Electric Motor Specifications

When shopping for RC electric motors, it's important to understand what all those numbers and specs really 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 ARF'S 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/lb is a darn good "estimate" for the power required for an average sports/aerobatic RC airplane.

If you plan to fly high alpha 3D maneuvers you'll 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.