Brushless Motor Basics

Brushless Motor Basics

Written by Greg Gimlick - Selecting the Correct Motor

This guide aims to equip park pilots with the essential knowledge needed for choosing or replacing a motor in their aircraft. While finding an exact replacement can be straightforward, the challenge arises when the specific motor is unavailable. This is where a basic understanding becomes invaluable. Longtime electric fliers often overwhelm newcomers with technical details like torque constants and winding types, but such in-depth knowledge isn’t necessary for most pilots. What’s crucial is knowing how much power you need. Resources like the chart from Common Sense RC offer a user-friendly starting point.

Understanding Power Requirements

For demonstration, let’s assume your aircraft weighs 3 pounds and you wish to achieve vigorous aerobatic performance. According to the chart, you'll require approximately 100 watts per pound. Therefore, for a 3-pound model, you would need about 300 watts of power (3 pounds x 100 watts per pound).

Calculating Power with Battery Specifications

Next, you want to fly around 6 minutes using a 3S LiPo battery. The equation for power is straightforward: Power (watts) = Current (amps) x Voltage (volts). A 3S pack typically provides 11.1 volts under load, so to achieve 300 watts, you’d calculate the required current as follows: 300 watts / 11.1 volts = approximately 27 amps. Typically, average current for a flight is about 66% of full throttle; hence, your expected draw would be approximately 18 amps.

Selecting the Appropriate Battery

When determining battery requirements, remember to not fully discharge your battery for safety. It’s advisable to use only 80% of the capacity, resulting in a calculation of 48 minutes (60 mAh x 80%). This will lead to a C-rate of 8C. To find your needed LiPo pack, divide the average current draw (18 amps) by the discharge rate (8C), yielding a requirement for a 2250 mAh LiPo that can adequately support a 6-minute flight.

For a motor, the Cobra motor line offers a suitable option, with a specific model capable of delivering 450 watts on a 3S pack. This elegant overcapacity ensures durability, with performance remaining well within safe limits.

Understanding Kv Ratings

Kv ratings, which indicate the motor's rpm per volt under no load, play a role in your selection but aren't the sole factor. For instance, a 1,390 Kv rating indicates you’d expect around 15,429 rpm from an unloaded motor powered by a 3S pack.

Key Considerations When Choosing a Motor

When selecting a motor for your RC project, various parameters must be taken into account. These include physical characteristics, operating and stall currents, and wattage. Always refer to manufacturer recommendations for optimal performance.

For more information, consider checking out Wing Flying.

A Comprehensive Overview of Electric Motor Selection

When upgrading your RC's motor or building from scratch, it's essential to understand the brushless motor's efficiency and operating characteristics, especially in comparing it to brushed motors, which may be a common inclination for those newer to the hobby. The efficiency brought by brushless motors can lead to excellent performance, although they do come at a higher cost.

To dive deeper into best practices, refer to our guide on brushless motor testing.

If you are curious about how to measure rpm of a dc motor, our expert consultation is just a click away!

Conclusion

Understanding effective motor selection combines various elements beyond just the motor type. This guide simplifies the initial approach, and we encourage further research and inquiries to refine your expertise in the field of RC motors.