Electric Power for Your R/C Aircraft - Part I: Electric Motors

Inrunners Vs Outrunner Brushless Motors

Brushless motors for r/c are available in 2 flavors: Inrunners & Outrunners. Inrunners have been out for a longer period of time and resemble the shape of brushed motors; they also function in a similar way with a rotating inner-shaft and generally require a gearbox to increase their torque in order to swing propellers. Outrunners on the other hand are quite a bit different than inrunner motors because the outer can of the motor is what spins and creates power while the inner shaft of the motor stays stationary. Because of their design outrunners offer more torque than inrunner motors and do not require a gearbox (however a gearbox can be used to add even more torque).

Today, outrunners are the dominant form of brushless motor for r/c aircraft as they allow for easier direct prop bolt-on operations and are also marketed by some manufactures as direct replacements for nitro/gas engines. Inrunners remain popular for the competitive modelers and are still regarded as more powerful then outrunners as they offer quicker acceleration (due to less rotating mass) and greater efficiency than outrunners despite having a gearbox. Inrunners are preferred in aerobatic and 3d type flying where rapid changes in throttle (power) are needed, while outrunners are used best in sport aircraft or applications throttle changes are less frequent.

To keep things easier to remember here is a small bulleted list comparison:

• Inrunners require a gearbox to operate (unless being used in specialized aircraft setups like ducted fans). Knowledge about Gear Ratios is needed to determine the right setup for your aircraft and gearbox maintenance is also required.
• Outrunners generally don’t require a gear box and can have propellers bolted on directly to the motor.
• Inrunners are generally used be experienced modelers and used in aerobatic & 3d aircraft as well as electric ducted fan jets.
• Outrunners can be used in a variety of aircraft from trainers to jets and have become the dominant form of brushless motor (however they don’t offer the same performance as inrunners).

I hope this article helped clear up some things about electric motors and electric powered R/C aircraft in general. Check back for part 2 where I will go over motor KV ratings and how to select your power source for your aircraft.