Both electromagnets and permanent
magnets have the so-called poles at either end. One is
called N (north) and the other S (south). When two magnets
get close together the N and the S poles attract, whereas the
same poles (N N or S S) will repel each other. The
electric motor functions according to the same principle.
There are two main different motor types used in model aircraft: The
brushed and the brushless.
brushed motor consists mainly of a cylindrical metal case
containing a stator and a rotor.
The rotor is part of the motor shaft, which rotates inside the
stator. The rotor has several coils (poles) that may
either have an iron core or are coreless.
The stator consists usually of two permanent magnets mounted
close to the metal case.
The rotor coils receive electric current via a so-called
commutator, which is connected to a DC voltage through two
brushes (hence the name). The commutator changes the
voltage polarity to the coils at a certain instant once every
turn of the motor shaft, thereby keeping the motor running.
The motor shaft is supported by two bearings, which may be of
plastic, porous brass bushes or ball bearings (more expensive).
The coreless motor has the rotor coils not wrapped around an iron core
but just fastened into shape with glue, which makes the rotor much
lighter and faster to accelerate and thus suitable for servos.
Since the coreless don't have iron core they have much less iron losses,
which make them more efficient than cored motors. However, the
coreless motors will not stand continuous high RPM and/or loads without
falling apart. That's why they are generally rather small, with
low speed and low power. As flight power motors the coreless are
only used with small indoor planes.
A DC motor converts the electric current into torque and the voltage
into rotations per minute (RPM). The power consumption of a
DC motor (Input Power) is equal to its terminal voltage times the
current. However, every motor has losses, which means that the
motor consumes more power than it delivers at its shaft.
The motor's Output Power is equal to the Input Power minus the Power
Loss. Most of Power Loss is equal to the sum of the Copper Loss
plus Iron Loss.
Copper Loss = Coil's Resistance
Rm * Current Iin2
Iron Loss = Vin * Idle Current
The following equation can be used to calculate the motor's Output
Pout = (Vin -
Iin * Rm) * (Iin - Io)
The motor's Efficiency (h) is the ratio of the Output Power to
the Input Power.
% h = 100 *
Pout / Pin
Assuming the same current, increasing the voltage increases the motor
efficiency until the RPM limit is reached above which the efficiency
A further parameter is the motor's Kv, which refers to the ratio
of the RPM to the Voltage at the motor's terminals minus the Voltage
loss inside the motor due to the coil's resistance Rm.
Kv = RPM / (Vin -
RPM = Kv
* (Vin - Vloss)
Iin * Rm
The RPM will
decrease as the current Iin (load) increases.
For instance, a motor with a Kv of 1000, a coil resistance Rm
of .04 ohms and with a terminal voltage of 8 volts at 12 amps will have
the following RPM,
1000 * (8 -
12 * .04) = 7520 RPM
8000 RPM (if the motor coils had no resistance Rm) that means a
loss of 480 RPM from the ideal in this case.
RPM Loss =
Kv * (Iin * Rm)
If a larger propeller is used the current will increase, thereby further
decreasing the motor's RPM. So, in high current applications a low
resistance Rm is needed in order to prevent too much loss of RPM.
In reality the coil's resistance Rm increases as the temperature
increases, which means that the RPM will decrease over time even if the
input voltage is constant.
If the motor shaft is held so that it cannot move at all, it is in
stalled condition. In such a condition the motor will draw the
maximum current possible from the battery and will most likely be
destroyed. The current drawn in a stalled condition is calculated
according to Ohm's Law.
Vin / Rm
Another parameter is the motor Torque as a function of Current.
It is called Kt and is expressed in inch-ounces per ampere
Kt = 1352 /
The amount of Torque per ampere depends on the motor's Kv.
The higher the Kv, the lower the Torque per ampere.
High Kv = Low Torque per ampere
Kv = High Torque per ampere
Like the actual RPM is less than the ideal due to the resistance Rm,
the actual torque is also less than the ideal due to the idle (no load)
current Io. The actual Torque is calculated as follows,
Kt * (Iin - Io)
For the same Torque,
High Kv - needs more Current
Kv - needs more Voltage
Kv is much dependent on the coils’ number of turns. A
high number of turns gives a low Kv and vice-versa. So one
may ask, which Kv is the best?
The answer is; it depends on the sort of plane and on the type of
For the same power, a lower Kv allows the use of a larger
diameter prop, giving higher thrust at the expense of top speed, whereas
a higher Kv requires a smaller prop, spinning it at higher RPM
resulting in a higher top speed but in lower thrust.
So, if you intend to hover, have fast climb, good acceleration, are able
to use a larger diameter prop and the top speed is not of concern, the
low Kv is preferable.
Increasing the current increases the RPM Loss, but decreases the Torque
Loss. Motor's maximum efficiency occurs when the RPM loss equals
the Torque loss.
Every motor type has an ideal voltage, current and RPM at which the
motor's max efficiency is obtained. These values are often shown
in the manufacturer's data sheets. Brushed motors' efficiencies
are normally between 30 and 80% depending on the type and price.
Most motors supplied in kits for beginners have the stator made of low
cost ferrite magnetic material. They are called ferrite or "can" motors.
motors are rather inefficient and cannot be opened and serviced
like other higher quality motors. However they are cheap
and most kits will fly just fine with these motors, so it's ok
to use a "can" motor for your first plane.
hearth" motors such as Cobalt and Neodymium are considered to be
far superior to ferrite motors, but they are also much more
Unlike ferrite magnets, the "rare earth" magnets withstand high
temperatures without losing their magnetic properties.
Electric motors have several designations such as 280, 300, 400, 480 and
600, which refer to the case length and also give an idea of their power
and weight. For example a 480 motor has about 48mm case length, is
heavier and is able to deliver more power than a 280 motor.
Generally a 280 motor is suitable to power models up to 400gr and a 480
motor may be suitable to models up to 800gr, while a 600 motor may power
models up to 1200gr, assuming direct drive (without gearbox reduction).
As a rule of thumb, the input power for a sports plane (no EDF) should
be about 110 W/kg (50 W/lb) in order to get reasonable flying
characteristics. Gliders and parkflyers may need much less power,
65 W/kg (30 W/lb), while the scale and aerobatics may need much more
power, e.g. > 200 W/kg (90 W/lb). This assuming that the motor has
about 75% efficiency.
However, the power to weight ratio by itself not enough to guarantee the
plane's performance in flight, as other factors have to be taken into
account, such as the pitch speed of the propeller, which refers to
propeller's rpm times the pitch. Note that the static rpm is lower
than when the model is flying.
pitch speed recommended is 2 to 3 times the plane's stall speed.
speed of an aircraft (both model and full-scale) is approximately equal
to four times the square root of the wing loading.
Another factor is the Static Thrust, which refers to how much the
aircraft is pulled or pushed forward by the power system when the
aircraft is stationary. The Static Thrust should be at least about
1/3 of the aircraft's weight. However, in order to be able to
hover with 3-D models, the Static Thrust should be greater than the
Note that the Static Thrust alone is not enough to predict how the
aircraft will fly, as other factors like the prop pitch speed should
also be considered.
Measuring and comparing the propellers' Static Thrust may be misleading,
as the blades of a given prop may stall, resulting in a low static
thrust on the test bench, while it may give excellent performance in
flight and even outperform others that have a better Static Thrust.
Output Power = Thrust * Pitch Speed
So, with a given power, the more thrust you have, the less top speed you
get. In other words, assuming the same power: Larger
diameter & less pitch = more thrust, less top speed (like low gear on a
car). Smaller diameter & more pitch = less thrust, more speed
(like high gear on a car).
The prop diameter-to-pitch ratio for sport models should be between 2:1
and 1:1 In case the pitch is too high, the prop becomes inefficient at
low forward speed and high rpm, as when during the take-off and/or
climbing. At the other end of the scale, a propeller designed for
greatest efficiency at take-off and climbing will accelerate the model
very quickly from standstill but will lose efficiency rapidly as the
forward speed increases.
Gearboxes are often used to reduce the motor's rpm at the
propeller shaft, increasing their torque and allowing the use of
Since the propeller blades also are more efficient at moderate
rpm, this combination is often worth-while despite the increased
Indoors and slow flier models have often a gearbox which allows
the use of relatively smaller and lighter motors improving the
slow flight performance and
prolonging the flight time. The drawback is that the top
speed is reduced.
High-speed models such as those powered by Electric Ducted Fans,
(EDF) require high Kv motors that have max
efficiency at high RPM (typical above 25.000).
Some factors have to be taken into account when designing an EDF
propulsion system, such as the intake (inlet) should have about the same
area as the Fan Swept Area FSA, in order to prevent efficiency loss.
Also care should be taken during the design of both the intake (inlet)
and the exhaust (outlet) ducting. In order to reduce efficiency
losses due to turbulence and drag, the duct internal surfaces should be
as smooth and straight as possible.
Circles are the best duct cross sections to minimise surface drag.
The exhaust area is usually about 85 to 95% of FSA for best performance.
Some examples of
ducting are shown in the photos.
For a given power, the EDF propulsion system has often lower
thrust/weight ratio compared with a conventional propeller system, and
some EDFs need to be hand-launched or bungee-launched since they can't
take-off the ground. Once in the air the EDF may reach rather high
The flight time of an electric powered model depends on some variables
Aircraft's flight characteristics (based on wing loading and lift), the
combination motor/propeller, the motor's efficiency (Pout/Pin)
and last but not the least, the batteries energy/weight ratio.
Time in minutes = (battery capacity / average current drawn) x
course, it's also quite possible to build much bigger electric
powered aircraft models.
As the motor rpm increases it requires the rotor coils to be energised
sooner so that they get the full magnetic field strength in time to
react with the stator's magnetic field. Also when the load
increases, the magnetic field in the rotor coils increases, which
interacts with the stator's magnetic field, producing a rotated
resultant magnetic field.
Some motors allow the brushes' angle to be changed by the same amount as
the field rotation, thereby increasing the motor's efficiency under a
given load. That's called for motor "timing".
An electric motor may be timed under load by slowly changing the brush
holder's angle while measuring the current. The ideal brush angle
is when the motor draws less current. There is no fixed ideal
timing angle, since the best timing angle changes as the motor load and
speed changes. If the motor has been timed at clockwise rotation
it has to be re-timed in case the rotation needs to be reversed.
The motor's direction of rotation may be reversed by inverting the
voltage polarity at the supply terminals. A timed motor gets
higher idle current (with no load).
Brushed motors need some maintenance, since both the brushes and the
commutator will wear after a while due to the friction. Most
quality motors allow brush replacement. The commutator itself also
needs cleaning as it gathers deposits of carbon and gunk due to the
graphite powder from the brushes.
It may be cleaned by a very light polishing action with scotchbrite or
with a so-called commutator stick. The gunk can also be cleaned
off while the motor is running manually, using a few drops of alcohol.
If commutator is pitted or shows brush skipping and chattering means
that it has been overheated and got deformed (out of round). It needs to
be repaired, as polishing will not cure the deformation.
Brushes are usually made of three different compounds :
of silver are normally used in competitive racing as they have low
resistance, but they produce the highest commutator wear and also have
medium brush wear and lubrication. Silver brushes produce sludge that
only can be removed by lathing the commutator.
Copper brushes don't produce sludge and work best at high rpm. These
brushes produce medium commutator wear and have high brush wear and low
Graphite brushes produce low commutator wear, have low brush wear and
high lubrication but have high resistance, which means that they are not
suitable for racing.
Usually it's necessary to "break-in" a new brushed motor so that the
flat brushes get a curved surface and thus increasing the contact area
with the commutator. Running a motor with new flat brushes at full
load will cause a lot of arcing, which pits the contact surfaces and
The "break-in" may be done by running the motor without load (without
prop), at about 1/2 its rated voltage for about a hour or two. The
brushes should get a curved surface without sparks/arcing.
Some high-quality motors do not need to be "broken-in". This will be
mentioned in the respective motor's manual. In case of doubt, just break
Sparks that occur between the brushes and the commutator can cause radio
interference. In order to prevent radio interference it is
recommended the use of ceramic capacitors soldered between each motor
terminal and the motor case. For extra security against
interference, a third capacitor should also be fitted between the motor
Note : Some
high quality motors may have the first 2 of these capacitors already
A common way to control the electric motor's speed is by using an
Electronic Speed Controller (ESC).
The Electronic Speed Controller is based on Pulse Width Modulation (PWM),
which means that the motor's rpm is regulated by varying the pulses'
duty-cycle according to the transmitter's throttle position.
example, with the throttle at the minimum position, there
will be no pulses, while moving the throttle to the middle will
produce 50% duty-cycle. With the throttle at the max
position the motor will get a continuous DC voltage.
Most ESCs have a facility known as Battery Eliminator Circuit (BEC).
These controllers include a 5V regulator to supply the receiver and
servos from the same battery that is used to power the motor, thereby
eliminating the weight of a second battery only to power the radio and
The motor power is cut-off when the battery voltage falls, for example
below 5V. This prevents the battery from getting totally flat
allowing the pilot to control the model when the motor stops. Some
controllers also include a brake function that prevents the propeller
from keeping spinning when the motor power is cut-off.
Electronic Speed Controllers are available in different sizes and
weights, which depends on their max output current capabilities.
Another important characteristic of an ESC is the on-resistance of the
output power switching transistor(s).
The on-resistance should be as low as possible, since its value is
proportional to the power loss dissipated by the output transistor(s),
P = R x I2
The on-resistance is normally between approx. 0.012 and 0.0010ohm. The
value depends on how many output parallel-connected transistors the actual ESC
has. The higher the current capability the lower the on-resistance should
be. These figures are normally shown on the ESC data sheet along with the
BEC voltage cut-off value and the max. output current to the receiver and
As a safety measure many ESCs have a function that won't allow the motor to
start running unless the throttle is initially set in the minimum position.
Another safety device is the so-called arming switch connected between the motor
and the controller. The arming switch should be off until the plane is
ready to taxi out on the runway or be hand-launched. After the flight, the
arming switch should be turned off as soon as possible. This will prevent
the motor from start running in case the throttle stick is moved forward
In order to keep the arming switch contacts in good shape (lowest resistance)
it's advisable to never switch it on/off under power. This means that the arming
switch should be only turned on/off when the throttle is in the minimum
The more powerful the motor, the more need for the safety of an arming switch.
A reasonable approach is using an arming switch on flight models larger than
speed 400 size (approximately 100 watts and above).
Large batteries are capable of delivering very high currents when shorted or
when the propeller gets blocked. Such high currents are enough to overheat
and melt components/wiring, which may lead to a fire.
Some organizations that provide insurance for modelers require a fuse in
electrically powered models.
To choose the correct rating for the fuse just put the largest and
highest-pitch prop that you expect to fly with. Measure the
current draw of your power system on the bench and multiply the value by
about 1.25. This 25% margin should prevent nuisance blows. Find
the fuse with a rating at or just above this current level.
Another type of electric motors for
model aircraft are the so-called brushless.
motors are rather expensive, but they have higher efficiency.
Typically between 80 to 90%. Since they have no brushes,
there is less friction and virtually no parts to wear, apart
from the bearings.
Unlike the DC brushed motor, the stator of the AC brushless
motor has coils and the rotor consists normally of permanent
stator of a conventional brushless motor is part of its outer
case, while the rotor rotates inside it. The metal case
acts as a heat-sink, radiating the heat generated by the stator
coils, thereby keeping the permanent magnets at lower
are 3-phase AC synchronous motors.
Three alternated voltages are applied to the stator's coils
sequentially (by phase shift) creating a rotating magnetic field
which is followed by the rotor.
It's required an electronic speed controller specially designed for the
brushless motors, which converts the battery's DC voltage into three
pulsed voltage lines that are 120o out of phase.
The brushless motor's max rpm is dependent on the 3-phase's frequency and on the
number of poles :
= 2 x frequency x 60/number of poles
number of poles will decrease the max rpm but increase the torque.
motor's direction of rotation can be reversed by just swapping two of
the three phases.
Earlier speed controllers needed an additional set of smaller wires
connected to the motors' internal sensors in order to determine the
rotor position to generate the right phase sequence. New
controllers read the so-called "back EMF" from each phase, which allows
the motor to be controlled without the need of the extra wires and
sensors. These new controllers are called "sensorless" and can be
used to control motors with or without internal sensors.
At less than full throttle the 3-phase pulses are chopped at a fixed
frequency with a duty-cycle depending on the throttle position. At full
throttle the phase pulses are no longer chopped giving the max rpm and
torque. The ESC's 3-phase actual output frequency and thus the
motor's rpm depend on motor's Kv (rpm / volt), the actual load and the
voltage applied, as the ESC needs the EMF positioning pulses back from
the motor before it sends the output pulses.
Many brushless ESC allow the user to set the Electronic Advance Timing.
High advance timing (hard timing) is suitable for high pole count motors
(above 6 poles, such as Jeti, Mega, Plettenberg).
timing gives more output power at expense of efficiency.
timing (soft timing) is suitable for low pole count motors. It gives
higher efficiency with some loss of output power and is recommended when
long run-time is the primary goal.
recent type of brushless motor is the so-called "outrunner".
These motors have the rotor "outside" as part of a rotating
outer case and the stator is located inside the rotor.
This arrangement generates much higher torque than the
conventional brushless motors, which means that the "outrunner"
are able to drive larger and more efficient propellers without
the need of gearboxes.