Helicopter Flying Handbook (2019)

The Helicopter Flying Handbook is designed as a technical manual for applicants who are preparing for their private,
commercial, or flight instructor pilot certificates with a helicopter class rating. Certificated flight instructors may find
this handbook a valuable training aid, since detailed coverage of aerodynamics, flight controls, systems, performance,
flight maneuvers, emergencies, and aeronautical decision-making is included. Topics such as weather, navigation, radio
navigation and communications, use of flight information publications, and regulations are available in other Federal
Aviation Administration (FAA) publications.

This handbook conforms to pilot training and certification concepts established by the FAA. There are different ways of
teaching, as well as performing, flight procedures and maneuvers, and many variations in the explanations of aerodynamic
theories and principles. This handbook adopts a selective method and concept to flying helicopters. The discussion and
explanations reflect the most commonly used practices and principles. Occasionally the word “must” or similar language
is used where the desired action is deemed critical. The use of such language is not intended to add to, interpret, or relieve
a duty imposed by Title 14 of the Code of Federal Regulations (14 CFR). Persons working towards a helicopter rating are
advised to review the references from the applicable practical test standards (FAA-S-8081-3 for recreational applicants,
FAA-S-8081-15 for private applicants, and FAA-S-8081-16 for commercial applicants). Resources for study include
FAA-H-8083-25, Pilot’s Handbook of Aeronautical Knowledge, and FAA-H-8083-1, Weight and Balance Handbook, as
these documents contain basic material not duplicated herein. All beginning applicants should refer to FAA-H-8083-25,
Pilot’s Handbook of Aeronautical Knowledge, for study and basic library reference.

It is essential for persons using this handbook to become familiar with and apply the pertinent parts of 14 CFR and the
Aeronautical Information Manual (AIM). The AIM is available online at www.faa.gov. The current Flight Standards
Service airman training and testing material and learning statements for all airman certificates and ratings can be obtained
from www.faa.gov.

This handbook supersedes FAA-H-8083-21A, Helicopter Flying Handbook, dated 2012. Gyroplane information can be
found in the FAA-H-8083-20, Gyroplane Flying Handbook.

This handbook is available for download, in PDF format, from www.faa.gov.

This handbook is published by the United States Department of Transportation, Federal Aviation Administration, Airman
Testing Branch, P.O. Box 25082, Oklahoma City, OK 73125.

Comments regarding this publication should be emailed to AFS630comments@faa.gov.

Preface

1-2

Figure 1-1.

Search and rescue helicopter conducting a pinnacle

approach.

Figure 1-2.

Search and rescue helicopter landing in a confined area.

As an aircraft, the primary advantages of the helicopter are
due to the rotor blades that revolve through the air, providing
lift without requiring the aircraft to move forward. This lift
allows the helicopter to hover in one area and to take off
and land vertically without the need for runways. For this
reason, helicopters are often used in congested or isolated
areas where fixed-wing aircraft are not able to take off or
land.

[Figures 1-1

and

1-2]

Piloting a helicopter requires adequate, focused and safety-
orientated training. It also requires continuous attention
to the machine and the operating environment. The pilot
must work in three dimensions and use both arms and
both legs constantly to keep the helicopter in a desired
state. Coordination, timing and control touch are all used
simultaneously when flying a helicopter.

Although helicopters were developed and built during the
first half-century of flight, some even reaching limited
production; it was not until 1942 that a helicopter designed by
Igor Sikorsky reached full-scale production, with 131 aircraft
built. Even though most previous designs used more than one
main rotor, it was the single main rotor with an antitorque
tail rotor configuration that would come to be recognized
worldwide as the helicopter.

Turbine Age

In 1951, at the urging of his contacts at the Department of
the Navy, Charles H. Kaman modified his K-225 helicopter
with a new kind of engine, the turbo-shaft engine. This
adaptation of the turbine engine provided a large amount of
horsepower to the helicopter with a lower weight penalty
than piston engines, heavy engine blocks, and auxiliary
components. On December 11, 1951, the K-225 became
the first turbine-powered helicopter in the world. Two years
later, on March 26, 1954, a modified Navy HTK-1, another
Kaman helicopter, became the first twin-turbine helicopter
to fly. However, it was the Sud Aviation Alouette II that
would become the first helicopter to be produced with a
turbine engine.

Reliable helicopters capable of stable hover flight were
developed decades after fixed-wing aircraft. This is largely
due to higher engine power density requirements than
fixed-wing aircraft. Improvements in fuels and engines
during the first half of the 20

century were critical factors

in helicopter development. The availability of lightweight
turbo-shaft engines in the second half of the 20

century led

to the development of larger, faster, and higher-performance
helicopters. While smaller and less expensive helicopters
still use piston engines, turboshaft engines are the preferred
powerplant for helicopters today.

The turbine engine has the following advantages over a
reciprocating engine:

•

Less vibration

•

Increased aircraft performance

• Reliability
•

Ease of operation

1-3

Figure 1-3.

The many uses for a helicopter include search and rescue

(top), firefighting (middle), and construction (bottom).

Hub

Mast

Rotor blades

Figure 1-4.

Basic components of the rotor system.

Uses

Due to the unique operating characteristics of the helicopter—
its ability to take off and land vertically, to hover for extended
periods of time, and the aircraft’s handling properties under
low airspeed conditions—it has been chosen to conduct tasks
that were previously not possible with other aircraft or were
too time- or work-intensive to accomplish on the ground.
Today, helicopters are used for transportation, construction,
firefighting, search and rescue, and a variety of other jobs
that require its special capabilities.

[Figure 1-3]

Rotor System

The helicopter rotor system is the rotating part of a
helicopter that generates lift. A rotor system may be mounted
horizontally, as main rotors are, providing lift vertically; and
it may be mounted vertically, such as a tail rotor, to provide
lift horizontally as thrust to counteract torque effect. In the
case of tilt rotors, the rotor is mounted on a nacelle that
rotates at the edge of the wing to transition the rotor from a
horizontal mounted position, providing lift horizontally as
thrust, to a vertical mounted position providing lift exactly
as a helicopter.

The rotor consists of a mast, hub, and rotor blades.

[Figure 1-4]

The mast is a hollow cylindrical metal shaft which extends
upwards from and is driven by the transmission. At the top
of the mast is the attachment point for the rotor blades called
the hub. The rotor blades are then attached to the hub by
several different methods. Main rotor systems are classified
according to how the main rotor blades are attached and
move relative to the main rotor hub. There are three basic
classifications: semirigid, rigid, or fully articulated, although
some modern rotor systems use an engineered combination
of these types. All three rotor systems are discussed with
greater detail in Chapter 4, Helicopter Components, Sections,
and Systems.

With a single main rotor helicopter, a torque effect is created
as the engine turns the rotor. This torque causes the body of
the helicopter to turn in the opposite direction of the rotor
(Newton’s Third Law: Every action has an equal and opposite
reaction, as explained in Chapter 2, Aerodynamics of Flight).
To eliminate this effect, some sort of antitorque control must
be used with a sufficient margin of power available to allow
the helicopter to maintain its heading and prevent the aircraft
from moving unsteadily. The three most common controls
used today are the traditional tail rotor, Fenestron (also called
a fantail), and the NOTAR

. All three antitorque designs will

be discussed in Chapter 4, Helicopter Components, Sections,
and Systems.

1-4

Figure 1-7.

Coaxial rotors.

Figure 1-5.

Igor Sikorsky designed the VS-300 helicopter

incorporating the tail rotor into the design.

Figure 1-6.

Tandem rotor helicopters.

Rotor Configurations

Most helicopters have a single, main rotor but require a
separate rotor to overcome torque which is a turning or
twisting force. This is accomplished through a variable
pitch, antitorque rotor or tail rotor. This is the design that
Igor Sikorsky settled on for his VS-300 helicopter shown
in

Figure 1-5.

It has become the recognized convention for

helicopter design, although designs do vary. Helicopter main
rotor designs from different manufacturers rotate in one of
two different directions (clockwise or counter-clockwise
when viewed from above). This can make it confusing when
discussing aerodynamic effects on the main rotor between
different designs, since the effects may manifest on opposite
sides of each aircraft. For clarity, throughout this handbook,
all examples use a counter-clockwise rotating main rotor
system when viewed from above.

For clarity, throughout this handbook, all examples use a
counter-clockwise rotating main rotor system when viewed
from above.

Tandem Rotor

Tandem rotor (sometimes referred to as dual rotor) helicopters
have two large horizontal rotor assemblies, instead of one
main assembly and a smaller tail rotor.

[Figure 1-6]

Single

rotor helicopters need a tail rotor to neutralize the twisting
momentum produced by the single large rotor. Tandem
rotor helicopters, however, use counter-rotating rotors, each
canceling out the other’s torque. Counter-rotating rotor
blades will not collide with and destroy each other if they
flex into the other rotor’s pathway. This configuration has
the advantage of being able to hold more weight with shorter
blades, since there are two blade sets. Also, all the power
from the engines can be used for lift, whereas a single rotor
helicopter must use some power to counter main rotor torque.
Because of this, tandem helicopters make up some of the
most powerful and fastest rotor system aircraft.

Coaxial Rotors

Coaxial rotors are a pair of rotors turning in opposite
directions, but mounted on a mast, with the same axis of
rotation, one above the other. This configuration is a noted
feature of helicopters produced by the Russian Kamov
helicopter design bureau.

[Figure 1-7]

Intermeshing Rotors

Intermeshing rotors on a helicopter are a set of two rotors
turning in opposite directions, with each rotor mast mounted
on the helicopter with a slight angle to the other so that
the blades intermesh without colliding.

[Figure 1-8]

This

arrangement allows the helicopter to function without the
need for a tail rotor. It has high stability and powerful lifting
capability. This configuration is sometimes referred to as a
synchropter. The arrangement was developed in Germany

1-5

Tail rotor driveshaft

located inside

of tail body

Tail rotor shaft

Tail rotor

Pitch change links

Cross Head

Figure 1-8.

HH-43 Huskie with intermeshing rotors.

Figure 1-9.

Basic tail rotor components.

for a small anti-submarine warfare helicopter, the Flettner
Fl 282 Kolibri. During the Cold War the American Kaman
Aircraft company produced the HH-43 Huskie, for USAF
firefighting purposes. The latest Kaman K-MAX model is
a dedicated sky crane design used for construction work.

Tail Rotor

The tail rotor is a smaller rotor mounted vertically or near-
vertically on the tail of a traditional single-rotor helicopter.
The tail rotor either pushes or pulls against the tail to counter
the torque. The tail rotor drive system consists of a drive shaft

powered from the main transmission and a gearbox mounted
at the end of the tail boom.

[Figure 1-9]

The drive shaft may

consist of one long shaft or a series of shorter shafts connected
at both ends with flexible couplings. The flexible couplings
allow the drive shaft to flex with the tail boom.

The gearbox at the end of the tail boom provides an angled
drive for the tail rotor and may also include gearing to adjust
the output to the optimum rotational speed typically measured
in revolutions per minute (rpm) for the tail rotor. On some
larger helicopters, intermediate gearboxes are used to angle
the tail rotor drive shaft from along the tail boom or tailcone
to the top of the tail rotor pylon, which also serves as a vertical
stabilizing airfoil to alleviate the power requirement for the
tail rotor in forward flight. The pylon (or vertical fin) may
also provide limited antitorque within certain airspeed ranges
if the tail rotor or the tail rotor flight controls fail.

Controlling Flight

A helicopter has four primary flight controls:

• Cyclic
• Collective
•

Antitorque pedals

• Throttle

Cyclic

The cyclic control is usually located between the pilot’s legs
and is commonly called the “cyclic stick” or simply “cyclic.”
On most helicopters, the cyclic is similar to a joystick;
however, Robinson helicopters have unique T-bar cyclic
control systems. A few helicopters have cyclic controls that
descend into the cockpit from overhead while others use side
cyclic controls.

The control is called the cyclic because it can vary the pitch
of the rotor blades throughout each revolution of the main
rotor system (i.e., through each cycle of rotation) to develop
unequal lift (thrust). The result is to tilt the rotor disk in a
particular direction, resulting in the helicopter moving in that
direction. If the pilot pushes the cyclic forward, the rotor disk
tilts forward, and the rotor produces a thrust in the forward
direction. If the pilot pushes the cyclic to the side, the rotor
disk tilts to that side and produces thrust in that direction,
causing the helicopter to hover sideways.

[Figure 1-10]

Collective

The collective pitch control, or collective, is located on the
left side of the pilot’s seat with a pilot-selected variable
friction control to prevent inadvertent movement. The
collective changes the pitch angle of all the main rotor blades

1-6

Horizontal stabilizer

100

Twist grip throttle

Collective control

Throttle cable

Throttle linkage

Fuel control or carburetor

Figure 1-11.

The throttle control mounted at the end of the collective

control.

Figure 1-12.

The horizontal stabilizer helps level the helicopter to

minimize drag during flight.

Swash plate

Figure 1-10.

Cyclic controls changing the pitch of the rotor blades.

collectively (i.e., all at the same time) and independently of
their positions. Therefore, if a collective input is made, all
the blades change equally, increasing or decreasing total
lift or thrust, with the result of the helicopter increasing or
decreasing in altitude or airspeed.

Antitorque Pedals

The antitorque pedals are located in the same position as the
rudder pedals in a fixed-wing aircraft and serve a similar
purpose, namely to control the direction in which the nose
of the aircraft is pointed. Application of the pedal in a given
direction changes the pitch of the tail rotor blades, increasing
or reducing the thrust produced by the tail rotor, causing the
nose to yaw in the direction of the applied pedal. The pedals
mechanically change the pitch of the tail rotor, altering the
amount of thrust produced.

Throttle

Helicopter rotors are designed to operate at a specific rpm.

The throttle controls the power produced by the engine, which
is connected to the rotor by a transmission. The purpose of
the throttle is to maintain enough engine power to keep the
rotor rpm within allowable limits to produce enough lift for
flight. In single-engine helicopters, if so equipped, the throttle
control is typically a twist grip mounted on the collective
control, but it can also be a lever mechanism in fully
governed systems. Multi-engine helicopters generally have
a power lever or mode switch for each engine.

[Figure 1-11]

Helicopter flight controls are discussed in greater detail
throughout Chapter 4, Helicopter Components, Sections
and Systems.

Flight Conditions

There are two basic flight conditions for a helicopter: hover
and forward 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 the
need for 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
direction that is to control forward and back, right and left.
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 so difficult, since an
adjustment in any one control requires an adjustment of the
other two, creating a cycle of constant correction.

Displacing the cyclic forward initially causes the nose to
pitch down, with a resultant increase in airspeed and loss
of altitude. Aft cyclic initially causes the nose to pitch up,
slowing the helicopter and causing it to climb; however, as
the helicopter reaches a state of equilibrium, the horizontal
stabilizer helps level the helicopter to minimize drag, unlike

2-2

Rotor thrust

Lift component of rotor thrust

Propulsive force

component of

rotor thrust

Drag

Weight

Resultant of drag and weight

Figure 2-3.

Four forces acting on a helicopter in forward flight.

Figure 2-2.

Profile of an airfoil.

20´

5´

Figure 2-1.

Area of a blade.

Gravity acting on the mass (the amount of matter) of an object
creates a force called weight. The rotor blade below weighs
100 lbs. It is 20 feet long (span) and is 1 foot wide (chord).
Accordingly, its surface area is 20 square feet.

[Figure 2-1]

The blade is perfectly balanced on a pinpoint stand, as you
can see in

Figure 2-2

from looking at it from the end (the

airfoil view). The goal is for the blade to defy gravity and
stay exactly where it is when we remove the stand. If we do
nothing before removing the stand, the blade will simply fall
to the ground. Can we exert a force (a push or pull) opposite
gravity that equals the 100 lb. weight of the blade? Yes, for
example, electromagnetic force could be used. In helicopters,
however, we use aerodynamic force to oppose weight and
to maneuver.

Every object in the atmosphere is surrounded by a gas that
exerts a static force of 2,116 lb per square foot (a force
times a unit area, called pressure) at sea level. However, that
pressure is exerted equally all over the blade (top and bottom)
and therefore does not create any useful force on the blade.
We need only create a difference of a single pound of static
pressure differential per square foot of blade surface to have a
force equal to the blade’s weight (100 lb of upward pressure
opposite 100 lb downward weight).

Total pressure consists of static pressure and, if the air is
moving, dynamic pressure (a pressure in the direction of the
air movement). As shown in

Figure 2-3

, if dynamic pressure

is increased the static pressure will decrease. Due to the
design of the airfoil, the velocity of the air passing over the
upper surface will be greater than that of the lower surface,
leading to higher dynamic pressure on the upper surface than
on the lower surface. The higher dynamic pressure on the
upper surface lowers the static pressure on the upper surface.
The static pressure on the bottom will now be greater than
the static pressure on the top. The blade will experience an
upward force. With just the right amount of air passing over
the blade the upward force will equal one pound per square
foot. This upward force is equal to, and acts opposite the
blade’s weight of 100 lb. So, if we now remove the stand, the
blade will defy gravity and remain in its position (ignoring
rearward drag for the moment).

The force created by air moving over an object (or moving
an object through the air) is called aerodynamic force. Aero
means air. Dynamic means moving or motion. Accordingly,
by moving the air over an airfoil we can change the static
pressures on the top and bottom thereby generating a useful
force (an aerodynamic force). The portion of the aerodynamic
force that is usually measured perpendicular to the air flowing
around the airfoil is called lift and is used to oppose weight.
Drag is the portion of aerodynamic force that is measured
as the resistance created by an object passing through the air
(or having the air passed over it). Drag acts in a streamwise
direction with the wind passing over the airfoil and retards
forward movement.

Forces Acting on the Aircraft

Once a helicopter leaves the ground, it is acted upon by
four aerodynamic forces; thrust, drag, lift, and weight.
Understanding how these forces work and knowing how to
control them with the use of power and flight controls are
essential to flight.

[Figure 2-3]

They are defined as follows:

2-3

Increased air

pressure underneath

uced

Upper

ber

efle

air

down

Mass of air deflected down

Figure 2-4.

Production of lift.

•

Lift—opposes the downward force of weight, is
produced by the dynamic effect of the air acting on the
airfoil and acts perpendicular to the flightpath through
the center of lift.

•

Weight—the combined load of the aircraft itself, the
crew, the fuel, and the cargo or baggage. Weight pulls
the aircraft downward because of the force of gravity.
It opposes lift and acts vertically downward through
the aircraft’s center of gravity (CG).

•

Thrust—the force produced by the power plant/
propeller or rotor. It opposes or overcomes the force
of drag. As a general rule, it acts parallel to the
longitudinal axis. However, this is not always the case,
as explained later.

•

Drag—a rearward, retarding force caused by
disruption of airflow by the wing, rotor, fuselage, and
other protruding objects. Drag opposes thrust and acts
rearward parallel to the relative wind.

For a more in-depth explanation of general aerodynamics,
refer to the Pilot’s Handbook of Aeronautical Knowledge.

Lift

Lift is generated when an object changes the direction of
flow of a fluid or when the fluid is forced to move by the
object passing through it. When the object and fluid move
relative to each other and the object turns the fluid flow in
a direction perpendicular to that flow, the force required to
do this work creates an equal and opposite force that is lift.
The object may be moving through a stationary fluid, or the
fluid may be flowing past a stationary object—these two are
effectively identical as, in principle, it is only the frame of
reference of the viewer which differs. The lift generated by
an airfoil depends on such factors as:

•

Speed of the airflow

•

Density of the air

•

Total area of the segment or airfoil

•

Angle of attack (AOA) between the air and the airfoil

The AOA is the angle at which the airfoil meets the oncoming
airflow (or vice versa). In the case of a helicopter, the object
is the rotor blade (airfoil) and the fluid is the air. Lift is
produced when a mass of air is deflected, and it always acts
perpendicular to the resultant relative wind. A symmetric
airfoil must have a positive AOA to generate positive lift. At
a zero AOA, no lift is generated. At a negative AOA, negative
lift is generated. A cambered or nonsymmetrical airfoil may
produce positive lift at zero, or even small negative AOA.

The basic concept of lift is simple. However, the details of how
the relative movement of air and airfoil interact to produce

the turning action that generates lift are complex. In any case
causing lift, an angled flat plate, revolving cylinder, airfoil,
etc., the flow meeting the leading edge of the object is forced to
split over and under the object. The sudden change in direction
over the object causes an area of low pressure to form behind
the leading edge on the upper surface of the object. In turn,
due to this pressure gradient and the viscosity of the fluid,
the flow over the object is accelerated down along the upper
surface of the object. At the same time, the flow forced under
the object is rapidly slowed or stagnated causing an area of
high pressure. This also causes the flow to accelerate along
the upper surface of the object. The two sections of the fluid
each leave the trailing edge of the object with a downward
component of momentum, producing lift.

[Figure 2-4]

Bernoulli’s Principle

Bernoulli’s principle describes the relationship between
internal fluid pressure and fluid velocity. It is a statement
of the law of conservation of energy and helps explain why
an airfoil develops an aerodynamic force. The concept of
conservation of energy states energy cannot be created or
destroyed and the amount of energy entering a system must
also exit. Specifically, in this case the “energy” referred
to is the dynamic pressure (the kinetic energy of the air—
more velocity, more kinetic energy) and static air pressure
(potential energy). These will change among themselves, but
the total pressure energy remains constant inside the tube.

A simple tube with a constricted portion near the center of its
length illustrates this principle. An example is running water
through a garden hose. The mass of flow per unit area (cross-
sectional area of tube) is the mass flow rate. In

Figure 2-5,

the flow into the tube is constant, neither accelerating nor
decelerating; thus, the mass flow rate through the tube must
be the same at stations 1, 2, and 3. If the cross-sectional area
at any one of these stations—or any given point—in the
tube is reduced, the fluid velocity must increase to maintain
a constant mass flow rate to move the same amount of fluid
through a smaller area. The continuity of mass flow causes
the air to move faster through the venturi. In other words,
fluid speeds up in direct proportion to the reduction in area.

2-4

WATER INPUT

WATER OUTPUT

Station 1

Station 2

Station 3

Velocity increased

Static pressure decreased

(compared to original)

mass

of a

Mass of air

Cross-section of cylinder

Total

= P

Dynamic

+ P

Static

PSF

2116

PSF

2109

PSF

Point 1

Point 2

= 2150

PSF

= 34

PSF

= 2116

PSF

V = 100

KTS

Point 1

= 2150

PSF

= 41

PSF

= 2109

PSF

V = 120

KTS

Point 2

Figure 2-5.

Water flow through a tube.

Figure 2-6.

Venturi effect.

Bernoulli (P

total

= P

dynamic

+ P

static

) states that the increase

in velocity will increase the streamwise dynamic pressure.
Since the total pressure in the tube must remain constant,
the static pressure on the sides of the venturi will decrease.
Venturi effect is the term used to describe this phenomenon.

Figure 2-6

illustrates plates of one square foot in the dynamic

flow and on the sides of the tube indicating static pressure,
with corresponding pressure. At point 2, it is easier to
visualize the static pressure reduction on the top of the airfoil
as compared to the bottom of the airfoil, which is depicted as
outside of the tube and therefore at ambient static pressure.
Keep in mind with actual blades it is not a simple as this
example because the bottom static pressure is influenced by
blade design and blade angle, among other things. However,
the basic idea is that it is the static pressure differential
between the top and bottom multiplied by the surface area
of the blade that generates the aerodynamic force.

Venturi Flow

While the amount of total energy within a closed system (the
tube) does not change, the form of the energy may be altered.
Pressure of flowing air may be compared to energy in that the
total pressure of flowing air always remains constant unless
energy is added or removed. Fluid flow pressure has two
components—static and dynamic pressure. Static pressure
is the pressure component measured in the flow but not
moving with the flow as pressure is measured. Static pressure
is also known as the force per unit area acting on a surface.
Dynamic pressure of flow is that component existing as a
result of movement of the air. The sum of these two pressures
is total pressure. As air flows through the constriction, static
pressure decreases as velocity increases. This increases
dynamic pressure.

Figure 2-7

depicts the bottom half of the

constricted area of the tube, which resembles the top half of
an airfoil. Even with the top half of the tube removed, the air

2-5

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

9
8
7
6
5
4
3
2
1
0

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

Aver

e rel

tive w

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/D

MAX

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

Cam

of up

r s

ber

lowe

ace

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 root

Section in center

Section near tip

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 knots

More 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

= 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

= 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

ation

Blade rot

Drift

Tail rotor thrust

Tail rotor

downwash

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

Dire

ctio

n o

f ro

tio

tat

ion

Figure 2-35.

Airflow in forward flight.

Figure 2-34.

Power versus airspeed chart.

Power required (horsepower)

Indicated airspeed (KIAS)

800

600

400

200

100

120

Minimum power

for level flight (V

)

Maximum

continuous

level

(horizontal)

flight

airspeed (V

)

Maximum continuous power available

Increasing power for

decreasing airspeed

Increasing power for

increasing airspeed

r r

uir

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

rot

ati

lad

e r

otat

ion

Relative wind

Forward flight at 100 knots

Relative wind

Direction of Flight

Advancing Side

Retreating 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

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 position

Upflap velocity

Resultant relative wind

Chord line

Angle of attack over nose

Resultant relative wind

Chord line

Angle of attack at 9 o’clock position

Downflap velocity

Resultant relative wind

Chord line

Angle of attack over tail

Resultant relative wind

Chord line

lad

e r

ota

tion

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

Helicopter Flying Handbook (2019) - page 1