is straightened before entering the
combustion area.
2. Some of this air is used to support combustion and the rest
used for cooling purpose.
COMBUSTOR
1. Although different types of combustors may be used, the
most popular for the turbo shaft is annular combustor using
reverse flow. (Figure)
2. At this point fuel is added to the air and is ignited.
3. This heats the air which rapidly expands and passes through
a nozzle assembly and is directed to the turbine wheel.
4. This wheel is rotated by the gases passing over it and turns
the compressor, which generates more air to repeat the process.
TURBINE
1. It is at this point the two turbo shafts vary in operation.
2. In the free turbine another turbine wheel is placed behind the
compressor turbine.
This turbine drives a shaft to power the helicopter.
3. The direct drive turbine may also have another turbine wheel
but this is fixed to the compressor and also drives a power shaft
used to power the helicopter.
4. The way the shaft and turbine are used for power varies from engine to engine.
Fig. Cold end drive turbo shaft
15 Sub Module : 17 Turbo Shaft Engines
Fig. Hot end drive turbo shaft
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5. Some engines drive from the compressor end(cold end) while others drive from the combustor
end (hot end). Both have advantages and disadvantages.
6. The main advantage of driving form the hot end is that less shafting is required. However, the
gear reduction is exposed to more heat in this area.
POWER TO WEIGHT RATIO
1. With the reciprocating engine, a weight ratio of one pound per horsepower was considered ideal
and today it is not unusual to produce a turbine with ratio excess of four horsepower per pound.
2. This increases the payload of the helicopter, provides extra power for emergency condition and
under certain atmospheric condition that require increased power.
3. SHAFT HORSE POWER (SHP) : The power the engine was specifically built to produce.
4. EQUIVALENT SHAFT HORSE POWER : Another term sometimes used to indicate the
power of a turbo shaft engine. ESHP takes into consideration both the shaft horsepower
delivered to
a) rotor shaft through gear box, and
b) the thrust developed at the engine exhaust and is calculated as additional power.
FLAT RATED
1. It is known that both the engine performance and the helicopter performance will deteriorate with
changes in atmospheric conditions.
2. A turbo-shaft engine whose allowable output power is less than the engine is physically capable
of producing.
TORQUE LIMITED
1. The term is not used in regards to the engine but, power the engine may deliver to the
transmission.
2. It is the transmission that is torque limited or capable of withstanding only so much of the engine’s
potential power output.
3. This can however, be of some benefit during operations when engine power deteriorates.
4. If all the power is not used, additional power may be available because of the torque limitations.
This often occurs with the single engine operation of a twin engine helicopter.
PARTICLE SEPARATORS
1. The turbo shaft engine require a greater airflow operation, and
one of the most critical requirements of this air is that it
be clear of foreign objects.
2. This can be difficult in helicopter operations where
landings are often conducted in unimproved areas, with
dust and sand being blown into the air by downwash of
the main rotors.
3. This often requires the use of a particle separator on the
inlet of the engine.
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LYCOMING T 53 SERIES TURBOSHAFT ENGINE
1.
2.
3.
4.
Is a turbo shaft engine with a two stage free-power turbine and a two-stage gas-producer turbine.
It has a combination axial-centrifugal compressor and an annular atomizing type combustor.
It is torque limited in horsepower to 1250 SHP for 5 minutes and 1100 SHP for max. cont. operation.
The first stage turbine turns counterclockwise. The second stage turbine and the out put shaft
turn clockwise.
5. The major sections : a) Air inlet b) Compressor rotor c) Diffuser d) combustion e) Exhaust
BASIC
OPERATION
1. The air is
compressed by a
five –stage axial
compressor and
single-stage
centrifugal
compressor.
2. The air then
passes
through the diffuser
where the high
velocity air is
converted to high
pressure and the
air flow is
returned to a radial
flow.
3. Provisions are
made on the
diffuser for
bleeding air
from the compressor
4. As the air leave the
diffuser, it enters the
combustion area
where the pressure is
reduced, the
velocity is decreased,
and the direction is
changed.
5. The fuel is introduced
into the chamber by 22
atomizers at the aft
end of the combustor.
6. As the combustion takes
place, the temperatures will reach 3,500oF.
7. Flowing out of the combustor, the gases again reverse direction and flow across the two-stage gas
producer turbine and two-stage power turbine.
8. The power turbines drive a shaft passing through the compressor turning the gear reduction, the
engine output shaft, and the N2 gear box.
9. The gas producer turbines turn the compressor which extracts approximately 2/3 of the energy to
produce more air for combustion.
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10. In addition to driving the compressor it also turns the N1 accessory gearbox on the bottom of the
engine.
BOEING CH-47 CHINOOK HELICOPTER
1. Tandem rotors are two horizontal main rotor assemblies mounted one behind the other.
2. However, it requires the expense of two large rotors rather than the more common one large
main rotor and a much smaller tail rotor.
3. The Boeing CH-47 Chinook is the most common tandem rotor helicopter today.
BOEING CH-47 R :
The Drive Shafting
1. The drive system
includes: five
transmissions (forward,
aft, combining, number
one and two engine
nose box), and aft
vertical shaft, nine
synchronizing shafts
(drive shafts), and two
engine drive
shafts(cross shafts).
2. Each transmission has
an independent oil
lubrication system that
is air cooled through a
heat exchanger.
3. Each transmission’s oil
pressure and temperature
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is monitored.
4. The maintenance panel has indicators for chip detectors and debris screens that latch when metal
particles are detected in the oil system. A tripped latched would indicate possible transmission failure.
1. The drive system provides power to turn the rotors.
2. Engine output is connected to the engine transmission, then to the combining transmission.
3. From the combining transmission the power is directed, through the synchronizing shafts, to
the forward and aft transmission.
4. The reduction in gear ratio between the engine and the rotor wing blades is 66.96 :1.
5. All synchronizing shafts are free floating at their inputs and secured to the transmission at
their outputs.
TYPICAL TAILROTOR TRAMSMISSION DRIVE
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