RC Electric Motors for Airplanes: Outrunners, kV, Sizing and Setup
Almost all RC planes now use brushless electric motors, mostly outrunners whose outer case spins, which give high torque to turn propellers directly. Motors are described by size codes (such as 2212 or 3536, the stator diameter and height in mm) and kV (rpm per volt). Choosing one means matching power to the plane’s weight (watts per kilogram), then picking kV, propeller and battery so the current stays within the motor and ESC ratings. Inrunners with gearboxes or ducted fans cover special cases like gliders and jets.
By Kyle HilsonUpdated: October 20263 min read
Motor types
| Type | Character | Typical use |
|---|---|---|
| Brushless outrunner | High torque, direct drive | Most planes |
| Brushless inrunner | High rpm; often with gearbox | Gliders, EDF jets, speed planes |
| Brushed/coreless | Light, cheap | Micro and toy planes |
How much power
A common guide uses input watts per kilogram of flying weight: about 100–150 W/kg for gentle trainers and gliders, 150–250 W/kg for sport planes and warbirds, and 250–400+ W/kg for 3D and aerobatic planes. A 1 kg sport plane therefore needs roughly 150–250 W. Full sizing steps are in how to size an electric motor for an RC airplane.
kV, prop and battery
kV tells you how fast a motor spins per volt without load. Low-kV motors turn large props slowly for thrust (trainers, 3D, scale); high-kV motors spin small props fast for speed. Pick the battery cell count first, then a motor kV and prop the maker recommends for that voltage. Measure current with a watt meter: it must stay below the motor’s and ESC’s ratings. Motor fundamentals are in understanding RC electric motors.
Choosing the ESC
- Current rating above the measured maximum, with 20–30% margin.
- Supports your cell count.
- BEC strong enough for your servos.
- Brake function for folding props on gliders.
Mounting and cooling
- Mount the motor on a firm firewall with the right thrust angles (down and side thrust as the plane needs).
- Use threadlock on mounting screws.
- Make sure the spinning case of an outrunner can’t touch wires or cowls.
- Provide airflow in through the cowl and out the back.
- Balance the propeller.
Testing
- Full-throttle current with a watt meter, prop installed, plane held securely.
- Check motor, ESC and battery temperatures after the first flights.
- Listen for vibration; balance the prop if needed.
- Re-check after changing props or batteries.
Electric vs glow
Electric motors are quiet, clean, start instantly and need little maintenance, which is why they replaced glow engines for most new planes. Battery cost and flight time are the trade-offs; large planes may still use gas engines for long flights.
A worked example
Say you have a 1.2 kg sport plane and want lively but not wild performance: about 200 W/kg means roughly 240 W. On a 3S battery (about 11.1 V nominal), that’s around 22 A. A motor rated for at least 300 W and 30 A, with a kV that turns a 10–11 inch prop at that voltage, would suit. Check the maker’s prop chart, then confirm with a watt meter and a test flight. This approach works for any size: weight, target W/kg, voltage, then motor, prop and ESC.
Record each setup’s current and temperatures for future reference.
FAQ
What does 2212 mean on a motor?
Stator about 22 mm diameter and 12 mm tall.
How many watts do I need?
About 150–250 W per kg for sport planes.
Outrunner or inrunner?
Outrunner for most planes; inrunner with gearbox for gliders and EDFs.
How do I know my prop is too big?
Current above ratings and a hot motor or ESC.
More on this topic: the complete RC Airplanes guide covers the basics behind this article.
About 20 years in RC. Mostly boats in the NAVIGA Mini Eco Expert class, with many self-made parts; also a King Motor Baja 5B, a home-built electric plane, home-built nitro planes he has flown, and an RC helicopter. More about SwellRC
