Electronics and Propulsion System in RC Flying - Part 8 - Propulsion

8. Propulsion
  • There are two main propulsion systems used by R/C models today:
  1. The Internal Combustion engines  (glow engines)
  2. The electric motors.
  • IC engines- has so far a higher energy/weight ratio than the batteries used to power the electric planes.
  • However, the combustion engines are usually more noisy and more prone to oil spillage and difficult to maintain than the electric motors.
  • There are two types of glow engines: The four-stroke and the two-stroke.
IC Engine
Ducted Fan IC Engine
Pulsejet Engine
Gas Turbine Engine
EDF (Electric Duct Fan)
Electric Motors
  • Motor is the power plant of your plane. The motor will turn your propeller at a high speed to propel it through the air.


  • 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 IC Engines. 
  • There are two main different motor types used in model aircraft: brushed, and brushless.
 Brushless Outrunner Motor (BLDC motor)
Brushed Motor
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 commutator, which is connected to a DC voltage through two brushes.(so it is called brushed motor).
  • 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).
  • Brushless motors are more common, more efficient, have faster speeds, have more torque, but a little bit more difficult to control.
  • Brushed motors run on DC power, and brushless motors run on AC power(ESC converts the DC power of the battery to AC).
  • The Brushless motors' speed are mentioned as KV-- RPMs per volt.
  • 1500kv motor will spin at 1500 rpm when supplied 1 volt, 11000 rpm when supplied by 11.1 volts, and so on.
Brushless motors are of two types :
  1. Outrunner Motors - as the name suggests outer case rotate with shaft - most of Rc-Aircrafts uses outrunner motor.
  2. Inrunner motors - as the name suggests only the shaft rotates - mostly used in duct fans (EDF systems)
Inrunner BLDC motor
Outrunner BLDC motor
Brushless DC Motors(BLDC).
  • These motors are little more expensive than normal brushed DC motors 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  brushless motor has coils while the rotor consists normally of permanent magnets.
  • The stator of a conventional (inrunner) 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.
  • To operate brushless motors an electronic speed controller specially designed for the brushless motors are required, which converts the battery's DC voltage into three pulsed voltage lines that are 120 deg 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 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.
  • The other type of brushless motor is the so-called "outrunner“.
  • These motors have the rotor "outside" as part of a rotating outer case while the stator is located inside the rotor.
  • This arrangement gives 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.
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.
  • 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.
Manufacturer's data sheet 
Turnigy D3536/6 1250KV Brushless Outrunner Motor
Battery: 2~4 Cell /7.4~14.8V,
RPM: 1250kv,
Max current: 34A , No load current: 3A
Max power: 500W
Internal resistance: 0.034 ohm,
Weight: 102g (including connectors)
Diameter of shaft: 5mm
Dimensions: 35x36m
Prop size: 7.4V/10x5,    14.8V/9x4.7
Max thrust: 1270g

MOTOR POWER CHOICE - Based on recommended AUW (All Up Weight) or Flying Weight of model choice
  • Vintage types and many non-aerobatic indoor flyers - 50w~70w per 1lb (1 lb = 455 gm)
  • Trainers, gliders and high wing scale - 70w~100w per 1lb
  • Sport flyer with general aerobatic performance - 100w per 1lb
  • War birds - 120w~150w per 1lb
  • Multi engine models - 100w per 1lb (thrust from Multiple props gives in effect, more than 100w per 1lb performance)
  • EDF Jets - 150w~200w per 1lb
  • 3D, F3A and high performance Models -  150w~200w per 1lb 
  • Above mentioned recommendations are given by experienced people in this field.
  • You can take deviations from this but the performance of your model will vary accordingly.
MOTOR CHOICE - KV or RPM per volt - Which actually means and what prop size.
  • If you are used to IC engines, the simple method is to treat low kv motors as 4 stroke engine equivalents and mid-high kv motors as 2 stroke engine equivalents.
  • If you are not used to IC then I can give you some examples of the approach to take, this is an important choice because you can define how your model flies, however, their are practical considerations, the most important is ground clearance, flight speed of your model, weight of motor,current consumption, etc..
  • Please refer to motors such as the NTM and Turnigy range, which give you prop data as well as power, dimension and weight data.
Example 1: Trainer/Sport Model, 1lb AUW, we want 100w motor,  (3s 20c Lipoy), mid kv for general flying, probably around 1200kv~1400kv - so around 8" prop
Example 2:  3D/F3A Model, 1lb AUW, we want 150w motor (3s 20c~30c Lipoy), low kv, 1000kv or under -  so around 10~11" prop. (highly efficient at low throttle openings giving lot's of prop wash over control surfaces at all times, high thrust for low rpm and low amps draw at higher throttle openings.)
Example 3: War bird/scale Model, 1lb AUW, 120w motor,  kv choice,  either of the above, it is personal choice
Example 4: High Speed Delta type model, 1lb AUW, 200w motor,  (3s 25c~30c Lipoly) ,  2200kv~3200kv motor - 5"~6" Prop. (high speed/low torque, low thrust at low throttle openings, high speed from high rpm at full throttle.)
How to select proper motor for your model aircraft.
  1. Go through all the above explanations about different motors carefully.
  2. Once you decided about the model you are going to construct, do an approximate estimation of AUW of your model (weight of each and every part is known or you weigh it), as per the model selection you know the speed at which you are going to fly (such as slow flying for Trainer or fast flying for aerobatic/jet models).
  3. Now you select the required power and KV rating of motor.
  4. Decide upon the type of motor "outrunner or inrunner", this depend upon how you are going to mount the motor on your model.
  5. Go through different manufactures data sheet and do a comparison study for quality and price.
  6. Carefully check for the accessories supplied ( such as mounting plates, screws, propeller adapter and bullet connectors, etc..). If the accessories are not supplied with the motor you have to purchase it separately, this will add up to actual cost. 
  7. Check for the availability and shipping, then do the purchasing.