How a brushless motor works
Brushless motors are the standard in RC models. Unlike brushed motors they have no mechanical commutator. Instead, an electronic speed controller (ESC) switches current through the stator windings in the correct sequence, creating a rotating magnetic field that drives a rotor with permanent magnets.
The lack of brushes means less friction, higher efficiency (75% to 90%), longer lifespan and less noise. Brushless motors come in two variants: inrunner (internal rotor) and outrunner (external rotor). The outrunner is the most common type in flying models because it produces high torque at low RPM, allowing direct propeller drive without a gearbox.
What the KV rating means
Motor KV is the number of revolutions per minute per volt applied to the motor with no load. For example, a 1000 KV motor powered by a 3S LiPo pack (11.1 V nominal) will reach approximately 11,100 RPM with no propeller.
Lower KV means slower rotation but higher torque. These motors suit larger propellers and slow-flying models (trainers, gliders, scale). Higher KV means faster rotation with less torque. High KV motors (above 2000 KV) are used with small propellers on fast models and racing drones.
When choosing a motor, do not focus on KV alone. KV combined with the supply voltage and propeller diameter determines the rotational speed, and that speed must fall within the optimal range for the chosen propeller.
Stator size and model weight
Brushless motor size is described by stator dimensions in the format XXYY, where XX is the stator diameter in millimeters and YY is the stator height. For example, a 2212 motor has a stator 22 mm in diameter and 12 mm tall.
A larger stator means more copper in the windings and stronger magnets, which translates to higher power and torque but also a heavier motor. Approximate stator size to model weight matching:
- 18xx to 22xx: models weighing 200 g to 800 g (mini gliders, small trainers),
- 28xx to 32xx: models weighing 800 g to 2000 g (1.2 to 1.6 m trainers, sport),
- 35xx to 42xx: models weighing 2000 g to 5000 g (1.8 m and larger trainers, aerobatic),
- 50xx and above: models over 5 kg (large scale, glider tugs).
Propeller matching
A propeller is described by two parameters: diameter and pitch. For example, a 10x6 prop has a 10-inch diameter and a 6-inch pitch. Diameter mainly affects static thrust, while pitch affects cruise speed.
A larger propeller (greater diameter) requires a lower KV motor and generates more thrust at lower RPM. It is better for slow-flying models. A smaller propeller with a high pitch suits fast models.
Every motor manufacturer specifies a recommended propeller range. Exceeding this range (too large a propeller) overloads the motor, draws excessive current, causes overheating and risks damage. A propeller that is too small wastes the motor's potential and produces insufficient thrust.
Power and model weight
A common rule of thumb for power sizing is 50 to 100 W per pound (0.45 kg) of takeoff weight. 50 W/lb is enough for gentle trainer flight. 75 W/lb gives a comfortable margin for ground takeoff and steady climbing. 100 W/lb and above allows aerobatic flight and vertical climbing.
The motor's electrical power is calculated as the product of voltage and current: P = U x I. If the motor draws 25 A from a 3S pack (11.1 V), the power is approximately 277 W. For a model weighing 1.5 kg (3.3 lb) that gives about 84 W/lb, which is a good sport setup.
ESC selection, BEC and current rating
The ESC (Electronic Speed Controller) converts the DC current from the LiPo pack into three-phase AC to drive the brushless motor. The most important ESC parameter is the maximum continuous current, which must exceed the motor's peak current draw by at least 20%.
Many ESCs include a built-in BEC (Battery Eliminator Circuit) that steps down the drive pack voltage (for example 11.1 V) to 5 V or 6 V to power the receiver and servos. A linear BEC is simpler but wastes energy as heat. A switching BEC (UBEC, Universal BEC) is more efficient and handles larger voltage differences better (4S packs and above).
When choosing an ESC, check: maximum continuous current, supported LiPo cell count (for example 2S to 4S), BEC type and its current capacity (how many servos it can power), and whether the ESC supports a brake and timer programming.
Common power system mistakes
- Oversized propeller: the motor draws current that exceeds the ESC limit, the controller overheats and shuts down in flight. Always check current draw with a watt meter before the first flight.
- Undersized ESC: a controller with too low a current limit will cut out under load. Apply the 20% headroom rule above the motor's maximum draw.
- Ignoring cooling: brushless motors and ESCs require airflow. In models with a closed fuselage, plan ventilation openings.
- Voltage mismatch: a motor matched to a 3S pack is not suitable for 4S without changing the propeller. Higher voltage increases RPM and current draw, which may exceed the limits of both the motor and the ESC.
- Not checking rotation direction: a brushless motor can spin either way. Before the first flight, make sure the propeller generates thrust in the correct direction. Swapping any two of the motor's three wires reverses the rotation.