a turboshaft engine acts as the
powerhouse of modern helicopters
they are also used for power generation
and marine propulsion
this video will illustrate the inner
workings of turboshaft engines in a
clear and logical way
the main function of a turboshaft engine
is to generate shaft power
this is contrary to the function of the
turbofan engines used in airliners where
the engine function is to produce thrust
force
in a helicopter shaft power produced by
the turboshaft engine is used to run the
rotor
let's examine how this shaft power is
produced along with the other
accessories needed to produce it
the power shaft which turns the rotor
receives power from a set of turbines
the turbine absorbs energy from hot and
high pressure gas flowing over the
blades
airfoil technology enables the turbine
blades to turn
in order to properly guide the flow a
set of stator blades is used
these are fixed to the engine cover
the stator blades also ensure that
sufficient flow velocity is maintained
at the turbine row inlet
now the question is how this high energy
gas is produced
the answer a combustion chamber
for effective combustion an atomized
fuel form is to be mixed with medium
temperature and high pressure air
this air is produced by a set of
compression stages
you can see this compressor is fitted on
a hollow shaft and is not connected to
the power shaft
the compressor requires energy for its
operation
this energy is provided by a new turbine
set
which is fitted on the same hollow shaft
the only purpose of this new turbine is
to supply energy to the compressor it is
referred to as a gas producer turbine
you can see how the gas producer turbine
unit and power turbine unit rotate
independently using a coaxial shaft
arrangement
thus the synchronized operation of the
compressor combustion chamber and
turbine produce necessary power output
at the power shaft
a centrifugal compressor is also
introduced in the compressor section to
effectively compress the gas
these are the basic parts and workings
of a turboshaft engine
still there are many excellent
engineering innovations hidden in this
turbine design
let's explore them
you may have noticed that the power
turbine and gas producer turbine rotate
in opposite directions
why is this so
this is to reduce the gyroscopic effects
associated with spinning objects
a spinning object has angular momentum
this angular momentum affects helicopter
motion in a quite complex manner and
makes controlling a helicopter quite
difficult by spinning the power and gas
generator units in opposite directions
we almost eliminate the angular momentum
of the engine
thus making helicopter navigation easier
another peculiar design feature is that
the fire in the combustion chamber flows
in the direction opposite to the major
airflow in the system
this kind of design is called the
reverse flow combustion chamber
you can also note that a major portion
of the compressed air is not used for
the combustion process
rather they simply engulf the combustion
chamber and are bypassed to the
combustion chamber
this is all to limit the combustion
chamber material temperature
the combustion produces flame
temperatures of up to 1700 degrees
celsius
no chamber material will be able to
withstand such high temperatures thus by
admitting colder secondary air via holes
and slots a protective shield is created
and proper insulation is given against
super hot flames
almost 80 percent of the air is used for
cooling purposes
a counter flow heat exchanger is the
most efficient in exchanging heat the
colder air flow counter to the flame
makes sure that maximum cooling is
guaranteed for the combustion chamber
this in effect helps in reducing the
combustion chamber length
a peculiar and disastrous problem that
helicopter and airline pilots face
occasionally is compressor stall
this results in sudden uncontrolled
explosions from the engine along with
total loss of shaft power output
compressor stall is basically caused by
flow separation at compressor airfoil
blades
bird strikes low airspeed or wear and
tear are frequent reasons for this flow
separation
to explain the detailed compressor stall
physics a dedicated video is needed we
will release this soon
the compressor stall can be effectively
overcome by controlling the throttle and
with the usage of variable pitch stator
blades the flow separation can be
prevented by using these techniques
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