Pitch Angle
Angle of Attack
Reference Plane
Axis of Rotation
Chord Line
Relative Wind
Center of pressure
Tip-path plane
Chord line
Angle of incidence
More vertical left vector
Lift
Induced flow velocity= 45 ft/sec
Axis of rotation
α = 10°
Reduced blade pitch angle
Angle of attack is the same
in- or out-of-ground effect
Pitch angle = 14°
Induced flow velocity = 45 ft/sec Reduced blade-tip vortexes
MAX VELOCITY = 90 ft/sec
Figure 2-23. Out of ground effect (OGE).
Figure 2-24. Angle of incidence. Figure 2-25. The AOA is the angle between the airfoil chord line
and resultant relative wind.
Angle of Attack
AOA is the angle between the airfoil chord line and resultant
relative wind. [Figure 2-25] It is an aerodynamic angle and
not easy to measure. It can change with no change in the blade
pitch angle (angle of incidence, discussed earlier).
When the AOA is increased, air flowing over the airfoil is
diverted over a greater distance, resulting in an increase of
air velocity and more lift. As the AOA is increased further,
it becomes more difficult for air to flow smoothly across the
top of the airfoil. At this point, the airflow begins to separate
from the airfoil and enters a burbling or turbulent pattern.
The turbulence results in a large increase in drag and loss of
lift in the area where it is taking place. Increasing the AOA
increases lift until the critical angle of attack is reached. Any
increase in the AOA beyond this point produces a stall and
a rapid decrease in lift (refer to the Low Rotor RPM and
Rotor Stall section of Chapter 11, Helicopter Emergencies
and Hazards).
Several factors may change the rotor blade AOA. The pilot
has little direct control over AOA except indirectly through
the flight control input. Collective and cyclic feathering
help to make these changes. Feathering is the rotation of the
blade about its longitudinal axis by collective/cyclic inputs
causing changes in blade pitch angle. Collective feathering
changes angle of incidence equally and in the same direction
on all rotor blades simultaneously. This action changes AOA,
which changes coefficient of lift (CL), and affects overall
lift of the rotor disk.
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
Blade rotation
Blade rotation
Drift
Tail rotor thrustTail rotor
downwash
Torque
Torque
Figure 2-27. A tail rotor is designed to produce thrust in a direction
opposite torque. The thrust produced by the tail rotor is sufficient
to move the helicopter laterally.
landing gear. Throughout the rest of this discussion, the term
“drag” includes induced, profile and parasite drag.
An important consequence of producing thrust is torque.
As discussed earlier, Newton’s Third Law states: for every
action there is an equal and opposite reaction. Therefore, as
the engine turns the main rotor disk in a counterclockwise
direction, the helicopter fuselage wants to turn clockwise.
The amount of torque is directly related to the amount of
engine power being used to turn the main rotor disk. As
power changes, torque changes.
To counteract this torque-induced turning tendency, an
antitorque rotor or tail rotor is incorporated into most
helicopter designs. A pilot can vary the amount of thrust
produced by the tail rotor in relation to the amount of torque
produced by the engine. As the engine supplies more power
to the main rotor, the tail rotor must produce more thrust to
overcome the increased torque effect. This control change
is accomplished through the use of antitorque pedals (See
page 3-4).
Translating Tendency (Drift)
During hovering flight, a single main rotor helicopter tends
to move in the direction of tail rotor thrust. This lateral
(or sideward) movement is called translating tendency.
[Figure 2-27]
To counteract this tendency, one or more of the following
features may be used. All examples are for a counterclockwise
rotating main rotor disk.
• The main transmission is mounted at a slight angle to
the left (when viewed from behind) so that the rotor
mast has a built-in tilt to oppose the tail rotor thrust.
• Flight controls can be rigged so that the rotor disk is
tilted to the left slightly when the cyclic is centered.
Whichever method is used, the tip-path plane is tilted
slightly to the left in the hover.
• The transmission is mounted so the rotor shaft is
vertical with respect to the fuselage, the helicopter
“hangs” left skid low in the hover. (The opposite is
true for rotor disks turning clockwise when viewed
from above.)
• The helicopter fuselage will also be tilted when the
tail rotor is below the main rotor disk and supplying
antitorque thrust. The fuselage tilt is caused by the
imperfect balance of the tail rotor thrust against the
main rotor torque in the same plane. The helicopter
tilts due to two separate forces, the main rotor disk tilt
to neutralize the translating tendency and the lower
tail rotor thrust below the plane of the torque action.
• In forward flight, the tail rotor continues to push
to the right, and the helicopter makes a small angle
with the wind when the rotors are level and the slip
ball is in the middle (See page 12-2). This is called
inherent sideslip. For some larger helicopters, the
vertical fin or stabilizer is often designed with the
tail rotor mounted on them to correct this side slip
and to eliminate some of the tilting at a hover. (By
mounting the tail rotor on top of the vertical fin or
pylon, the antitorque is more in line with or closer
to the horizontal plane of torque, resulting in less
airframe (or body) lean from the tail rotor.) Also,
having the tail rotor higher off the ground reduces
the risk of objects coming in contact with the blades,
but at the cost of increased weight and complexity.
Pendular Action
Since the fuselage of the helicopter, with a single main rotor,
is suspended from a single point and has considerable mass, it
is free to oscillate either longitudinally or laterally in the same
way as a pendulum. This pendular action can be exaggerated
by overcontrolling; therefore, control movements should be
smooth and not exaggerated. [Figure 2-28]
The horizontal stabilizer helps to level the helicopter in
forward flight. However, in rearward flight, the horizontal
stabilizer can press the tail downward, resulting in a tail
strike if the helicopter is moved rearward into the wind.
Normally, with the helicopter mostly into the wind, the
horizontal stabilizer experiences less headwind component
as the helicopter begins rearward travel (downwind). When
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).
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
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
Resultant
Figure 2-33. To transition to forward flight, more lift and thrust must
be generated to overcome the forces of weight and drag.
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
Knots
Knots
Knots
Rotational
Velocity
Aircraft
Airspeed
Wind
Velocity
=
=
=
–
Knots
Knots
Knots
Rotational
Velocity
Aircraft
Airspeed
Wind
Velocity
=
=
=
+
Direction of rotation
Rotational velocity = 480 knots
Figure 2-35. Airflow in forward flight.
Figure 2-34. Power versus airspeed chart.
Power required (horsepower)
Indicated airspeed (KIAS)
0 40 60 80 100 120
Minimum power
for level flight (VY)
Maximum
continuous
level
(horizontal)
flight
airspeed (VH)
Maximum continuous power available
Increasing power for
decreasing airspeed
Increasing power for
increasing airspeed
Power required to hover OGE
any changes in power or in cyclic movement, the helicopter
begins either to climb or to descend. Once straight-and-level
flight is obtained, the pilot should make note of the power
(torque setting) required and not make major adjustments to
the flight controls. [Figure 2-34]
Airflow in Forward Flight
Airflow across the rotor disk in forward flight varies from
airflow at a hover. In forward flight, air flows opposite the
aircraft’s flightpath. The velocity of this air flow equals the
helicopter’s forward speed. Because the rotor blades turn
in a circular pattern, the velocity of airflow across a blade
depends on the position of the blade in the plane of rotation
at a given instant, its rotational velocity, and airspeed of the
helicopter. Therefore, the airflow meeting each blade varies
continuously as the blade rotates. The highest velocity of
airflow occurs over the right side (3 o’clock position) of
the helicopter (advancing blade in a rotor disk that turns
counterclockwise) and decreases to rotational velocity over
the nose. It continues to decrease until the lowest velocity of
airflow occurs over the left side (9 o’clock position) of the
helicopter (retreating blade). As the blade continues to rotate,
velocity of the airflow then increases to rotational velocity
over the tail. It continues to increase until the blade is back
at the 3 o’clock position.
The advancing blade in Figure 2-35, position A, moves in
the same direction as the helicopter. The velocity of the air
meeting this blade equals rotational velocity of the blade
plus wind velocity resulting from forward airspeed. The
retreating blade (position C) moves in a flow of air moving in the opposite direction of the helicopter. The velocity of
airflow meeting this blade equals rotational velocity of the
blade minus wind velocity resulting from forward airspeed.
The blades (positions B and D) over the nose and tail move
essentially at right angles to the airflow created by forward
airspeed; the velocity of airflow meeting these blades equals
the rotational velocity. This results in a change to velocity
of airflow all across the rotor disk and a change to the lift
pattern of the rotor disk.
Advancing Blade
As the relative wind speed of the advancing blade increases,
the blade gains lift and begins to flap up. It reaches its
maximum upflap velocity at the 3 o’clock position, where the
wind velocity is the greatest. This upflap creates a downward
flow of air and has the same effect as increasing the induced
flow velocity by imposing a downward vertical velocity
vector to the relative wind which decreases the AOA.
Retreating Blade
As relative wind speed of the retreating blade decreases,
the blade loses lift and begins to flap down. It reaches its
maximum downflap velocity at the 9 o’clock position, where
Blade rotation
Blade rotation
Relative wind
Forward flight at 100 knots
Relative wind
Direction of Flight
Advancing SideRetreating Side
Blade tip
speed
plus
helicopter
speed
(500 knots)
Blade tip
speed
minus
helicopter
speed
(300 knots)
Figure 2-36. The blade tip speed of this helicopter is approximately
400 knots. If the helicopter is moving forward at 100 knots, the
relative windspeed on the advancing side is 500 knots. On the
retreating side, it is only 300 knots. This difference in speed causes
a dissymmetry of lift.
Pilots can avoid retreating blade stall by not exceeding the
never-exceed speed. This speed is designated V NE and is
indicated on a placard and marked on the airspeed indicator
by a red line.
Blade flapping compensates for dissymmetry of lift in the
following way. At a hover, equal lift is produced around the
rotor disk with equal pitch (AOI) on all the blades and at
all points in the rotor disk (disregarding compensation for
translating tendency). The rotor disk is parallel to the horizon.
To develop a thrust force, the rotor disk must be tilted in the
desired direction of movement. Cyclic feathering changes
the angle of incidence differentially around the rotor disk.
For a counterclockwise rotation, forward cyclic movement
decreases the angle of incidence on the right of the rotor disk
and increases it on the left.
When transitioning to forward flight either from a hover or
taking off from the ground, pilots must be aware that as the
helicopter speed increases, translational lift becomes more
effective and causes the nose to rise or pitch up (sometimes
referred to as blowback). This tendency is caused by the
combined effects of dissymmetry of lift and transverse flow.
Pilots must correct for this tendency by maintaining a constant
rotor disk attitude that will move the helicopter through the
speed range in which blowback occurs. If the nose is permitted
to pitch up while passing through this speed range, the aircraft
may also tend to roll to the right. To correct for this tendency,
the pilot must continuously move the cyclic forward as velocity
of the helicopter increases until the takeoff is complete, and
the helicopter has transitioned into forward flight.
wind velocity is the least. This downflap creates an upward
flow of air and has the same effect as decreasing the induced
flow velocity by imposing an upward velocity vertical vector
to the relative wind which increases the AOA.
Dissymmetry of Lift
Dissymmetry of lift is the differential (unequal) lift between
advancing and retreating halves of the rotor disk caused by the
different wind flow velocity across each half. This difference
in lift would cause the helicopter to be uncontrollable in any
situation other than hovering in a calm wind. There must
be a means of compensating, correcting, or eliminating this
unequal lift to attain symmetry of lift.
When the helicopter moves through the air, the relative
airflow through the main rotor disk is different on the
advancing side from the retreating side. The relative wind
encountered by the advancing blade is increased by the
forward speed of the helicopter, while the relative wind speed
acting on the retreating blade is reduced by the helicopter’s
forward airspeed. Therefore, as a result of the relative wind
speed, the advancing blade side of the rotor disk can produce
more lift than the retreating blade side. [Figure 2-36]
If this condition were allowed to exist, a helicopter with a
counterclockwise main rotor blade rotation would roll to the
left because of the difference in lift. In reality, the main rotor
blades flap and feather automatically to equalize lift across
the rotor disk. Articulated rotor disks, usually with three or
more blades, incorporate a horizontal hinge (flapping hinge)
to allow the individual rotor blades to move, or flap up and
down as they rotate. A semi-rigid rotor disk (two blades)
utilizes a teetering hinge, which allows the blades to flap as
a unit. When one blade flaps up, the other blade flaps down.
As shown in Figure 2-37, as the rotor blade reaches the
advancing side of the rotor disk (A), it reaches its maximum
up flap velocity. When the blade flaps upward, the angle
between the chord line and the resultant relative wind
decreases. This decreases the AOA, which reduces the
amount of lift produced by the blade. At position (C), the
rotor blade is now at its maximum down flapping velocity.
Due to down flapping, the angle between the chord line and
the resultant relative wind increases. This increases the AOA
and thus the amount of lift produced by the blade.
The combination of blade flapping and slow relative wind
acting on the retreating blade normally limits the maximum
forward speed of a helicopter. At a high forward speed,
the retreating blade stalls because of a high AOA and slow
relative wind speed. This situation is called retreating blade
stall and is evidenced by a nose pitch up, vibration, and a
rolling tendency—usually to the left in helicopters with
counterclockwise blade rotation.