Part 2: FAA… | FAA Helicopter Flying…

2-5

0° 10° 20° 30° 40° 50° 60° 70° 80° 90°

Load factor - (in Gs)

Bank angle (in degrees)

Station 1

Station 2

Station 3

Upper layers act to restrict flow

Figure 2-7. Venturi flow.

Figure 2-8. The load factor diagram allows a pilot to calculate

the amount of “G” loading exerted with various angles of bank.

still accelerates over the curved area because the upper air

layers restrict the flow—just as the top half of the constricted

tube did. This acceleration causes decreased static pressure

above the curved portion and creates a pressure differential

caused by the variation of static and dynamic pressures.

Newton’s Third Law of Motion

Additional lift is provided by the rotor blade’s lower surface

as air striking the underside is deflected downward. 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 that 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 disk must

generate enough lift to overcome or offset the total weight of

the helicopter and its occupants. Newton’s First Law states:

“Every object in a state of uniform motion tends to remain

in that state of motion unless an external force is applied

to it.” In this case, the object is the helicopter whether at a

hover or on the ground and the external force applied to it

is lift, which is accomplished by increasing the pitch angle

of the main rotor blades. This action forces the helicopter

into a state of motion, without it the helicopter would either

remain on the ground or at a hover.

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. The load factor is the actual load on the rotor

blades at any time, divided by the normal load or gross

weight (weight of the helicopter and its contents). Any time

a helicopter flies in a constant altitude curved flightpath, the

load supported by the rotor blades is greater than the total

weight of the helicopter. The tighter the curved flightpath

is, the steeper the bank is; the more rapid the flare or pullout

from a dive is, the greater the load supported by the rotor.

Therefore, the greater the load factor must be. [Figure 2-8]

To overcome this additional load factor, the helicopter must

be able to produce more lift. If excess engine power is not

available, the helicopter either descends or has to decelerate in

order to maintain the same altitude. The load factor and, hence,

2-6

FLAT PLATE

SPHERE

SPHERE WITH

A FAIRING

SPHERE INSIDE

A HOUSING

Form drag

Figure 2-9. 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.

apparent gross weight increase is relatively small in banks up

to 30°. Even so, under the right set of adverse circumstances,

such as high-density altitude, turbulent air, high gross weight,

and poor pilot technique, sufficient or excess power may not

be available to maintain altitude and airspeed. Pilots must take

all of these factors into consideration throughout the entire

flight from the point of ascending to a hover to landing. Above

30° of bank, the apparent increase in gross weight soars. At

30° of bank, or pitch, the apparent increase is only 16 percent,

but at 60°, it is twice the load on the wings and rotor disk.

For example, if the weight of the helicopter is 1,600 pounds,

the weight supported by the rotor disk in a 30° bank at a

constant altitude would be 1,856 pounds (1,600 + 16 percent

(or 256)). In a 60° bank, it would be 3,200 pounds; in an 80°

bank, it would be almost six times as much, or 8,000 pounds.

It is important to note that each rotor blade must support a

percentage of the gross weight. In a two-bladed system, each

blade of the 1,600-pound helicopter as stated above would

have to lift 50 percent or 800 pounds. If this same helicopter

had three rotor blades, each blade would have to lift only 33

percent, or 533 pounds. One additional cause of large load

factors is rough or turbulent air. The severe vertical gusts

produced by turbulence can cause a sudden increase in AOA,

resulting in increased rotor blade loads that are resisted by the

inertia of the helicopter.

Each type of helicopter has its own limitations that are based

on the aircraft structure, size, and capabilities. Regardless

of how much weight one can carry or the engine power

that it may have, they are all susceptible to aerodynamic

overloading. Unfortunately, if the pilot attempts to push

the performance envelope the consequence can be fatal.

Aerodynamic forces effect every movement in a helicopter,

whether it is increasing the collective or a steep bank

angle. Anticipating results from a particular maneuver or

adjustment of a flight control is not good piloting technique.

Instead pilots need to truly understand the capabilities of the

helicopter under any and all circumstances and plan never to

exceed the flight envelope for any situation.

Thrust

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

rotor disk. In a helicopter, thrust can be forward, rearward,

sideward, or vertical. The resultant 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 disk area. This ratio provides a means to measure

the potential for a rotor disk to provide thrust and lift. The

mathematical calculations needed to calculate the solidity ratio

for each helicopter may not be of importance to most pilots but

what should be are the capabilities of the rotor disk to produce

and maintain lift. Many helicopter accidents are caused from

the rotor disk being overloaded. Simply put, pilots attempt

maneuvers that require more lift than the rotor disk can

produce or more power than the helicopter’s powerplant can

provide. Trying to land with a nose high attitude along with

any other unfavorable condition (i.e., high gross weight or

wind gusts) is most likely to end in disaster.

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.

Drag

The force that resists the movement of a helicopter through the

air and is produced when lift is developed is called drag. Drag

must be overcome by the engine to turn the rotor. 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 AOA 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 2-9]

2-7

Drag

Forward speed

Total drag Parasite drag

Induced drag

Profile drag

Average relative wind

Total lift

Vertical lift

Induced drag

Figure 2-10. The formation of induced drag is associated with the

downward deflection of the airstream near the rotor blade.

Figure 2-11. The total drag curve represents the combined forces of

parasite, profile, and induced drag and is plotted against airspeed.

Skin friction is caused by surface roughness. Even 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 area 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

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 2-10]

As the air pressure differential increases with an increase in

AOA, stronger vortices form, and induced drag increases.

Since the blade’s AOA 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.

Non-lifting 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; therefore,

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 2-11] 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 an important factor in helicopter performance.

Airfoil

Helicopters are able to fly due to aerodynamic forces

produced when air passes around the airfoil. An airfoil is

any surface producing more lift than drag when passing

through the air at a suitable angle. Airfoils are most often

associated with production of lift. Airfoils are also used for

stability (fin), control (elevator), and thrust or propulsion

(propeller or rotor). Certain airfoils, such as rotor blades,

combine some of these functions. The main and tail rotor

blades of the helicopter are airfoils, and air is forced to pass

around the blades by mechanically powered rotation. In

some conditions, parts of the fuselage, such as the vertical

and horizontal stabilizers, can become airfoils. Airfoils are

carefully structured to accommodate a specific set of flight

characteristics.

Airfoil Terminology and Definitions

• Blade span—the length of the rotor blade from center

of rotation to tip of the blade.

2-8

Camber of upper surface

Camber of lower surface

Trailing edge

Leading edge

Mean camber line

Chord line

Nonsymmetrical

Symmetrical

Figure 2-12. Aerodynamic terms of an airfoil.

Figure 2-13. The upper and lower curvatures are the same on a

symmetrical airfoil and vary on a nonsymmetrical airfoil.

• Chord line—a straight line intersecting leading and

trailing edges of the airfoil. [Figure 2-12]

• Chord—the length of the chord line from leading edge

to trailing edge; it is the characteristic longitudinal

dimension of the airfoil section.

• Mean camber line—a line drawn halfway between the

upper and lower surfaces of the airfoil. [Figure 2-12]

The chord line connects the ends of the mean camber

line. Camber refers to curvature of the airfoil and

may be considered as curvature of the mean camber

line. The shape of the mean camber is important for

determining aerodynamic characteristics of an airfoil

section. Maximum camber (displacement of the mean

camber line from the chord line) and its location help

to define the shape of the mean camber line. The

location of maximum camber and its displacement

from the chord line are expressed as fractions or

percentages of the basic chord length. By varying the

point of maximum camber, the manufacturer can tailor

an airfoil for a specific purpose. The profile thickness

and thickness distribution are important properties of

an airfoil section.

• Leading edge—the front edge of an airfoil.

[Figure 2-12]

• Flightpath velocity—the speed and direction of

the airfoil passing through the air. For airfoils on

an airplane, the flightpath velocity is equal to true

airspeed (TAS). For helicopter rotor blades, flightpath

velocity is equal to rotational velocity, plus or minus

a component of directional airspeed. The rotational

velocity of the rotor blade is lowest closer to the hub

and increases outward towards the tip of the blade

during rotation.

• Relative wind—defined as the airflow relative to

an airfoil and is created by movement of an airfoil

through the air. This is rotational relative wind for

rotary-wing aircraft and is covered in detail later. As

an induced airflow may modify flightpath velocity,

relative wind experienced by the airfoil may not be

exactly opposite its direction of travel.

• Trailing edge—the rearmost edge of an airfoil.

• Induced flow—the downward flow of air through the

rotor disk.

• Resultant relative wind—relative wind modified by

induced flow.

• AOA—the angle measured between the resultant

relative wind and chord line.

• Angle of incidence (AOI)—the angle between the

chord line of a blade and rotor hub. It is usually

referred to as blade pitch angle. For fixed airfoils,

such as vertical fins or elevators, angle of incidence

is the angle between the chord line of the airfoil and

a selected reference plane of the helicopter.

• Center of pressure—the point along the chord line of

an airfoil through which all aerodynamic forces are

considered to act. Since pressures vary on the surface

of an airfoil, an average location of pressure variation is

needed. As the AOA changes, these pressures change,

and the center of pressure moves along the chord line.

Airfoil Types

Symmetrical Airfoil

The symmetrical airfoil is distinguished by having identical

upper and lower surfaces. [Figure 2-13] The mean camber

line and chord line are the same on a symmetrical airfoil,

and it produces no lift at zero AOA. Most light helicopters

incorporate symmetrical airfoils in the main rotor blades.

Nonsymmetrical Airfoil (Cambered)

The nonsymmetrical airfoil has different upper and lower

surfaces, with a greater curvature of the airfoil above the

chord line than below. [Figure 2-13] The mean camber line

and chord line are different. The nonsymmetrical airfoil design

can produce useful lift at zero AOA. A nonsymmetrical design

2-9

Section near rootA

Section in centerB

Section near tipC

A B C

Tip

Trim tab

Root

Note: “More nose-down” tilt to blade section closer to tip

Figure 2-14. Blade twist.

has advantages and disadvantages. The advantages are more

lift production at a given AOA than a symmetrical design,

an improved lift-to-drag ratio, and better stall characteristics.

The disadvantages are center of pressure travel of up to 20

percent of the chord line (creating undesirable torque on the

airfoil structure) and greater production costs.

Blade Twist

Because of lift differential due to differing rotational relative

wind values along the blade, the blade should be designed

with a twist to alleviate internal blade stress and distribute

the lifting force more evenly along the blade. Blade twist

provides higher pitch angles at the root where velocity is

low and lower pitch angles nearer the tip where velocity

is higher. This increases the induced air velocity and blade

loading near the inboard section of the blade. [Figure 2-14]

Rotor Blade and Hub Definitions

• Hub—on the mast, the attaching point for the root of

the blade, and the axis about which the blades rotate.

[See Figure 1-7]

• Tip—the farthest outboard section of the rotor blade

• Root—the inner end of the blade and is the point that

attaches to the hub

• Twist—the change in blade incidence from the root

to the outer blade

The angular position of the main rotor blades (as viewed from

above, as they rotate about the vertical axis of the mast) is

measured from the helicopter’s longitudinal axis, and usually

from its nose. The radial position of a segment of the blade is

the distance from the hub as a fraction of the total distance.

Airflow and Reactions in the Rotor Disk

Relative Wind

Knowledge of relative wind is essential for an understanding

of aerodynamics and its practical flight application for the

pilot. Relative wind is airflow relative to an airfoil. Movement

of an airfoil through the air creates relative wind. Relative

wind moves in a direction parallel to but opposite of the

movement of the airfoil. [Figure 2-15]

There are two parts to wind passing a rotor blade:

• Horizontal part—caused by the blades turning

plus movement of the helicopter through the air

[Figure 2-16]

• Vertical part—caused by the air being forced down

through the rotor blades plus any movement of the air

relative to the blades caused by the helicopter climbing

or descending [Figures 2-17 and 2-18]

Rotational Relative Wind (Tip-Path Plane)

The rotation of rotor blades as they turn about the mast

produces rotational relative wind (tip-path plane). The

term rotational refers to the method of producing relative

wind. Rotational relative wind flows opposite the physical

flightpath of the airfoil, striking the blade at 90° to the

leading edge and parallel to the plane of rotation; and it is

constantly changing in direction during rotation. Rotational

relative wind velocity is highest at blade tips, decreasing

uniformly to zero at the axis of rotation (center of the

mast). [Figure 2-19]

2-10

Results in

Relative wind

Airfoil direction

Results in

Relative wind

Airfoil direction

Results in

Relative wind

Airfoil direction

387

290

183

Airspeed knots

Axis of rotation

Figure 2-15. Relative wind.

Figure 2-16. Horizontal component of relative wind.

Figure 2-17. Induced flow.

Figure 2-18. Normal induced flow velocities along the blade span

during hovering flight. Downward velocity is highest at the blade

tip where blade speed is highest. As blade speed decreases nearer

the center of the disk, downward velocity is less.

2-11

Center of pressure

Rotational relative wind

Flightpath of airfoil

Chord line

Angle of incidence

Center of pressure

Chord line

Angle of attack

Rotational relative wind

Induced flow

Resultant relative wind

10–20 knotsMore horizontal

flow of air

Downward velocity of air molecules used by aft section of rotor

Fore

Aft

Rotational relative

wind

Less induced flow

Angle of attack

Resultant relative

wind

Chord line

Rotational relative

wind

Greater induced flow

Angle of attack

Resultant relative wind

Chord line

Figure 2-19. Rotational relative wind. Figure 2-20. Resultant relative wind.

Figure 2-21. A helicopter in forward flight, or hovering with a headwind or crosswind, has more molecules of air entering the aft portion

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

forward velocity results in decreased induced flow velocity.

This change results in an improved efficiency (additional lift)

being produced from a given blade pitch setting.

Induced Flow (Downwash)

At flat pitch, air leaves the trailing edge of the rotor blade

in the same direction it moved across the leading edge; no

lift or induced flow is being produced. As blade pitch angle

is increased, the rotor disk induces a downward flow of air

through the rotor blades creating a downward component of

air that is added to the rotational relative wind. Because the

blades are moving horizontally, some of the air is displaced

downward. The blades travel along the same path and pass a

given point in rapid succession. Rotor blade action changes the

still air to a column of descending air. Therefore, each blade has

a decreased AOA due to the downwash. This downward flow

of air is called induced flow (downwash). It is most pronounced

at a hover under no-wind conditions. [Figure 2-21]

Resultant Relative Wind

The resultant relative wind at a hover is rotational relative

wind modified by induced flow. This is inclined downward

at some angle and opposite the effective flightpath of the

airfoil, rather than the physical flightpath (rotational relative

wind). The resultant relative wind also serves as the reference

plane for development of lift, drag, and total aerodynamic

force (TAF) vectors on the airfoil. [Figure 2-20] When the

helicopter has horizontal motion, airspeed further modifies

the resultant relative wind. The airspeed component of

relative wind results from the helicopter moving through

the air. This airspeed component is added to, or subtracted

from, the rotational relative wind depending on whether

the blade is advancing or retreating in relation to helicopter

movement. Introduction of airspeed relative wind also

modifies induced flow. Generally, the downward velocity

of induced flow is reduced. The pattern of air circulation

through the disk changes when the aircraft has horizontal

motion. As the helicopter gains airspeed, the addition of

2-12

Lift vector inclined well to rear

Lift

Induced flow velocity= 60 ft/sec

Axis of rotation

α = 10°

Large blade pitch angle

Pitch angle = 18°

Induced flow velocity = 60 ft/sec Large blade-tip vortexes

V+

P−

MAX VELOCITY =

120 ft/sec

Figure 2-22. In ground effect (IGE).

In Ground Effect (IGE)

Ground effect is the increased efficiency of the rotor

disk caused by interference of the airflow when near the

ground. The air pressure or density is increased, which

acts to decrease the downward velocity of air. Ground

effect permits relative wind to be more horizontal, lift

vector to be more vertical, and induced drag to be reduced.

These conditions allow the rotor disk to be more efficient.

Maximum ground effect is achieved when hovering over

smooth hard surfaces. When hovering over surfaces as tall

grass, trees, bushes, rough terrain, and water, maximum

ground effect is reduced. Rotor efficiency is increased

by ground effect to a height of about one rotor diameter

(measured from the ground to the rotor disk) for most

helicopters. Since the induced flow velocities are decreased,

the AOA is increased, which requires a reduced blade pitch

angle and a reduction in induced drag. This reduces the

power required to hover IGE. [Figure 2-22]

Out of Ground Effect (OGE)

The benefit of placing the helicopter near the ground is lost

above IGE altitude. Above this altitude, the power required

to hover remains nearly constant, given similar conditions

(such as wind). Induced flow velocity is increased, resulting

in a decrease in AOA and a decrease in lift. Under the correct

circumstances, this downward flow can become so localized

that the helicopter and locally disturbed air will sink at

alarming rates. This effect is called vortex ring state (formerly

referenced as settling-with-power) and is discussed at length

in Chapter 11, Helicopter Emergencies and Hazards. A higher

blade pitch angle is required to maintain the same AOA as in

IGE hover. The increased pitch angle also creates more drag.

This increased pitch angle and drag requires more power to

hover OGE than IGE. [Figure 2-23]

Rotor Blade Angles

There are two angles that enable a rotor disk to produce the

lift required for a helicopter to fly: angle of incidence and

angle of attack.

Angle of Incidence

Angle of incidence is the angle between the chord line of a

main or tail rotor blade and its rotor disk. It is a mechanical

angle rather than an aerodynamic angle and is sometimes

referred to as blade pitch angle. [Figure 2-24] In the absence

of induced flow, AOA and angle of incidence are the same.

Whenever induced flow, up flow (inflow), or airspeed modifies

the relative wind, the AOA is different from the angle of

incidence. Collective input and cyclic feathering (see page

2-12) change the angle of incidence. A change in the angle of

incidence changes the AOA, which changes the coefficient of

lift, thereby changing the lift produced by the airfoil.

2-13

Pitch Angle

Angle of Attack

Reference Plane

Axis of Rotation

Chord Line

Relative Wind

Center of pressure

Tip-path plane

Chord line

Angle of incidence

More vertical left vector

Lift

Induced flow velocity= 45 ft/sec

Axis of rotation

α = 10°

Reduced blade pitch angle

Angle of attack is the same

in- or out-of-ground effect

Pitch angle = 14°

Induced flow velocity = 45 ft/sec Reduced blade-tip vortexes

MAX VELOCITY = 90 ft/sec

Figure 2-23. Out of ground effect (OGE).

Figure 2-24. Angle of incidence. Figure 2-25. The AOA is the angle between the airfoil chord line

and resultant relative wind.

Angle of Attack

AOA is the angle between the airfoil chord line and resultant

relative wind. [Figure 2-25] It is an aerodynamic angle and

not easy to measure. It can change with no change in the blade

pitch angle (angle of incidence, discussed earlier).

When the AOA is increased, air flowing over the airfoil is

diverted over a greater distance, resulting in an increase of

air velocity and more lift. As the AOA is increased further,

it becomes more difficult for 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 AOA

increases lift until the critical angle of attack is reached. Any

increase in the AOA beyond this point produces a stall and

a rapid decrease in lift (refer to the Low Rotor RPM and

Rotor Stall section of Chapter 11, Helicopter Emergencies

and Hazards).

Several factors may change the rotor blade AOA. The pilot

has little direct control over AOA except indirectly through

the flight control input. Collective and cyclic feathering

help to make these changes. Feathering is the rotation of the

blade about its longitudinal axis by collective/cyclic inputs

causing changes in blade pitch angle. Collective feathering

changes angle of incidence equally and in the same direction

on all rotor blades simultaneously. This action changes AOA,

which changes coefficient of lift (CL), and affects overall

lift of the rotor disk.

2-14

Lift

Weight

Thrust Drag

Figure 2-26. To maintain a hover at a constant altitude, the lift

must equal the weight of the helicopter. Thrust must equal any

wind and tail rotor thrust to maintain position. The power must be

sufficient to turn the rotors and overcome the various drags and

frictions involved.

Cyclic feathering changes the blade’s AOA differentially

around the rotor disk and creates a differential lift. Aviators

use cyclic feathering to control attitude of the rotor disk. It

is the means to control rearward tilt of the rotor (blowback)

caused by flapping action and (along with blade flapping)

counteract dissymmetry of lift (discussed in chapter 3). Cyclic

feathering causes attitude of the rotor disk to change but does

not change the amount of net lift the rotor disk is producing.

Most of the changes in AOA come from change in airspeed

and rate of climb or descent; others such as flapping occur

automatically due to the rotor system design. Flapping is the

up and down movement of rotor blades about a hinge on a

fully articulated rotor system. A semi-rigid system does not

have a hinge but flap as a unit. A rigid rotor system has no

vertical or horizontal hinges, so the blades cannot flap or

drag, but they can flex. By flexing, the blades themselves

compensate for the forces which previously required rugged

hinges. It occurs in response to changes in lift due to changing

velocity or cyclic feathering. No flapping occurs when the tip-

path plane is perpendicular to the mast. The flapping action

alone, or along with cyclic feathering, controls dissymmetry

of lift. Flapping is the primary means of compensating for

dissymmetry of lift.

Pilots adjust AOA through normal control manipulation of

the pitch angle of the blades. If the pitch angle is increased,

the AOA increases; if the pitch angle is reduced, the AOA

is reduced.

Powered Flight

In powered flight (hovering, vertical, forward, sideward,

or rearward), the total lift and thrust forces of a rotor are

perpendicular to the rotor disk.

Hovering Flight

Hovering is the most challenging part of flying a helicopter.

This is because a helicopter generates its own gusty air

while in a hover, which acts against the fuselage and flight

control surfaces. The end result is constant control inputs

and corrections by the pilot to keep the helicopter where it is

required to be. Despite the complexity of the task, the control

inputs in a hover are simple. The cyclic is used to eliminate

drift in the horizontal plane, controlling forward, backward,

right and left movement or travel. The throttle, if not governor

controlled, is used to control revolutions per minute (rpm). The

collective is used to maintain altitude. The pedals are used to

control nose direction or heading. It is the interaction of these

controls that makes hovering difficult, since an adjustment

in any one control requires an adjustment of the other two,

creating a cycle of constant correction. During hovering flight,

a helicopter maintains a constant position over a selected

point, usually a few feet above the ground. The ability of the

helicopter to hover comes from the both the lift component,

which is the force developed by the main rotor(s) to overcome

gravity and aircraft weight, and the thrust component, which

acts horizontally to accelerate or decelerate the helicopter in

the desired direction. Pilots direct the thrust of the rotor disk

by using the cyclic to rotate the rotor disk plane relative to the

horizon. They do this in order to induce travel or compensate

for the wind and hold a position. At a hover in a no-wind

condition, all opposing forces (lift, thrust, drag, and weight)

are in balance; they are equal and opposite. Therefore, lift

and weight are equal, resulting in the helicopter remaining at

a stationary hover. [Figure 2-26]

While hovering, the amount of main rotor thrust can be

adjusted to maintain the desired hovering height. This is done

by changing the angle of incidence (by moving the collective)

of the rotor blades, and hence their AOA. Changing the AOA

changes the drag on the rotor blades, and the power delivered

by the engine must change as well to keep the rotor speed

constant.

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

helicopter and its occupants. If the amount of lift is greater

than the actual weight, the helicopter accelerates upwards

until the lift force equals the weight of the helicopter; if lift

is less than weight, the helicopter accelerates downward.

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

and a small amount of parasite drag from the non-lift-producing

surfaces of the helicopter, such as the rotor hub, cowlings, and

2-15

Blade rotation

Drift

Tail rotor thrustTail rotor

downwash

Torque

Figure 2-27. 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.

landing gear. Throughout the rest of this discussion, the term

“drag” includes induced, profile and parasite drag.

An important consequence of producing thrust is torque.

As discussed earlier, Newton’s Third Law states: for every

action there is an equal and opposite reaction. Therefore, as

the engine turns the main rotor disk in a counterclockwise

direction, the helicopter fuselage wants to turn clockwise.

The amount of torque is directly related to the amount of

engine power being used to turn the main rotor disk. As

power changes, torque changes.

To counteract this torque-induced turning tendency, an

antitorque rotor or tail rotor is incorporated into most

helicopter designs. A pilot 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

to the main rotor, the tail rotor must produce more thrust to

overcome the increased torque effect. This control change

is accomplished through the use of antitorque pedals (See

page 3-4).

Translating Tendency (Drift)

During hovering flight, a single main rotor helicopter tends

to move in the direction of tail rotor thrust. This lateral

(or sideward) movement is called translating tendency.

[Figure 2-27]

To counteract this tendency, one or more of the following

features may be used. All examples are for a counterclockwise

rotating main rotor disk.

• The main transmission is mounted at a slight angle to

the left (when viewed from behind) so that the rotor

mast has a built-in tilt to oppose the tail rotor thrust.

• Flight controls can be rigged so that the rotor disk is

tilted to the left slightly when the cyclic is centered.

Whichever method is used, the tip-path plane is tilted

slightly to the left in the hover.

• The transmission is mounted so the rotor shaft is

vertical with respect to the fuselage, the helicopter

“hangs” left skid low in the hover. (The opposite is

true for rotor disks turning clockwise when viewed

from above.)

• The helicopter fuselage will also be tilted when the

tail rotor is below the main rotor disk and supplying

antitorque thrust. The fuselage tilt is caused by the

imperfect balance of the tail rotor thrust against the

main rotor torque in the same plane. The helicopter

tilts due to two separate forces, the main rotor disk tilt

to neutralize the translating tendency and the lower

tail rotor thrust below the plane of the torque action.

• In forward flight, the tail rotor continues to push

to the right, and the helicopter makes a small angle

with the wind when the rotors are level and the slip

ball is in the middle (See page 12-2). This is called

inherent sideslip. For some larger helicopters, the

vertical fin or stabilizer is often designed with the

tail rotor mounted on them to correct this side slip

and to eliminate some of the tilting at a hover. (By

mounting the tail rotor on top of the vertical fin or

pylon, the antitorque is more in line with or closer

to the horizontal plane of torque, resulting in less

airframe (or body) lean from the tail rotor.) Also,

having the tail rotor higher off the ground reduces

the risk of objects coming in contact with the blades,

but at the cost of increased weight and complexity.

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 overcontrolling; therefore, control movements should be

smooth and not exaggerated. [Figure 2-28]

The horizontal stabilizer helps to level the helicopter in

forward flight. However, in rearward flight, the horizontal

stabilizer can press the tail downward, resulting in a tail

strike if the helicopter is moved rearward into the wind.

Normally, with the helicopter mostly into the wind, the

horizontal stabilizer experiences less headwind component

as the helicopter begins rearward travel (downwind). When

2-16

Initial rearward flight Initial forward flight

Calm wind hover

Figure 2-28. 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.

Before takeoff

During takeoff

Lift

Centrifugal

force

Resultant

blade

angle

Figure 2-29. During takeoff, the combination of centrifugal force

and lift cause the rotor disk to cone upward.

As lift on the blades is increased (in a takeoff, for example),

two major forces are acting at the same time—centrifugal

force acting outward, and lift acting upward. The result of

these two forces is that the blades assume a conical path

instead of remaining in the plane perpendicular to the mast.

This can be seen in any helicopter when it takes off; the rotor

disk changes from flat to a slight cone shape. [Figure 2-29]

If the rotor rpm is allowed to go too low (below the minimum

power-on rotor rpm, for example), the centrifugal force

becomes smaller and the coning angle becomes much

larger. In other words, should the rpm decrease too much,

at some point the rotor blades fold up with no chance

of recovery.

Coriolis Effect (Law of Conservation of Angular

Momentum)

The Coriolis Effect is also referred to as the law of

conservation of angular momentum. It states that the value

of angular momentum of a rotating body does not change

unless an external force is applied. In other words, a rotating

body continues to rotate with the same rotational velocity

until some external force is applied to change the speed of

rotation. Angular momentum is the moment of inertia (mass

times distance from the center of rotation squared) multiplied

by the speed of rotation.

Changes in angular velocity, known as angular acceleration

and deceleration, take place as the mass of a rotating body

is moved closer to or farther away from the axis of rotation.

The speed of the rotating mass varies proportionately with

the square of the radius.

An excellent example of this principle in action is a figure

skater performing a spin on ice skates. The skater begins

rotation on one foot, with the other leg and both arms

extended. The rotation of the skater’s body is relatively

slow. When a skater draws both arms and one leg inward,

the moment of inertia (mass times radius squared) becomes

rearward flight groundspeed equals the windspeed, then

the helicopter is merely hovering in a no-wind condition.

However, rearward hovering into the wind requires

considerable care and caution to prevent tail strikes.

It is important to note that there is a difference in the amount

of pendular action between a semirigid system and a fully

articulated system. Because of the hard connection (offset)

of the latter, the centrifugal force pulling out on the blades is

transferred to the fuselage, and the fuselage tends to follow

the rotor attitude. The semirigid system is a true pendulum,

with thrust required to create a moment around the fuselage

CG to allow for control of the fuselage. This comes into play

later when mast bumping is discussed.

Coning

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

must be turning. Rotor disk rotation drives the blades into

the air, creating a relative wind component without having

to move the airframe through the air as with an airplane or

glider. Depending on the motion of the blades and helicopter

airframe, many factors cause the relative wind direction to

vary. The rotation of the rotor disk creates centrifugal force

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

from the main rotor hub: the faster the rotation, the greater

the centrifugal force, the slower the rotation, the smaller

the centrifugal force. This force gives the rotor blades their

rigidity and, in turn, the strength to support the weight of

the helicopter. The maximum centrifugal force generated

is determined by the maximum operating rotor revolutions

per minute (rpm).

2-17

Forward

Downward movement

response here

Upward movement

Downward

force

applied

here

Upward

force

applied

here

Figure 2-30. Gyroscopic precession.

much smaller and the body is rotating almost faster than the

eye can follow. Because the angular momentum must, by

law of nature, remain the same (no external force applied),

the angular velocity must increase.

The rotor blade rotating about the rotor hub possesses angular

momentum. As the rotor begins to cone due to G-loading

maneuvers, the diameter of the rotor disk shrinks. Due to

conservation of angular momentum, the blades increase

speed even though the blade tips have a shorter distance to

travel due to reduced disk diameter. The action results in an

increase in rotor rpm which causes a slight increase in lift.

Most pilots arrest this increase of rpm with an increase in

collective pitch. This increase in blade rpm lift is somewhat

negated by the slightly smaller disk area as the blades cone

upward.

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 (or 90° later in the rotation cycle). [Figure 2-30]

Examine a two-bladed rotor disk to see how gyroscopic

precession affects the movement of the tip-path plane.

Moving the cyclic pitch control increases the angle of

incidence of one rotor blade with the result of a greater lifting

force being applied at that point in the plane of rotation.

This same control movement simultaneously decreases the

angle of incidence 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 incidence tends

to flap up; the blade with the decreased angle of incidence

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. Figure 2-31 illustrates the

result of a forward cyclic input. The retreating blade angle

of incidence is increased, and the advancing blade angle of

incidence 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 disk using three or more blades, the movement of

the cyclic pitch control changes the angle of incidence 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 incidence of the rotor blades (pitch) while

keeping their rotation speed constant generates additional

lift and the helicopter ascends. Decreasing the pitch causes

the helicopter to descend. In a no-wind condition in which

lift and thrust are less than weight and drag, the helicopter

descends vertically. If lift and thrust are greater than weight

and drag, the helicopter ascends vertically. [Figure 2-32]

Forward Flight

In steady forward flight, with no change in airspeed or vertical

speed, the four forces of lift, thrust, drag, and weight must

be in balance. Once the tip-path plane is tilted forward, the

total lift-thrust force is also tilted forward. 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 force opposing the

motion of an airfoil through the air). [Figure 2-33]

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

and-level flight is flight with a constant heading and at a

constant altitude), lift equals weight and thrust equals drag.

If lift exceeds weight, the helicopter accelerates vertically

until the forces are in balance; if thrust is less than drag, the

helicopter slows down until the forces are in balance. As a

helicopter initiates a move forward, it begins to lose altitude

because lift is lost as thrust is diverted forward. However,

as the helicopter begins to accelerate from a hover, the rotor

disk becomes more efficient due to translational lift (see

translational lift on page 2-19). The result is excess power

over that which is required to hover. Continued acceleration

causes an even larger increase in airflow through the rotor

disk (up to a maximum determined by drag and the engine’s

limit of power), and more efficient flight. In order to maintain

unaccelerated flight, the pilot must understand that with

2-18

Angle of attack decreased

Blade rotation

Maximum upward deflection

Maximum downward deflection

Angle of attack increased

Blade rotation

Direction of travel

Figure 2-31. As each blade passes the 90° position on the left in a counterclockwise main rotor blade rotation, the maximum increase

in angle of incidence occurs. As each blade passes the 90° position to the right, the maximum decrease in angle of incidence 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.

Weight/drag

Lift/thrust

Vertical/ascent

Figure 2-32. Balanced forces: hovering in a no-wind condition.

Helicopter movement

Thrust

Drag

Weight

Lift

Resultant

Figure 2-33. To transition to forward flight, more lift and thrust must

be generated to overcome the forces of weight and drag.

2-19

Airspeed = 120 Knots

Relative wind as a

result of aircraft

movement at 120 knots

Relative wind as a

result of aircraft

movement at 120 knots

480

Knots

120

Knots

360

Knots

Rotational

Velocity

Aircraft

Airspeed

Wind

Velocity

–

480

Knots

120

Knots

600

Knots

Rotational

Velocity

Aircraft

Airspeed

Wind

Velocity

Direction of rotation

Rotational velocity = 480 knots

Figure 2-35. Airflow in forward flight.

Figure 2-34. Power versus airspeed chart.

Power required (horsepower)

Indicated airspeed (KIAS)

800

600

400

200

0 40 60 80 100 120

Minimum power

for level flight (VY)

Maximum

continuous

level

(horizontal)

flight

airspeed (VH)

Maximum continuous power available

Increasing power for

decreasing airspeed

Increasing power for

increasing airspeed

Power required to hover OGE

any changes in power or in cyclic movement, the helicopter

begins either to climb or to descend. Once straight-and-level

flight is obtained, the pilot should make note of the power

(torque setting) required and not make major adjustments to

the flight controls. [Figure 2-34]

Airflow in Forward Flight

Airflow across the rotor disk in forward flight varies from

airflow at a hover. In forward flight, air flows opposite the

aircraft’s flightpath. The velocity of this air flow equals the

helicopter’s forward speed. Because the rotor blades turn

in a circular pattern, the velocity of airflow across a blade

depends on the position of the blade in the plane of rotation

at a given instant, its rotational velocity, and airspeed of the

helicopter. Therefore, the airflow meeting each blade varies

continuously as the blade rotates. The highest velocity of

airflow occurs over the right side (3 o’clock position) of

the helicopter (advancing blade in a rotor disk that turns

counterclockwise) and decreases to rotational velocity over

the nose. It continues to decrease until the lowest velocity of

airflow occurs over the left side (9 o’clock position) of the

helicopter (retreating blade). As the blade continues to rotate,

velocity of the airflow then increases to rotational velocity

over the tail. It continues to increase until the blade is back

at the 3 o’clock position.

The advancing blade in Figure 2-35, position A, moves in

the same direction as the helicopter. The velocity of the air

meeting this blade equals rotational velocity of the blade

plus wind velocity resulting from forward airspeed. The

retreating blade (position C) moves in a flow of air moving in the opposite direction of the helicopter. The velocity of

airflow meeting this blade equals rotational velocity of the

blade minus wind velocity resulting from forward airspeed.

The blades (positions B and D) over the nose and tail move

essentially at right angles to the airflow created by forward

airspeed; the velocity of airflow meeting these blades equals

the rotational velocity. This results in a change to velocity

of airflow all across the rotor disk and a change to the lift

pattern of the rotor disk.

Advancing Blade

As the relative wind speed of the advancing blade increases,

the blade gains lift and begins to flap up. It reaches its

maximum upflap velocity at the 3 o’clock position, where the

wind velocity is the greatest. This upflap creates a downward

flow of air and has the same effect as increasing the induced

flow velocity by imposing a downward vertical velocity

vector to the relative wind which decreases the AOA.

Retreating Blade

As relative wind speed of the retreating blade decreases,

the blade loses lift and begins to flap down. It reaches its

maximum downflap velocity at the 9 o’clock position, where

2-20

Blade rotation

Relative wind

Forward flight at 100 knots

Relative wind

Direction of Flight

Advancing SideRetreating Side

Blade tip

speed

plus

helicopter

speed

(500 knots)

Blade tip

speed

minus

helicopter

speed

(300 knots)

Figure 2-36. The blade tip speed of this helicopter is approximately

400 knots. If the helicopter is moving forward at 100 knots, the

relative windspeed on the advancing side is 500 knots. On the

retreating side, it is only 300 knots. This difference in speed causes

a dissymmetry of lift.

Pilots can avoid retreating blade stall by not exceeding the

never-exceed speed. This speed is designated V NE and is

indicated on a placard and marked on the airspeed indicator

by a red line.

Blade flapping compensates for dissymmetry of lift in the

following way. At a hover, equal lift is produced around the

rotor disk with equal pitch (AOI) on all the blades and at

all points in the rotor disk (disregarding compensation for

translating tendency). The rotor disk is parallel to the horizon.

To develop a thrust force, the rotor disk must be tilted in the

desired direction of movement. Cyclic feathering changes

the angle of incidence differentially around the rotor disk.

For a counterclockwise rotation, forward cyclic movement

decreases the angle of incidence on the right of the rotor disk

and increases it on the left.

When transitioning to forward flight either from a hover or

taking off from the ground, pilots must be aware that as the

helicopter speed increases, translational lift becomes more

effective and causes the nose to rise or pitch up (sometimes

referred to as blowback). This tendency is caused by the

combined effects of dissymmetry of lift and transverse flow.

Pilots must correct for this tendency by maintaining a constant

rotor disk attitude that will move the helicopter through the

speed range in which blowback occurs. If the nose is permitted

to pitch up while passing through this speed range, the aircraft

may also tend to roll to the right. To correct for this tendency,

the pilot must continuously move the cyclic forward as velocity

of the helicopter increases until the takeoff is complete, and

the helicopter has transitioned into forward flight.

wind velocity is the least. This downflap creates an upward

flow of air and has the same effect as decreasing the induced

flow velocity by imposing an upward velocity vertical vector

to the relative wind which increases the AOA.

Dissymmetry of Lift

Dissymmetry of lift is the differential (unequal) lift between

advancing and retreating halves of the rotor disk caused by the

different wind flow velocity across each half. This difference

in lift would cause the helicopter to be uncontrollable in any

situation other than hovering in a calm wind. There must

be a means of compensating, correcting, or eliminating this

unequal lift to attain symmetry of lift.

When the helicopter moves through the air, the relative

airflow through the main rotor disk is different on the

advancing side from 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 disk can produce

more lift than the retreating blade side. [Figure 2-36]

If this condition were 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 disk. Articulated rotor disks, 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 semi-rigid rotor disk (two blades)

utilizes a teetering hinge, which allows the blades to flap as

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

As shown in Figure 2-37, as the rotor blade reaches the

advancing side of the rotor disk (A), it reaches its maximum

up flap velocity. When the blade flaps upward, the angle

between the chord line and the resultant relative wind

decreases. This decreases the AOA, which reduces the

amount of lift produced by the blade. At position (C), the

rotor blade is now at its maximum down flapping velocity.

Due to down flapping, the angle between the chord line and

the resultant relative wind increases. This increases the AOA

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 AOA 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.

2-21

Figure 2-38. To compensate for blowback, you must move the

cyclic forward.

Figure 2-37. 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 disk, counteracting dissymmetry of lift.

Angle of attack at 3 o’clock positionA

Upflap velocity

Resultant relative wind

Chord line

Angle of attack over noseB

Resultant relative wind

Chord line

Angle of attack at 9 o’clock positionC

Downflap velocity

Resultant relative wind

Chord line

Angle of attack over tailD

Resultant relative wind

Chord line

Blade rotation

or surface wind enters the rotor disk, turbulence and vortices

are left behind and the flow of air becomes more horizontal.

In addition, the tail rotor becomes more aerodynamically

efficient during the transition from hover to forward flight.

Figures 2-39 and 2-40 show the different airflow patterns

at different speeds and how airflow affects the efficiency of

the tail rotor.

Figure 2-38 illustrates the tilting forward of the rotor disk,

which is the result of a change in pitch angle with forward

cyclic. At a hover, the cyclic is centered and the pitch angle on

the advancing and retreating blades is the same. At low forward

speeds, moving the cyclic forward reduces pitch angle on the

advancing blade and increases pitch angle on the retreating

blade. This causes a slight rotor disk tilt. At higher forward

speeds, the pilot must continue to move the cyclic forward.

This further reduces pitch angle on the advancing blade and

further increases pitch angle on the retreating blade. As a result,

there is even more tilt to the rotor disk than at lower speeds.

A horizontal lift component (thrust) generates higher

helicopter airspeed. The higher airspeed induces blade

flapping to maintain symmetry of lift. The combination of

flapping and cyclic feathering maintains symmetry of lift and

desired attitude on the rotor disk and helicopter.

Translational Lift

Improved rotor efficiency resulting from directional flight is

called translational lift. The efficiency of the hovering rotor

disk is greatly improved with each knot of incoming wind

gained by horizontal movement of the aircraft or surface

wind. As the incoming wind produced by aircraft movement

2-22

Downward velocity of air molecules used by aft section of rotor

1–5 knots

10–15 knots

Airflow pattern just prior to effective translational lift

Figure 2-39. The airflow pattern for 1–5 knots of forward airspeed. Note how the downwind vortex is beginning to dissipate and induced

flow down through the rear of the rotor disk is more horizontal.

Figure 2-40. An airflow pattern at a speed of 10–15 knots. At this increased airspeed, the airflow continues to become more horizontal.

The leading edge of the downwash pattern is being overrun and is well back under the nose of the helicopter.

As speed increases, translational lift becomes more effective,

nose rises or pitches up, and aircraft rolls to the right.

The combined effects of dissymmetry of lift, gyroscopic

precession, and transverse flow effect cause this tendency.

It is important to understand these effects and anticipate

correcting for them. Once the helicopter is transitioning

through ETL, the pilot needs to apply forward and left

lateral cyclic input to maintain a constant rotor-disk attitude.

[Figure 2-41]

Translational Thrust

Translational thrust occurs when the tail rotor becomes more

aerodynamically efficient during the transition from hover

to forward flight. As the tail rotor works in progressively

less turbulent air, this improved efficiency produces more

antitorque thrust, causing the nose of the aircraft to yaw left

Effective Translational Lift (ETL)

While transitioning to forward flight at about 16 to 24 knots,

the helicopter goes through effective translational lift (ETL).

As mentioned earlier in the discussion on translational lift,

the rotor blades become more efficient as forward airspeed

increases. Between 16 and 24 knots, the rotor disk completely

outruns the recirculation of old vortices and begins to work

in relatively undisturbed air. The flow of air through the rotor

disk is more horizontal, which reduces induced flow and

drag with a corresponding increase in angle of attach and lift.

The additional lift available at this speed is referred to as the

ETL, which makes the rotor disk operate more efficiently.

This increased efficiency continues with increased airspeed

until the best climb airspeed is reached, and total drag is at

its lowest point.

2-23

Helicopter movement

Thrust

Drag

Weight

Lift

Resultant

Figure 2-42. Forces acting on the helicopter during sideward flight.

Figure 2-41. Effective translational lift is easily recognized in actual

flight by a transient induced aerodynamic vibration and increased

performance of the helicopter.

16–24 knots

No recirculation

of air

More horizontal

flow of air

Reduced induced flow

increases angle of attack

Tail rotor operates in

relatively clean air

Transverse flow effect is recognized by increased vibrations

of the helicopter at airspeeds around 12 to 15 knots and can

be produced by forward flight or from the wind while in a

hover. This vibration happens at an airspeed just below ETL

on takeoff and after passing through ETL during landing. The

vibration happens close to the same airspeed as ETL because

that is when the greatest lift differential exists between the

front and rear portions of the rotor system. As such, some

pilots confuse the vibration felt by transverse flow effect with

passing through ETL. To counteract transverse flow effect,

a cyclic input to the left may be needed.

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 2-42]

Sideward flight can be a very unstable condition due to the

parasitic drag of the fuselage combined with the lack of

horizontal stabilizer for that direction of flight. Increased

altitudes help with control and the pilot must always scan in

the direction of flight. Movement of the cyclic in the intended

direction of flight causes the helicopter to move, controls the

rate of speed, and ground track, but the collective and pedals

are key to successful sideward flight. Just as in forward flight,

the collective keeps the helicopter from contacting the ground

and the pedals help maintain the correct heading; even in

sideward flight, the tail of the helicopter should remain behind

you. Inputs to the cyclic should be smooth and controlled,

and the pilot should always be aware of the tip-path plane in

relation to the ground. [Figure 2-43]

(with a main rotor turning counterclockwise) and forces the

pilot to apply right pedal (decreasing the AOA in the tail

rotor blades) in response. In addition, during this period, the

airflow affects the horizontal components of the stabilizer

found on most helicopters which tends to bring the nose of

the helicopter to a more level attitude.

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 disk. This downwash or induced flow can significantly

change the efficiency of the rotor disk. Rotational relative

wind combines with induced flow to form the resultant

relative wind. As induced flow increases, resultant relative

wind becomes less horizontal. Since AOA is determined

by measuring the difference between the chord line and the

resultant relative wind, as the resultant relative wind becomes

less horizontal, AOA decreases. [See Figure 2-21]

Transverse Flow Effect

As the helicopter accelerates in forward flight, induced flow

drops to near zero at the forward disk area and increases at the

aft disk area. These differences in lift between the fore and

aft portions of the rotor disk are called transverse flow effect.

[Figure 2-41] This increases the AOA at the front disk area

causing the rotor blade to flap up and reduces AOA at the aft

disk 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 accelerates through approximately

20 knots or if the headwind is approximately 20 knots.

2-24

Helicopter movement

Thrust

Drag

Weight Lift

Resultant

Downward force from

the horizontal stabilizer

Ground track required

Forward reference

Side reference

Figure 2-44. Forces acting on the helicopter during rearward flight.

Figure 2-43. Forces acting on the helicopter during sideward flight.

Contacting the ground with the skids during sideward flight

will most likely result in a dynamic rollover event before the

pilot has a chance to react. Extreme caution should be used

when maneuvering the helicopter sideways to avoid such

hazards from happening. Refer to Chapter 11, Helicopter

Hazards and Emergencies.

Rearward Flight

For rearward flight, the tip-path plane is tilted rearward,

which, in turn, tilts the lift-thrust vector rearward. Drag now

acts forward with the lift component straight up and weight

straight down. [Figure 2-44]

Pilots must be aware of the hazards of rearward flight.

Because of the position of the horizontal stabilizer, the tail

end of the helicopter tends to pitch downward in rearward

flight, causing the probability of hitting the ground to be

greater than in forward flight. Another factor to consider

in rearward flight is skid design. Most helicopter skids are

not turned upward in the back, and any contact with the

ground during rearward flight can put the helicopter in an

uncontrollable position leading to tail rotor contact with the

ground. Pilots must do a thorough scan of the area before

attempting to hover rearward, looking for obstacles and

terrain changes. Slower airspeeds can help mitigate risk and

maintain a higher-than-normal hover altitude.

Turning Flight

In forward flight, the rotor disk is tilted forward, which also

tilts the total lift-thrust force of the rotor disk forward. When

the helicopter is banked, the rotor disk 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 2-45]

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

2-25

Normal Powered Flight Autorotation

Direction of flight

Figure 2-46. 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 2-45. Forces acting on the helicopter during turning flight.

90 80

Centrifugal force (inertia)

Weight

Resultant lift

Centripetal force

(horizontal component of lift)

Vertical

component

of lift

Bank

angle

vertically is decreased. To compensate for this decreased

vertical lift, the AOA of the rotor blades must be increased in

order to maintain altitude. The steeper the angle of bank is,

the greater the AOA of the rotor blades required to maintain

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

resultant lifting force increases, and the rate of turn is higher.

Simply put, collective pitch must be increased in order to

maintain altitude and airspeed while turning. Collective pitch

controls the angle of incidence and along with other factors,

determines the overall AOA in the rotor disk.

Autorotation

Autorotation is the state of flight where the main rotor disk

of a helicopter is being turned by the action of air moving

up through the rotor rather than engine power driving

the rotor. In normal, powered flight, air is drawn into the

main rotor disk from above and exhausted downward, but

during autorotation, air moves up into the rotor disk from

below as the helicopter descends. Autorotation is permitted

mechanically by a freewheeling unit, which is a special

clutch mechanism that allows the main rotor to continue

turning even if the engine is not running. If the engine fails,

the freewheeling unit automatically disengages the engine

from the main rotor allowing the main rotor to rotate freely.

It is the means by which a helicopter can be landed safely in

the event of an engine failure; consequently, all helicopters

must demonstrate this capability in order to be certified.

[Figure 2-46] If a decision is made to attempt an engine

restart in flight (the parameters for this emergency procedure

will be different for each helicopter and must be precisely

followed) the pilot must reengage the engine starter switch to

start the engine. Once the engine is started, the freewheeling

unit will reengage the engine with the main rotor.

Vertical Autorotation

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 disk is divided into

three regions (as illustrated in Figure 2-47): driven region,

2-26

Blade rotation

Stall region 25%

Driving region 45%

Driven region 30%

Figure 2-47. Blade regions during autorotational descent.

driving region, and stall region. Figure 2-48 shows three

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 AOA 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 percent of

the radius. In the driven region, part A of Figure 2-48, the

TAF acts behind the axis of rotation, resulting in an 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 rpm. When changing

autorotative rpm 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, TAF 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 2-48 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 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 rpm.

By controlling the size of this region, a pilot can adjust

autorotative rpm. 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 rpm to decrease. A constant

rotor rpm 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.

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

stall region and operates above its maximum AOA (stall

angle), causing drag, which tends to slow rotation of the

blade. Part E of Figure 2-48 depicts the stall region.

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 disk, all three regions move

outboard along the blade span on the retreating side of the

disk where AOA is larger. [Figure 2-49] With lower AOA

on the advancing side blade, more of the 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.

Prior to landing from an autorotative descent (or autorotation),

the pilot must flare the helicopter in order to decelerate. The

pilot initiates the flare by applying aft cyclic. As the helicopter

flares back, the airflow patterns change around the blades

causing the rpm to increase. Pilots must adjust the collective

as necessary to keep the rpm within operating limits.

Chapter Summary

This chapter introduced the basics of aerodynamic

fundamentals and theory and how they relate to flying a

helicopter. This chapter also explained how aerodynamics

affect helicopter flight and how important it is for pilots to

understand aerodynamic principles and be prepared to react

to these effects. For additional information on aerodynamics,

refer to the aerodynamics of flight portion of the Pilot’s

Handbook of Aeronautical Knowledge.

2-27

Figure 2-48. Force vectors in vertical autorotation descent.

Inflow up through rotor

Resultant RW

Rotational

relative

wind

Chord line

Axis of rotation

Lift

TAF

Drag

B & D

Inflow

Lift

TAF

Drag

AOA

Inflow

AOA

Lift

TAF

Drag

Inflow

Lift TAF

Drag

AOA

(blade stall)

Driven region

Drag

Point of equilibrium

Driving region

Stall region

Drag

Autorotative force

A Driven region

Equilibrium

Driving region

Stall region

Axis of rotation

2-28

Blade rotation

Stall region

Driving region

Driven region

Advancing sideRetreating side

Direction of Flight

Figure 2-49. Blade regions in forward autorotation descent.

3-1

Introduction

There are three major controls in a helicopter that the pilot

must use during flight. They are the collective pitch control,

the cyclic pitch control, and the antitorque pedals or tail rotor

control. In addition to these major controls, the pilot must also

use the throttle control, which is usually mounted directly

to the collective pitch control in order to fly the helicopter.

In this chapter, the control systems described are not limited

to the single main rotor type helicopter but are employed

in one form or another in most helicopter configurations.

All examples in this chapter refer to a counterclockwise

main rotor blade rotation as viewed from above. If flying a

helicopter with a clockwise rotation, left and right references

must be reversed, particularly in the areas of rotor blade pitch

change, antitorque pedal movement, and tail rotor thrust.

Helicopter Flight Controls

Chapter 3

3-2

Figure 3-1. Raising the collective pitch control increases the pitch angle, or angle of incidence, by the same amount on all blades.

Collective Pitch Control

The collective pitch control (or simply “collective” or “thrust

lever”) is located on the left side of the pilot’s seat and is

operated with the left hand. The collective is used to make

changes to the pitch angle of the main rotor blades and does

this 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 3-1]

An adjustable friction control helps prevent inadvertent

collective pitch movement.

Changing the pitch angle on the blades changes the angle

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

incidence comes a change in drag, which affects the speed

or revolutions per minute (rpm) of the main rotor. As the

pitch angle increases, angle of incidence increases, drag

increases, and rotor rpm decreases. Decreasing pitch angle

decreases both angle of incidence and drag, while rotor rpm

increases. In order to maintain a constant rotor rpm, 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 governor,

which automatically adjusts engine power.

Throttle Control

The function of the throttle is to regulate engine rpm. If

the correlator or governor system does not maintain the

desired rpm when the collective is raised or lowered, or if

those systems are not installed, the throttle must be moved

manually with the twist grip in order to maintain rpm. In

most helicopters, rotating the twist-grip throttle away from

the pilot (counter-clockwise), increases engine rpm; rotating

the twist-grip throttle towards the pilot (clockwise) decreases

engine rpm. [Figure 3-2]

Governor/Correlator

A governor is a sensing device that senses rotor and engine

rpm and makes the necessary adjustments in order to keep

rotor rpm constant. In normal operations, once the rotor

rpm is set, the governor keeps the rpm constant, and there

is no need to make any throttle adjustments. Governors are

common on all turbine helicopters (as it is a function of the

fuel control system of the turbine engine) and used on some

piston powered helicopters.

A correlator is a mechanical connection between the

collective lever and the engine throttle. When the collective

lever is raised, power is automatically increased; when

lowered, power is decreased. This system maintains rpm

close to the desired value, but still requires adjustment of

the throttle for fine tuning.

FAA Helicopter Flying Handbook: Chapter 1 — Introduction to the Helicopter — Part 2

More from this manual