"That definitely isn't good." I had just landed and unplugged my RC airplane, a Flyzone Rapide. As I carried the model back to the pit area and absentmindedly turned its propeller by hand, I heard and felt metal-on-metal clicking…which is not something you should ever have with a brushless motor. There had been no noticeable symptoms of a problem in flight, and the motor was not seized. So I was hopeful that I would be able to find and repair the problem without too much time or money (spoiler: mission accomplished!).
A Little About Brushless Motors
Brushless motors are very simple devices. In high-level terms, a brushless motor breaks down into two coaxial parts, the rotor and stator. The rotor is the spinning part and usually has permanent magnets attached to it. The stator does not move. It typically has copper windings that create magnetic fields when energized. Brushless motors require a dedicated Electronic Speed Control that rapidly toggles the polarity of the magnetic fields in the windings to make the rotor spin.
There are two basic types of brushless motors, inrunners and outrunners. On inrunners, the stator is integrated into the outer housing of the motor. Other than the exposed drive shaft, the rotor is fully enclosed within the stator. Outrunners are nearly the exact opposite. The rotor comprises the outer casing of the motor, while the stator is mostly hidden inside. Parts of the stator are accessible to permit mounting the motor and connecting power wires. Most electric-powered RC airplanes, including the Rapide, use outrunner brushless motors.
The sheer simplicity of brushless motors makes them practically maintenance-free. You just need to check on the ball bearings from time to time. They also tend to be very reliable. There just isn't much that can go wrong. The most common problems I see stem from people pushing their motors too hard and burning out the windings or overheating the permanent magnets…at which point, they're no longer permanent.
Although it was a hot day, I had not worked the Rapide's motor very hard on any flights. So I did not suspect an overheating issue. I pondered the short list of other potential problems with my motor and decided that I would not attempt any field surgery. This was a problem best solved on the workbench.
Back in my shop, I set about removing the motor from the airframe. The spinner and propeller were first to go. I then unbolted the motor from the firewall. The Rapide is a little unique due to its adjustable motor mount that allows you to tune the thrust angle. It's a very clever design. This mount requires one extra component on the stator that I had to detach. It was all smooth sailing.
With the motor unbolted from the airplane, I gave it a quick inspection to see if I could detect any foreign objects within. Small magnetic components such as washers or clips can sometimes get sucked in though a cooling hole in the rotor. These parts invariably cuddle up to a magnet and get wedged in the narrow gap between the rotor and stator. However, I did not see any cling-ons inside this motor.
Small outrunner motors such as this often have only a tiny e-clip or snap ring securing the stator to the rotor. This motor utilizes a snap ring. Using snap ring pliers is the only sane way to remove snap rings without damaging them. I used my set here without any difficulty. There were also several very thin brass shims which I carefully removed with needle-nose pliers.
I then separated the rotor and stator by pulling them apart with my hands. I had to fight the pull of the rotor's magnets on the stator's iron core (at least I think it's iron). It demands a surprising amount of force to get them apart. So make sure you have a good grip on both pieces when doing this in your shop!
Having the rotor and stator separated allowed me to perform a more though visual inspection for foreign objects or anything else that might appear out of place. I didn't find any stray washers, but I did uncover the problem rather quickly. One of the permanent magnets in the rotor had come unglued and shifted out of position. Although the magnet was only a millimeter out of place (and only on one end), that was sufficient to cause the magnet to rub on the stator as the rotor spun.
My first order of business was to ensure that the magnet had not been broken. A cracked magnet would probably be a deal-breaker. Having no specs on the magnets used in this motor, I doubt that I would ever find a suitable replacement (in terms of exact size and magnet grade). Thankfully, my magnet was still in good shape and only needed to be moved back into position.
To maintain the magnet's unblemished condition, I avoided using any metal tools on it. A wooden craft stick worked just fine to push the magnet back into place. A ridge of glue residue helped me to relocate the magnet to the exact spot it had been before. I then temporarily rejoined the rotor to the stator and spun the motor by hand to ensure that there was no more interference.
My next step was to secure the magnet with glue. I think there are numerous adhesive that could work for this task. I used cyanoacrylate (CA – aka super glue). Medium or thin viscosity CA will work fine. I applied thin CA with a small disposable pipette. I just added a drop of glue in each of the gaps between the subject magnet and its neighbors. The watery glue found its way into all of the spots it needed to be.
Just to be safe, I also applied thin CA in the gaps between all of the other magnets. If you use medium CA, make sure that none of the glue dries on the face of any magnets. Otherwise the glue could rub on the stator. Also keep in mind that the rotor is a spinning part and is sensitive to balance. So make sure that you're not loading up one side of the rotor with a lot of glue.
Once the magnet was affixed back into place, it was time to reassemble the motor and get it back into the Rapide (don't forget the shims!). A quick safety tip: the same magnetic force that makes the rotor somewhat difficult to separate from the stator also works to rejoin them with equal force. If you are gripping the forward edge of the rotor or stator when mating them, expect to have the offending digits pinched without mercy. I've worked on dozens of brushless motors and this still gets me almost every time.
I made sure to apply thread-locker to each of the machine screws while installing the motor back into the airframe. I've seen instances of thread locker causing plastic to become brittle, so I was careful to avoid getting any on the motor mount, firewall, or spinner.
It is never a good idea to test a motor in your workshop with the propeller on. So I completed all of the mechanical and electrical connections for the motor before installing the prop. A quick run-up ensured that everything was working properly. So I deactivated the system, installed the prop and spinner, and declared the Rapide ready for a test flight the next day. The whole process of breakdown to completion required only about 20 minutes.
My subsequent flights with the Rapide have been trouble-free. The motor appears to be working as well as it ever has. A little bit of shop time and a few drops of glue were all I needed to avoid the $45 price tag of a replacement motor.
The only lingering question is what caused the magnet to shift in the first place. There was no trauma to the motor and all of the other magnets were still solidly attached. Maybe that particular glue joint was sub-par. I'll probably never know for sure, and I'm okay with that now that the motor is back in service. Brushless motors rarely have problems. But when they do, a DIY fix is often worth considering.
Terry is a freelance writer living in Buffalo, NY. Visit his website at TerryDunn.org and follow him on Twitter and Facebook. You can also hear Terry talk about RC hobbies as one of the hosts of the RC Roundtable podcast.