How to Size an Electric Motor for an RC Airplane
To size an electric motor for an RC plane, start from the flying weight and the kind of flying you want, turn that into a power target in watts, then pick a motor, propeller, battery and ESC that deliver it without overheating. This guide walks through each step with worked examples.
By Kyle HilsonUpdated: October 20269 min read
Step 1: set a power target
The most reliable starting point is power per unit of flying weight. Weigh the plane ready to fly, including battery, or use the weight the plans or manufacturer give. Then choose a target from the kind of flying you want. These are widely used rules of thumb, not physics constants, but they get you close:
| Type of flying | Watts per kg | Watts per lb | Thrust : weight |
|---|---|---|---|
| Gliders, slow flyers, scale cruising | 110–150 | 50–70 | about 0.5 : 1 |
| Trainers, sport flying | 150–220 | 70–100 | 0.7–1 : 1 |
| Aerobatics, warbirds | 220–330 | 100–150 | 1–1.3 : 1 |
| 3D, hovering, unlimited vertical | 330 and more | 150 and more | 1.5–2 : 1 |
A 1.2 kg trainer therefore wants roughly 180–260 W at full throttle. Heavier planes need more absolute power but not necessarily more per kilogram; draggy shapes such as biplanes and big radial cowls need the upper end of each range. If you are new to planes, the RC airplanes guide explains why a moderate power level makes learning easier.
Step 2: read motor specs
Brushless outrunners are described by a few numbers. Understanding them saves a lot of trial and error.
- Size code (e.g. 2826): usually the stator diameter and height in millimetres, sometimes the case size. Bigger stators handle more power.
- kV: rpm per volt without load. A 1,000 kV motor on 11.1 V spins about 11,000 rpm unloaded. Low kV swings big props; high kV suits small props and ducted fans.
- Max continuous current and power: what the motor handles without overheating. Stay below it at full throttle.
- Recommended cells and props: the manufacturer’s table is the best starting point you will get.
- Weight: heavier motors of the same type usually handle more power; check that the plane can carry it without becoming nose-heavy.
kV is not a measure of power. Two motors with the same kV can differ hugely in how much power they handle; that depends on size, build and cooling. For more on motors, ESCs and batteries in general, see RC basics.
Step 3: match prop, battery and ESC
Power is voltage times current. The battery sets the voltage; the propeller decides how much current the motor pulls at that voltage. So the motor, prop and battery have to be chosen together.
Battery
Pick the cell count the motor is designed for. Under load a LiPo sits around 3.5–3.7 V per cell, so a 3S pack delivers about 10.5–11 V in flight. Divide the power target by that voltage to get the expected current: 220 W on 3S is about 20 A. The pack must deliver that comfortably: its capacity in Ah times its C rating should be well above the full-throttle current.
Propeller
Props are given as diameter × pitch in inches, for example 10 × 6. More diameter gives more thrust at lower speed, good for trainers and 3D. More pitch gives more speed, good for sport and racing planes. Always check ground clearance and that the prop is within the motor’s recommended range.
ESC
Choose an ESC rated at least 20–30 % above the full-throttle current, for the same cell count. Check whether its BEC can power your servos; larger planes with strong servos often need a separate BEC or receiver pack.
Worked examples
| High-wing trainer | Sport aerobatic | Electric glider | |
|---|---|---|---|
| Flying weight | 1.2 kg | 1.8 kg | 0.9 kg |
| Target | ~180 W/kg → ~220 W | ~260 W/kg → ~470 W | ~130 W/kg → ~120 W |
| Battery | 3S 2200 mAh | 4S 3300 mAh | 3S 1300 mAh |
| Expected current | ~20 A | ~32 A | ~11 A |
| Motor | ~900–1,100 kV, 28 mm class | ~700–900 kV, 35 mm class | ~1,000–1,300 kV, 28 mm class |
| Prop to start with | 10 × 6 | 12 × 6 | 10 × 6 folding |
| ESC | 30 A | 40–50 A | 20 A |
These combinations are typical starting points, not guaranteed results: the real current depends on the exact motor and prop. That is why the next step matters.
Step 4: measure and adjust
A plug-in watt meter between battery and ESC is the most useful tool for this job. Hold the plane securely or restrain it, stand behind the propeller, run full throttle for a few seconds and read the current and watts.
- If the current is above the motor’s or ESC’s rating, go down a prop size (diameter or pitch).
- If you are well below your power target, go up a prop size and measure again.
- After a normal flight, feel the motor, ESC and battery. Warm is fine; too hot to hold means the setup is overloaded.
Safety: a full-throttle test turns the prop into a blade. Restrain the plane, keep hands and bystanders out of the prop arc, and remove the prop when you only need to check directions or settings.
Common mistakes
- Choosing by kV alone. kV says nothing about power handling.
- Too big a prop “for more thrust”. It overloads motor, ESC and battery together.
- Ignoring the CG. A heavier motor and battery move the balance point forward; re-balance before the first flight.
- Undersized ESC or BEC. An ESC at its limit overheats; a weak BEC can brown out the receiver under heavy servo load.
- No airflow. Motors and ESCs inside cowls need cooling air in and out.
FAQ
How many watts do I need for my RC plane?
Multiply the flying weight in kg by about 150–220 W for a trainer or sport plane, 220–330 W for aerobatics and over 330 W for 3D. In pounds, that is roughly 70–100, 100–150 and 150+ W per lb.
What does motor size like 2212 or 3536 mean?
Usually the stator diameter and height in millimetres (22 × 12 mm, 35 × 36 mm). Some brands use the case dimensions instead, so compare power ratings, not just the numbers.
Higher or lower kV for my plane?
Lower kV for large propellers and slow, heavy or 3D planes; higher kV for small props, fast sport planes and ducted fans. Use the cell count and prop range the motor maker recommends.
What size motor for a large 12 lb (5.4 kg) plane?
At sport power levels that is roughly 800–1,200 W, which usually means a 6S–8S setup with a 50 mm class motor, a matching 80–100 A ESC and a separate BEC. Check the motor maker’s prop table and measure with a watt meter.
More on this topic: choosing a first plane, building and learning to fly are covered in the complete RC airplanes guide.
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
