Liner
N2
N1
Inlet Air

Compressor Discharge Air

Combustion Gasses

Exhaust Gasses
Compression Section Gearbox Section Turbine Section Combustion Section
Stator
Rotor
Figure 5-2. Many helicopters use a turboshaft engine to drive the main transmission and rotor systems. The main difference
between a turboshaft and a turbojet engine is that most of the energy produced by the expanding gases is used to drive a turbine
rather than producing thrust through the expulsion of exhaust gases.
5-3
If the first and second stage turbines are mechanically coupled
to each other, the system is said to be a direct-drive
engine or fixed turbine. These engines share a common
shaft, which means the first and second stage turbines, and
thus the compressor and output shaft, are connected.
On most turbine assemblies used in helicopters, the
first stage and second stage turbines are not mechanically
connected to each other. Rather, they are mounted
on independent shafts and can turn freely with respect to
each other. This is referred to as a “free turbine.” When
the engine is running, the combustion gases pass
through the first stage turbine to drive the compressor
rotor, and then past the independent second stage turbine,
which turns the gearbox to drive the output shaft.
TRANSMISSION SYSTEM
The transmission system transfers power from the
engine to the main rotor, tail rotor, and other accessories.
The main components of the transmission system
are the main rotor transmission, tail rotor drive
system, clutch, and freewheeling unit. Helicopter transmissions
are normally lubricated and cooled with their
own oil supply. A sight gauge is provided to check the
oil level. Some transmissions have chip detectors
located in the sump. These detectors are wired to warning
lights located on the pilot’s instrument panel that
illuminate in the event of an internal problem.
MAIN ROTOR TRANSMISSION
The primary purpose of the main rotor transmission
is to reduce engine output r.p.m. to optimum rotor
r.p.m. This reduction is different for the various helicopters,
but as an example, suppose the engine r.p.m. of
a specific helicopter is 2,700. To achieve a rotor speed of
450 r.p.m. would require a 6 to 1 reduction. A 9 to 1
reduction would mean the rotor would turn at
300 r.p.m.
Most helicopters use a dual-needle tachometer to show
both engine and rotor r.p.m. or a percentage of engine
and rotor r.p.m. The rotor r.p.m. needle normally is
used only during clutch engagement to monitor rotor
acceleration, and in autorotation to maintain r.p.m.
within prescribed limits. [Figure 5-3]
Chip Detector—A chip detector is
a warning device that alerts you to
any abnormal wear in a transmission
or engine. It consists of a
magnetic plug located within the
transmission. The magnet attracts
any ferrous metal particles that
have come loose from the bearings
or other transmission parts. Most
chip detectors send a signal to
lights located on the instrument
panel that illuminate when ferrous
metal particles are picked up.
In helicopters with horizontally mounted engines,
another purpose of the main rotor transmission is to
change the axis of rotation from the horizontal axis of
the engine to the vertical axis of the rotor shaft.
TAIL ROTOR DRIVE SYSTEM
The tail rotor drive system consists of a tail rotor drive
shaft powered from the main transmission and a tail
rotor transmission mounted at the end of the tail boom.
The drive shaft may consist of one long shaft or a series
of shorter shafts connected at both ends with flexible
couplings. This allows the drive shaft to flex with the
tail boom. The tail rotor transmission provides a right
angle drive for the tail rotor and may also include gearing
to adjust the output to optimum tail rotor r.p.m.
[Figure 5-4]
Figure 5-3. There are various types of dual-needle tachometers,
however, when the needles are superimposed or married,
the ratio of the engine r.p.m. is the same as the gear reduction
ratio.
Figure 5-4. The typical components of a tail rotor drive system
are shown here.
Tail Rotor

Transmission
Tail Rotor
Drive Shaft
Main

Transmission
5-4
CLUTCH
In a conventional airplane, the engine and propeller are
permanently connected. However, in a helicopter there
is a different relationship between the engine and the
　
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