Once a helicopter leaves the ground, it is acted upon by
the four aerodynamic forces. In this chapter, we will
examine these forces as they relate to flight maneuvers.
POWERED FLIGHT
In powered flight (hovering, vertical, forward, side-
ward, or rearward), the total lift and thrust forces of a
rotor are perpendicular to the tip-path plane or plane of
rotation of the rotor.
HOVERING FLIGHT
For standardization purposes, this discussion assumes
a stationary hover in a no-wind condition. During hov-
ering flight, a helicopter maintains a constant position
over a selected point, usually a few feet above the
ground. For a helicopter to hover, the lift and thrust
produced by the rotor system act straight up and must
equal the weight and drag, which act straight down.
While hovering, you can change the amount of main
rotor thrust to maintain the desired hovering altitude.
This is done by changing the angle of attack of the main
rotor blades and by varying power, as needed. In this
case, thrust acts in the same vertical direction as lift.
[Figure 3-1]
The weight that must be supported is the total weight of the
helicopter and its occupants. If the amount of thrust is
greater than the actual weight, the helicopter gains altitude;
if thrust is less than weight, the helicopter loses altitude.
The drag of a hovering helicopter is mainly induced drag
incurred while the blades are producing lift. There is,
however, some profile drag on the blades as they rotate
through the air. Throughout the rest of this discussion,
the term “drag” includes both induced and profile drag.
An important consequence of producing thrust is
torque. As stated before, for every action there is an
equal and opposite reaction. Therefore, as the engine
turns the main rotor system in a counterclockwise
direction, the helicopter fuselage turns clockwise. The
amount of torque is directly related to the amount of
engine power being used to turn the main rotor system.
Remember, as power changes, torque changes.
To counteract this torque-induced turning tendency, an
antitorque rotor or tail rotor is incorporated into most
helicopter designs. You can vary the amount of thrust
produced by the tail rotor in relation to the amount of
torque produced by the engine. As the engine supplies
more power, the tail rotor must produce more thrust.
This is done through the use of antitorque pedals.
TRANSLATING TENDENCY OR DRIFT
During hovering flight, a single main rotor helicopter tends
to drift in the same direction as antitorque rotor thrust. This
drifting tendency is called translating tendency. [Figure 3-2]
Thrust
Lift
Weight
Drag
Figure 3-1. To maintain a hover at a constant altitude, enough
lift and thrust must be generated to equal the weight of the
helicopter and the drag produced by the rotor blades.
Blade Rotation
Torque
Torque
Drift
Tail Rotor Thrust
Figure 3-2. A tail rotor is designed to produce thrust in a
direction opposite torque. The thrust produced by the tail
rotor is sufficient to move the helicopter laterally.
greater the centrifugal force. This force gives the rotor
blades their rigidity and, in turn, the strength to support
the weight of the helicopter. The centrifugal force gen-
erated determines the maximum operating rotor r.p.m.
due to structural limitations on the main rotor system.
As a vertical takeoff is made, two major forces are act-
ing at the same time—centrifugal force acting outward
and perpendicular to the rotor mast, and lift acting
upward and parallel to the mast. The result of these two
forces is that the blades assume a conical path instead
of remaining in the plane perpendicular to the mast.
[Figure 3-4]
CORIOLIS EFFECT
(LAW OF CONSERVATION
OF ANGULAR MOMENTUM)
Coriolis Effect, which is sometimes referred to as con-
servation of angular momentum, might be compared to
spinning skaters. When they extend their arms, their
rotation slows down because the center of mass moves
farther from the axis of rotation. When their arms are
retracted, the rotation speeds up because the center of
mass moves closer to the axis of rotation.
When a rotor blade flaps upward, the center of mass of
that blade moves closer to the axis of rotation and blade
acceleration takes place in order to conserve angular
momentum. Conversely, when that blade flaps down-
ward, its center of mass moves further from the axis of
Before Takeoff
During Takeoff
Lift
Centrifugal□
Force
Resultant□
Blade□
Angle
Figure 3-4. Rotor blade coning occurs as the rotor blades
begin to lift the weight of the helicopter. In a semirigid and
rigid rotor system, coning results in blade bending. In an
articulated rotor system, the blades assume an upward angle
through movement about the flapping hinges.
Centrifugal Force—The apparent
force that an object moving along
a circular path exerts on the body
constraining the obect and that
acts outwardy away from the cen-
ter of rotation.
To counteract this drift, one or more of the following
features may be used:
• The main transmission is mounted so that the rotor
mast is rigged for the tip-path plane to have a built-
in tilt opposite tail thrust, thus producing a small
sideward thrust.
• Flight control rigging is designed so that the rotor
disc is tilted slightly opposite tail rotor thrust when
the cyclic is centered.
• The cyclic pitch control system is designed so that
the rotor disc tilts slightly opposite tail rotor thrust
when in a hover.
Counteracting translating tendency, in a helicopter with a
counterclockwise main rotor system, causes the left skid
to hang lower while hovering. The opposite is true for
rotor systems turning clockwise when viewed from above.
PENDULAR ACTION
Since the fuselage of the helicopter, with a single main
rotor, is suspended from a single point and has consider-
able mass, it is free to oscillate either longitudinally or
laterally in the same way as a pendulum. This pendular
action can be exaggerated by over controlling; therefore,
control movements should be smooth and not exagger-
ated. [Figure 3-3]
CONING
In order for a helicopter to generate lift, the rotor blades
must be turning. This creates a relative wind that is
opposite the direction of rotor system rotation. The
rotation of the rotor system creates centrifugal force
(inertia), which tends to pull the blades straight outward
from the main rotor hub. The faster the rotation, the
Hover
Rearward□
Flight
Forward□
Flight
Figure 3-3. Because the helicopter’s body has mass and is
suspended from a single point (the rotor mast head), it tends
to act much like a pendulum.
rotation and blade deceleration takes place. [Figure 3-5]
Keep in mind that due to coning, a rotor blade will not
flap below a plane passing through the rotor hub and
perpendicular to the axis of rotation. The acceleration
and deceleration actions of the rotor blades are absorbed
by either dampers or the blade structure itself, depend-
ing upon the design of the rotor system.
Two-bladed rotor systems are normally subject to
Coriolis Effect to a much lesser degree than are articu-
lated rotor systems since the blades are generally
“underslung” with respect to the rotor hub, and the
change in the distance of the center of mass from the
axis of rotation is small. [Figure 3-6] The hunting
action is absorbed by the blades through bending. If a
two-bladed rotor system is not “underslung,” it will be
subject to Coriolis Effect comparable to that of a fully
articulated system.
GROUND EFFECT
When hovering near the ground, a phenomenon known
as ground effect takes place. [Figure 3-7] This effect
usually occurs less than one rotor diameter above the
surface. As the induced airflow through the rotor disc is
reduced by the surface friction, the lift vector increases.
This allows a lower rotor blade angle for the same
amount of lift, which reduces induced drag. Ground
effect also restricts the generation of blade tip vortices
due to the downward and outward airflow making a
larger portion of the blade produce lift. When the heli-
copter gains altitude vertically, with no forward air-
speed, induced airflow is no longer restricted, and the
blade tip vortices increase with the decrease in outward
airflow. As a result, drag increases which means a
Axis of□
Rotation
Blade□
Flapping
Center of Mass
Figure 3-5. The tendency of a rotor blade to increase or
decrease its velocity in its plane of rotation due to mass
movement is known as Coriolis Effect, named for the mathe-
matician who made studies of forces generated by radial
movements of mass on a rotating disc.
Large Blade□
Tip Vortex
No Wind Hover
Blade Tip□
Vortex
OUT OF GROUND EFFECT (OGE) IN GROUND EFFECT (IGE)
Downwash Pattern□
Equidistant 360ϒ
Figure 3-7. Air circulation patterns change when hovering out of ground effect (OGE) and when hovering in ground effect (IGE).
This elbow moves away from□
the mast as the rotor is tilted.
This elbow moves toward□
the mast as the rotor is tilted.
Mast□
Axis
CM CM
Figure 3-6. Because of the underslung rotor, the center of
mass remains approximately the same distance from the
mast after the rotor is tilted.
higher pitch angle, and more power is needed to move
the air down through the rotor.
Ground effect is at its maximum in a no-wind condition
over a firm, smooth surface. Tall grass, rough terrain,
revetments, and water surfaces alter the airflow pattern,
causing an increase in rotor tip vortices.
GYROSCOPIC PRECESSION
The spinning main rotor of a helicopter acts like a gyro-
scope. As such, it has the properties of gyroscopic
action, one of which is precession. Gyroscopic preces-
sion is the resultant action or deflection of a spinning
object when a force is applied to this object. This action
occurs approximately 90° in the direction of rotation
from the point where the force is applied. [Figure 3-8]
Let us look at a two-bladed rotor system to see how
gyroscopic precession affects the movement of the tip-
path plane. Moving the cyclic pitch control increases
the angle of attack of one rotor blade with the result
that a greater lifting force is applied at that point in the
plane of rotation. This same control movement simul-
taneously decreases the angle of attack of the other
blade the same amount, thus decreasing the lifting force
applied at that point in the plane of rotation. The blade
with the increased angle of attack tends to flap up; the
blade with the decreased angle of attack tends to flap
down. Because the rotor disk acts like a gyro, the
blades reach maximum deflection at a point approxi-
mately 90° later in the plane of rotation. As shown in
figure 3-9, the retreating blade angle of attack is
increased and the advancing blade angle of attack is
decreased resulting in a tipping forward of the tip-path
plane, since maximum deflection takes place 90° later
when the blades are at the rear and front, respectively.
In a rotor system using three or more blades, the move-
ment of the cyclic pitch control changes the angle of
attack of each blade an appropriate amount so that the
end result is the same.
VERTICAL FLIGHT
Hovering is actually an element of vertical flight.
Increasing the angle of attack of the rotor blades (pitch)
while their velocity remains constant generates addi-
tional vertical lift and thrust and the helicopter ascends.
Decreasing the pitch causes the helicopter to descend.
In a no wind condition when lift and thrust are less than
weight and drag, the helicopter descends vertically. If
90ϒ
Axis
Upward□
Force□
Applied□
Here
Reaction□
Occurs□
Here
New Axis
Gyro Tips□
Down Here
Gyro Tips□
Up Here
Old Axis
Figure 3-8. Gyroscopic precession principle—when a force is applied to a spinning gyro, the maximum reaction occurs approx-
imately 90° later in the direction of rotation.
Blade□
Rotation
Angle of Attack□
Decreased
Maximum□
Upward□
Deflection
Maximum□
Downward□
Deflection Angle of Attack□
Increased
Figure 3-9. With a counterclockwise main rotor blade rota-
tion, as each blade passes the 90° position on the left, the
maximum increase in angle of attack occurs. As each blade
passes the 90° position to the right, the maximum decrease
in angle of attack occurs. Maximum deflection takes place
90° later—maximum upward deflection at the rear and maxi-
mum downward deflection at the front—and the tip-path
plane tips forward.
lift and thrust are greater than weight and drag, the hel-
icopter ascends vertically. [Figure 3-10]
FORWARD FLIGHT
In or during forward flight, the tip-path plane is tilted for-
ward, thus tilting the total lift-thrust force forward from
the vertical. This resultant lift-thrust force can be resolved
into two components—lift acting vertically upward and
thrust acting horizontally in the direction of flight. In
addition to lift and thrust, there is weight (the downward
acting force) and drag (the rearward acting or retarding
force of inertia and wind resistance). [Figure 3-11]
In straight-and-level, unaccelerated forward flight, lift
equals weight and thrust equals drag (straight-and-level
flight is flight with a constant heading and at a constant
altitude). If lift exceeds weight, the helicopter climbs;
if lift is less than weight, the helicopter descends. If
thrust exceeds drag, the helicopter speeds up; if thrust
is less than drag, it slows down.
As the helicopter moves forward, it begins to lose alti-
tude because of the lift that is lost as thrust is diverted
forward. However, as the helicopter begins to acceler-
ate, the rotor system becomes more efficient due to the
increased airflow. The result is excess power over that
which is required to hover. Continued acceleration
causes an even larger increase in airflow through the
rotor disc and more excess power.
TRANSLATIONAL LIFT
Translational lift is present with any horizontal flow of
air across the rotor. This increased flow is most notice-
able when the airspeed reaches approximately 16 to 24
knots. As the helicopter accelerates through this speed,
the rotor moves out of its vortices and is in relatively
undisturbed air. The airflow is also now more horizontal,
which reduces induced flow and drag with a correspon-
ding increase in angle of attack and lift. The additional
lift available at this speed is referred to as “effective
translational lift” (ETL). [Figure 3-12]
When a single-rotor helicopter flies through translational
lift, the air flowing through the main rotor and over the
tail rotor becomes less turbulent and more aerodynami-
cally efficient. As the tail rotor efficiency improves,
more thrust is produced causing the aircraft to yaw left
in a counterclockwise rotor system. It will be necessary
to use right torque pedal to correct for this tendency on
takeoff. Also, if no corrections are made, the nose rises
or pitches up, and rolls to the right. This is caused by
combined effects of dissymmetry of lift and transverse
flow effect, and is corrected with cyclic control.
Resultant
Resultant
Lift
Thrust
Helicopter□
Movement
Weight
Drag
Figure 3-11. To transition into forward flight, some of the ver-
tical thrust must be vectored horizontally. You initiate this by
forward movement of the cyclic control.
No Recirculation□
of Air More Horizontal□
Flow of Air
Reduced□
Induced Flow□
Increases□
Angle of Attack
Tail Rotor Operates in□
Relatively Clean Air
16 to 24□
Knots
Figure 3-12. Effective translational lift is easily recognized in
actual flight by a transient induced aerodynamic vibration
and increased performance of the helicopter.
Thrust
Lift
Weight
Drag
Vertical Ascent
Figure 3-10. To ascend vertically, more lift and thrust must be
generated to overcome the forces of weight and the drag.
Translational lift is also present in a stationary hover if
the wind speed is approximately 16 to 24 knots. In nor-
mal operations, always utilize the benefit of translational
lift, especially if maximum performance is needed.
INDUCED FLOW
As the rotor blades rotate they generate what is called
rotational relative wind. This airflow is characterized
as flowing parallel and opposite the rotor’s plane of
rotation and striking perpendicular to the rotor blade’s
leading edge. This rotational relative wind is used to
generate lift. As rotor blades produce lift, air is acceler-
ated over the foil and projected downward. Anytime a
helicopter is producing lift, it moves large masses of air
vertically and down through the rotor system. This
downwash or induced flow can significantly change
the efficiency of the rotor system. Rotational relative
wind combines with induced flow to form the resultant
relative wind. As induced flow increases, resultant rel-
ative wind becomes less horizontal. Since angle of
attack is determined by measuring the difference
between the chord line and the resultant relative wind,
as the resultant relative wind becomes less horizontal,
angle of attack decreases. [Figure 3-13]
TRANSVERSE FLOW EFFECT
As the helicopter accelerates in forward flight, induced
flow drops to near zero at the forward disc area and
increases at the aft disc area. This increases the angle
of attack at the front disc area causing the rotor blade to
flap up, and reduces angle of attack at the aft disc area
causing the rotor blade to flap down. Because the rotor
acts like a gyro, maximum displacement occurs 90° in
the direction of rotation. The result is a tendency for
the helicopter to roll slightly to the right as it acceler-
ates through approximately 20 knots or if the headwind
is approximately 20 knots.
You can recognize transverse flow effect because of
increased vibrations of the helicopter at airspeeds just
below effective translational lift on takeoff and after
passing through effective translational lift during land-
ing. To counteract transverse flow effect, a cyclic input
needs to be made.
DISSYMMETRY OF LIFT
When the helicopter moves through the air, the relative
airflow through the main rotor disc is different on the
advancing side than on the retreating side. The relative
wind encountered by the advancing blade is increased
by the forward speed of the helicopter, while the rela-
tive wind speed acting on the retreating blade is
reduced by the helicopter’s forward airspeed.
Therefore, as a result of the relative wind speed, the
advancing blade side of the rotor disc produces more
lift than the retreating blade side. This situation is
defined as dissymmetry of lift. [Figure 3-14]
If this condition was allowed to exist, a helicopter with
a counterclockwise main rotor blade rotation would roll
to the left because of the difference in lift. In reality, the
main rotor blades flap and feather automatically to
equalize lift across the rotor disc. Articulated rotor sys-
tems, usually with three or more blades, incorporate a
horizontal hinge (flapping hinge) to allow the individ-
ual rotor blades to move, or flap up and down as they
rotate. A semirigid rotor system (two blades) utilizes a
teetering hinge, which allows the blades to flap as a
unit. When one blade flaps up, the other flaps down.
Figure 3-13. A helicopter in forward flight, or hovering with a headwind or crosswind, has more molecules of air entering the aft
portion of the rotor blade. Therefore, the angle of attack is less and the induced flow is greater at the rear of the rotor disc.
Resultant Relative Wind
Resultant Relative Wind
10 to 20□
Knots
Induced□
Flow
Induced□
Flow
Angle of□
Attack Angle of□
Attack
Rotational Relative Wind Rotational Relative Wind
As shown in figure 3-15, as the rotor blade reaches the
advancing side of the rotor disc (A), it reaches its max-
imum upflap velocity. When the blade flaps upward,
the angle between the chord line and the resultant rela-
tive wind decreases. This decreases the angle of attack,
which reduces the amount of lift produced by the blade.
At position (C) the rotor blade is now at its maximum
downflapping velocity. Due to downflapping, the angle
between the chord line and the resultant relative wind
increases. This increases the angle of attack and thus
the amount of lift produced by the blade.
The combination of blade flapping and slow relative wind
acting on the retreating blade normally limits the maxi-
mum forward speed of a helicopter. At a high forward
speed, the retreating blade stalls because of a high angle of
attack and slow relative wind speed. This situation is
called retreating blade stall and is evidenced by a nose
pitch up, vibration, and a rolling tendency—usually to the
left in helicopters with counterclockwise blade rotation.
You can avoid retreating blade stall by not exceeding
the never-exceed speed. This speed is designated VNE
and is usually indicated on a placard and marked on the
airspeed indicator by a red line.
During aerodynamic flapping of the rotor blades as they
compensate for dissymmetry of lift, the advancing blade
Relative Wind
Relative Wind
Direction□
of Flight
Advancing□
Side
Blade Tip□
Speed Plus□
Helicopter□
Speed□
(400 KTS)
Blade Tip□
Speed Minus□
Helicopter□
Speed□
(200 KTS)
Retreating□
Side
Forward Flight□
100 KTS
Blade□
Rotation
Figure 3-14. The blade tip speed of this helicopter is approxi-
mately 300 knots. If the helicopter is moving forward at 100
knots, the relative wind speed on the advancing side is 400
knots. On the retreating side, it is only 200 knots. This differ-
ence in speed causes a dissymmetry of lift.
Figure 3-15. The combined upward flapping (reduced lift) of the advancing blade and downward flapping (increased lift) of the
retreating blade equalizes lift across the main rotor disc counteracting dissymmetry of lift.
Direction of Rotation
Chord Line
Resultant RW
Chord Line
Resultant RW
Chord Line
Downflap Velocity
Resultant RW
Chord Line
Resultant RW
Upflap Velocity
Angle of Attack at□
9 O'Clock Position
Angle of Attack at□
3 O'Clock Position
Angle of Attack over□
Tail
Angle of Attack over□
Nose
RW = Relative Wind□
= Angle of Attack
VNE —The speed beyond which an aircraft should never be
operated. VNE can change with altitude, density altitude, and
weight.
ward. Drag now acts forward with the lift component
straight up and weight straight down. [Figure 3-18]
TURNING FLIGHT
In forward flight, the rotor disc is tilted forward, which
also tilts the total lift-thrust force of the rotor disc for-
ward. When the helicopter is banked, the rotor disc is
tilted sideward resulting in lift being separated into two
components. Lift acting upward and opposing weight is
called the vertical component of lift. Lift acting hori-
zontally and opposing inertia (centrifugal force) is the
horizontal component of lift (centripetal force) .
[Figure 3-19]
As the angle of bank increases, the total lift force is tilted
more toward the horizontal, thus causing the rate of turn
to increase because more lift is acting horizontally. Since
the resultant lifting force acts more horizontally, the
effect of lift acting vertically is deceased. To compen-
sate for this decreased vertical lift, the angle of attack of
the rotor blades must be increased in order to maintain
altitude. The steeper the angle of bank, the greater the
angle of attack of the rotor blades required to maintain
altitude. Thus, with an increase in bank and a greater
angle of attack, the resultant lifting force increases and
the rate of turn is faster.
AUTOROTATION
Autorotation is the state of flight where the main rotor
system is being turned by the action of relative wind
Centripetal Force—The force
opposite centrifugal force and
attracts a body toward its axis of
rotation.
ResultantLift
Thrust
Drag
Resultant
Weight
Helicopter□
Movement
Figure 3-18. Forces acting on the helicopter during rearward
flight.
achieves maximum upflapping displacement over the
nose and maximum downflapping displacement over the
tail. This causes the tip-path plane to tilt to the rear and is
referred to as blowback. Figure 3-16 shows how the rotor
disc was originally oriented with the front down follow-
ing the initial cyclic input, but as airspeed is gained and
flapping eliminates dissymmetry of lift, the front of the
disc comes up, and the back of the disc goes down. This
reorientation of the rotor disc changes the direction in
which total rotor thrust acts so that the helicopter’s for-
ward speed slows, but can be corrected with cyclic input.
SIDEWARD FLIGHT
In sideward flight, the tip-path plane is tilted in the direc-
tion that flight is desired. This tilts the total lift-thrust
vector sideward. In this case, the vertical or lift compo-
nent is still straight up and weight straight down, but the
horizontal or thrust component now acts sideward with
drag acting to the opposite side. [Figure 3-17]
REARWARD FLIGHT
For rearward flight, the tip-path plane is tilted rear-
ward, which, in turn, tilts the lift-thrust vector rear-
Helicopter□
Movement
Weight
Drag
Resultant
Thrust
Lift
Figure 3-17. Forces acting on the helicopter during sideward
flight.
Figure 3-16. To compensate for blowback, you must move
the cyclic forward. Blowback is more pronounced with higher
airspeeds.