Electric Motors - The Beginners' RC Planes Guide

Electric powered model aircraft has gained popularity, mainly because the electric motors are more quiet, clean and often easier to start and operate than the combustion motors.

They need batteries to operate and despite some developments in this area; the batteries still are somewhat heavier as energy source compared with the gas fuel.

Thus, the electric flier has to strive to build the model as light as possible in order to obtain a reasonable wing loading and/or a reasonable flight time.

The electric motor's operation is based on the electromagnetic principle. When electric current flows through a coil it creates a magnetic field with a strength proportional to the current's value, the number of windings of the coil and is inversely proportional to the coil's length. The strength of the magnetic field will further increase by introducing a so-called ferromagnetic material inside the coil.


An electromagnetic device only gets magnetic when electric current is applied, whereas a permanent magnet doesn't need electric power to be magnetic.

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.

A 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 corless 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 Io

The following equation can be used to calculate the motor's Output Power: 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 falls.

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 - Vloss)

Thus:

RPM = Kv * (Vin - Vloss)

And since:

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 instead of 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:

Istall = 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 (imperial units): Kt = 1352 / Kv

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 Low 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:

Torque = Kt * (Iin - Io)

For the same Torque:

High Kv - needs more Current

Low Kv - needs more Voltage

The motor's 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 flying. For instance: 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. "Can" 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.

"Rare hearth" motors such as Cobalt and Neodymium are considered to be far superior to ferrite motors, but they are also much more expensive.

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.

Power/Weight : Estimated Performance (Sports Plane) :

Watts / lb

Watts / kg

For wingloading about 60g/sq.dm (20oz/sq.ft) However, the power to weight ratio recommended above is 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. The minimum pitch speed recommended is 2 to 3 times the plane's stall speed. The stall speed of an aircraft (both model and full-scale) is approximately equal to four times the square root of the wing loading. To calculate the aircraft's approximate stall speed click here To calculate the aircraft's approximate level flight speed click here

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 plane's weight.

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 larger propellers. Since the propeller blades also are more efficient at moderate rpm, this combination is often worth- while despite the increased weight.

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 pics on right.

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 speed though.

The flight time of an electric powered model depends on some variables like: 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.

Flight time in minutes = (battery capacity / average current drawn) x 60.

Electric flight models may be built small and lightweight enough to fly inside a sports hall. They are the so-called Indoor Models, having approx. 75cm wingspan (30") with a weight less than 200gr (7oz) and flying no faster than 8-16Km/h (5-10mph).

The so-called Park Fliers are somewhat faster. They are often made of foam material and may fly at speeds anywhere from 25Km/h up to about 40Km/h (16 to 25mph). They are rather sensitive to strong winds, so it's recommended to fly them during calm weather.

Of 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 comm. 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: Graphite, Copper and Silver. Brushes made 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 lubrication. 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 degrades performance. 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 it in.

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 terminals.

Note: many Graupner Speed xxx motors have the first 2 of these capacitors already fitted internally.

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.

For 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 servos. 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 servos.

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 unintentionally.

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 position.

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 organisations that provide insurance for modellers 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. These 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 magnets. The 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 temperature.

They 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: rpm = 2 x frequency x 60/number of poles. Increasing the number of poles will decrease the max rpm but increase the torque.

A brushless 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). High advance timing gives more output power at expense of efficiency. Low advance 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.

A 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 "outrunners" are able to drive larger and more efficient propellers without the need of gearboxes.