Rise of the Multicopter - 2012/12

If you look up the defi nition of a helicopter in a HarperCollins dictionary, it states that a helicopter is
“an aircraft that derives its lift from blades that rotate about an approximately vertical central axis.”
If you view Merriam-Webster’s online dictionary, it defi nes a helicopter as “an aircraft whose lift is
derived from the aerodynamic forces acting on one or more powered rotors turning about substantially
vertical axes.”
Both of these dictionaries also make reference to the origin of the word “helicopter,” which derives
from the French word, “hélicoptère,” which was fi rst used in roughly 1887.
By those defi nitions, a multicopter is a helicopter. Then why is a multicopter so special that it needs
its own name?Multicopters are special and a
breed of their own and don’t rely
on mechanical swashplates, tail
rotors, or coaxial rotors to achieve
controlled fl ight.
Multicopters do not employ all of
the characteristics that Igor Sikorksy
captured in the modern helicopter
design of the 20th century. If you
are thinking that there have been
multirotored aircraft such as the
V-22 Osprey in service for years,
you are correct. However, you
would also be wrong. The V-22 is a
multirotor, but not a multicopter.
The Osprey has its own characteristics and is classifi ed
as a tilt-rotor aircraft. Although tilt rotors are part of the
multicopter history, they have their own set of design
criteria.
Tilt rotors are similar to a mix between an airplane and
a helicopter, using rotors for lift and using fi xed wings and
control surfaces during forward fl ight. There is no dictionary
defi nition for multicopter, so let’s create one.
mul·ti·copter, (noun) ’m l-t -käp-t r’ A heavier-than-air
aircraft that has two or more usually symmetrically placed rotors
and whose control of pitch, roll, yaw, and lift are achieved solely
through the variation of the speed (rpm) of each rotor and whose
 ight stabilization is through a combination of electro/mechanical
sensors and computing devices. Related words: tricopter,
quadcopter, hexacopter, octocopter, pluscopter, xcopter, hcopter—
referring to speci c con gurations of the rotors on a multicopter.Flight Theory
Although the defi nition of the multicopter may appear
straightforward, the physics of how it fl ies can be complex.
It may not seem that way because only a few components
make up a multicopter.
Compared to an airplane or a helicopter, the mechanical
complexity of a modern multicopter is far less. However,
this is where the simplicity ends. There is complex software
embedded in the fl ight controller.
Attaching four or more propellers to some motors and
some sticks does not work. I have tried it and am sure that
many have tried something like that. The reason it does not
work is that without a fl ight controller, there is no balance
or control of the forces that are being generated by each
motor and propeller.
A multicopter at rest has a net
sum of zero forces. Each motor
and propeller combination,
or rotor, can create thrust
and rotational torque. When
combined, these two forces allow
the multicopter to achieve lift,
yaw, and roll.
Lift on the multicopter is
generated by the thrust of each
rotor. If one were to eliminate
gravity, drag, wind, and any other
forces and equally generate thrust
from all rotors, a multicopter
should fl y up (the opposite
direction of the force of the
thrust) and continue straight
up. If gravity is added into the
equation, then the thrust must
overcome the force of gravity
on the aircraft. When thrust and
gravity are equal, you can hover
level.
Directional fl ight of a
multicopter is achieved by
varying the thrust produced by
one or more of the rotors. For a tricopter to fl y forward, for
example, the front rotor spins slower and the rear rotors
spin faster, thus changing the attitude and pitch of the
aircraft. The thrust of all of the rotors would be at an angle
and push it forward.
On a traditional helicopter, the tail rotor is the
counterbalance for the rotational torque generated by the
main rotor. On a coaxial helicopter, the opposite spinning
rotors balance the rotational torque forces.
The sum of the rotational torque generated by all of
the rotors controls the rotation of a multicopter. Rotating
or yawing a multicopter requires changing the rotational
torque of each rotor so there is a positive amount of total
rotational torque in the direction the pilot wants to yaw.
The rpm of the rotors must be changed to allow the
effects of torque to take place. When a multicopter is
hovering level and still, the sum of all of the rotational
torque is zero. If you want to rotate the aircraft left or right,
you need to speed up and slow down each rotor.
Slowing down certain rotors also reduces thrust.
Changing the rpm of a rotor alters the thrust generated by
the rotor. Anyone who tries to quickly spin a multicopter
will notice a drop in altitude until either thrust is added, or
the fl ight controller adjusts the thrust.
To stay in fl ight and yaw at the same time, certain rotors
need to spin fast enough to compensate for the lost thrust
of the rotors that are slowing down. During a typical fl ight,
the aircraft pitches, yaws, and rolls at the same time—
increasing the complexity.
Something is missing: the fl ight control loop. A
multicopter tries to fall out of the air every moment. Why
are muticopters so stable in fl ight? The answer is in the fl ight
controller, which continuously evaluates all of the forces
acting on the multicopter and receives control commands.
The control loop has to make these evaluations and
adjustments faster than the effects of each of the forces.
The speed at which the control can repeat is called the
control loop resolution, and is apparent in the altitude drop
I previously mentioned. When a multicopter yaws, the
compensation needed in thrust happens when the control
loop senses a change in the forces affecting altitude.
If the change is too slow, the multicopter can lose altitude
and crash. If the change is too fast, the muticopters do a
weird oscillation and continuously try to readjust.
Components and Confi guration
Multicopters come in all shapes and sizes. Although there
are confi guration differences, they all have the same basic
components: pilot command and control, motors, propellers,
Paul Michael Gentile, the author’s son, uses a MacBook,
an Xbox controller, and Drone Station so ware to fly the
AR.Drone. Pilots have plenty of choices to control their
aircra .
ESCs, a frame, and a fl ight control module.
Depending on how advanced you want to be,
multicopters may also have video cameras, GPS, compasses,
barometers, sonar sensors, and telemetry. Often, many of
the sensors and secondary options are built into the fl ight
controller, which also houses the electronic gyros and
accelerometers found on most multicopters.
According to our defi nition of a multicopter, they have
two or more symmetrically placed rotors. The amount of
rotors and the fl ight confi guration of the multicopter will
also determine the subclass of multicopter.
For example, a four rotor is a quadcopter and it can fl y
in “+,” “X,” or “H” confi guration. Rotors also can be set in
single or coaxial, upper, and lower confi gurations. A sixbladed
hexacoptor could be in a “Y” confi guration or in a
symmetrical confi guration. Begin adding or subtracting rotors
and you will see that a picture is worth a thousand words.
I have included a diagram of some multicopter rotor
confi gurations. Exact rotor placement and orientation
depend on the fl ight controller being used.
Although most confi gurations limit multicopters from
two to eight rotors, the Distributed Flight Array (DFA)
may change that paradigm. The Swiss Federal Institute of
Technology Zurich is experimenting with the DFA in its
Institute for Dynamic Systems and Control.
According to the institute’s website, the “Distributed
Flight Array is a fl ying platform consisting of multiple
autonomous single propeller vehicles that are able to drive,
dock with their peers, and fl y in a coordinated fashion.”
The DFA could have two, three, four, or 40 or more
rotors. Each rotor can operate independently and, when
needed, the rotors can link up and fl y as a unit. The institute
has been experimenting with the DFA for several years and
has already produced several versions.
Pilot command and control uses the same radio
transmitter confi guration that has been used for
approximately the last 60 years. RC technology has evolved
from one channel to multichannel, and from AM, FM, and
PPM to 2.4 GHz, but the same “sticks in a box” remains.
Multicopters have introduced a host of new options for
pilot control. Multicopters such as the Parrot AR.Drone
allow a pilot to use a smartphone, a tablet, a laptop, or a
video game controller such as those used for the Nintendo
Wii, Sony PlayStation 3, or Microsoft XBOX 360.
Young pilots who have grown up with these devices are
fi nding them more familiar and easier to use. Smartphones,
tablets, and laptops allow pilots to control advanced features
such as photography or video recording, telemetry, and
autopilot functionality. The RC transmitter still reigns king,
though.
Weight is the enemy of any aircraft and multicopters are
no exception. Airframes for multicopters need to be strong
to avoid twisting under the load of all the opposing forces,
and lightweight enough to not impact fl ight characteristics.
Frame options range from the do-it-yourself, made
from off-the-shelf components found in the local home
Diagrams of some common multicopter rotor con gurations. improvement shop, to high-end, carbon-fi ber designs.
Multicopter: Quadcopter “+” Con guration
Multicopter: Quadcopter “X” Con guration
Multicopter: Quadcopter “H” Con guration
Multicopter: Hexacopter
34 Model Aviation DECEMBER 2012 www.ModelAviation.comThere is a range of multicopter frames available for a
reasonable price from your favorite hobby centers such as BP
Hobbies or Atlanta Hobby. During my multicopter control
boards testing, BP Hobbies supplied me with laser-cut wood
frames. To my surprise, the frames survived my lack of skill.
They were lightweight, yet rigid enough in flight and soft
enough for some rough
landings.
Motors, ESCs, batteries,
and propeller choices for
multicopters require skill
and research. Although
most airplanes fly with a
wide range of propeller
choices, multicopters are
more peculiar. Pitch and
diameter are important
in determining thrust. A
propeller that is too big
requires more energy to
spin up or down.
The amount of power
needed to spin the
propellers will impact
flight time and flight
characteristics. LiPo
batteries and ESCs must
be matched to the motor
and propeller selection or
you could risk damaging
components or losing your
multicopter in midair.
Unlike an airplane
that can dead-stick or
a helicopter that can
autorotate, a multicopter
with no power has the
flight characteristics of a
brick. Most multicopters
have enough flight history
that choosing these components is as easy as reading pilot
recommendations.
The most important choice for your multicopter is the
flight controller. Many feature and function choices for doit-
yourself multicopters are available in a wide price range.
Every flight controller has a set of gyros in at least three
axes: X, Y, and Z. At the heart of the flight controller is
a microcontroller or computer chip that receives all the
inputs from the pilot and the onboard sensors, which
determine each rotor’s function.
Let’s review the functions offered by most flight
controllers. Bill at oddCopter.com has done a great job of
defining the key features that are available today, and has
posted a chart online. See “Sources” for his website address.
• Gyro stabilization: The ability to easily keep the copter
stable and level under the pilot’s control. This is a standard
feature of all flight control boards.
• Self-leveling: The ability to let go of the pitch and roll
stick on the transmitter and have the copter stay level.
• Carefree: The pilot can control the copter as if it is
pointing in its original direction, as the orientation of the
copter changes.
• Altitude hold: The
ability to hover a certain
distance from the ground
without needing to
manually adjust the
throttle.
• Position hold: The
ability to hover at a
specific location.
• Return home: The ability
to automatically return to
the point where the copter
initially took off.
• Waypoint navigation:
The ability to set specific
points on a map that the
copter will follow as part
of a flight plan.
Component selection
can be a daunting task.
Begin with a plan,
conduct research, and ask
questions. Many in the
multicopter community
are willing to help.
An octocopter with
GPS and 14-inch
propellers is not for the
living room, and a micro
quadcopter that weighs
2 ounces with no selfleveling
is not a good
outdoor camera platform.
As with any tool, you need the right one for the job.
To read more about the history of the multicopter, visit
www.ModelAviation.com/multicopter.
—Paul Gentile
[email protected]

If you look up the defi nition of a helicopter in a HarperCollins dictionary, it states that a helicopter is
“an aircraft that derives its lift from blades that rotate about an approximately vertical central axis.”
If you view Merriam-Webster’s online dictionary, it defi nes a helicopter as “an aircraft whose lift is
derived from the aerodynamic forces acting on one or more powered rotors turning about substantially
vertical axes.”
Both of these dictionaries also make reference to the origin of the word “helicopter,” which derives
from the French word, “hélicoptère,” which was fi rst used in roughly 1887.
By those defi nitions, a multicopter is a helicopter. Then why is a multicopter so special that it needs
its own name?Multicopters are special and a
breed of their own and don’t rely
on mechanical swashplates, tail
rotors, or coaxial rotors to achieve
controlled fl ight.
Multicopters do not employ all of
the characteristics that Igor Sikorksy
captured in the modern helicopter
design of the 20th century. If you
are thinking that there have been
multirotored aircraft such as the
V-22 Osprey in service for years,
you are correct. However, you
would also be wrong. The V-22 is a
multirotor, but not a multicopter.
The Osprey has its own characteristics and is classifi ed
as a tilt-rotor aircraft. Although tilt rotors are part of the
multicopter history, they have their own set of design
criteria.
Tilt rotors are similar to a mix between an airplane and
a helicopter, using rotors for lift and using fi xed wings and
control surfaces during forward fl ight. There is no dictionary
defi nition for multicopter, so let’s create one.
mul·ti·copter, (noun) ’m l-t -käp-t r’ A heavier-than-air
aircraft that has two or more usually symmetrically placed rotors
and whose control of pitch, roll, yaw, and lift are achieved solely
through the variation of the speed (rpm) of each rotor and whose
 ight stabilization is through a combination of electro/mechanical
sensors and computing devices. Related words: tricopter,
quadcopter, hexacopter, octocopter, pluscopter, xcopter, hcopter—
referring to speci c con gurations of the rotors on a multicopter.Flight Theory
Although the defi nition of the multicopter may appear
straightforward, the physics of how it fl ies can be complex.
It may not seem that way because only a few components
make up a multicopter.
Compared to an airplane or a helicopter, the mechanical
complexity of a modern multicopter is far less. However,
this is where the simplicity ends. There is complex software
embedded in the fl ight controller.
Attaching four or more propellers to some motors and
some sticks does not work. I have tried it and am sure that
many have tried something like that. The reason it does not
work is that without a fl ight controller, there is no balance
or control of the forces that are being generated by each
motor and propeller.
A multicopter at rest has a net
sum of zero forces. Each motor
and propeller combination,
or rotor, can create thrust
and rotational torque. When
combined, these two forces allow
the multicopter to achieve lift,
yaw, and roll.
Lift on the multicopter is
generated by the thrust of each
rotor. If one were to eliminate
gravity, drag, wind, and any other
forces and equally generate thrust
from all rotors, a multicopter
should fl y up (the opposite
direction of the force of the
thrust) and continue straight
up. If gravity is added into the
equation, then the thrust must
overcome the force of gravity
on the aircraft. When thrust and
gravity are equal, you can hover
level.
Directional fl ight of a
multicopter is achieved by
varying the thrust produced by
one or more of the rotors. For a tricopter to fl y forward, for
example, the front rotor spins slower and the rear rotors
spin faster, thus changing the attitude and pitch of the
aircraft. The thrust of all of the rotors would be at an angle
and push it forward.
On a traditional helicopter, the tail rotor is the
counterbalance for the rotational torque generated by the
main rotor. On a coaxial helicopter, the opposite spinning
rotors balance the rotational torque forces.
The sum of the rotational torque generated by all of
the rotors controls the rotation of a multicopter. Rotating
or yawing a multicopter requires changing the rotational
torque of each rotor so there is a positive amount of total
rotational torque in the direction the pilot wants to yaw.
The rpm of the rotors must be changed to allow the
effects of torque to take place. When a multicopter is
hovering level and still, the sum of all of the rotational
torque is zero. If you want to rotate the aircraft left or right,
you need to speed up and slow down each rotor.
Slowing down certain rotors also reduces thrust.
Changing the rpm of a rotor alters the thrust generated by
the rotor. Anyone who tries to quickly spin a multicopter
will notice a drop in altitude until either thrust is added, or
the fl ight controller adjusts the thrust.
To stay in fl ight and yaw at the same time, certain rotors
need to spin fast enough to compensate for the lost thrust
of the rotors that are slowing down. During a typical fl ight,
the aircraft pitches, yaws, and rolls at the same time—
increasing the complexity.
Something is missing: the fl ight control loop. A
multicopter tries to fall out of the air every moment. Why
are muticopters so stable in fl ight? The answer is in the fl ight
controller, which continuously evaluates all of the forces
acting on the multicopter and receives control commands.
The control loop has to make these evaluations and
adjustments faster than the effects of each of the forces.
The speed at which the control can repeat is called the
control loop resolution, and is apparent in the altitude drop
I previously mentioned. When a multicopter yaws, the
compensation needed in thrust happens when the control
loop senses a change in the forces affecting altitude.
If the change is too slow, the multicopter can lose altitude
and crash. If the change is too fast, the muticopters do a
weird oscillation and continuously try to readjust.
Components and Confi guration
Multicopters come in all shapes and sizes. Although there
are confi guration differences, they all have the same basic
components: pilot command and control, motors, propellers,
Paul Michael Gentile, the author’s son, uses a MacBook,
an Xbox controller, and Drone Station so ware to fly the
AR.Drone. Pilots have plenty of choices to control their
aircra .
ESCs, a frame, and a fl ight control module.
Depending on how advanced you want to be,
multicopters may also have video cameras, GPS, compasses,
barometers, sonar sensors, and telemetry. Often, many of
the sensors and secondary options are built into the fl ight
controller, which also houses the electronic gyros and
accelerometers found on most multicopters.
According to our defi nition of a multicopter, they have
two or more symmetrically placed rotors. The amount of
rotors and the fl ight confi guration of the multicopter will
also determine the subclass of multicopter.
For example, a four rotor is a quadcopter and it can fl y
in “+,” “X,” or “H” confi guration. Rotors also can be set in
single or coaxial, upper, and lower confi gurations. A sixbladed
hexacoptor could be in a “Y” confi guration or in a
symmetrical confi guration. Begin adding or subtracting rotors
and you will see that a picture is worth a thousand words.
I have included a diagram of some multicopter rotor
confi gurations. Exact rotor placement and orientation
depend on the fl ight controller being used.
Although most confi gurations limit multicopters from
two to eight rotors, the Distributed Flight Array (DFA)
may change that paradigm. The Swiss Federal Institute of
Technology Zurich is experimenting with the DFA in its
Institute for Dynamic Systems and Control.
According to the institute’s website, the “Distributed
Flight Array is a fl ying platform consisting of multiple
autonomous single propeller vehicles that are able to drive,
dock with their peers, and fl y in a coordinated fashion.”
The DFA could have two, three, four, or 40 or more
rotors. Each rotor can operate independently and, when
needed, the rotors can link up and fl y as a unit. The institute
has been experimenting with the DFA for several years and
has already produced several versions.
Pilot command and control uses the same radio
transmitter confi guration that has been used for
approximately the last 60 years. RC technology has evolved
from one channel to multichannel, and from AM, FM, and
PPM to 2.4 GHz, but the same “sticks in a box” remains.
Multicopters have introduced a host of new options for
pilot control. Multicopters such as the Parrot AR.Drone
allow a pilot to use a smartphone, a tablet, a laptop, or a
video game controller such as those used for the Nintendo
Wii, Sony PlayStation 3, or Microsoft XBOX 360.
Young pilots who have grown up with these devices are
fi nding them more familiar and easier to use. Smartphones,
tablets, and laptops allow pilots to control advanced features
such as photography or video recording, telemetry, and
autopilot functionality. The RC transmitter still reigns king,
though.
Weight is the enemy of any aircraft and multicopters are
no exception. Airframes for multicopters need to be strong
to avoid twisting under the load of all the opposing forces,
and lightweight enough to not impact fl ight characteristics.
Frame options range from the do-it-yourself, made
from off-the-shelf components found in the local home
Diagrams of some common multicopter rotor con gurations. improvement shop, to high-end, carbon-fi ber designs.
Multicopter: Quadcopter “+” Con guration
Multicopter: Quadcopter “X” Con guration
Multicopter: Quadcopter “H” Con guration
Multicopter: Hexacopter
34 Model Aviation DECEMBER 2012 www.ModelAviation.comThere is a range of multicopter frames available for a
reasonable price from your favorite hobby centers such as BP
Hobbies or Atlanta Hobby. During my multicopter control
boards testing, BP Hobbies supplied me with laser-cut wood
frames. To my surprise, the frames survived my lack of skill.
They were lightweight, yet rigid enough in flight and soft
enough for some rough
landings.
Motors, ESCs, batteries,
and propeller choices for
multicopters require skill
and research. Although
most airplanes fly with a
wide range of propeller
choices, multicopters are
more peculiar. Pitch and
diameter are important
in determining thrust. A
propeller that is too big
requires more energy to
spin up or down.
The amount of power
needed to spin the
propellers will impact
flight time and flight
characteristics. LiPo
batteries and ESCs must
be matched to the motor
and propeller selection or
you could risk damaging
components or losing your
multicopter in midair.
Unlike an airplane
that can dead-stick or
a helicopter that can
autorotate, a multicopter
with no power has the
flight characteristics of a
brick. Most multicopters
have enough flight history
that choosing these components is as easy as reading pilot
recommendations.
The most important choice for your multicopter is the
flight controller. Many feature and function choices for doit-
yourself multicopters are available in a wide price range.
Every flight controller has a set of gyros in at least three
axes: X, Y, and Z. At the heart of the flight controller is
a microcontroller or computer chip that receives all the
inputs from the pilot and the onboard sensors, which
determine each rotor’s function.
Let’s review the functions offered by most flight
controllers. Bill at oddCopter.com has done a great job of
defining the key features that are available today, and has
posted a chart online. See “Sources” for his website address.
• Gyro stabilization: The ability to easily keep the copter
stable and level under the pilot’s control. This is a standard
feature of all flight control boards.
• Self-leveling: The ability to let go of the pitch and roll
stick on the transmitter and have the copter stay level.
• Carefree: The pilot can control the copter as if it is
pointing in its original direction, as the orientation of the
copter changes.
• Altitude hold: The
ability to hover a certain
distance from the ground
without needing to
manually adjust the
throttle.
• Position hold: The
ability to hover at a
specific location.
• Return home: The ability
to automatically return to
the point where the copter
initially took off.
• Waypoint navigation:
The ability to set specific
points on a map that the
copter will follow as part
of a flight plan.
Component selection
can be a daunting task.
Begin with a plan,
conduct research, and ask
questions. Many in the
multicopter community
are willing to help.
An octocopter with
GPS and 14-inch
propellers is not for the
living room, and a micro
quadcopter that weighs
2 ounces with no selfleveling
is not a good
outdoor camera platform.
As with any tool, you need the right one for the job.
To read more about the history of the multicopter, visit
www.ModelAviation.com/multicopter.
—Paul Gentile
[email protected]