When designing a power plant for your aircraft there are many RC electric motors from which to choose – but only two types.
The two types of RC electric motors are brushed and brushless. Brushless motors are further separated into the inrunning and outrunning varieties.
Which Motor Type is Best?
The choice of which type of motor, and the specific type of brushless motor, will depend on several factors. These factors include:
- Propeller size and pitch
- Duration (flight time)
- Type of flying (i.e.: aggressive or efficient/gliding)
- Battery characteristics
- Mounting location on airframe
RC Electric Motors.
This post will provide an explanation of the differences between RC electric motors, how motors are classified, and a recommendation on where to purchase your equipment.
Brushed RC Electric Motors
A brushed electric motor contains two small permanent magnets (called the stator), usually three brushes (made of metal or carbon) and electromagnets (called the rotor) that ultimately causes rotation of the shaft in a stationary magnetic field.
Brushed electric motors have several qualities which reduce their usefulness in RC electric aircraft, including:
- Brushes that wear out with use
- Increased electrical noise and possible sparking due to brushes contacting and disconnecting from the spinning armature (rotor)
- Balance issues associated with a large spinning mass
- Frictional loses due to brush contact
Due to the above limitations and the development of faster, cheaper computers and speed controllers, brushless motors replaced brushed motors in many RC aircraft designs.
Brushless Outrunner L: Bell with shaft, R: Stator with electromagnets.
Brushless RC Electric Motors
The development of brushless electric motors represented an improvement over the brushed type.
A brushless motor is an inverted brushed motor, with the permanent magnets located on the rotor and the electromagnets moved to the stator.
As the rotor rotates, a computer controller energizes and de-energizes the electromagnets. There are many advantages to this type of setup, which include:
- Increased precision and efficiency due to computer control instead of mechanical (brush) control; most notable at low loads
- Less electrical noise due to lack of mechanical contact between rotor and stator
- Conduction cooling of the electromagnets to the case reduces heat build up
- Increased space on stator for more electromagnets results in option for increased control precision
- Speed limiting and holding torque capabilities available
Inrunners versus Outrunners
An RC electric brushless motor can be designed as either an inrunner or an outrunner.
An inrunner brushless motor has a rotor that is contained wholly within the motor case or can and looks, at least from the outside, like a typical brushed motor.
Inrunners typically spin at high RPM, often 10000 RPM or more per volt. Often times inrunners require a gearbox to step down the RPMs to a useful rate. Inrunners are best suited for smaller propellers, and have high RPM and low torque.
Outrunners on the other hand have a bell with the magnets glued inside. This bell is mounted to the shaft and spins around the electromagnets on the stator. The bell acts as a flywheel and allows the outrunner brushless motor to produce a high torque but lower RPM as compared to an inrunner brushless motor.
Outrunners are best used with larger propellers on larger aircraft.
Also, outrunner motors require more space for mounting. An inrunner motor, having no spinning bell or moving parts outside the case, can be mounted in tighter locations. Depending on your setup, this can be a deciding factor in your choice of motor type.
Brushless Motor Classification Labels L: Outrunner; R: Inrunner.
How Motors are Classified
RC electric motors are classified by a few parameters. Usually the manufacturer will stamp the motor parameters on the outside of the case.
Let’s look at a typical label to determine what each of the numbers mean in classifying the electric motor.
In the image to the left, you can see several numbers stamped on the motor casing. The four digit number (3530 on the outrunner and 2815 on the inrunner) is a combination of the diameter and length of the motor casing, respectively. The first two digits represent the motor casing diameter in millimeters. The second two digits represent the motor casing length in millimeters.
Another important dimension to know is the shaft diameter. In this case, the outrunner motor has a shaft diameter of 5 mm, but you could only tell that from looking at the specifications sheet online. It’s important to know the shaft size so that you can buy the proper size propeller attachment.
The 1700 kV description means that for every volt of battery potential, the shaft will spin 1700 RPM. This measurement is made under no-load conditions, so adding a propeller to the motor shaft will reduce the true RPM per volt readings.
Determining which motor to use with your FPV setup is an inexact science. You could use a calculator to test various equipment configurations, but it’s probably best to start out with an RC electric motor/ESC/propeller/battery combination that has been tested by many users and known to work OK.
Where to Buy Your RC Electric Motors?
Although much information may be found on message boards and scattered around the web, there’s a better place to get ideas on what motor to select and where to purchase it.
Lee at Crash Test Hobby designs and sells EPP flying wings and planes. He has split up his portfolio of aircraft into large and small types and produced a recommended parts list for both categories of aircraft.
I highly recommend checking out his build tutorials and recommended parts list for ideas on what motor may work for your project or plane.