直升机飞行手册Rotorcraft flying handbook

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CORRELATOR / GOVERNOR

A correlator is a mechanical connection between the

collective lever and the engine throttle. When the collective lever is raised, power is automatically increased

and when lowered, power is decreased. This system

maintains r.p.m. close to the desired value, but still

requires adjustment of the throttle for fine tuning.

A governor is a sensing device that senses rotor and

engine r.p.m. and makes the necessary adjustments in

order to keep rotor r.p.m. constant. In normal operations,

once the rotor r.p.m. is set, the governor keeps the r.p.m.

constant, and there is no need to make any throttle adjustments. Governors are common on all turbine helicopters

and used on some piston powered helicopters.

Some helicopters do not have correlators or governors

and require coordination of all collective and throttle

movements. When the collective is raised, the throttle

must be increased; when the collective is lowered, the

throttle must be decreased. As with any aircraft control,

large adjustments of either collective pitch or throttle

should be avoided. All corrections should be made

through the use of smooth pressure.

CYCLIC PITCH CONTROL

The cyclic pitch control tilts the main rotor disc by

changing the pitch angle of the rotor blades in their

cycle of rotation. When the main rotor disc is tilted, the

horizontal component of lift moves the helicopter in

the direction of tilt. [Figure 4-4]

Figure 4-2. A twist grip throttle is usually mounted on the end

of the collective lever. Some turbine helicopters have the

throttles mounted on the overhead panel or on the floor in

the cockpit.

If

Manifold

Pressure

is

and

R.P.M.

is

Solution

Low

Low

Low

Low High

High

High

High

Increasing the throttle increases manifold

pressure and r.p.m.

Lowering the collective pitch decreases

manifold pressure and increases r.p.m.

Raising the collective pitch increases

manifold pressure and decreases r.p.m.

Reducing the throttle decreases manifold

pressure and r.p.m.

Figure 4-3. Relationship between manifold pressure, r.p.m.,

collective, and throttle.

Figure 4-4. The cyclic pitch control may be mounted vertically between the pilot’s knees or on a teetering bar from a

single cyclic located in the center of the helicopter. The cyclic

can pivot in all directions.

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The rotor disc tilts in the direction that pressure is applied

to the cyclic pitch control. If the cyclic is moved forward,

the rotor disc tilts forward; if the cyclic is moved aft, the

disc tilts aft, and so on. Because the rotor disc acts like a

gyro, the mechanical linkages for the cyclic control rods

are rigged in such a way that they decrease the pitch angle

of the rotor blade approximately 90° before it reaches the

direction of cyclic displacement, and increase the pitch

angle of the rotor blade approximately 90° after it passes

the direction of displacement. An increase in pitch angle

increases angle of attack; a decrease in pitch angle

decreases angle of attack. For example, if the cyclic is

moved forward, the angle of attack decreases as the rotor

blade passes the right side of the helicopter and increases

on the left side. This results in maximum downward

deflection of the rotor blade in front of the helicopter and

maximum upward deflection behind it, causing the rotor

disc to tilt forward.

ANTITORQUE PEDALS

The antitorque pedals, located on the cabin floor by the

pilot’s feet, control the pitch, and therefore the thrust,

of the tail rotor blades. [Figure 4-5] . The main purpose

of the tail rotor is to counteract the torque effect of the

main rotor. Since torque varies with changes in power,

the tail rotor thrust must also be varied. The pedals are

connected to the pitch change mechanism on the tail

rotor gearbox and allow the pitch angle on the tail rotor

blades to be increased or decreased.

HEADING CONTROL

Besides counteracting torque of the main rotor, the tail

rotor is also used to control the heading of the helicopter

while hovering or when making hovering turns. Hovering

turns are commonly referred to as “pedal turns.”

In forward flight, the antitorque pedals are not used to

control the heading of the helicopter, except during portions of crosswind takeoffs and approaches. Instead they

are used to compensate for torque to put the helicopter in

longitudinal trim so that coordinated flight can be maintained. The cyclic control is used to change heading by

making a turn to the desired direction.

The thrust of the tail rotor depends on the pitch angle of

the tail rotor blades. This pitch angle can be positive, negative, or zero. A positive pitch angle tends to move the tail

to the right. A negative pitch angle moves the tail to the

left, while no thrust is produced with a zero pitch angle.

With the right pedal moved forward of the neutral position, the tail rotor either has a negative pitch angle or a

small positive pitch angle. The farther it is forward, the

larger the negative pitch angle. The nearer it is to neutral, the more positive the pitch angle, and somewhere

in between, it has a zero pitch angle. As the left pedal is

moved forward of the neutral position, the positive pitch

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angle of the tail rotor increases until it becomes maximum with full forward displacement of the left pedal.

If the tail rotor has a negative pitch angle, tail rotor

thrust is working in the same direction as the torque of

the main rotor. With a small positive pitch angle, the

tail rotor does not produce sufficient thrust to overcome

the torque effect of the main rotor during cruise flight.

Therefore, if the right pedal is displaced forward of

neutral during cruising flight, the tail rotor thrust does

not overcome the torque effect, and the nose yaws to

the right. [Figure 4-6]

With the antitorque pedals in the neutral position, the tail

rotor has a medium positive pitch angle. In medium positive pitch, the tail rotor thrust approximately equals the

torque of the main rotor during cruise flight, so the helicopter maintains a constant heading in level flight.

Figure 4-5. Antitorque pedals compensate for changes in

torque and control heading in a hover.

Tail Moves Tail Moves

Negative or Low

Positive Pitch

Medium

Positive Pitch

High Positive

Pitch

Figure 4-6. Tail rotor pitch angle and thrust in relation to pedal positions during cruising flight.

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If the left pedal is in a forward position, the tail rotor

has a high positive pitch position. In this position, tail

rotor thrust exceeds the thrust needed to overcome

torque effect during cruising flight so the helicopter

yaws to the left.

The above explanation is based on cruise power and airspeed. Since the amount of torque is dependent on the

amount of engine power being supplied to the main rotor,

the relative positions of the pedals required to counteract

torque depend upon the amount of power being used at

any time. In general, the less power being used, the

greater the requirement for forward displacement of the

right pedal; the greater the power, the greater the forward

displacement of the left pedal.

The maximum positive pitch angle of the tail rotor is

generally somewhat greater than the maximum negative pitch angle available. This is because the primary

purpose of the tail rotor is to counteract the torque of

the main rotor. The capability for tail rotors to produce

thrust to the left (negative pitch angle) is necessary,

because during autorotation the drag of the transmission tends to yaw the nose to the left, or in the same

direction the main rotor is turning.

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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 understanding of what to do to correct the situation.

ENGINES

The two most common types of engines used in helicopters 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 tremendous 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 compressed. 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 providing 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 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

main rotor system through an output shaft. The

Figure 5-1. The arrows in this illustration indicate the direction of motion of the crankshaft and piston during the fourstroke cycle.

Intake Compression

Power Exhaust

Intake

Valve

Exhaust

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Valve

Spark

Plug

Piston

Connecting

Rod

Crankshaft

1 2

3 4

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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, 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.

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 N1 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 components driven by the N1 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

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.

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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

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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

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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 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 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 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, 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 consists 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 synchronized, 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

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rotor r.p.m. This allows the main rotor to continue turning

at normal in-flight speeds. The most common freewheeling 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 systems 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.

5-5

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 dissymetry 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 movement is called lead-lag, dragging, or hunting.

Dampers are usually used to prevent excess back

and forth movement around the drag hinge. The purpose 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 direction of the main rotor disc.

SEMIRIGID ROTOR SYSTEM

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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 principles of the rotor systems mentioned above. Some

rotor hubs incorporate a flexible hub, which allows

for blade bending (flexing) without the need for bearings 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 helicopter applications. Flextures and elastomeric bearings require no lubrication and, therefore, require less

maintenance. They also absorb vibration, which

means less fatigue and longer service life for the helicopter 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 flapping 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.

5-6

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 vertically. The rotating swash plate is mounted to the stationary 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 rotating swash plate is connected to the pitch horns by the

pitch links.

FUEL SYSTEMS

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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 center of gravity. 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 for fuel to expand without rupturing 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 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.

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 outside 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

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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 transmitted 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.

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RECIPROCATING ENGINES

Fuel is delivered to the cylinders by either a carburetor

or fuel injection system.

CARBURETOR

In a carburetor system, air is mixed with vaporized fuel as

it passes through a venturi in the carburetor. The metered

fuel/air mixture is then delivered to the cylinder intake.

Carburetors are calibrated at sea level, and the correct

fuel-to-air mixture ratio is established at that altitude

with the mixture control set in the FULL RICH position. However, as altitude increases, the density of air

entering the carburetor decreases while the density of

the fuel remains the same. This means that at higher

altitudes, the mixture becomes progressively richer. To

maintain the correct fuel/air mixture, you must be able

to adjust the amount of fuel that is mixed with the

incoming air. This is the function of the mixture control. This adjustment, often referred to as “leaning the

mixture,” varies from one aircraft to another. Refer to

the FAA-Approved Rotocraft Flight Manual (RFM) to

determine specific procedures for your helicopter. Note

that most manufacturers do not recommend leaning helicopters in-flight.

Most mixture adjustments are required during changes of

altitude or during operations at airports with field elevations well above sea level. A mixture that is too rich can

result in engine roughness and reduced power. The roughness normally is due to spark plug fouling from excessive carbon buildup on the plugs. This occurs because

the excessively rich mixture lowers the temperature inside

the cylinder, inhibiting complete combustion of the fuel.

This condition may occur during the pretakeoff runup at

high elevation airports and during climbs or cruise flight

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at high altitudes. Usually, you can correct the problem by

leaning the mixture according to RFM instructions.

If you fail to enrich the mixture during a descent from

high altitude, it normally becomes too lean. High

engine temperatures can cause excessive engine wear

or even failure. The best way to avoid this type of situation is to monitor the engine temperature gauges regularly and follow the manufacturer’s guidelines for

maintaining the proper mixture.

CARBURETOR ICE

The effect of fuel vaporization and decreasing air pressure in the venturi causes a sharp drop in temperature

in the carburetor. If the air is moist, the water vapor in

the air may condense. When the temperature in the carburetor is at or below freezing, carburetor ice may form

on internal surfaces, including the throttle valve.

[Figure 5-10] Because of the sudden cooling that takes

place in the carburetor, icing can occur even on warm

days with temperatures as high as 38°C (100°F) and

the humidity as low as 50 percent. However, it is more

likely to occur when temperatures are below 21°C

(70°F) and the relative humidity is above 80 percent.

The likelihood of icing increases as temperature

decreases down to 0°C (32°F), and as relative humidity

increases. Below freezing, the possibility of carburetor

icing decreases with decreasing temperatures.

Although carburetor ice can occur during any phase of

flight, it 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

r.p.m. or manifold pressure, the carburetor air temperature gauge indicating a temperature outside the safe

operating range, and engine roughness. Since changes

in r.p.m. or manifold pressure can occur for a number

of reasons, it is best to 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. You should refer to the FAA-Approved Rotorcraft

Flight Manual for the specific procedure as to when

and how to apply carburetor heat. However, in most

cases, you should 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

To Engine

Incoming Air

Ice

Ice

Venturi

Fuel/Air

Mixture

Ice

Figure 5-10. Carburetor ice reduces the size of the air passage to the engine. This restricts the flow of the fuel/air

mixture, and reduces power.

5-8

routing air across a heat source, such as an exhaust

帅哥

manifold, before it enters the carburetor. [Figure 5-11].

FUEL INJECTION

In a fuel injection system, fuel and air are metered at

the fuel control unit but are not mixed. The fuel is

injected directly into the intake port of the cylinder

where it is mixed with the air just before entering the

cylinder. This system ensures a more even fuel distribution in the cylinders and better vaporization, which

in turn, promotes more efficient use of fuel. Also, the

fuel injection system eliminates the problem of carburetor icing and the need for a carburetor heat system.

TURBINE ENGINES

The fuel control system on the turbine engine is fairly

complex, as it monitors and adjusts many different

parameters on the engine. These adjustments are done

automatically and no action is required of the pilot

other than starting and shutting down. No mixture

adjustment is necessary, and operation is fairly simple

as far as the pilot is concerned. New generation fuel

controls incorporate the use of a full authority digital

engine control (FADEC) computer to control the

engine’s fuel requirements. The FADEC systems

increase efficiency, reduce engine wear, and also

reduce pilot workload. The FADEC usually incorporates back-up systems in the event of computer failure.

ELECTRICAL SYSTEMS

The electrical systems, in most helicopters, reflect the

increased use of sophisticated avionics and other electrical accessories. More and more operations in today’s

flight environment are dependent on the aircraft’s electrical system; however, all helicopters can be safely

flown without any electrical power in the event of an

electrical malfunction or emergency.

Helicopters have either a 14- or 28-volt, direct-current electrical system. On small, piston powered

helicopters, electrical energy is supplied by an engine-

driven alternator. These alternators have advantages

over older style generators as they are lighter in

weight, require lower maintenance, and maintain a

uniform electrical output even at low engine r.p.m.

[Figure 5-12]

Turbine powered helicopters use a starter/generator

system. The starter/generator is permanently coupled

to the engine gearbox. When starting the engine, electrical power from the battery is supplied to the

starter/generator, which turns the engine over. Once the

engine is running, the starter/generator is driven by the

engine and is then used as a generator.

Current from the alternator or generator is delivered

through a voltage regulator to a bus bar. The voltage

regulator maintains the constant voltage required by

the electrical system by regulating the output of the

alternator or generator. An over-voltage control may be

Avionic

Bus

Bar