直升机飞行手册Rotorcraft flying handbook

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the cyclic is centered.

• The cyclic pitch control system is designed so that

the rotor disc tilts slightly opposite tail rotor thrust

when in a hover.

Counteracting translating tendency, in a helicopter with a

counterclockwise main rotor system, causes the left skid

to hang lower while hovering. The opposite is true for

rotor systems turning clockwise when viewed from above.

PENDULAR ACTION

Since the fuselage of the helicopter, with a single main

rotor, is suspended from a single point and has considerable mass, it is free to oscillate either longitudinally or

laterally in the same way as a pendulum. This pendular

action can be exaggerated by over controlling; therefore,

control movements should be smooth and not exaggerated. [Figure 3-3]

CONING

In order for a helicopter to generate lift, the rotor blades

must be turning. This creates a relative wind that is

opposite the direction of rotor system rotation. The

rotation of the rotor system creates centrifugal force

(inertia), which tends to pull the blades straight outward

from the main rotor hub. The faster the rotation, the

Hover

Rearward

Flight

Forward

Flight

Figure 3-3. Because the helicopter’s body has mass and is

suspended from a single point (the rotor mast head), it tends

to act much like a pendulum.

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rotation and blade deceleration takes place. [Figure 3-5]

Keep in mind that due to coning, a rotor blade will not

flap below a plane passing through the rotor hub and

perpendicular to the axis of rotation. The acceleration

and deceleration actions of the rotor blades are absorbed

by either dampers or the blade structure itself, depending upon the design of the rotor system.

Two-bladed rotor systems are normally subject to

Coriolis Effect to a much lesser degree than are articulated rotor systems since the blades are generally

“underslung” with respect to the rotor hub, and the

change in the distance of the center of mass from the

axis of rotation is small. [Figure 3-6] The hunting

action is absorbed by the blades through bending. If a

two-bladed rotor system is not “underslung,” it will be

subject to Coriolis Effect comparable to that of a fully

articulated system.

GROUND EFFECT

When hovering near the ground, a phenomenon known

as ground effect takes place. [Figure 3-7] This effect

usually occurs less than one rotor diameter above the

surface. As the induced airflow through the rotor disc is

reduced by the surface friction, the lift vector increases.

This allows a lower rotor blade angle for the same

amount of lift, which reduces induced drag. Ground

effect also restricts the generation of blade tip vortices

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In powered flight (hovering, vertical, forward, sideward, or rearward), the total lift and thrust forces of a

rotor are perpendicular to the tip-path plane or plane of

rotation of the rotor.

HOVERING FLIGHT

For standardization purposes, this discussion assumes

a stationary hover in a no-wind condition. During hovering flight, a helicopter maintains a constant position

over a selected point, usually a few feet above the

ground. For a helicopter to hover, the lift and thrust

produced by the rotor system act straight up and must

equal the weight and drag, which act straight down.

While hovering, you can change the amount of main

rotor thrust to maintain the desired hovering altitude.

This is done by changing the angle of attack of the main

rotor blades and by varying power, as needed. In this

case, thrust acts in the same vertical direction as lift.

[Figure 3-1]

The weight that must be supported is the total weight of the

helicopter and its occupants. If the amount of thrust is

greater than the actual weight, the helicopter gains altitude;

if thrust is less than weight, the helicopter loses altitude.

The drag of a hovering helicopter is mainly induced drag

incurred while the blades are producing lift. There is,

however, some profile drag on the blades as they rotate

through the air. Throughout the rest of this discussion,

the term “drag” includes both induced and profile drag.

An important consequence of producing thrust is

torque. As stated before, for every action there is an

equal and opposite reaction. Therefore, as the engine

turns the main rotor system in a counterclockwise

direction, the helicopter fuselage turns clockwise. The

amount of torque is directly related to the amount of

engine power being used to turn the main rotor system.

Remember, as power changes, torque changes.

To counteract this torque-induced turning tendency, an

antitorque rotor or tail rotor is incorporated into most

helicopter designs. You can vary the amount of thrust

produced by the tail rotor in relation to the amount of

torque produced by the engine. As the engine supplies

more power, the tail rotor must produce more thrust.

This is done through the use of antitorque pedals.

TRANSLATING TENDENCY OR DRIFT

During hovering flight, a single main rotor helicopter tends

to drift in the same direction as antitorque rotor thrust. This

drifting tendency is called translating tendency. [Figure 3-2]

Thrust

Lift

Weight

Drag

Figure 3-1. To maintain a hover at a constant altitude, enough

lift and thrust must be generated to equal the weight of the

helicopter and the drag produced by the rotor blades.

Blade Rotation

Torque

Torque

Drift

Tail Rotor Thrust

Figure 3-2. A tail rotor is designed to produce thrust in a

direction opposite torque. The thrust produced by the tail

rotor is sufficient to move the helicopter laterally.

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greater the centrifugal force. This force gives the rotor

blades their rigidity and, in turn, the strength to support

the weight of the helicopter. The centrifugal force generated determines the maximum operating rotor r.p.m.

due to structural limitations on the main rotor system.

As a vertical takeoff is made, two major forces are acting at the same time—centrifugal force acting outward

and perpendicular to the rotor mast, and lift acting

upward and parallel to the mast. The result of these two

forces is that the blades assume a conical path instead

of remaining in the plane perpendicular to the mast.

[Figure 3-4]

CORIOLIS EFFECT

(LAW OF CONSERVATION

OF ANGULAR MOMENTUM)

Coriolis Effect, which is sometimes referred to as conservation of angular momentum, might be compared to

spinning skaters. When they extend their arms, their

rotation slows down because the center of mass moves

farther from the axis of rotation. When their arms are

retracted, the rotation speeds up because the center of

mass moves closer to the axis of rotation.

When a rotor blade flaps upward, the center of mass of

that blade moves closer to the axis of rotation and blade

acceleration takes place in order to conserve angular

momentum. Conversely, when that blade flaps downward, its center of mass moves further from the axis of

Before Takeoff

During Takeoff

Lift

Centrifugal

Force

Resultant

Blade

Angle

Figure 3-4. Rotor blade coning occurs as the rotor blades

begin to lift the weight of the helicopter. In a semirigid and

rigid rotor system, coning results in blade bending. In an

articulated rotor system, the blades assume an upward angle

through movement about the flapping hinges.

Centrifugal Force—The apparent

force that an object moving along

a circular path exerts on the body

constraining the obect and that

acts outwardy away from the center of rotation.

To counteract this drift, one or more of the following

features may be used:

• The main transmission is mounted so that the rotor

mast is rigged for the tip-path plane to have a builtin tilt opposite tail thrust, thus producing a small

sideward thrust.

• Flight control rigging is designed so that the rotor

disc is tilted slightly opposite tail rotor thrust when

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though the surface appears smooth, it may be quite

rough when viewed under a microscope. A thin layer of

air clings to the rough surface and creates small eddies

that contribute to drag.

INDUCED DRAG

Induced drag is generated by the airflow circulation

around the rotor blade as it creates lift. The high-pressure area beneath the blade joins the low-pressure air

above the blade at the trailing edge and at the rotor tips.

This causes a spiral, or vortex, which trails behind each

blade whenever lift is being produced. These vortices

deflect the airstream downward in the vicinity of the

blade, creating an increase in downwash. Therefore,

the blade operates in an average relative wind that is

inclined downward and rearward near the blade.

Because the lift produced by the blade is perpendicular

Aircraft Yaw—The movement of

the helicopter about its vertical

axis.

THRUST

Thrust, like lift, is generated by the rotation of the

main rotor system. In a helicopter, thrust can be forward, rearward, sideward, or vertical. The resultant of

lift and thrust determines the direction of movement of

the helicopter.

The solidity ratio is the ratio of the total rotor blade

area, which is the combined area of all the main rotor

blades, to the total rotor disc area. This ratio provides a

means to measure the potential for a rotor system to

provide thrust.

The tail rotor also produces thrust. The amount of

thrust is variable through the use of the antitorque pedals and is used to control the helicopter’s yaw.

Figure 2-11. The load factor diagram allows you to calculate

the amount of “G” loading exerted with various angle of

bank.

Load Factor - "G's"

Bank Angle (in Degrees)

0 10 20 30 40 50 60 70 80 90

9

8

7

6

5

4

3

2

1

0

Figure 2-12. It is easy to visualize the creation of form drag by examining the airflow around a flat plate. Streamlining decreases

form drag by reducing the airflow separation.

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to the relative wind, the lift is inclined aft by the same

amount. The component of lift that is acting in a rearward direction is induced drag. [Figure 2-13]

As the air pressure differential increases with an

increase in angle of attack, stronger vortices form, and

induced drag increases. Since the blade’s angle of

attack is usually lower at higher airspeeds, and higher

at low speeds, induced drag decreases as airspeed

increases and increases as airspeed decreases. Induced

drag is the major cause of drag at lower airspeeds.

PARASITE DRAG

Parasite drag is present any time the helicopter is moving

through the air. This type of drag increases with airspeed.

Nonlifting components of the helicopter, such as the

cabin, rotor mast, tail, and landing gear, contribute to parasite drag. Any loss of momentum by the airstream, due

to such things as openings for engine cooling, creates

additional parasite drag. Because of its rapid increase

with increasing airspeed, parasite drag is the major cause

of drag at higher airspeeds. Parasite drag varies with the

square of the velocity. Doubling the airspeed increases

the parasite drag four times.

TOTAL DRAG

Total drag for a helicopter is the sum of all three drag

forces. [Figure 2-14] As airspeed increases, parasite

drag increases, while induced drag decreases. Profile

drag remains relatively constant throughout the speed

range with some increase at higher airspeeds.

Combining all drag forces results in a total drag curve.

The low point on the total drag curve shows the airspeed at which drag is minimized. This is the point

where the lift-to-drag ratio is greatest and is referred to

as L/Dmax. At this speed, the total lift capacity of the

helicopter, when compared to the total drag of the helicopter, is most favorable. This is important in helicopter

performance.

Figure 2-14. The total drag curve represents the combined

forces of parasite, profile, and induced drag; and is plotted

against airspeed.

0 25 50 75 100 125 150

Speed

Drag

Parasite

Drag

Profile

Drag

Induced

Drag

Total Drag

Minimum

Drag or

L/D max

L/Dmax—The maximum ratio

between total lift (L) and the total

drag (D). This point provides the

best glide speed. Any deviation

from best glide speed increases

drag and reduces the distance you

can glide.

Induced Drag

Figure 2-13. The formation of induced drag is associated with

the downward deflection of the airstream near the rotor

blade.

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Once a helicopter leaves the ground, it is acted upon by

the four aerodynamic forces. In this chapter, we will

examine these forces as they relate to flight maneuvers.

POWERED FLIGHT

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A

Compare the upper surface of an airfoil with the constriction in a venturi tube that is narrower in the middle

than at the ends. [Figure 2-9]

The upper half of the venturi tube can be replaced by

layers of undisturbed air. Thus, as air flows over the

upper surface of an airfoil, the camber of the airfoil

causes an increase in the speed of the airflow. The

increased speed of airflow results in a decrease in pressure on the upper surface of the airfoil. At the same

time, air flows along the lower surface of the airfoil,

building up pressure. The combination of decreased

pressure on the upper surface and increased pressure

on the lower surface results in an upward force.

[Figure 2-10]

As angle of attack is increased, the production of lift is

increased. More upwash is created ahead of the airfoil

as the leading edge stagnation point moves under the

leading edge, and more downwash is created aft of the

trailing edge. Total lift now being produced is perpendicular to relative wind. In summary, the production of

lift is based upon the airfoil creating circulation in the

airstream (Magnus Effect) and creating differential

pressure on the airfoil (Bernoulli’s Principle).

NEWTON’S THIRD LAW OF MOTION

Additional lift is provided by the rotor blade’s lower

surface as air striking the underside is deflected down-

Figure 2-10. Lift is produced when there is decreased pressure above and increased pressure below an airfoil.

Lift

Decreased Pressure

Increased Pressure

Increased Velocity

Decreased Pressure

Figure 2-9. The upper surface of an airfoil is similar to the

constriction in a venturi tube.

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DRAG

The force that resists the movement of a helicopter

through the air and is produced when lift is developed

is called drag. Drag always acts parallel to the relative

wind. Total drag is composed of three types of drag:

profile, induced, and parasite.

PROFILE DRAG

Profile drag develops from the frictional resistance of

the blades passing through the air. It does not change

significantly with the airfoil’s angle of attack, but

increases moderately when airspeed increases. Profile

drag is composed of form drag and skin friction.

Form drag results from the turbulent wake caused by

the separation of airflow from the surface of a structure. The amount of drag is related to both the size and

shape of the structure that protrudes into the relative

wind. [Figure 2-12]

Skin friction is caused by surface roughness. Even

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with distance from the cylinder. On a cylinder, which is

rotating in such a way that the top surface area is rotating

in the same direction as the airflow, the local velocity at

the surface is high on top and low on the bottom.

As shown in figure 2-7, at point “A,” a stagnation point

exists where the airstream line that impinges on the surface splits; some air goes over and some under. Another

stagnation point exists at “B,” where the two air

streams rejoin and resume at identical velocities. We

now have upwash ahead of the rotating cylinder and

downwash at the rear.

The difference in surface velocity accounts for a difference in pressure, with the pressure being lower on the

top than the bottom. This low pressure area produces

an upward force known as the “Magnus Effect.” This

mechanically induced circulation illustrates the relationship between circulation and lift.

An airfoil with a positive angle of attack develops air

circulation as its sharp trailing edge forces the rear

stagnation point to be aft of the trailing edge, while the

front stagnation point is below the leading edge.

[Figure 2-8]

BERNOULLI’S PRINCIPLE

Air flowing over the top surface accelerates. The airfoil

is now subjected to Bernoulli’s Principle or the “venturi

effect.” As air velocity increases through the constricted

portion of a venturi tube, the pressure decreases.

Axis of Rotation

Reference Plane

Pitch

Angle

ChordLine

Angleof

Attack

RELATIVE WIND

Figure 2-6. Angle of attack may be greater than, less than, or

the same as the pitch angle.

Figure 2-5. As the angle of attack is increased, the separation

point starts near the trailing edge of the airfoil and progresses forward. Finally, the airfoil loses its lift and a stall

condition occurs.

LIFT

STALL

8°

12-16°

Figure 2-7. Magnus Effect is a lifting force produced when a

rotating cylinder produces a pressure differential. This is the

same effect that makes a baseball curve or a golf ball slice.

B A

Increased Local Velocity

(Decreased pressure)

Decreased Local Velocity

Downwash Upwash

Figure 2-8. Air circulation around an airfoil occurs when the

front stagnation point is below the leading edge and the aft

stagnation point is beyond the trailing edge.

Leading Edge

Stagnation Point

Trailing Edge

Stagnation Point

B

A

Steady-State Flight—A condition

when an aircraft is in straightand-level, unaccelerated flight,

and all forces are in balance.

2-4

ward. According to Newton’s Third Law of Motion,

“for every action there is an equal and opposite reaction,” the air that is deflected downward also produces

an upward (lifting) reaction.

Since air is much like water, the explanation for this

source of lift may be compared to the planing effect of

skis on water. The lift which supports the water skis

(and the skier) is the force caused by the impact pressure and the deflection of water from the lower surfaces

of the skis.

Under most flying conditions, the impact pressure and

the deflection of air from the lower surface of the rotor

blade provides a comparatively small percentage of the

total lift. The majority of lift is the result of decreased

pressure above the blade, rather than the increased

pressure below it.

WEIGHT

Normally, weight is thought of as being a known, fixed

value, such as the weight of the helicopter, fuel, and

occupants. To lift the helicopter off the ground vertically, the rotor system must generate enough lift to

overcome or offset the total weight of the helicopter

and its occupants. This is accomplished by increasing

the pitch angle of the main rotor blades.

The weight of the helicopter can also be influenced by

aerodynamic loads. When you bank a helicopter while

maintaining a constant altitude, the “G” load or load

factor increases. Load factor is the ratio of the load supported by the main rotor system to the actual weight of

the helicopter and its contents. In steady-state flight,

the helicopter has a load factor of one, which means the

main rotor system is supporting the actual total weight

of the helicopter. If you increase the bank angle to 60°,

while still maintaining a constant altitude, the load factor increases to two. In this case, the main rotor system

has to support twice the weight of the helicopter and its

contents. [Figure 2-11]

Disc loading of a helicopter is the ratio of weight to the

total main rotor disc area, and is determined by dividing the total helicopter weight by the rotor disc area,

which is the area swept by the blades of a rotor. Disc

area can be found by using the span of one rotor blade

as the radius of a circle and then determining the area

the blades encompass during a complete rotation. As

the helicopter is maneuvered, disc loading changes.

The higher the loading, the more power you need to

maintain rotor speed.

Leading Edge

Stagnation Point

B

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air to flow smoothly across the top of the airfoil. At this

point the airflow begins to separate from the airfoil and

enters a burbling or turbulent pattern. The turbulence

results in a large increase in drag and loss of lift in the

area where it is taking place. Increasing the angle of

attack increases lift until the critical angle of attack is

reached. Any increase in the angle of attack beyond this

point produces a stall and a rapid decrease in lift.

[Figure 2-5]

Angle of attack should not be confused with pitch

angle. Pitch angle is determined by the direction of the

relative wind. You can, however, change the angle of

attack by changing the pitch angle through the use of

the flight controls. If the pitch angle is increased, the

angle of attack is increased, if the pitch angle is

reduced, the angle of attack is reduced. [Figure 2-6]

Axis-of-Rotation—The imaginary

line about which the rotor rotates.

It is represented by a line drawn

through the center of, and perpendicular to, the tip-path plane.

Tip-Path Plane—The imaginary

circular plane outlined by the

rotor blade tips as they make a

cycle of rotation.

Aircraft Pitch—When referenced

to a helicopter, is the movement of

the helicopter about its lateral, or

side to side axis. Movement of the

cyclic forward or aft causes the

nose of the helicopter to move up

or down.

Aircraft Roll—Is the movement of

the helicopter about its longitudinal, or nose to tail axis. Movement

of the cyclic right or left causes the

helicopter to tilt in that direction.

Figure 2-3. Aerodynamic terms of an airfoil.

Trailing

Edge

ChordLine

Angleof

Attack

FLIGHT PATH

RELATIVE WIND

Upper

Camber

Lower

Camber

Leading

Edge

Axis of Rotation

Reference Plane

Pitch

Angle

ChordLine

Figure 2-4. Do not confuse the axis of rotation with the rotor

mast. The only time they coincide is when the tip-path plane

is perpendicular to the rotor mast.

2-3

LIFT

MAGNUS EFFECT

The explanation of lift can best be explained by looking

at a cylinder rotating in an airstream. The local velocity

near the cylinder is composed of the airstream velocity

and the cylinder’s rotational velocity, which decreases

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Figure 2-1. Four forces acting on a helicopter in forward flight.

2-2

Twisting a rotor blade causes it to produce a more even

amount of lift along its span. This is necessary because

rotational velocity increases toward the blade tip. The

leading edge is the first part of the airfoil to meet the

oncoming air. [Figure 2-3] The trailing edge is the aft

portion where the airflow over the upper surface joins

the airflow under the lower surface. The chord line is

an imaginary straight line drawn from the leading to

the trailing edge. The camber is the curvature of the airfoil’s upper and lower surfaces. The relative wind is the

wind moving past the airfoil. The direction of this wind

is relative to the attitude, or position, of the airfoil and

is always parallel, equal, and opposite in direction to

the flight path of the airfoil. The angle of attack is the

angle between the blade chord line and the direction of

the relative wind.

RELATIVE WIND

Relative wind is created by the motion of an airfoil

through the air, by the motion of air past an airfoil, or by

a combination of the two. Relative wind may be

affected by several factors, including the rotation of the

rotor blades, horizontal movement of the helicopter,

flapping of the rotor blades, and wind speed and direction.

For a helicopter, the relative wind is the flow of air with

respect to the rotor blades. If the rotor is stopped, wind

blowing over the blades creates a relative wind. When

the helicopter is hovering in a no-wind condition, relative wind is created by the motion of the rotor blades

through the air. If the helicopter is hovering in a wind,

the relative wind is a combination of the wind and the

motion of the rotor blades through the air. When the

helicopter is in forward flight, the relative wind is a

combination of the rotation of the rotor blades and the

forward speed of the helicopter.

BLADE PITCH ANGLE

The pitch angle of a rotor blade is the angle between its

chord line and the reference plane containing the rotor

hub. [Figure 2-4] You control the pitch angle of the blades

with the flight controls. The collective pitch changes each

rotor blade an equal amount of pitch no matter where it is

located in the plane of rotation (rotor disc) and is used to

change rotor thrust. The cyclic pitch control changes the

pitch of each blade as a function of where it is in the plane

of rotation. This allows for trimming the helicopter in

pitch and roll during forward flight and for maneuvering

in all flight conditions.

ANGLE OF ATTACK

When the angle of attack is increased, air flowing over

the airfoil is diverted over a greater distance, resulting

in an increase of air velocity and more lift. As angle of

attack is increased further, it becomes more difficult for

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aware of certain aerodynamic terms that describe an

airfoil and the interaction of the airflow around it.

An airfoil is any surface, such as an airplane wing or a

helicopter rotor blade, which provides aerodynamic

force when it interacts with a moving stream of air.

Although there are many different rotor blade airfoil

designs, in most helicopter flight conditions, all airfoils

perform in the same manner.

Engineers of the first helicopters designed relatively

thick airfoils for their structural characteristics.

Because the rotor blades were very long and slender, it

was necessary to incorporate more structural rigidity

into them. This prevented excessive blade droop when

the rotor system was idle, and minimized blade twisting while in flight. The airfoils were also designed to

be symmetrical, which means they had the same camber (curvature) on both the upper and lower surfaces.

Symmetrical blades are very stable, which helps keep

blade twisting and flight control loads to a minimum.

[Figure 2-2] This stability is achieved by keeping the

center of pressure virtually unchanged as the angle of

attack changes. Center of pressure is the imaginary

point on the chord line where the resultant of all aerodynamic forces are considered to be concentrated.

Today, designers use thinner airfoils and obtain the

required rigidity by using composite materials. In addition, airfoils are asymmetrical in design, meaning the

upper and lower surface do not have the same camber.

Normally these airfoils would not be as stable, but this

can be corrected by bending the trailing edge to produce

the same characteristics as symmetrical airfoils. This is

called “reflexing.” Using this type of rotor blade allows

the rotor system to operate at higher forward speeds.

One of the reasons an asymmetrical rotor blade is not

as stable is that the center of pressure changes with

changes in angle of attack. When the center of pressure

lifting force is behind the pivot point on a rotor blade, it

tends to cause the rotor disc to pitch up. As the angle of

attack increases, the center of pressure moves forward.

If it moves ahead of the pivot point, the pitch of the

rotor disc decreases. Since the angle of attack of the

rotor blades is constantly changing during each cycle

of rotation, the blades tend to flap, feather, lead, and

lag to a greater degree.

When referring to an airfoil, the span is the distance

from the rotor hub to the blade tip. Blade twist refers to

a changing chord line from the blade root to the tip.

Figure 2-2. The upper and lower curvatures are the same on a

symmetrical airfoil and vary on an asymmetrical airfoil.

Asymmetrical

Symmetrical

Lift

Weight

Drag

Thrust

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A typical small helicopter has a reciprocating engine,

which is mounted on the airframe. The engine can be

mounted horizontally or vertically with the transmission supplying the power to the vertical main rotor

shaft. [Figure 1-6]

Another engine type is the gas turbine. This engine is

used in most medium to heavy lift helicopters due to its

large horsepower output. The engine drives the main

transmission, which then transfers power directly to the

main rotor system, as well as the tail rotor.

FLIGHT CONTROLS

When you begin flying a helicopter, you will use four

basic flight controls. They are the cyclic pitch control;

the collective pitch control; the throttle, which is

usually a twist grip control located on the end of the

collective lever; and the antitorque pedals. The collective and cyclic controls the pitch of the main rotor

blades. The function of these controls will be explained

in detail in Chapter 4—Flight Controls. [Figure 1-7]

Figure 1-5. While in a hover, Coanda Effect supplies approximately two-thirds of the lift necessary to maintain directional

control. The rest is created by directing the thrust from the

controllable rotating nozzle.

Main Rotor

Wake

Rotating

Nozzle

Downwash

Air

Jet

Lift

Air Intake

Main

Rotor

Main

Transmission

Antitorque

Rotor

Engine

Figure 1-6. Typically, the engine drives the main rotor through

a transmission and belt drive or centrifugal clutch system.

The antitorque rotor is driven from the transmission.

Cyclic

Throttle

Collective

Antitorque

Pedals

Figure 1-7. Location of flight controls.

1-4

2-1

There are four forces acting on a helicopter in flight.

They are lift, weight, thrust, and drag. [Figure 2-1] Lift

is the upward force created by the effect of airflow as it

passes around an airfoil. Weight opposes lift and is

caused by the downward pull of gravity. Thrust is the

force that propels the helicopter through the air.

Opposing lift and thrust is drag, which is the retarding

force created by development of lift and the movement

of an object through the air.

AIRFOIL

Before beginning the discussion of lift, you need to be

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TAIL ROTOR

Most helicopters with a single, main rotor system

require a separate rotor to overcome torque. This is

accomplished through a variable pitch, antitorque rotor

or tail rotor. [Figure 1-3]. You will need to vary the

thrust of the antitorque system to maintain directional

control whenever the main rotor torque changes, or to

make heading changes while hovering.

FENESTRON

Another form of antitorque rotor is the fenestron or

“fan-in-tail” design. This system uses a series of rotating blades shrouded within a vertical tail. Because the

blades are located within a circular duct, they are less

likely to come into contact with people or objects.

[Figure 1-4]

NOTAR®

The NOTAR® system is an alternative to the antitorque

rotor. The system uses low-pressure air that is forced

into the tailboom by a fan mounted within the helicopter. The air is then fed through horizontal slots, located

on the right side of the tailboom, and to a controllable

rotating nozzle to provide antitorque and directional

control. The low-pressure air coming from the horizontal slots, in conjunction with the downwash from the

main rotor, creates a phenomenon called “Coanda

Effect,” which produces a lifting force on the right side

of the tailboom. [Figure 1-5]

LANDING GEAR

The most common landing gear is a skid type gear,

which is suitable for landing on various types of surfaces. Some types of skid gear are equipped with

dampers so touchdown shocks or jolts are not transmitted to the main rotor system. Other types absorb the

shocks by the bending of the skid attachment arms.

Landing skids may be fitted with replaceable heavyduty skid shoes to protect them from excessive wear

and tear.

Helicopters can also be equipped with floats for water

operations, or skis for landing on snow or soft terrain.

Wheels are another type of landing gear. They may be

in a tricycle or four point configuration. Normally, the

Tail Rotor Thrust

to Compensate for Torque

Torque

Torque

Blade Rotation

Figure 1-3. The antitorque rotor produces thrust to oppose

torque and helps prevent the helicopter from turning in the

opposite direction of the main rotor.

Figure 1-4. Compared to an unprotected tail rotor, the fenestron antitorque system provides an improved margin of

safety during ground operations.

1-3

nose or tail gear is free to swivel as the helicopter is

taxied on the ground.

POWERPLANT