Brushed vs. brushless, outrunner vs. inrunner, KV rating explained — and how to size a motor for your plane without the guesswork.
Most RC planes use brushless outrunner motors. KV is RPM per volt — lower KV suits larger props and more thrust; higher KV suits smaller props and higher speed. To size a motor: use 75–100W per pound for sport planes and 50–75W/lb for trainers, then find a motor with a published thrust table that delivers that wattage on your battery voltage. Match ESC amperage to the motor's peak draw plus 20–30% headroom.
Brushed motors transfer power to the rotating windings through physical carbon contacts — the brushes. They are mechanically simple, inexpensive, and require only a basic speed controller. The trade-off: brushes wear and require periodic replacement, RPM is limited by friction and heat, and efficiency is significantly lower than brushless designs. In current RC planes, brushed motors appear only in toy-grade aircraft at the very low end of the price range. They are not worth considering for any build intended to last.
Brushless motors have no physical contacts. The stator (fixed windings) creates a rotating magnetic field; the rotor (with permanent magnets) follows it. The ESC switches current through the stator phases in sequence to maintain rotation — electronically, not mechanically. The result is higher efficiency, longer lifespan, higher achievable RPM, and significantly better power-to-weight ratio. All serious RC plane builds use brushless motors.
In an outrunner, the outer bell (case) rotates around the stator. The prop mounts to the spinning bell directly — no gearbox needed. Outrunners produce high torque at moderate RPM, making them the natural choice for direct-drive prop applications. Almost all RC plane motors sold today are outrunners. The rotating can is an advantage for prop mounting and a minor disadvantage in dirt or dust environments where the exposed bearing can accumulate contamination faster.
In an inrunner, the inner shaft rotates inside a fixed outer case — like a conventional motor. Inrunners spin faster (higher natural RPM) and were common in earlier electric designs that used a gearbox to step down RPM for the prop. For most current fixed-wing builds, outrunners are simpler and equally capable. Inrunners remain relevant in speed-specific applications or designs that already incorporate a drive system requiring high-RPM input.
KV is the motor's RPM per volt under no load. A 1000KV motor on 11.1V (3S) theoretically spins at 11,100 RPM before a prop creates load. Real RPM under load drops substantially — the useful figure is what the motor delivers at your operating voltage with the specified prop attached, which is why motor manufacturers publish thrust tables.
KV guides prop selection:
A 1000KV motor is not better or worse than a 2000KV motor. Both can produce the same wattage at different prop and voltage combinations. KV describes the motor's operating point, not its capability.
The starting point is watts per pound of all-up weight (battery, motor, all electronics, airframe):
| Plane Type | W/lb | Characteristic |
|---|---|---|
| Trainer / glider | 50–75 | Docile, efficient cruise, easy landing |
| Sport / scale | 75–100 | Good climb, moderate speed, responsive |
| Aerobatic | 100–150 | Vertical capability, snap roll authority |
| 3D / unlimited | 150+ | Hover, prop hang, high torque demand |
Multiply your target weight by the W/lb figure to get the target power output. Then find a motor whose published thrust table delivers that wattage at your battery voltage with an appropriate prop. Most reputable motor manufacturers (T-Motor, Sunnysky, Emax, Cobra) publish full tables — use them, not KV alone.
A sport plane at 48oz (3 lbs) all-up weight at 90W/lb = 270W target. A 1200KV motor on 3S pulling 270W at full throttle with an 8×4.5" prop meets the spec. The published table confirms the combination is within the motor's rated current.
Brushless motors are sized by stator dimensions, written as a four-digit number: the first two digits are stator diameter in mm, the last two are stator height (stack length). A 2212 motor has a 22mm diameter, 12mm tall stator. Larger stator = more copper = more power capacity. Height adds torque; diameter adds RPM capability at a given KV.
Common size ranges for fixed-wing planes:
Do not run a motor above its rated current. Motor ratings include continuous and burst current — both are thermal limits. Exceeding them demagnetises the rotor magnets permanently. Always verify with the motor's published specification sheet, not just the product listing title.
Outrunners mount via the front face (can-forward, bell-forward mounting). The prop adapter attaches to the spinning bell — use the correct adapter thread for your prop size. Tighten the prop nut to the motor manufacturer's specification; a loose prop on an outrunner causes vibration that works loose further under centrifugal load.
Motor direction: connect all three phase wires from motor to ESC in any order. If the motor spins the wrong direction at startup, swap any two phase wires. The order of the remaining wire determines rotation direction; swapping two corrects it without re-wiring the whole bundle.
See RC ESC guide for matching ESC amperage to motor peak draw.
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