RC Electric Motors for your RC Airplane

Choosing RC Electric Motors for your
radio control airplane isn't as hard as you think!

A few short years ago in this hobby, electric RC airplanes were looked upon as mere toys. There were plenty of parkflyers tinker around with, but the "real" RC airplanes were those slime covered glow powered models. Light weight energy dense Lipo batteries along with powerful efficient brushless motors can now run neck-to-neck with their glow powered cousins when it comes to power and longevity.

Choosing an RC electric motor for your RC airplane isn't quite as straight forward as ordering the recommended size glow engine. All the different brands, sizes, and specifications can make choosing a motor quite intimidating if you're new to the hobby. Understanding the basic fundamentals outlined here is essential for choosing a suitable motor for your airplane. Like everything in life, the more experience you have tinkering with different setups, the more confident you will be when choosing RC electric motors for different types and sizes of RC airplanes...

If you're completely new to the hobby, now's a good time to get familiar with the basic components of electric powered RC airplanes. It is also a good idea to have a basic understanding of the difference of brushed vs. brushless motors. You also need to have a basic understand of lipo battery packs.

If you're looking to get into electric flight without having to worry about sizing the components, there are many ready to fly electric airplanes on the market. ParkZone is known for their high quality ready to fly models. Hobbico makes an excellent electric powered ready-to-fly trainer . I personally recommend going with an RTF for your first electric airplane as it gives you hands on experience with the various components of an electric power system.

This page goes over the basic fundamentals and thought process of choosing an RC electric motor for a specific airplane.

Motor Specifications

When shopping for RC electric motors, you will need to understand what the following specifications mean.

Voltage Constant

Kv is the "Voltage Constant". This is how many RPM's the motor turns for each volt applied. Actually, it's a tad less because motors aren't 100% efficient.

For example, if you apply 12volts to a 200Kv motor, it will turn at just under (12 x 200) 2400 RPM's.

Torque Constant

Kt is the "Torque Constant". For all motors, Kt= 1355/Kv. You'll have to ask someone smarter than me where the 1355 comes from, cause I don't have a clue! Just accept it for what it is.

In a nut shell, this means the faster the motor spins for a given voltage (Kv), the lower the output torque will be (Kt).

Outrunners brushless motors generally have a lower Kv, which in turn produces more torque at a slower speed for spinning those larger props. The opposite is true with inrunners.

No Load Current

Io is the "No Load Current". This is the amount of current it takes to spin the RC electric motor with no prop.

For example, if your motor is pulling 25Amps, and the Io is 2Amps, then you really only have 23Amps turning the prop.

Terminal Resistance

Rm is the "Terminal Resistance". This is the internal resistance of the motor measured in Ohms. The higher the Rm, the less efficient the motor is.

Current and Power

The maximum current and power is what determines how large of a prop and what size plane can be used with the motor. Simply multiply the current by the battery voltage to get power.

The larger the propeller's diameter and pitch, the more current the RC electric motor will draw for a given RPM. For example, the 200Kv motor will run at 2400 RPM's regardless of whether it has a 10x6 prop or a 11x5 prop. But, the 11x5 prop will cause the motor to draw more current. Drawing too much current will destroy the motor.

How much power do you need?

The manual for most ARFS and kits these days make it really easy for you by recommending a specific motor. For marketing purposes, they will obviously recommend the specific brand of RC electric motors that they are affiliated with. If you'd rather use a different brand all you have to do is find another motor with comparable power and KV rating.

If you don't have this reference, or are working on a glow to electric conversion, then you'll need to estimate how much power you will need. It is accepted by most in the hobby that 100 Watts/1b is a good "estimate" for the power required for an average sports/aerobatic RC airplane.

If you plan to fly high alpha 3D maneuvers you would want to bump that up to maybe 150-200 Watts/lb. You may want to lower this number for a high lift trainer type airplane. But 100 watts/lb is a good starting point if you going at it blind.

What ESC to use?

Once you have chosen a suitable RC electric motor for your plane you will need to choose the battery and electronic speed controller. Choosing an ESC is rather straight forward. Remember that an ESC made for brushed motors can't be used with brushless motors, and vice versa.

You need to choose an ESC that has a higher power rating that the maximum current the motor can pull. The ESC you choose also must be rated for the voltage of the battery. More than likely you'll be using Lipo batteries, so you will want to make sure the ESC has a Low Voltage Cutoff.

More Details here for choosing an appropriate ESC

Sizing the Battery

One of the quickest ways to damage a battery is to pull too much current from it. Every battery pack is rated for a maximum amount of current which is determined by the C rating. The C rating multiplied by the capacity of the battery gives you the maximum discharge rate of the battery.

When choosing a battery it is crucial that you choose a battery pack that has a higher maximum discharge rate than the maximum current draw of the motor.

More in depth explanation of C rating vs. Maximum Discharge Rate

Electric RC Motor Manufacturers

Brushless motors have become a lot more affordable since they first came out. Their efficiency and high power to weight ratio are quickly making brushed motored a thing of the past.

The following is a list of some of the more popular or well known manufacturers of brushless motors. By following the links below you will find that many of these companies offer free software and charts to help you choose the correct motor for your RC airplane.

Feel free to ask questions or add comments!

KMS Brushless Motors KMS brushless motors and ESC's are made in Hong Kong China by Kinetic Motor Systems.

The performance and quality of KMS motors are comparable to ...

Emax Brushless Motors Emax Brushless Motors are designed and manufactured by the Chinese company Yinyan Model Tech .

There are currently four different models of Emax ...

Electrifly Brushless Motors Electrifly Brushless Motors are top-of-the line quality motors. Electrifly is a division of Great Planes Great Planes Model Manufacturing, one of the ...

E-Flite Brushless Motors E-Flite has a solid reputation for being on the leading edge of electric powered flight technology. E-flite has a wide selection of electric RC airplanes ...

AXI Outrunner Brushless Motors AXI brushless motors are very efficient high quality outrunner motors manufactured by Models Motors in the Czech Republic. Since all AXI motors are ...