There are 3 basic ways to determine which electric motor you should pick for your airplane. The first is a calculation method developed many years ago by Dr. Keith Shaw. The second uses recommendations from similar airplanes and the third uses a computer program.
Method 1: Calculation Method
This method requires to you to know some basic information about your plane. We will use a rule of thumb that says an airplane will require a certain wattage per pound to operate at a given performance level. There are three performance levels that we'll work with.
- 65 Watts - Trainer or scale type flying requires 65 watts per pound. This will enable leisurely climbs, can loop from a dive, and will have a mild performance level.
- 100 Watts - Sport planes require 100 watts per pound. This will have good power, can loop from level flight, but will lose speed in a vertical climb and fall out.
- 150+ Watts - 3D or High Performance planes require 150+ watts per pound. This is crazy power and the airplane will climb vertically with no limits.
Now that you know the rule of thumb, you can begin to calculate the power requirements of your airplane for the performance level that you desire. You will need to look at the airplanes specifications to get its flying weight and you need to look up the specs of the motors you are considering.
Example: Lets say you have a trainer type plane that weighs 5.5lbs. We need 65 Watts per pound so multiply 5.5 X 65 and you get 357.5. You need a motor that will deliver ~360 watts to fly it.
Watts = Volts X Amps. We need to know the volts and amps that will be used. Look at the motor specs to help find this information. The Axi 2826/12 motor runs on 3 Lipo cells that equal ~11.1 Volts nominal, and pulls about 33Amps at full throttle on the recommended prop. Multiply 11.1 x 33 and you get 366 Watts. That's right in the power class you'd want for that airplane.
Method 2: Copy a similar planes power system
There are many reviews both on-line and in print magazines that you can use to help you choose a motor for your airplane. Simply find a plane that is similar in weight and type to the one you have and copy the power system the reviewer used.
Method 3: Computer Program
You can use a computer program to assist you in selecting a motor for your airplane. ElectricCalc and MotoCalc can give you a reasonable starting point or verify the merit of your chosen system.
We hope these three methods will make choosing your next motor easier and more fun. If all else fails, you can always give us a call on the phone and we'll do our best to help you pick the right motor.