AIRFORCE OF ZIMBABWE

SCHOOL OF TECHNICAL TRAINING

NUMBER 15747
RANK SAC
NAME RUBINO L

COURSE 17 ND

MODULE ADVANCED
HELICOPTERS

TRADE AIRFRAMES

DUE DATE: 14 MARCH 2025

FACILITATOR MT B BARE
TOPIC WITH THE AID OF DIAGRAMS EXPLAIN THE
VECTORS ON THE AEROFOIL DURING
HOVER, TRANSLATION AND AUTO
ROTATION
MARKS
Vectors on the Aerofoil during Hover, Translation, and Auto Rotation in Helicopters

Vectors on the aerofoil during hover, the helicopter remains stationary relative to the ground, and
the rotor blades generate lift to counteract the weight of the helicopter. The key vectors acting on
the aerofoil include, lift vector which acts perpendicular to the relative airflow and upward
balancing the weight of the helicopter. Weight vector, it acts downward due to gravity. Drag
vector it acts opposite to the direction of rotation, opposing the motion of the rotor blades and
thrust vector which is generated by the rotor system, acts upward to balance weight, for steady
hover, the net force must be zero. Vectors on the aerofoil during translation, the helicopter moves
horizontally, transitioning from hover to forward flight. The vectors on the aerofoil change due
to the introduction of horizontal motion, lift vector still acts perpendicular to the relative airflow
but may tilt slightly forward due to the cyclic pitch control. Weight vector continues to act
downward, drag vector Increases due to the forward motion and acts opposite to the direction of
flight. Thrust Vector has a horizontal component to overcome drag and a vertical component to
maintain lift. For uniform translation, the relative airflow combines the rotational motion of the
rotor blades and the forward motion of the helicopter, resulting in a skewed airflow
vector .Vectors on the Aerofoil during auto rotation during an engine failure, where the rotor
blades are driven by the upward flow of air. The vectors on the aerofoil are significantly
different, lift Vector acts perpendicular to the relative airflow but is now primarily generated by
the upward flow of air rather than engine power, weight vector acts downward and drag vector
acts opposite to the direction of rotation, but its magnitude is reduced due to the lack of engine
power. Inertial Force acts due to the deceleration of the helicopter. For controlled auto rotation
rotor speed is maintained by converting potential energy into kinetic energy The relati ve airflow
during auto rotation is primarily upward, driving the rotor blades and allowing the helicopter to
descend safe.
Vectors in Hover

hover is a critical aspect of understanding the behavior of aircraft, particularly helicopters and
drones, when they are stationary relative to the ground. In hover, the aircraft maintains a stable
position by balancing the forces acting on it, primarily through the manipulation of aerodynamic
vectors.

Fundamental forces in hover

In hover, four primary forces act on the aircraft which are lift generated by the rotor blades to
counteract the weight of the aircraft. During hover lift must exactly balance the weight of the
aircraft to maintain a stable position in the air. Weight The gravitational force acting downward
which must be equal to the lift force. Thrust is the force produced by the rotor system to maintain
lift. Drag is the resistance encountered by the rotor blades as they move through the air. For a
stable hover, lift must equal weight, and thrust must counteract drag.
Hovering is the term applied when a helicopter maintains a constant position at a selected point,

usually a few feet above the ground (but not always, helicopters can hover high in the air, given
sufficient power). For a helicopter to hover, the main rotor must supply lift equal to the total
weight

of the helicopter. With the blades rotating at high velocity, an increase of blade pitch (angle of

attack) would induce the necessary lift for a hover. The forces of lift and weight reach a state of

balance during a stationary hover. Hovering is actually an element of vertical flight. Assuming a
no-wind condition, the tip-path plane of the blades will remain horizontal. If the angle of attack
of the blades is increased while their velocity remains constant, additional vertical thrust is
obtained. Thus, by upsetting the vertical balance of forces, helicopters can climb or descend
vertically. Airflow during hovering At a hover, the rotor tip vortex (air swirl at the tip of the rotor
blades) reduces the effectiveness of the outer blade portions. Also, the vortexes of the preceding
blade severely affect the lift of the following blades. If the vortex made by one passing blade
remains a vicious swirl for some number of seconds, then two blades operating at 350 RPM
create 700 longlasting vortex patterns per minute. This continuous creation of new vortexes and
ingestion of existing vortexes is a primary cause of high power requirements for hovering.

During hover, the rotor blades move large volumes of air in a downward direction. This pumping

process uses lots of horsepower and accelerates the air to relatively high velocities. Air velocity

under the helicopter may reach 60 to 100 knots, depending on the size of the rotor and the gross

weight of the helicopter. The air flow pattern of a hovering helicopter is illustrated here

Note how the downwash (induced flow) of air has introduced another element into the relative
wind which alters the angle of attack of the airfoil. When there is no induced flow, the relative
wind is opposite and parallel to the flightpath of the airfoil. In the hovering case, the downward
airflow alters the relative wind and changes the angle of attack so less aerodynamic force is
produced. This condition requires the pilot to increase collective pitch to produce enough
aerodynamic force to sustain a hover. Although this does increase the lift, it also increases the
induced drag, and so total power required is higher.
Ground effect

The high power requirement needed to hover out of ground effect is reduced when operating in

ground effect. Ground effect is a condition of improved performance encountered when perating
near (within 1/2 rotor diameter) of the ground. It is due to the interference of the surface with the

airflow pattern of the rotor system, and it is more pronounced the nearer the ground is pproached.

Increased blade efficiency while operating in ground effect is due to two separate and distinct

phenomena. First and most important is the reduction of the velocity of the induced airflow.
Since the ground interrupts the airflow under the helicopter, the entire flow is altered.
This reduces downward velocity of the induced flow. The result is less induced drag and a more
vertical lift vector. The lift needed to sustain a hover can be produced with a reduced angle of
attack and less power because of the more vertical lift vector. The second phenomena is a
reduction of the rotor tip vortex: When operating in ground effect, the downward and outward
airflow pattern tends to restrict vortex generation. This makes the outboard portion of the rotor
blade more efficient and reduces overall system turbulence caused by ingestion and recirculation
of the vortex swirls. Rotor efficiency is increased by ground effect up to a height of about one
rotor diameter for most helicopters. This figure illustrates the percent increase in rotor thrust
experienced at various rotor heights: At a rotor height of one-half rotor diameter, the thrust is
increased about 7 percent. At rotor heights above one rotor diameter, the thrust increase is small
and decreases to zero at a height of about 1 1/4 rotor diameters. Maximum ground effect is
accomplished when hovering over smooth paved surfaces. While hovering over tall grass, rough
terrain, revetments, or water, ground effect may be seriously reduced. This phenomena is due to
the partial breakdown and cancellation of ground effect and the return of large vortex patterns
with increased downwash angles.

Two identical airfoils with equal blade pitch angles are compared in the following figure the top
airfoil is out-of-ground-effect while the bottom airfoil is in-ground-effect. The airfoil that is in-
ground-effect is more efficient because it operates at a larger angle of attack and produces a more
vertical lift vector. Its increased efficiency results from a smaller downward induced wind
velocity which increases angle of attack. The airfoil operating out-of-ground-effect is less
efficient because of increased induced wind velocity which reduces angle of attack. If a
helicopter hovering out-of-ground-effect descends into a ground-effect hover, blade efficiency
increases because of the more favorable induced flow. As efficiency of the rotor system
increases, the pilot reduces blade pitch angle to remain in the ground-effect hover. Less power is
required to maintain however in-ground-effect than for the out-of-ground-effect hover
.Rotor Blade Aerodynamics

The rotor blades are the primary source of aerodynamic forces in hover. Key concepts include:

Angle of Attack the angle between the chord line of the blade and the relative airflow. Adjusting
the AoA changes the lift generated, Blade Pitch is the angle at which the rotor blades are set
relative to the rotor hub. Collective pitch control adjusts this angle uniformly across all blades to
increase or decrease lift and

Tip Vortices, turbulent air patterns created at the tips of the rotor blades, which can reduce
efficiency and increase drag. Aerodynamic Vectors in Hover aerodynamic vectors describe the
direction and magnitude of forces acting on the aircraft. In hover, the following vectors are
crucial. Lift Vector acts vertically upward, opposing the weight vector and thrust vector is
aligned with the rotor axis, providing the necessary force to maintain altitude. Drag Vector it acts
opposite to the direction of rotor rotation, reducing efficiency and torque Vector the rotational
force generated by the rotor system, which must be counteracted by the tail rotor or other anti-
torque mechanisms.
Control Mechanisms in Hover

To maintain stability and control in hover, the following mechanisms are employed, collective
Pitch Control which adjusts the pitch of all rotor blades simultaneously to control lift. Cyclic
Pitch Control varies the pitch of individual blades during rotation to tilt the rotor disk and control
direction. Tail Rotor which provides anti-torque to counteract the rotational force generated by
the main rotor.

Challenges in Hover

Hovering presents unique aerodynamic challenges, including ground Effect, the increased lift
and reduced drag experienced when the aircraft is close to the ground, which can affect stability.
Vortex Ring State a dangerous condition where the rotor operates in its own downwash, reducing
lift and causing instability. Power Requirements Hovering requires significant power, as the rotor
must generate enough thrust to counteract both weight and drag.

Applications of Hover Aerodynamics, understanding aerodynamic vectors in hover is essential

for Helicopter Design Optimizing rotor systems for efficiency and stability. Drone Technology
enhancing the performance of multirotor UAVs. Search and Rescue Operations enabling precise
control in stationary or low-speed flight. Aerodynamics in hover is a complex interplay of forces
and vectors that requires precise control and understanding. By mastering these principles,
engineers and pilots can design and operate aircraft that achieve stable and efficient hover,
enabling a wide range of applications in aviation and beyond.
Vectors during Translation in Helicopters

Vectors during translation in helicopters describe the motion of the helicopter as it moves from
one position to another in a straight line. Translation is a critical phase of helicopter flight,
involving the transition from hover to forward flight or vice versa. Vectors are essential for
representing both the magnitude and direction of displacement, velocity, and acceleration.
Understanding translation vectors in helicopters is crucial for applications such as search and
rescue, military operations, and urban air mobility.

Fundamental Forces during Translation in Helicopters

During translation, several forces act on a helicopter which are lift generated by the rotor blades
to counteract the weight of the helicopter. Weight it is the gravitational force acting downward
which is the weight of the aircraft. Thrust is the force produced by the rotor system to move the
helicopter forward and drag which the resistance encountered as the helicopter moves through
the air. For uniform translation, the net force must be zero, meaning thrust must balance drag and
lift must equal weight.

Translational lift

The efficiency of the hovering rotor system is improved with each knot of incoming wind gained
by horizontal movement or surface wind. As the incoming wind enters the rotor system,
turbulence and vortexes are left behind and the flow of air becomes more horizontal. All of these
changes improve the efficiency of the rotor system and improve aircraft performance.

Improved rotor efficiency resulting from directional flight is called translational lift. The
following picture shows an airflow pattern at airspeeds between 1-5 knots:
Note how the downwind vortex is beginning to dissipate and induced flow down through the rear
of the rotor disk is more horizontal than at a hover. This next picture shows the airflow pattern at
a speed of 10-15 knots. Airflow is much more horizontal than at a hover. The leading edge of the
downwash pattern is being overrun and is well back under the helicopter nose. At about 16 to 24
knots (depending upon the size, blade area, and RPM of the rotor system) the rotor completely
outruns the recirculation of old vortexes, and begins to work in relatively clean air. The air
passing through the rotor system is nearly horizontal, depending on helicopter forward air speed.
As the helicopter speed increases, translational lift becomes more effective and causes the nose
to rise, or pitch up (sometimes called blowback). This tendency is caused by the combined
effects of dissymmetry of lift and transverse flow. Pilots must correct for this tendency in order
to maintain a constant rotor disk attitude that will move the helicopter through the speed range
where 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. When the single main rotor helicopter
transitions from hover to forward flight, the tail rotor becomes more aerodynamically efficient.
Efficiency increases because the tail rotor works in progressively less turbulent air as speed
increases. As tail rotor efficiency improves, more thrust is produced. This causes the aircraft
nose to yaw left if the main rotor turns counterclockwise. During a takeoff where power is
constant, the pilot must apply right pedal as speed increases to correct for

The left yaw tendency

Types of Translation Motion in Helicopters

Translation in helicopters can occur in various forms which are Linear Translation, which is
movement along a straight line. Curvilinear Translation which is movement along a curved path,
Uniform Translation, constant velocity with no acceleration and Non-Uniform Translation
which is variable velocity with acceleration. Each type has unique characteristics and
applications, from simple maneuvers to complex flight paths.

Control Mechanisms during Translation in Helicopters

Control during translation in helicopters is achieved through collective pitch control by adjusting
the pitch of all rotor blades simultaneously to control lift. Cyclic pitch control it varies the pitch
of individual blades during rotation to tilt the rotor disk and control direction. Tail rotor, provides
anti-torque to counteract the rotational force generated by the main rotor. Fly-by-Wire System it
is a system which use electronic controls to enhance stability and responsiveness. Advanced
control systems, such as autopilots and stability augmentation systems, are increasingly used to
improve translation performance and safety.
Environmental Effects on Translation Stability in Helicopters

Environmental factors can impact translation stability in helicopters, these includes the wind
which can cause turbulence and disrupt lift. Gusty conditions require precise control inputs to
maintain stability. Temperature it affects air density, altering lift and power requirements. Hotter
temperatures reduce air density, requiring more power for translation. Altitude this means higher
altitudes reduce air density, requiring more power for translation. This is particularly important
for operations in mountainous regions. Ground Effect it increases lift and reduces drag when the
helicopter is close to the ground, but can cause instability if not managed properly.
Understanding these effects is crucial for safe and efficient translation operations.
Vectors during Auto Rotation of a Helicopter

Auto rotation is a critical emergency maneuver in helicopters, allowing for a controlled descent
and landing in the event of engine failure. During auto rotation, the rotor system is driven by the
upward flow of air, converting potential energy into kinetic energy to maintain rotor speed.
Vectors during auto rotation describe the motion of the helicopter as it descends and the forces
acting on the rotor system. Understanding these vectors is essential for pilot training and
ensuring safe emergency landings.
Fundamental Forces during Auto Rotation in Helicopters

During auto rotation, several forces act on a helicopter which are lift which is generated by the
rotor blades to slow the descent, there is weight which is the gravitational force acting
downward. Drag is the resistance encountered as the helicopter descends through the air and

Centripetal force which is the inward force that keeps the rotor blades rotating and lastly Inertial
Force which is the force due to the helicopters mass and deceleration. For a controlled descent,
lift must balance weight, and drag must be managed to maintain rotor speed. The pilot must
carefully adjust these forces to ensure a safe landing.
Aerodynamics of Autorotation

During powered flight, the rotor drag is overcome with engine power. When the engine fails, or
is deliberately disengaged from the rotor system, some other force must be used to sustain rotor
RPM so controlled flight can be continued to the ground. This force is generated by adjusting the

Collective pitch to allow a controlled descent. Airflow during helicopter descent provides the
energy to overcome blade drag and turn the rotor. When the helicopter is descending in this
manner, it is said to be in a state of autorotation. In effect the pilot gives up altitude at a
controlled rate in return for energy to turn the rotor at an RPM which provides aircraft control.
Stated another way, the helicopter has potential energy by virtue of its altitude. As altitude
decreases, potential energy is converted to kinetic energy and stored in the turning rotor. The
pilot uses this kinetic energy to cushion the touchdown when near the ground.

Most autorotation’s are performed with forward airspeed. For simplicity, the following
aerodynamic explanation is based on a vertical autorotative descent (no forward airspeed) in still
air. Under these conditions, the forces that cause the blades to turn are similar for all blades
regardless of their position in the plane of rotation. Dissymmetry of lift resulting from helicopter
airspeed is therefore not a factor, but will be discussed later. During vertical autorotation, the
rotor disk is divided into three regions:

• The driven region, also called the propeller region, is nearest to the blade tips and normally

Consists of about 30 percent of the radius. The total aerodynamic force in this region is

Inclined slightly behind the rotating axis. This results in a drag force which tends to slow the

Rotation for the blade.

• The driving region or autorotative region, normally lies between about 25 to 70 percent of

The blade radius. Total aerodynamic force in this region is inclined slightly forward of the

Axis of rotation. This inclination supplies thrust which tends to accelerate the rotation of the
Blade.

• The stall region includes the inboard 25 percent of the blade radius. It operates above the

stall angle of attack and causes drag which tends to slow the rotation of the blade.

The following figure shows three blade sections that illustrate force vectors in the driven region
"A", a region of equilibrium "B" and the driving region "C".

The force vectors are different in each region, because the rotational relative wind is slower near
the blade root and increases continually toward the blade tip. When the inflow up through the
rotor combines with rotational relative wind, it produces different combinations of aerodynamic
force at every point along the blade. In the driven region, the total aerodynamic force acts behind
the axis of rotation, resulting in an overall dragging force. This area produces lift but it also
opposes rotation and continually tends to decelerate the blade. The size of this region varies with
blade pitch setting, rate of descent, and rotor RPM. When the pilot takes action to change
autorotative RPM, blade pitch, or rate of descent, he is in effect changing the size of the driven
region in relation to the other regions. Between the driven region and the driving region is a point
of equilibrium. At this point on the blade, total aerodynamic force is aligned with the axis of
rotation. Lift and drag are produced, but the total effect produces neither acceleration nor
deceleration of the rotor RPM. Point "D" is also an area of equilibrium in regard to thrust and
drag. Area "C" is the driving region of the blade and produces the forces needed to turn the
blades during autorotation. Total aerodynamic force in the driving region is inclined forward of
the axis of rotation and produces a continual acceleration force. Driving region size varies with
blade pitch setting, rate of descent and rotor RPM. The pilot controls the size of this region in
relation to the driven and stall regions in order to adjust autorotative RPM. For example, if the
collective pitch stick is raised, the pitch angle will increase in all regions. This causes the point
of equilibrium "B" to move toward the blade tip, decreasing the size of the driven region. The
entire driving region also moves toward the blade tip. The stall region becomes larger and the
total blade drag is increased, causing RPM decrease. A constant rotor RPM is achieved by
adjusting the collective pitch control so blade acceleration forces from the driving region are
balanced with the deceleration forces from the driven and stall regions. Aerodynamics of
autorotation in forward flight Autorotative force in forward flight is produced in exactly the same
manner as when the helicopter is descending vertically in still air. However, because forward
speed changes the inflow of air up through the rotor disk, the driving region and stall region
move toward the retreating side of the disk where angle of attack is larger:

Because of lower angles of attack on the advancing side blade, more of that blade falls into the

driven region. On the retreating side blade, more of the blade is in the stall region, and a small

section near the root experiences a reversed flow. The size of the driven region on the retreating

side is reduced. Autorotations may be divided into three distinct phases; the entry, the steady
state descent, and the deceleration and touchdown. Each of these phases is aerodynamically
different than the others. The following discussion describes forces pertinent to each phase.

Entry into autorotation is performed following loss of engine power. Immediate indications of

power loss are rotor RPM decay and an out-of-trim condition. Rate of RPM decay is most rapid

when the helicopter is at high collective pitch settings. In most helicopters it takes only seconds
for the RPM decay to reach a minimum safe range. Pilots must react quickly and initiate a
reduction in collective pitch that will prevent excessive RPM decay. A cyclic flare will help
prevent excessive decay if the failure occurs at thigh speed. This technique varies with the model
helicopter. Pilots should consult and follow the appropriate aircraft Operator's Manual.

The following figure shows the airflow and force vectors for a blade in powered flight at high
speed:

Note that the lift and drag vectors are large and the total aerodynamic force is inclined well to the

rear of the axis of rotation. If the engine stops when the helicopter is in this condition, rotor RPM

decay is rapid. To prevent RPM decay, the pilot must promptly lower the collective pitch control
to reduce drag and incline the total aerodynamic force vector forward so it is near the axis of
rotation. The following figure shows the airflow and force vectors for a helicopter just after
power loss. The collective pitch has been reduced, but the helicopter has not started to descend.
Note that lift and drag are reduced and the total aerodynamic force vector is inclined further
forward than it was in powered flight. As the helicopter begins to descend, the airflow changes.
This causes the total aerodynamic force to incline further forward. It will reach an equilibrium
that maintains a safe operating RPM. The pilot establishes a glide at the proper airspeed which is
50 to 75 knots, depending on the helicopter and its gross weight. Rotor RPM should be stabilized
at autorotative RPM which is normally a few turns higher than normal operating RPM.

The following figure shows the helicopter in a steady state descent. Airflow is now upward
through the rotor disk due the descent. Changed airflow creates a larger angle of attack although
blade pitch angle is the same as it was in the previous picture, before the descent began. Total
aerodynamic force is increased and inclined forward so equilibrium is established. Rate of
descent and RPM are stabilized, and the helicopter is descending at a constant angle. Angle of
descent is normally 17 degrees to 20 degrees, depending on airspeed, density altitude, wind, the
particular helicopter design, and other variables. The following figure illustrates the
aerodynamics of autorotative deceleration: Types of Auto Rotation Motion in Helicopters

Auto rotation in helicopters can occur in various forms. Steady-State Auto Rotation which is
constant descent rate with balanced forces. Transient Auto Rotation which is variable descent
rate as the helicopter adjusts to the loss of engine power. Autorotative Landing which is a
controlled descent and landing using auto rotation. High-Speed Auto Rotation it occurs at higher
forward speeds, requiring precise control inputs to manage descent and rotor speed. Each type
has unique characteristics and applications, from emergency procedures to training scenarios.

Control Mechanisms During Auto Rotation in Helicopters

Control during auto rotation in helicopters is achieved through, collective pitch control, Adjusts
the pitch of all rotor blades simultaneously to control lift and descent rate. Cyclic Pitch Control
varies the pitch of individual blades during rotation to control direction and stability. Rotor RPM
Management is the one which ensures the rotor maintains sufficient speed to generate lift. Pilot
Input Requires precise control inputs to manage descent and landing. Advanced Control Systems
the system use electronic controls to enhance stability and responsiveness during auto rotation.

Advanced control systems, such as autopilots and stability augmentation systems, can assist in
managing auto rotation.

Environmental Effects on Auto Rotation Stability in Helicopters

Environmental factors can impact auto rotation stability in helicopters, wind can cause
turbulence and disrupt lift. Gusty conditions require precise control inputs to maintain stability
Temperature it affects air density, altering lift and descent rate. Hotter temperatures reduce air
density, requiring more careful management of rotor speed. Altitude the higher altitudes reduce
air density, requiring more careful management of rotor speed. This is particularly important for
operations in mountainous regions.Ground Effect it increases lift and reduces drag when the
helicopter is close to the ground, but can cause instability if not managed properly. Terrain, the
type of terrain (e.g., water, forest, urban) can affect the landing site and the pilot's ability to
execute a safe landing. Understanding these effects is crucial for safe and efficient auto rotation
operations.

Conclusion
The analysis of vectors acting on the aerofoil (rotor blade) during hover, translation and auto
rotation provides a comprehensive understanding of the aerodynamic principles governing
helicopter flight. Each phase of flight involves unique forces and their interactions, which are
critical for safe and efficient operation. Hover, balancing forces for Stability during hover, the
helicopter remains stationary relative to the ground, and the rotor system generates lift to
counteract the weight of the aircraft. The key vectors include, Lift which acts upward, balancing
the weight, drag which is the force which opposes the rotation of the rotor blades and is balanced
by engine torque and thrust which provides the necessary upward force to maintain hover. The
equilibrium of these forces ensures stability during hover, making it a fundamental phase for
takeoff, landing, and precise maneuvering. During translation, the helicopter moves horizontally,
transitioning from hover to forward flight. The vectors on the aerofoil change due to the
introduction of horizontal motion, Lift tilts slightly forward due to cyclic pitch control, enabling
forward motion and drag increases with forward speed and must be overcome by the horizontal
component of thrust .Weight continues to act downward, balanced by the vertical component of
lift. The interplay of these vectors allows the helicopter to achieve controlled forward flight,
demonstrating the importance of cyclic pitch control and thrust management. Auto rotation is a
critical emergency maneuver where the rotor blades are driven by the upward flow of air rather
than engine power. The vectors during this phase include:Lift, generated by the upward airflow,
balancing the weight. Drag Reduced due to the lack of engine power but still opposes rotation
and

Inertial Force acts due to the deceleration of the helicopter. The conversion of potential energy
into kinetic energy drives the rotor blades, enabling a controlled descent and safe landing. This
phase highlights the importance of pilot skill and aerodynamic understanding in emergency
situations.

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by rotating at a constant rotor speed

 Aris, R. (1989), Vectors, Tensors, and the basic Equations of Fluid Mechanics, Dover
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