直升机飞行手册Rotorcraft flying handbook

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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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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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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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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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due to the downward and outward airflow making a

larger portion of the blade produce lift. When the helicopter gains altitude vertically, with no forward airspeed, induced airflow is no longer restricted, and the

blade tip vortices increase with the decrease in outward

airflow. As a result, drag increases which means a

Axis of

Rotation

Blade

Flapping

Center of Mass

Figure 3-5. The tendency of a rotor blade to increase or

decrease its velocity in its plane of rotation due to mass

movement is known as Coriolis Effect, named for the mathematician who made studies of forces generated by radial

movements of mass on a rotating disc.

Large Blade

Tip Vortex

No Wind Hover

Blade Tip

Vortex

OUT OF GROUND EFFECT (OGE) IN GROUND EFFECT (IGE)

Downwash Pattern

Equidistant 360°

Figure 3-7. Air circulation patterns change when hovering out of ground effect (OGE) and when hovering in ground effect (IGE).

This elbow moves away from

the mast as the rotor is tilted.

This elbow moves toward

the mast as the rotor is tilted.

Mast

Axis

CM CM

CM

CM

Figure 3-6. Because of the underslung rotor, the center of

mass remains approximately the same distance from the

mast after the rotor is tilted.

3-4

higher pitch angle, and more power is needed to move

the air down through the rotor.

Ground effect is at its maximum in a no-wind condition

over a firm, smooth surface. Tall grass, rough terrain,

revetments, and water surfaces alter the airflow pattern,

causing an increase in rotor tip vortices.

GYROSCOPIC PRECESSION

The spinning main rotor of a helicopter acts like a gyroscope. As such, it has the properties of gyroscopic

action, one of which is precession. Gyroscopic precession is the resultant action or deflection of a spinning

object when a force is applied to this object. This action

occurs approximately 90° in the direction of rotation

from the point where the force is applied. [Figure 3-8]

Let us look at a two-bladed rotor system to see how

gyroscopic precession affects the movement of the tippath plane. Moving the cyclic pitch control increases

the angle of attack of one rotor blade with the result

that a greater lifting force is applied at that point in the

plane of rotation. This same control movement simultaneously decreases the angle of attack of the other

blade the same amount, thus decreasing the lifting force

applied at that point in the plane of rotation. The blade

with the increased angle of attack tends to flap up; the

blade with the decreased angle of attack tends to flap

down. Because the rotor disk acts like a gyro, the

blades reach maximum deflection at a point approximately 90° later in the plane of rotation. As shown in

figure 3-9, the retreating blade angle of attack is

increased and the advancing blade angle of attack is

decreased resulting in a tipping forward of the tip-path

plane, since maximum deflection takes place 90° later

when the blades are at the rear and front, respectively.

In a rotor system using three or more blades, the movement of the cyclic pitch control changes the angle of

attack of each blade an appropriate amount so that the

end result is the same.

VERTICAL FLIGHT

Hovering is actually an element of vertical flight.

Increasing the angle of attack of the rotor blades (pitch)

while their velocity remains constant generates additional vertical lift and thrust and the helicopter ascends.

Decreasing the pitch causes the helicopter to descend.

In a no wind condition when lift and thrust are less than

weight and drag, the helicopter descends vertically. If

90°

Axis

Upward

Force

Applied

Here

Reaction

Occurs

Here

New Axis

Gyro Tips

Down Here

Gyro Tips

Up Here

Old Axis

Figure 3-8. Gyroscopic precession principle—when a force is applied to a spinning gyro, the maximum reaction occurs approximately 90° later in the direction of rotation.

Blade

Rotation

Angle of Attack

Decreased

Maximum

Upward

Deflection

Maximum

Downward

Deflection

Angle of Attack

Increased

Figure 3-9. With a counterclockwise main rotor blade rotation, as each blade passes the 90° position on the left, the

maximum increase in angle of attack occurs. As each blade

passes the 90° position to the right, the maximum decrease

in angle of attack occurs. Maximum deflection takes place

90° later—maximum upward deflection at the rear and maximum downward deflection at the front—and the tip-path

plane tips forward.

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lift and thrust are greater than weight and drag, the helicopter ascends vertically. [Figure 3-10]

FORWARD FLIGHT

In or during forward flight, the tip-path plane is tilted forward, thus tilting the total lift-thrust force

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forward from

the vertical. This resultant lift-thrust force can be resolved

into two components—lift acting vertically upward and

thrust acting horizontally in the direction of flight. In

addition to lift and thrust, there is weight (the downward

acting force) and drag (the rearward acting or retarding

force of inertia and wind resistance). [Figure 3-11]

In straight-and-level, unaccelerated forward flight, lift

equals weight and thrust equals drag (straight-and-level

flight is flight with a constant heading and at a constant

altitude). If lift exceeds weight, the helicopter climbs;

if lift is less than weight, the helicopter descends. If

thrust exceeds drag, the helicopter speeds up; if thrust

is less than drag, it slows down.

As the helicopter moves forward, it begins to lose altitude because of the lift that is lost as thrust is diverted

forward. However, as the helicopter begins to accelerate, the rotor system becomes more efficient due to the

increased airflow. The result is excess power over that

which is required to hover. Continued acceleration

causes an even larger increase in airflow through the

rotor disc and more excess power.

TRANSLATIONAL LIFT

Translational lift is present with any horizontal flow of

air across the rotor. This increased flow is most noticeable when the airspeed reaches approximately 16 to 24

knots. As the helicopter accelerates through this speed,

the rotor moves out of its vortices and is in relatively

undisturbed air. The airflow is also now more horizontal,

which reduces induced flow and drag with a corresponding increase in angle of attack and lift. The additional

lift available at this speed is referred to as “effective

translational lift” (ETL). [Figure 3-12]

When a single-rotor helicopter flies through translational

lift, the air flowing through the main rotor and over the

tail rotor becomes less turbulent and more aerodynamically efficient. As the tail rotor efficiency improves,

more thrust is produced causing the aircraft to yaw left

in a counterclockwise rotor system. It will be necessary

to use right torque pedal to correct for this tendency on

takeoff. Also, if no corrections are made, the nose rises

or pitches up, and rolls to the right. This is caused by

combined effects of dissymmetry of lift and transverse

flow effect, and is corrected with cyclic control.

Resultant

Resultant

Lift

Thrust

Helicopter

Movement

Weight

Drag

Figure 3-11. To transition into forward flight, some of the vertical thrust must be vectored horizontally. You initiate this by

forward movement of the cyclic control.

No Recirculation

of Air

More Horizontal

Flow of Air

Reduced

Induced Flow

Increases

Angle of Attack

Tail Rotor Operates in

Relatively Clean Air

16 to 24

Knots

Figure 3-12. Effective translational lift is easily recognized in

actual flight by a transient induced aerodynamic vibration

and increased performance of the helicopter.

Thrust

Lift

Weight

Drag

Vertical Ascent

Figure 3-10. To ascend vertically, more lift and thrust must be

generated to overcome the forces of weight and the drag.

3-6

Translational lift is also present in a stationary hover if

the wind speed is approximately 16 to 24 knots. In normal operations, always utilize the benefit of translational

lift, especially if maximum performance is needed.

INDUCED FLOW

As the rotor blades rotate they generate what is called

rotational relative wind. This airflow is characterized

as flowing parallel and opposite the rotor’s plane of

rotation and striking perpendicular to the rotor blade’s

leading edge. This rotational relative wind is used to

generate lift. As rotor blades produce lift, air is accelerated over the foil and projected downward. Anytime a

helicopter is producing lift, it moves large masses of air

vertically and down through the rotor system. This

downwash or induced flow can significantly change

the efficiency of the rotor system. Rotational relative

wind combines with induced flow to form the resultant

relative wind. As induced flow increases, resultant relative wind becomes less horizontal. Since angle of

attack is determined by measuring the difference

between the chord line and the resultant relative wind,

as the resultant relative wind becomes less horizontal,

angle of attack decreases. [Figure 3-13]

TRANSVERSE FLOW EFFECT

As the helicopter accelerates in forward flight, induced

flow drops to near zero at the forward disc area and

increases at the aft disc area. This increases the angle

of attack at the front disc area causing the rotor blade to

flap up, and reduces angle of attack at the aft disc area

causing the rotor blade to flap down. Because the rotor

acts like a gyro, maximum displacement occurs 90° in

the direction of rotation. The result is a tendency for

the helicopter to roll slightly to the right as it acceler-

ates through approximately 20 knots or if the headwind

is approximately 20 knots.

You can recognize transverse flow effect because of

increased vibrations of the helicopter at airspeeds just

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below effective translational lift on takeoff and after

passing through effective translational lift during landing. To counteract transverse flow effect, a cyclic input

needs to be made.

DISSYMMETRY OF LIFT

When the helicopter moves through the air, the relative

airflow through the main rotor disc is different on the

advancing side than on the retreating side. The relative

wind encountered by the advancing blade is increased

by the forward speed of the helicopter, while the relative wind speed acting on the retreating blade is

reduced by the helicopter’s forward airspeed.

Therefore, as a result of the relative wind speed, the

advancing blade side of the rotor disc produces more

lift than the retreating blade side. This situation is

defined as dissymmetry of lift. [Figure 3-14]

If this condition was allowed to exist, a helicopter with

a counterclockwise main rotor blade rotation would roll

to the left because of the difference in lift. In reality, the

main rotor blades flap and feather automatically to

equalize lift across the rotor disc. Articulated rotor systems, usually with three or more blades, incorporate a

horizontal hinge (flapping hinge) to allow the individual rotor blades to move, or flap up and down as they

rotate. A semirigid rotor system (two blades) utilizes a

teetering hinge, which allows the blades to flap as a

unit. When one blade flaps up, the other flaps down.

Figure 3-13. A helicopter in forward flight, or hovering with a headwind or crosswind, has more molecules of air entering the aft

portion of the rotor blade. Therefore, the angle of attack is less and the induced flow is greater at the rear of the rotor disc.

ResultantRelativeWind

ResultantRelativeWind

10 to 20

Knots

A

A

B

B

Induced

Flow

Induced

Flow

Angle of

Attack

Angle of

Attack

Rotational Relative Wind Rotational Relative Wind

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As shown in figure 3-15, as the rotor blade reaches the

advancing side of the rotor disc (A), it reaches its maximum upflap velocity. When the blade flaps upward,

the angle between the chord line and the resultant relative wind decreases. This decreases the angle of attack,

which reduces the amount of lift produced by the blade.

At position (C) the rotor blade is now at its maximum

downflapping velocity. Due to downflapping, the angle

between the chord line and the resultant relative wind

increases. This increases the angle of attack and thus

the amount of lift produced by the blade.

The combination of blade flapping and slow relative wind

acting on the retreating blade normally limits the maximum forward speed of a helicopter. At a high forward

speed, the retreating blade stalls because of a high angle of

attack and slow relative wind speed. This situation is

called retreating blade stall and is evidenced by a nose

pitch up, vibration, and a rolling tendency—usually to the

left in helicopters with counterclockwise blade rotation.

You can avoid retreating blade stall by not exceeding

the never-exceed speed. This speed is designated VNE

and is usually indicated on a placard and marked on the

airspeed indicator by a red line.

During aerodynamic flapping of the rotor blades as they

compensate for dissymmetry of lift, the advancing blade

Relative Wind

Relative Wind

Direction

of Flight

Advancing

Side

Blade Tip

Speed Plus

Helicopter

Speed

(400 KTS)

Blade Tip

Speed Minus

Helicopter

Speed

(200 KTS)

Retreating

Side

Forward Flight

100 KTS

Blade

Rotation

Figure 3-14. The blade tip speed of this helicopter is approximately 300 knots. If the helicopter is moving forward at 100

knots, the relative wind speed on the advancing side is 400

knots. On the retreating side, it is only 200 knots. This difference in speed causes a dissymmetry of lift.

Figure 3-15. The combined upward flapping (reduced lift) of the advancing blade and downward flapping (increased lift) of the

retreating blade equalizes lift across the main rotor disc counteracting dissymmetry of lift.

Direction of Rotation

ChordLine

Resultant RW

ChordLine

Resultant RW

ChordLine

DownflapVelocity

ResultantRW

ChordLine

ResultantRW

Upflap Velocity

Angle of Attack at

9 O'Clock Position

Angle of Attack at

3 O'Clock Position

Angle of Attack over

Tail

Angle of Attack over

Nose

A

B

C

D

D

A

B

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C

RW = Relative Wind

= Angle of Attack

VNE—The speed beyond which an aircraft should never be

operated. VNE can change with altitude, density altitude, and

weight.

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ward. Drag now acts forward with the lift component

straight up and weight straight down. [Figure 3-18]

TURNING FLIGHT

In forward flight, the rotor disc is tilted forward, which

also tilts the total lift-thrust force of the rotor disc forward. When the helicopter is banked, the rotor disc is

tilted sideward resulting in lift being separated into two

components. Lift acting upward and opposing weight is

called the vertical component of lift. Lift acting horizontally and opposing inertia (centrifugal force) is the

horizontal component of lift (centripetal force).

[Figure 3-19]

As the angle of bank increases, the total lift force is tilted

more toward the horizontal, thus causing the rate of turn

to increase because more lift is acting horizontally. Since

the resultant lifting force acts more horizontally, the

effect of lift acting vertically is deceased. To compensate for this decreased vertical lift, the angle of attack of

the rotor blades must be increased in order to maintain

altitude. The steeper the angle of bank, the greater the

angle of attack of the rotor blades required to maintain

altitude. Thus, with an increase in bank and a greater

angle of attack, the resultant lifting force increases and

the rate of turn is faster.

AUTOROTATION

Autorotation is the state of flight where the main rotor

system is being turned by the action of relative wind

Centripetal Force—The force

opposite centrifugal force and

attracts a body toward its axis of

rotation.

Resultant

Lift

Thrust

Drag

Resultant

Weight

Helicopter

Movement

Figure 3-18. Forces acting on the helicopter during rearward

flight.

achieves maximum upflapping displacement over the

nose and maximum downflapping displacement over the

tail. This causes the tip-path plane to tilt to the rear and is

referred to as blowback. Figure 3-16 shows how the rotor

disc was originally oriented with the front down following the initial cyclic input, but as airspeed is gained and

flapping eliminates dissymmetry of lift, the front of the

disc comes up, and the back of the disc goes down. This

reorientation of the rotor disc changes the direction in

which total rotor thrust acts so that the helicopter’s forward speed slows, but can be corrected with cyclic input.

SIDEWARD FLIGHT

In sideward flight, the tip-path plane is tilted in the direction that flight is desired. This tilts the total lift-thrust

vector sideward. In this case, the vertical or lift component is still straight up and weight straight down, but the

horizontal or thrust component now acts sideward with

drag acting to the opposite side. [Figure 3-17]

REARWARD FLIGHT

For rearward flight, the tip-path plane is tilted rearward, which, in turn, tilts the lift-thrust vector rear-

Helicopter

Movement

Weight

Drag

Resultant

Thrust

Lift

Figure 3-17. Forces acting on the helicopter during sideward

flight.

Figure 3-16. To compensate for blowback, you must move

the cyclic forward. Blowback is more pronounced with higher

airspeeds.

3-9

rather than engine power. It is the means by which a

helicopter can be landed safely in the event of an

engine failure. In this case, you are using altitude as

potential energy and converting it to kinetic energy during the descent and touchdown. All helicopters must

have this capability in order to be certified.

Autorotation is permitted mechanically because of a

freewheeling unit, which allows the main rotor to con-

tinue turning even if the engine is not running. In normal powered flight, air is drawn into the main rotor system from above and exhausted downward. During

autorotation, airflow enters the rotor disc from below

as the helicopter descends. [Figure 3-20]

AUTOROTATION (VERTICAL FLIGHT)

Most autorotations are performed with forward speed.

For simplicity, the following aerodynamic explanation

is based on a vertical autorotative descent (no forward

speed) in still air. Under these conditions, the forces

that cause the blades to turn are similar for all blades

regardless of their position in the plane of rotation.

Therefore, dissymmetry of lift resulting from helicopter airspeed is not a factor.

During vertical autorotation, the rotor disc is divided

into three regions as illustrated in figure 3-21—the

Figure 3-20. During an autorotation, the upward flow of relative wind permits the main rotor blades to rotate at their normal

speed. In effect, the blades are “gliding” in their rotational plane.

Figure 3-21. Blade regions in vertical autorotation descent.

Figure 3-19. The horizontal component of lift accelerates the

helicopter toward the center of the turn.

Centripetal Force

(Horizontal Component of Lift)

Vertical

Component

of Lift

Bank

Angle

帅哥

Resultant

Lift

Weight

Centrifugal

Force (Inertia)

Normal Powered Flight Autorotation

Direction

ofFlight

Direction

ofFlight

Stall

Region 25%

Driven

Region 30% Driving

Region 45%

3-10

driven region, the driving region, and the stall region.

Figure 3-22 shows four blade sections that illustrate

force vectors. Part A is the driven region, B and D are

points of equilibrium, part C is the driving region, and

part E is the stall region. Force vectors are different in

each region because rotational relative wind is slower

near the blade root and increases continually toward

the blade tip. Also, blade twist gives a more positive

angle of attack in the driving region than in the driven

region. The combination of the inflow up through the

rotor with rotational relative wind produces different

combinations of aerodynamic force at every point

along the blade.

The driven region, also called the propeller region, is

nearest the blade tips. Normally, it consists of about 30

Figure 3-22. Force vectors in vertical autorotation descent.

B & Rotational

Relative Wind

Lift

TAF

TAF

Total

Aerodynamic

Force Aft

of Axis of

Rotation

Total

Aerodynamic

Force Forward

of Axis of

Rotation

Angle of

Attack 2° Drag

Chord Line

Inflow Up

Through Rotor

Resultant

Relative Wind

Equilibrium

Drag

Inflow

TAF

Angle of

Attack 6°

Drag Driving

Region

Inflow

Axis of

Rotation

Angle of

Attack 24°

(Blade is Stalled)

TAF

Drag

Stall

Region

Inflow

Driven

Range

C

Driven

Region

Drag

Point of

Equilibrium

Point of

Equilibrium

Driving

Region

Stall

Region

Drag

Autorotative Force

A

D

C

E

E

D

B

A

Lift

Lift

Lift

3-11

percent of the radius. In the driven region, part A of figure 3-22, the total aerodynamic force acts behind the

axis of rotation, resulting in a overall drag force. The

driven region produces some lift, but that lift is offset

by drag. The overall result is a deceleration in the rotation of the blade. The size of this region varies with the

blade pitch, rate of descent, and rotor r.p.m. When

changing autorotative r.p.m., blade pitch, or rate of

descent, the size of the driven region in relation to the

other regions also changes.

There are two points of equilibrium on the blade—one

between the driven region and the driving region, and

one between the driving region and the stall region. At

points of equilibrium, total aerodynamic force is

aligned with the axis of rotation. Lift and drag are produced, but the total effect produces neither acceleration

nor deceleration.

The driving region, or autorotative region, normally

lies between 25 to 70 percent of the blade radius. Part

C of figure 3-22 shows the driving region of the blade,

which produces the forces needed to turn the blades

during autorotation. Total aerodynamic force in the

driving region is inclined slightly forward of the axis of

rotation, producing a continual acceleration force. This

帅哥

inclination supplies thrust, which tends to accelerate

the rotation of the blade. Driving region size varies

with blade pitch setting, rate of descent, and rotor r.p.m.

By controlling the size of this region you can adjust

autorotative r.p.m. For example, if the collective pitch

is raised, the pitch angle increases in all regions. This

causes the point of equilibrium to move inboard along

the blade’s span, thus increasing the size of the driven

region. The stall region also becomes larger while the

driving region becomes smaller. Reducing the size of

the driving region causes the acceleration force of the

driving region and r.p.m. to decrease.

The inner 25 percent of the rotor blade is referred to as

the stall region and operates above its maximum angle

of attack (stall angle) causing drag which tends to slow

rotation of the blade. Part E of figure 3-22 depicts the

stall region.

A constant rotor r.p.m. is achieved by adjusting the collective pitch so blade acceleration forces from the driving region are balanced with the deceleration forces

from the driven and stall regions.

AUTOROTATION (FORWARD FLIGHT)

Autorotative force in forward flight is produced in

exactly the same manner as when the helicopter is

descending vertically in still air. However, because forward speed changes the inflow of air up through the

rotor disc, all three regions move outboard along the

blade span on the retreating side of the disc where angle

of attack is larger, as shown in figure 3-23. With lower

angles of attack on the advancing side blade, more of

that blade falls in the driven region. On the retreating

side, more of the blade is in the stall region. A small

section near the root experiences a reversed flow, therefore the size of the driven region on the retreating side

is reduced.

Figure 3-23. Blade regions in forward autorotation descent.

Forward

Driven

Region

Driving

Region

Retreating

Side

Stall

Region

Advancing

Side

3-12

4-1

Note: In this chapter, it is assumed that the helicopter has

a counterclockwise main rotor blade rotation as viewed

from above. If flying a helicopter with a clockwise rotation, you will need to reverse left and right references,

particularly in the areas of rotor blade pitch change, antitorque pedal movement, and tail rotor thrust.

There are four basic controls used during flight. They

are the collective pitch control, the throttle, the cyclic

pitch control, and the antitorque pedals.

COLLECTIVE PITCH CONTROL

The collective pitch control, located on the left side of

the pilot’s seat, changes the pitch angle of all main rotor

blades simultaneously, or collectively, as the name

implies. As the collective pitch control is raised, there

is a simultaneous and equal increase in pitch angle of

all main rotor blades; as it is lowered, there is a simultaneous and equal decrease in pitch angle. This is done

through a series of mechanical linkages and the amount

of movement in the collective lever determines the

amount of blade pitch change. [Figure 4-1] An

adjustable friction control helps prevent inadvertent

collective pitch movement.

Changing the pitch angle on the blades changes the

angle of attack on each blade. With a change in angle

of attack comes a change in drag, which affects the

speed or r.p.m. of the main rotor. As the pitch angle

increases, angle of attack increases, drag increases,

and rotor r.p.m. decreases. Decreasing pitch angle

decreases both angle of attack and drag, while rotor

r.p.m. increases. In order to maintain a constant rotor

r.p.m., which is essential in helicopter operations, a

proportionate change in power is required to compensate for the change in drag. This is accomplished

with the throttle control or a correlator and/or governor, which automatically adjusts engine power.

THROTTLE CONTROL

The function of the throttle is to regulate engine r.p.m.

If the correlator or governor system does not maintain

the desired r.p.m. when the collective is raised or lowered, or if those systems are not installed, the throttle

Figure 4-1. Raising the collective pitch control increases the pitch angle the same amount on all blades.

4-2

has to be moved manually with the twist grip in order

to maintain r.p.m. Twisting the throttle outboard

increases r.p.m.; twisting it inboard decreases r.p.m.

[Figure 4-2]

COLLECTIVE PITCH / THROTTLE

COORDINATION

When the collective pitch is raised, the load on the

engine is increased in order to maintain desired r.p.m.

The load is measured by a manifold pressure gauge

in piston helicopters or by a torque gauge in turbine

helicopters.

In piston helicopters, the collective pitch is the primary

control for manifold pressure, and the throttle is the primary control for r.p.m. However, the collective pitch

control also influences r.p.m., and the throttle also

influences manifold pressure; therefore, each is considered to be a secondary control of the other’s function.

Both the tachometer (r.p.m. indicator) and the manifold

pressure gauge must be analyzed to determine which

control to use. Figure 4-3 illustrates this relationship.