By knowing the various systems on a helicopter, you
will be able to more easily recognize potential problems,
and if a problem arises, you will have a better under-
standing of what to do to correct the situation.
ENGINES
The two most common types of engines used in heli-
copters are the reciprocating engine and the turbine
engine. Reciprocating engines, also called piston
engines, are generally used in smaller helicopters. Most
training helicopters use reciprocating engines because
they are relatively simple and inexpensive to operate.
Turbine engines are more powerful and are used in a
wide variety of helicopters. They produce a tremen-
dous amount of power for their size but are generally
more expensive to operate.
RECIPROCATING ENGINE
The reciprocating engine consists of a series of pistons
connected to a rotating crankshaft. As the pistons move
up and down, the crankshaft rotates. The reciprocating
engine gets its name from the back-and-forth movement
of its internal parts. The four-stroke engine is the most
common type, and refers to the four different cycles the
engine undergoes to produce power. [Figure 5-1]
When the piston moves away from the cylinder head on
the intake stroke, the intake valve opens and a mixture
of fuel and air is drawn into the combustion chamber.
As the cylinder moves back towards the cylinder head,
the intake valve closes, and the fuel/air mixture is com-
pressed. When compression is nearly complete, the
spark plugs fire and the compressed mixture is ignited
to begin the power stroke. The rapidly expanding gases
from the controlled burning of the fuel/air mixture
drive the piston away from the cylinder head, thus pro-
viding power to rotate the crankshaft. The piston then
moves back toward the cylinder head on the exhaust
stroke where the burned gasses are expelled through
the opened exhaust valve.
Even when the engine is operated at a fairly low speed,
the four-stroke cycle takes place several hundred times
each minute. In a four-cylinder engine, each cylinder
operates on a different stroke. Continuous rotation of a
crankshaft is maintained by the precise timing of the
power strokes in each cylinder.
TURBINE ENGINE
The gas turbine engine mounted on most helicopters is
made up of a compressor, combustion chamber, turbine,
and gearbox assembly. The compressor compresses the
air, which is then fed into the combustion chamber
where atomized fuel is injected into it. The fuel/air
mixture is ignited and allowed to expand. This com-
bustion gas is then forced through a series of turbine
wheels causing them to turn. These turbine wheels
provide power to both the engine compressor and the
main rotor system through an output shaft. The
Figure 5-1. The arrows in this illustration indicate the direc-
tion of motion of the crankshaft and piston during the four-
stroke cycle.
Intake Compression
Power Exhaust
Intake□
Valve
Exhaust□
Valve
Spark□
Plug
Piston
Connecting□
Rod
Crankshaft
1 2
3 4
combustion gas is finally expelled through an exhaust
outlet. [Figure 5-2]
COMPRESSOR
The compressor may consist of an axial compressor, a
centrifugal compressor, or both. An axial compressor
consists of two main elements, the rotor and the stator.
The rotor consists of a number of blades fixed on a
rotating spindle and resembles a fan. As the rotor
turns, air is drawn rearwards. Stator vanes are arranged
in fixed rows between the rotor blades and act as a
diffuser at each stage to decrease air velocity and
increase air pressure. There may be a number of rows
of rotor blades and stator vanes. Each row constitutes
a pressure stage, and the number of stages depends on
the amount of air and pressure rise required for the
particular engine.
A centrifugal compressor consists of an impeller, dif-
fuser, and a manifold. The impeller, which is a forged
disc with integral blades, rotates at a high speed to
draw air in and expel it at an accelerated rate. The air
then passes through the diffuser which slows the air
down. When the velocity of the air is slowed, static
pressure increases, resulting in compressed, high-pres-
sure air. The high pressure air then passes through the
compressor manifold where it is distributed to the
combustion chamber.
COMBUSTION CHAMBER
Unlike a piston engine, the combustion in a turbine
engine is continuous. An igniter plug serves only to
ignite the fuel/air mixture when starting the engine.
Once the fuel/air mixture is ignited, it will continue to
burn as long as the fuel/air mixture continues to be
present. If there is an interruption of fuel, air, or both,
combustion ceases. This is known as a “flame-out,” and
the engine has to be restarted or re-lit. Some helicopters
are equipped with auto-relight, which automatically
activates the igniters to start combustion if the engine
flames out.
TURBINE
The turbine section consists of a series of turbine
wheels that are used to drive the compressor section
and the rotor system. The first stage, which is usually
referred to as the gas producer or N 1 may consist of
one or more turbine wheels. This stage drives the
components necessary to complete the turbine cycle
making the engine self-sustaining. Common compo-
nents driven by the N 1 stage are the compressor, oil
pump, and fuel pump. The second stage, which may
also consist of one or more wheels, is dedicated to
driving the main rotor system and accessories from
the engine gearbox. This is referred to as the power
turbine (N2 or Nr).
Compressor Discharge Air Tube
Exhaust Air Outlet
Igniter Plug
Fuel Nozzle
Air
Inlet
Output Shaft
Gear
Compressor Rotor Turbine to Compressor Coupling Combustion□
Liner
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 tur-
bine rather than producing thrust through the expulsion of exhaust gases.
If the first and second stage turbines are mechanically cou-
pled 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 mechani-
cally 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 tur-
bine, 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 acces-
sories. The main components of the transmission sys-
tem are the main rotor transmission, tail rotor drive
system, clutch, and freewheeling unit. Helicopter trans-
missions 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 warn-
ing 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 heli-
copters, 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 transmis-
sion 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 gear-
ing to adjust the output to optimum tail rotor r.p.m.
[Figure 5-4]
Figure 5-3. There are various types of dual-needle tachome-
ters, 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 sys-
tem are shown here.
Tail Rotor□
Transmission
Tail Rotor
Drive Shaft
Main□
Transmission
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
rotor. Because of the greater weight of a rotor in rela-
tion to the power of the engine, as compared to the
weight of a propeller and the power in an airplane, the
rotor must be disconnected from the engine when you
engage the starter. A clutch allows the engine to be
started and then gradually pick up the load of the rotor.
On free turbine engines, no clutch is required, as the
gas producer turbine is essentially disconnected from
the power turbine. When the engine is started, there is
little resistance from the power turbine. This enables
the gas producer turbine to accelerate to normal idle
speed without the load of the transmission and rotor
system dragging it down. As the gas pressure increases
through the power turbine, the rotor blades begin to
turn, slowly at first and then gradually accelerate to
normal operating r.p.m.
On reciprocating helicopters, the two main types of
clutches are the centrifugal clutch and the belt drive clutch.
CENTRIFUGAL CLUTCH
The centrifugal clutch is made up of an inner assembly
and a outer drum. The inner assembly, which is con-
nected to the engine driveshaft, consists of shoes lined
with material similar to automotive brake linings. At
low engine speeds, springs hold the shoes in, so there is
no contact with the outer drum, which is attached to the
transmission input shaft. As engine speed increases,
centrifugal force causes the clutch shoes to move out-
ward and begin sliding against the outer drum. The
transmission input shaft begins to rotate, causing the
rotor to turn, slowly at first, but increasing as the friction
increases between the clutch shoes and transmission
drum. As rotor speed increases, the rotor tachometer
needle shows an increase by moving toward the engine
tachometer needle. When the two needles are superim-
posed, the engine and the rotor are synchronized,
indicating the clutch is fully engaged and there is no
further slippage of the clutch shoes.
BELT DRIVE CLUTCH
Some helicopters utilize a belt drive to transmit power
from the engine to the transmission. A belt drive con-
sists of a lower pulley attached to the engine, an upper
pulley attached to the transmission input shaft, a belt
or a series of V-belts, and some means of applying
tension to the belts. The belts fit loosely over the
upper and lower pulley when there is no tension on
the belts. This allows the engine to be started without
any load from the transmission. Once the engine is
running, tension on the belts is gradually increased.
When the rotor and engine tachometer needles are
superimposed, the rotor and the engine are synchro-
nized, and the clutch is then fully engaged.
Advantages of this system include vibration isolation,
simple maintenance, and the ability to start and warm
up the engine without engaging the rotor.
FREEWHEELING UNIT
Since lift in a helicopter is provided by rotating airfoils,
these airfoils must be free to rotate if the engine fails. The
freewheeling unit automatically disengages the engine
from the main rotor when engine r.p.m. is less than main
rotor r.p.m. This allows the main rotor to continue turning
at normal in-flight speeds. The most common freewheel-
ing unit assembly consists of a one-way sprag clutch
located between the engine and main rotor transmission.
This is usually in the upper pulley in a piston helicopter
or mounted on the engine gearbox in a turbine helicopter.
When the engine is driving the rotor, inclined surfaces in
the spray clutch force rollers against an outer drum. This
prevents the engine from exceeding transmission r.p.m. If
the engine fails, the rollers move inward, allowing the
outer drum to exceed the speed of the inner portion. The
transmission can then exceed the speed of the engine. In
this condition, engine speed is less than that of the drive
system, and the helicopter is in an autorotative state.
MAIN ROTOR SYSTEM
Main rotor systems are classified according to how the
main rotor blades move relative to the main rotor hub.
As was described in Chapter 1—Introduction to the
Helicopter, there are three basic classifications: fully
articulated, semirigid, or rigid. Some modern rotor sys-
tems use a combination of these types.
FULLY ARTICULATED ROTOR SYSTEM
In a fully articulated rotor system, each rotor blade is
attached to the rotor hub through a series of hinges,
which allow the blade to move independently of the
others. These rotor systems usually have three or more
blades. [Figure 5-5]
Pitch Change□
Axis□
(Feathering)
Flapping□
Hinge
Damper
Drag Hinge
Pitch Horn
Figure 5-5. Each blade of a fully articulated rotor system can
flap, drag, and feather independently of the other blades.
The horizontal hinge, called the flapping hinge, allows
the blade to move up and down. This movement is
called flapping and is designed to compensate for dis-
symetry of lift. The flapping hinge may be located at
varying distances from the rotor hub, and there may be
more than one hinge.
The vertical hinge, called the lead-lag or drag hinge,
allows the blade to move back and forth. This move-
ment is called lead-lag, dragging, or hunting.
Dampers are usually used to prevent excess back
and forth movement around the drag hinge. The pur-
pose of the drag hinge and dampers is to compensate
for the acceleration and deceleration caused by
Coriolis Effect.
Each blade can also be feathered, that is, rotated around
its spanwise axis. Feathering the blade means changing
the pitch angle of the blade. By changing the pitch
angle of the blades you can control the thrust and direc-
tion of the main rotor disc.
SEMIRIGID ROTOR SYSTEM
A semirigid rotor system is usually composed of two
blades which are rigidly mounted to the main rotor hub.
The main rotor hub is free to tilt with respect to the
main rotor shaft on what is known as a teetering
hinge. This allows the blades to flap together as a
unit. As one blade flaps up, the other flaps down.
Since there is no vertical drag hinge, lead-lag forces
are absorbed through blade bending. [Figure 5-6]
RIGID ROTOR SYSTEM
In a rigid rotor system, the blades, hub, and mast are
rigid with respect to each other. There are no vertical or
horizontal hinges so the blades cannot flap or drag, but
they can be feathered. Flapping and lead/lag forces are
absorbed by blade bending.
COMBINATION ROTOR SYSTEMS
Modern rotor systems may use the combined princi-
ples of the rotor systems mentioned above. Some
rotor hubs incorporate a flexible hub, which allows
for blade bending (flexing) without the need for bear-
ings or hinges. These systems, called flextures, are
usually constructed from composite material.
Elastomeric bearings may also be used in place of
conventional roller bearings. Elastomeric bearings are
bearings constructed from a rubber type material and
have limited movement that is perfectly suited for hel-
icopter applications. Flextures and elastomeric bear-
ings require no lubrication and, therefore, require less
maintenance. They also absorb vibration, which
means less fatigue and longer service life for the heli-
copter components. [Figure 5-7]
SWASH PLATE ASSEMBLY
The purpose of the swash plate is to transmit control
inputs from the collective and cyclic controls to the main
rotor blades. It consists of two main parts: the stationary
Teetering□
Hinge
Feathering Hinge
Static Stops
Pitch Horn
Figure 5-6. On a semirigid rotor system, a teetering hinge
allows the rotor hub and blades to flap as a unit. A static flap-
ping stop located above the hub prevents excess rocking
when the blades are stopped. As the blades begin to turn,
centrifugal force pulls the static stops out of the way.
Figure 5-7. Rotor systems, such as Eurocopter’s Starflex or
Bell’s soft-in-plane, use composite material and elastomeric
bearings to reduce complexity and maintenance and,
thereby, increase reliability.
swash plate and the rotating swash plate. [Figure 5-8]
The stationary swash plate is mounted around the main
rotor mast and connected to the cyclic and collective
controls by a series of pushrods. It is restrained from
rotating but is able to tilt in all directions and move verti-
cally. The rotating swash plate is mounted to the sta-
tionary swash plate by means of a bearing and is
allowed to rotate with the main rotor mast. Both swash
plates tilt and slide up and down as one unit. The rotat-
ing swash plate is connected to the pitch horns by the
pitch links.
FUEL SYSTEMS
The fuel system in a helicopter is made up of two
groups of components: the fuel supply system and the
engine fuel control system.
FUEL SUPPLY SYSTEM
The supply system consists of a fuel tank or tanks, fuel
quantity gauges, a shut-off valve, fuel filter, a fuel line
to the engine, and possibly a primer and fuel pumps.
[Figure 5-9]
The fuel tanks are usually mounted to the airframe as
close as possible to the center of gravity. This way, as
fuel is burned off, there is a negligible effect on the cen-
ter of gravity. A drain valve located on the bottom of
the fuel tank allows the pilot to drain water and sedi-
ment that may have collected in the tank. A fuel vent
prevents the formation of a vacuum in the tank, and an
overflow drain allows for fuel to expand without rup-
turing the tank. A fuel quantity gauge located on the
pilot’s instrument panel shows the amount of fuel
measured by a sensing unit inside the tank. Some
gauges show tank capacity in both gallons and pounds.
The fuel travels from the fuel tank through a shut-off
valve, which provides a means to completely stop fuel
flow to the engine in the event of an emergency or fire.
The shut-off valve remains in the open position for all
normal operations.
Most non-gravity feed fuel systems contain both an
electric pump and a mechanical engine driven pump.
The electrical pump is used to maintain positive fuel
pressure to the engine pump and also serves as a
backup in the event of mechanical pump failure. The
electrical pump is controlled by a switch in the cockpit.
The engine driven pump is the primary pump that sup-
plies fuel to the engine and operates any time the
engine is running.
A fuel filter removes moisture and other sediment from
the fuel before it reaches the engine. These contami-
nants are usually heavier than fuel and settle to the bot-
tom of the fuel filter sump where they can be drained
out by the pilot.
Some fuel systems contain a small hand-operated pump
called a primer. A primer allows fuel to be pumped
directly into the intake port of the cylinders prior to
engine start. The primer is useful in cold weather when
fuel in the carburetor is difficult to vaporize.
ENGINE FUEL CONTROL SYSTEM
The purpose of the fuel control system is to bring out-
side air into the engine, mix it with fuel in the proper
proportion, and deliver it to the combustion chamber.
Throttle
Low Level□
Warning□
Light
Vent
Fuel Quantity□
Gauge
Mixture□
Control
Fuel□
Shutoff
Primer
Tank
Shut-off□
Valve
Carburetor
Fuel□
Strainer
Primer Nozzle□
at Cylinder
Figure 5-9. A typical gravity feed fuel system, in a helicopter
with a reciprocating engine, contains the components
shown here.
Stationary□
Swash□
Plate
Pitch□
Link
Rotating□
Swash□
Plate
Control□
Rod
Figure 5-8. Collective and cyclic control inputs are transmit-
ted to the stationary swash plate by control rods causing it to
tilt or to slide vertically. The pitch links attached from the
rotating swash plate to the pitch horns on the rotor hub
transmit these movements to the blades.