Electric RC outrunner motors come in a variety of sizes, picking the right one for your plane can be tricky. One must take factors like type of plane, weight, battery type and desired performance into account. What's right for a 3D plane is not going to be a good choice for a trainer of the same size. There are some simple formulas that can help narrow your choices.
- Skill level:
Select an outrunner that matches the performance of the equivalent nitro-powered engine. This is a great method to use when you are converting a nitro-powered plane to electric. Nitro-powered planes use the planes ready-to-fly weight to determine the engine size, this method can be used for outrunner motors as well.
Select the wattage of the outrunner based on the type of plane it will be used in. The simple rule of thumb is based on watts per pound. A trainer flies well with 65 to 80 watts per pound. A sport plane should have at least 100 watts per pound; 3D and aerobatic planes need 150 watts or more per pound. An outrunner motor's watts can usually be found on the motor's data sheet or online at the manufacturer's website.
Select an outrunner motor based on how long a flight you want. Motor and prop combinations affect the amount of amperage that is drawn. Select a battery that supplies at least two times the amperage of the motor and prop combination draw. This should give you about eight minutes of flight time. A higher amperage battery will give you longer flight times.
Tips and warnings
- Post a question at an RC forum site, chances are there are other RC pilots that have the same plane and have recommendations based on their experiences.
- There are web-based and standalone applications that can help you make this decision, the best known is MotorCalc.
- To calculate battery amperage multiply the milliamps by the discharge rate. For example, a 3000mAh, 30C (discharge rate) battery generates 90 amperes (3000mAh X 30C = 90Ah).
- LiPo batteries can be damaged and catch fire if the motor ampere draw far exceeds the batteries capacity.
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