Brushless motors in modelling — a complete guide

What is a brushless motor?

Brushless DC motors (BLDC) are now the standard in RC modelling. Unlike brushed motors, they have no commutator or brushes — commutation is handled electronically by the ESC. This makes them more efficient, quieter, longer-lasting and capable of far more power at the same weight.

Two main types are found in modelling: inrunners (rotor inside) — used mainly in EDF (ducted fan) and fast aircraft, and outrunners (rotor outside) — the most popular, used in the majority of RC models from trainers to aerobats.

The KV parameter — what does it mean?

KV is the number of RPM per volt of supply voltage (unloaded). For example, a 1000 KV motor on a 3S battery (11.1 V) will reach about 11,100 RPM without a propeller. This is a theoretical value — under load (with a propeller) the RPM will be lower.

General rule: lower KV = more torque, larger propeller, slower RPM. Higher KV = smaller propeller, faster RPM, less torque. For a calm trainer pick 800–1100 KV (with a large prop). For a 3D aerobatic model — 300–500 KV with a very large prop. For a fast EDF jet — 3000–4000 KV.

Choosing the right power for your model

The basic power-selection rule is watts per kilogram of take-off weight:

• 80–100 W/kg — gentle flying, trainer, powered glider

• 150–200 W/kg — sport aerobatics

• 250+ W/kg — 3D aerobatics, unlimited power

Example: a 1.5 kg model for sport flying needs 225–300 W. A 28-26 1400 KV motor on 3S will deliver about 280 W — perfect.

Propeller selection

A propeller is described by two parameters: diameter × pitch (e.g. 10×6). 10-inch diameter, 6-inch pitch. Larger diameter = more thrust at lower RPM. Larger pitch = more speed, but the motor needs torque to spare.

For gentle flying choose a larger diameter with smaller pitch (e.g. 11×5.5). For fast flying — smaller diameter, larger pitch (e.g. 8×8). Motor manufacturers usually list recommended prop sizes — this is the best starting point.

Warning: never fit a propeller larger than recommended! Overloading the motor leads to overheating and, in extreme cases, burning out the windings or the ESC.

The ESC

The ESC (Electronic Speed Controller) is the 'brain' of the electric drive. It converts the control signal from the receiver into three-phase AC to power the motor. Key ESC parameters are: maximum continuous current (e.g. 30 A, 40 A, 60 A), supported LiPo cell count (e.g. 2–4S) and commutation frequency.

Selection rule: the ESC should have at least 20% current headroom above the motor's maximum draw. If the motor draws max 35 A, choose a 40–45 A ESC. An undersized ESC will overheat and shut down in flight — meaning a forced landing.

Modern ESCs also offer programmable features: brake (useful for folding-prop models), soft start, various throttle curves and low-voltage cut-off (LVC).

Summary

Choosing the drive system is a crucial step in building an RC model. Motor, ESC, battery and propeller must all work together. Start from the model's weight and desired flying style, select power (W/kg), then look for a motor with the right KV. Choose the prop per the manufacturer's recommendations and verify with an ammeter on a test stand. Good luck!