Finding the brushless motor that is the best choice for your plane or helicopter can seem to be a daunting task due to the large number that are available. There are a few important considerations you should keep in mind when choosing. This article will help you identify these issues so you can spend more time flying and less time trying to find that “perfect” motor.
Ultimately, you want to swing a certain size prop at a certain RPM. In fact, the freedom you have in choosing propeller size and operating RPM can lead to huge performance gains over comparable glow motors used in many remote control airplanes. Prop and RPM selection determines how much power you need, it is important to choose a motor that is almost at its limits when running at that power level. A motor that is too small will overheat and ruin itself, a motor that is too large will be a detriment to performance, due to the added weight.
Translating propellor size and RPM into power requires some help. This help can come from a computer-based prop simulator, such as the Slough RC Model Club Prop Power, Thrust and Efficiency Calculations web site by Rod Badcock. You can also find data posted by someone who has done what you are trying to do; find out what prop they used, what RPM it spun, and how much power was used.
Your list of potential motors should now only contain motors that can comfortably (but not ‘in their sleep’) put out the power you need. Now you’ll have to make decisions on the other things: battery voltage and capacity, direct drive or geared, outrunner or inrunner, and kV.
The easiest choice is whether to use direct drive or a gear box, so make that one first. If you want to turn high RPM (greater than 10,000 RPM) you’ll probably want a direct drive inrunner. For lower RPM, you can run an outrunner in direct drive or an inrunner through a gearbox. The outrunner is simpler and quieter, but the inrunner in a gearbox can be more adjustable and slightly more efficient. In some cases, the outrunner can be quite a bit cheaper. Each has its advantages, so consider them both.
At this point you know what RPM your motor needs to turn. It is either the same as you want the prop to turn(direct drive), or at a ratio faster than the prop when using a gearbox. Motor RPM is going to determine your specific motor and battery choice, by the following approximate formula (assuming lithium polymer batteries).
Motor RPM = 0.8 x 3.5V x Series Cell Count x Motor kV Rating
You need to select the right motor and battery combination that will satisfy the motor RPM formula. You can do it with a low kV motor and a high series cell count battery, or vice versa.
Lithium polymer battery packs, such as the ones made by Thunder Power, are ideal for use with brushless motors in radio controlled airplanes and helicopters due to their low weight and high capacity compared to NiMH and NiCd packs. Along with a brushless motor and battery, you will also need a brushless speed control (ESC) with an amp rating equal to or greater than the peak current drawn by your motor.
Make your choice, order the parts, put them together, and test it with an Eagle Tree watt meter. You want to make sure that you are near the RPM and power levels you were aiming for. Remember, though, the most important test is how it performs in the air. Fly it, and fine tune with prop selection. Hopefully this article has brought you close enough that a motor or battery change isn’t required.
In the event you require an upgrade, Draganfly Innovations Inc. is your best choice for Hacker Brushless Controllers and Motors, Castle Creations Brushless Controllers, APC Landing Products electric RC airplane propellers, Eagle Tree flight data e-loggers and Thunder Power RC Lipo Battery Packs.
|» Next||Choosing the Right Electronic Speed Control (ESC) For Your Electric RC Airplane|
|« Previous||A Guide to Charging Lithium Polymer Batteries|