Output Shaft
Air inlet
Centrifugal Compression Section Turbine Section Combustion SectionGearbox
Section
Inlet air
Compressor discharge air
Combustion gases
Exhaust gases
Combustion liner
Exhaust air outlet
Compressor rotor
Fuel nozzle
Igniter plug
N1 RotorN2 Rotor Stator
Gear
Main rotor gearbox
Drive belts
Input drives sun wheel
Tail rotor drive shaft
Power and accessory gearbox
Main drive shaft with freewheeling unit
Tail rotor gearbox
Tail rotor
Engine
Figure 4-16. The tail rotor driveshaft is connected to both the main transmission and the tail rotor transmission.
Figure 4-17. Many helicopters use a turboshaft engine as shown above 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.
Although this design significantly reduces the ingestion
of foreign objects into the engine, it is important for pilots
to be aware of how much debris is actually being filtered.
Operating in the sand, dust, or even in grassy type materials
can choke an engine in just minutes. The compressed air is
directed to the combustion section through discharge tubes
where atomized fuel is injected into it. The fuel/air mixture
is ignited and allowed to expand. This combustion 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 accessory gearbox. Depending on model
and manufacturer, the rpm can vary from 20,000 to 51,600.
Power is provided to the main rotor and tail rotor systems
through the freewheeling unit which is attached to the
accessory gearbox power output gear shaft. The combustion
gas is finally expelled through an exhaust outlet. The
temperature of gas is measured at different locations and is
referenced differently by each manufacturer. Some common
terms are inter-turbine temperature (ITT), exhaust gas
temperature (EGT), measured gas temperature (MGT), or
turbine outlet temperature (TOT). TOT is used throughout
this discussion for simplicity. [Figure 4-17]
Compressor
The compressor may consist of an axial compressor, a
centrifugal compressor, or combination of the two.
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 inward. 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, diffuser,
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 pressure air. The high-pressure air
then passes through the compressor manifold where it is
distributed to the combustion chamber via discharge tubes.
If the airflow through the compressor is disturbed, a
condition called surge, or compressor stall, may take effect.
This phenomenon is a periodic stalling of the compressor
blades. When this occurs, the pressure at the compressor
is reduced and the combustion pressure may cause reverse
flow into the compressor output. As the airflow through
the compressor is reduced, the air pressure then increases
temporarily correcting the condition until it occurs again.
This is felt throughout the airframe as vibrations and is
accompanied by power loss and an increase in TOT as the
fuel control adds fuel in an attempt to maintain power. This
condition may be corrected by activating the bleed air system
which vents excess pressure to the atmosphere and allows
a larger volume of air to enter the compressor to unstall the
compressor blades.
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 continues 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 “flameout,”
and the engine must 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 two-stage turbine section consists of a series of turbine
wheels that are used to drive the compressor section and
other components attached to the accessory gearbox. Both
stages may consist of one or more turbine wheels. The first
stage is usually referred to as the gas producer (N1 or NG)
while the second stage is commonly called the power turbine
(N2 or NP). (The letter N is used to denote rotational speed.)
If the first and second stage turbines are mechanically
coupled to each other, the system is said to be a fixed turbine
(turboshaft). 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, one inside the other, and can turn
freely with respect to each other. This is referred to as a
“free turbine.” When a free turbine engine is running, the
combustion gases pass through the first stage turbine (N1)
to drive the compressor and other components, and then past
the independent second stage turbine (N2), which turns the
power and accessory gearbox to drive the output shaft, as
well as other miscellaneous components.
Accessory Gearbox
The accessory gearbox of the engine houses all of the
necessary gears to drive the numerous components of the
helicopter. Power is provided to the accessory gearbox
through the independent shafts connected to the N1 and
N2 turbine wheels. The N1 stage drives the components
necessary to complete the turbine cycle, making the engine
self-sustaining. Common components driven by the N1
stage are the compressor, oil pump, fuel pump, and starter/
generator. The N2 stage is dedicated to driving the main
rotor and tail rotor drive systems and other accessories such
as generators, alternators, and air conditioning.
Transmission System
The transmission system transfers power from the engine to
the main rotor, tail rotor, and other accessories during normal
flight conditions. The main components of the transmission
system are the main rotor transmission, tail rotor drive
system, clutch, and freewheeling unit. The freewheeling unit
or autorotative clutch allows the main rotor transmission to
drive the tail rotor drive shaft during autorotation. In some
helicopter designs, such as the Bell BH-206, the freewheeling
unit is located in the accessory gearbox. Because it is part
of the transmission system, the transmission lubricates it to
E R
% RPM
% RPM
NR NP
RPM
ROTOR
TURBINE
ROTOR
PEEVER
TURBINEA
PERCENT
RPM
Figure 4-18. Various types of dual-needle tachometers.
E R
% RPM
Figure 4-19. A “split” or divided needle condition is a result of a
sudden loss of engine power.
ensure free rotation. 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, to detect loose pieces of
metal. These detectors are wired to warning lights located
on the pilot’s instrument panel that illuminate in the event
of an internal problem. Some chip detectors on modern
helicopters have a “burn off” capability and attempt to correct
the situation without pilot action. If the problem cannot be
corrected on its own, the pilot must refer to the emergency
procedures for that particular helicopter.
Main Rotor Transmission
The primary purpose of the main rotor transmission is
to reduce engine output rpm to optimum rotor rpm. This
reduction is different for the various helicopters. As an
example, suppose the engine rpm of a specific helicopter
is 2,700. A rotor speed of 450 rpm would require a 6:1
reduction. A 9:1 reduction would mean the rotor would turn
at 300 rpm.
Dual Tachometers
Most helicopters use a dual-needle tachometer or a vertical
scale instrument to show both engine and rotor rpm or a
percentage of engine and rotor rpm. The rotor rpm indicator is
used during clutch engagement to monitor rotor acceleration,
and in autorotation to maintain rpm within prescribed limits.
It is vital to understand that rotor rpm is paramount, and that
engine rpm is secondary. If the rotor tachometer fails, rotor
rpm can still be determined indirectly by the engine rpm
during powered flight, because the engine drives the rotor at
a fixed, one-to-one ratio (by virtue of the sprag clutch). There
have been many accidents where the pilot responded to the
rotor rpm tachometer failure and entered into autorotation
while the engine was still operating.
Look closer at the markings on the gauges in Figure 4-18. All
gauges shown are dual tachometer gauges. The two on the
left have two needles each, one marked with the letter ‘T’
(turbine) the other marked with the letter ‘R’ (rotor). The
lower left gauge shows two arced areas within the same
needle location. In this case, both needles should be nearly
together or superimposed during normal operation. Note the
top left gauge shows two numerical arcs. The outer arc, with
larger numbers, applies one set of values to engine rpm. The
inner arc, or smaller numbers, represents a separate set of
values for rotor rpm. Normal operating limits are shown when
the needles are married or appear superimposed. The top
right gauge shows independent needles, focused toward the
middle of the gauge, with colored limitation areas respective
to the needle head. The left side represents engine operational
parameters; the right, rotor operational parameters.
In normal conditions when the rotor is coupled to the engine,
both needles move together in the same direction. However,
with a sudden loss in engine power the needles “split”
showing that the engine and rotor are no longer coupled as
the clutch has disconnected. [Figure 4-19]
Many newer aircraft have what is referred to as a glass
cockpit, meaning the instrumentation is digital and displayed
Upper pulley
V-Belts
Lower pulley
Figure 4-21. Idler or manual clutch.
Gearbox
Main transmission
To engine
Main rotor
Antitorque rotor
Figure 4-20. The main rotor transmission reduces engine output
rpm to optimum rotor rpm.
to the pilot on digital screens and vertical scale instruments.
The bottom right gauge in Figure 4-18 replicates a vertical
scale instrument. The dual tachometer shown displays rotor
rpm (NR) on the left and engine rpm (NP) on the right side
of the vertical scale. Corresponding color limits are present
for each component parameter.
Structural Design
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. [Figure 4-20] This differs
from airplanes, which have their propellers mounted directly
to the crankshaft or to a shaft that is geared to the crankshaft.
Maintaining main rotor rpm is essential for adequate lift.
RPM within normal limits produces adequate lift for normal
maneuvering. Therefore, it is imperative not only to know
the location of the tachometers, but also to understand the
information they provide. If rotor rpm is allowed to go below
normal limits, the outcome could be catastrophic.
Clutch
In a conventional airplane, the engine and propeller are
permanently connected. However, in a helicopter they are
not. Because of the greater weight of a rotor in relation 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 the starter is engaged. A clutch allows
the engine to be started and then gradually pick up the load
of the rotor.
Freewheeling turbine engines do not require a separate clutch
since the air coupling between the gas producer turbine and
the power (takeoff) turbine functions as an air clutch for
starting purposes. 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 rpm.
On reciprocating and fixed turbine engines, a clutch is
required to enable engine start. Air, or windmilling starts,
are not possible. The two main types of clutches are the
centrifugal clutch and the idler or manual clutch.
How the clutch engages the main rotor system during engine
start differs between helicopter design. Piston-powered
helicopters have a means of engaging the clutch manually
just as a manual clutch in an automobile. This may be by
means of an electric motor that positions a pulley when the
engine is at the proper operating condition (oil temperature
and pressure in the appropriate range), but which is controlled
by a cockpit mounted switch.
Belt Drive Clutch
Some helicopters utilize a belt drive to transmit power from
the engine to the transmission. A belt drive consists of a lower
pulley attached to the engine, an upper pulley attached to the
transmission input shaft, a belt or a set 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. [Figure 4-21]
Fuel quantity
Fuel tank
FUEL SHOT OFF
LOW FUEL LEVEL
WARNING LIGHT
PULL LEAN
Carburetor
Throttle
Shut-off valve
Fuel strainer
Primer
Primer nozzle at cylinder
Figure 4-22. A typical gravity feed fuel system, in a helicopter with
a reciprocating engine, contains the components shown here.
Some aircraft utilize a clutch for starting. This allows the
engine to be started without requiring power to turn the
transmission. One advantage this concept has is that without
a load on the engine starting may be accomplished with
minimal throttle application. However, caution should also
be used during starting, since rapid or large throttle inputs
may cause overspeeds.
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 synchronized, and
the clutch is then fully engaged. Advantages of this system
include vibration isolation, simple maintenance. When the
clutch is not engaged, engines are very easy to overspeed,
resulting in costly inspections and maintenance. Power, or
throttle control, is very important in this phase of engine
operation.
Centrifugal Clutch
A centrifugal clutch is made up of an inner assembly and
an outer drum. The inner assembly, which is connected 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 outward 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 superimposed (in the case of a
coaxial-type gage), the engine and the rotor are synchronized,
indicating the clutch is fully engaged and there is no further
slippage of the clutch shoes.
The turbine engine engages the clutch through centrifugal
force, as stated above. Unless a rotor brake is used to
separate the automatic engagement of the main driveshaft and
subsequently the main rotor, the drive shaft turns at the same
time as the engine and the inner drum of the freewheeling unit
engages gradually to turn the main rotor system.
Fuel Systems
The fuel system in a helicopter is made up of two components:
supply and control.
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 4-22] The fuel
tanks are usually mounted to the airframe as close as possible
to the CG. This way, as fuel is burned off, there is a negligible
effect on the CG. A drain valve located on the bottom of the
fuel tank allows the pilot to drain water and sediment that may
have collected in the tank. A fuel vent prevents the formation
of a vacuum in the tank, and an overflow drain allows fuel to
expand without rupturing the tank.
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 may also serve 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 supplies 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 contaminants are usually heavier than fuel and settle to
the bottom of the fuel filter sump where they can be drained
out by the pilot.
Manifold pipe is connected to exhaust manifold
Filter
Carburetor heat collector
To carburetor
Carburetor Heat Off
Carburetor Heat On
Figure 4-23. When the carburetor heat is turned ON, normal air flow
is blocked, and heated air from an alternate source flows through
the filter to the carburetor.
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.
A fuel quantity gauge located on the pilot’s instrument panel
shows the amount of fuel measured by a sensing unit inside
the tank. Most fuel gauges will indicate in gallons or pounds
and must be accurate only when empty.
It is worth noting that in accordance with Title 14 of the Code
of Federal Regulations (14 CFR) section 27.1337(b)(1), fuel
quantity indicators “must be calibrated to read ‘zero’ during
level flight when the quantity of fuel remaining in the tank
is equal to the unusable fuel supply.” Therefore, it is of the
utmost importance that the pilot or operator determine an
accurate means of verifying partial or full fuel loads. It is
always a good habit, if possible, to visually verify the fuel on
board prior to flight and determine if adequate fuel is present
for the duration of the flight.
Additionally, 14 CFR section 27.1305(l)(1) requires newer
helicopters to have warning systems “provide a warning
to the flight crew when approximately 10 minutes of
usable fuel remains in the tank.” Caution should be used
to eliminate unnecessary or erratic maneuvering that could
cause interruption of fuel flow to the engine. Although these
systems must be calibrated, never assume the entire amount
is available. Many pilots have not reached their destinations
due to poor fuel planning or faulty fuel indications.
Engine Fuel Control System
Regardless of the device, the reciprocating engine and the
turbine engine both use the ignition and combustion of the
fuel/air mix to provide the source of their power. Engine
fuel control systems utilize several components to meter
the proper amount of fuel necessary to produce the required
amount of power. The fuel control system, in concert with
the air induction components, combines the proper amount of
fuel and air to be ignited in the combustion chamber. Refer
to the Pilot’s Handbook of Aeronautical Knowledge for a
detailed explanation and illustration.
Carburetor Ice
The effect of fuel vaporization and/or a decrease of air
pressure in the venturi causes a rapid decrease in air
temperature in the carburetor. If the air is moist, the water
vapor in the air may condense causing ice to form in the
carburetor. If ice is allowed to form inside the carburetor,
engine failure is a very real possibility and the ability to
restart the engine is greatly reduced. Carburetor icing can
occur during any phase of flight but is particularly dangerous
when you are using reduced power, such as during a descent.
You may not notice it during the descent until you try to add
power. Indications of carburetor icing are a decrease in engine
rpm or manifold pressure, the carburetor air temperature
gauge indicating a temperature outside the safe operating
range, and engine roughness. A reciprocating engine with a
governor may mask the formation of carburetor ice since it
will maintain a constant manifold pressure and rpm.
Since changes in rpm or manifold pressure can occur
for a number of reasons, closely check the carburetor
air temperature gauge when in possible carburetor icing
conditions. Carburetor air temperature gauges are marked
with a yellow caution arc or green operating arcs. In most
cases, it is best to keep the needle out of the yellow arc or
in the green arc. This is accomplished by using a carburetor
heat system, which eliminates the ice by routing air across