RC Plane Motors Guide: Kv, Power, Sizing and Real Examples

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Choosing the wrong brushless motor is one of the most common and most expensive building mistakes. The result is a plane that barely climbs, a motor that gets hot in seconds, or a propeller spinning so fast it sounds impressive and produces little thrust. Getting it right is not hard once a handful of numbers make sense, and that is what this guide covers.

It explains Kv, how to read a motor spec sheet, how to match a motor to a propeller, cell count and ESC, and how a motor differs from a servo. Then it looks at real motors using the data their makers publish. This is a researched guide based on manufacturer specs and documentation, not on our own bench or flight testing, and thrust and power figures are manufacturer claims. If you are still choosing between hobbies, the RC plane versus drone guide compares them. If you have not picked a first plane yet, the best RC trainer planes and the best RC planes for beginners guides are better places to start.

How Brushless Outrunners Work

A brushless outrunner has a rotating outer bell with magnets and a fixed inner stator with wound coils. The propeller bolts to the bell. The ESC energizes the coils in sequence and the rotor follows, so there are no brushes to wear. An inrunner has the rotor inside the stator, spins faster for its size and is the usual motor inside an EDF fan unit. The EDF jet guide covers those. For an open propeller, an outrunner is the normal choice.

Kv is the motor's speed constant, in rpm per volt with no load. Multiply Kv by battery voltage for the unloaded speed. A 900Kv motor on 3S (nominally 11.1V) spins at about 9,990 rpm with no propeller, and less once it drives one. Low Kv means slower rpm and more torque, which suits larger propellers. High Kv means faster rpm, which suits smaller ones. A small park flyer on a small propeller needs rpm. A warbird on a large propeller needs torque. That is why Kv follows propeller size and cell count, not the other way around.

Kv does not tell you power. Two motors with the same Kv can differ a lot in how much current and power they can take, which depends on stator size and winding.

Reading a Spec Sheet

The four-digit code

Many outrunners use a four-digit code such as 2814 or 4120. For the makers checked here, the first two digits are the stator diameter in millimeters and the last two are the stator thickness. SunnySky's X2814 table lists a 28 mm stator diameter and 14 mm stator thickness. Cobra's C-4120/12 lists 41.0 mm and 20.0 mm. Not every brand follows the same convention, and some code the outer case instead of the stator, so compare Kv, weight and rated power across brands rather than trusting the code alone.

The numbers that matter

• Maximum continuous current and power. The ceiling for normal flying. Makers often attach a time limit, such as SunnySky's 40A for 30 seconds on the 900Kv X2814.
• Cell count range. SunnySky lists 3 to 4S for the X2814, and T-Motor lists 4 to 6S for the AT3520 KV550.
• Recommended propellers. Treat the largest listed propeller as a limit.
• Recommended ESC. Makers publish this, and it is the safest starting point.
• Weight. It matters most on small airplanes.
• Shaft diameter and mounting. These determine the propeller adapter and mount you need.
• Maker application rating. Some makers state what airframe weight a motor suits. Spektrum does this explicitly, as the examples below show.

Matching Motor, Propeller, Cells and ESC

Start from the airplane. Weigh it complete, with the flight battery. Then find a motor that the maker rates for that weight and flying style, check its recommended propellers and cell count, and pick the ESC the maker recommends.

Examples of maker pairings:

An undersized ESC is one of the most damaging beginner mistakes, so follow the maker's figure, and read the ESC guide for how current ratings work. Use a watt meter between battery and ESC on the bench, with the propeller secured and the plane held, to read peak current. The propeller guide covers sizing, and the LiPo battery guide covers packs. If a plane is already built and weak, the underpowered plane guide and the motor cutting out guide cover diagnosis.

Motor vs Servo

"RC plane servo motor" is a common search because servos contain small motors. A servo is a different device. It is a closed-loop position actuator with a small motor, a gear train and a feedback sensor, and its job is to move a control surface to an angle and hold it. A brushless outrunner spins the propeller, draws large current and is driven by an ESC. They are not interchangeable and are not sized the same way. The servo guide covers servos.

Motors Checked Against Maker Data

Small airplanes: Spektrum Avian parkflyer motors

Search for the Spektrum Avian 2830-950Kv motor on Amazon

Horizon Hobby now sells its fixed-wing motors under the Spektrum Avian name. Two small ones show how a maker pairs power with weight.

Both were listed in stock. They are the kind of motor chosen for a light foam plane or a small balsa model. Horizon ties airframe weight to power class across the range: the Avian 3536-1200Kv is rated for sport airplanes up to 4 lb (1815 g) at 250 to 499 W, and the Avian 4260-800Kv for sport airplanes up to 7.5 lb (3402 g) at 750 to 999 W.

Mid-size sport planes: Spektrum Avian 4250-800Kv and 3542-800Kv

Search for the Spektrum Avian 4250-800Kv motor on Amazon

The Avian 4250-800Kv is rated for sport airplanes up to 6.5 lb (2950 g) and 3D airplanes up to 4 lb (1814 g), on 3-4S, with 500 to 749 W constant power, a 1480 W burst and 925 W maximum continuous. It weighs 7 oz (198 g), has a 5 mm shaft and takes 12x6 to 13x8 electric propellers. The Avian 3542-800Kv is a 14-pole motor with 3.5 mm bullet connectors, 90 mm leads and 16 AWG wire, and Horizon lists it as in stock. Its page notes a prop adapter used on the Timber 10-year model, and that the bolt-on adapter is not included.

The SunnySky X2814

Search for the SunnySky X2814 brushless motor on Amazon

SunnySky describes the X series as CNC-milled with a hollow body for heat dissipation, a stainless steel shaft, aluminum end caps, NMB bearings and hand-wound coils. Per SunnySky, every version has a 28 mm stator, a 4 mm shaft and a weight of 108 g.

SunnySky's recommended propellers range from APC 13x6.5 and 13x8 on the 900Kv down to APC 9x6 and 8x6 on the 1250Kv. Its test table shows the 900Kv with an APC 13x6.5 propeller on 11.1 V drawing 29.5 A for 1750 gf of thrust and 327.45 W. Older guides sometimes list a 5 mm shaft and higher power for this motor, which is why checking the maker's current table matters.

Large sport planes and warbirds: T-Motor AT3520

Search for the T-Motor AT3520 long shaft motor on Amazon

T-Motor sells the AT3520 as a fixed-wing UAV motor in KV550, KV720 and KV850 versions, with a heat-dissipating structure. For the KV550, T-Motor lists:

The store text gives the maximum thrust as 4.3 kg and recommends a T-Motor T13x6.5 propeller and an AT 115A ESC. It suits fixed-wing airframes in the 1.8 to 2.0 kg class, per T-Motor. The KV720 lists 70 A and 1300 W peaks on 4-5S, and the KV850 lists 70 A and 1100 W on 3-4S.

Giant scale: Cobra C-4120/12 and big Avians

Search for the Cobra C-4120/12 brushless motor on Amazon

Cobra's C-4120/12 is an 850Kv outrunner with a 41.0 mm stator diameter and 20.0 mm stator thickness, 12 slots and 14 poles. Cobra lists a maximum continuous current of 75 amps and a maximum continuous power of 830 W on 3 cells, 1110 W on 4 cells and 1390 W on 5 cells. It weighs 293 grams, has a 6.00 mm shaft and uses bolt-on prop adapters sold separately. At the time of research the Innov8tive Designs page showed it as out of stock.

For big airframes, Spektrum offers larger Avians. The Avian 5065-450Kv (BackOrder) takes 18.5 to 22.1 V, 50 A continuous, weighs 14.1 oz (400 g) and has an 8 mm shaft. The Avian 6362-250Kv (in stock) takes 33.3 to 44.4 V, 1500 to 1999 W constant, 45 A continuous, weighs 22.2 oz (634 g) and also has an 8 mm shaft. Both recommend an 80A speed control.

A note on other names. Several motors that appear in older recommendations could not be verified from maker pages for this guide, including A2212 kits, certain D2826 and 4250 motors sold under import brands, and EMAX's GT2218. EMAX USA's current product guide lists a GT2215 family (1100KV and 1180KV) but no GT2218. If a motor has no maker page, treat any spec as unconfirmed.

Calculators

Two tools help with setup. eCalc offers airplane propeller, setup finder and performance calculators, and MotoCalc 8 is software for predicting electric flight performance. Neither replaces a bench test with a watt meter.

Which Motor Fits

For airframe-specific motor and power choices on scale models, the warbird guide covers what pilots typically fit.

FAQ