Most modern, electric-powered RC airplanes utilize brushless motors for propulsion. And why not? Brushless motors are much more efficient than their brushed cousins (not to mention lighter and sometimes even less expensive). Similarly, airplane models that were designed to use brushed motors can enjoy a huge boost in performance with an upgrade to brushless power.
Some brushed-to-brushless upgrade projects are just a simple matter of swapping out the relevant components (motor, Electronic Speed Control (ESC), propeller). Others require a bit more planning and modification. My most recent brushless upgrade is a good example of the more-complex type. I'll show you the hurdles I encountered during this project and how I cleared them.
The recipient of this power system transplant is a small electric sailplane called the Skimmer 400. Kits for this balsa airplane were popular during the late 1990s and early 2000s. Like other electric-powered models of that era, the Skimmer 400 was designed around the brushed "can" motors and NiCad batteries of the day. There was never a surplus of power with that gear. So models that performed well, did so by virtue of excellent airframe design. I've found that these types of model airplanes adapt particularly well to brushless upgrades.
My Skimmer appears to have led a hard-knock life. I bought the airframe in very-used condition at a recent RC event. Despite model's wrinkled and tattered covering, the balsa structure seemed to be built relatively well and I did not notice any obvious crash damage. It may be an ugly duckling, but the asking price was only $5. What was I supposed to do…just leave it there?
As originally designed, the Skimmer 400 was powered with a Speed 400 brushed motor spinning a 6-inch propeller. "Speed 400" is just an RC-centric term for the generic Mabuchi RS-380 motor that is used in all types of different applications around the world. It has a diameter of 28mm (1.1"), is 38mm (1.5") long, and weighs 65 grams (2.3 ounces). The intended battery pack for the Skimmer 400 consists of seven 2/3A-sized NiCad cells of 600mAh capacity. Altogether, this system produced about 60-80 watts of power…enough to fly the Skimmer 400 in a leisurely fashion.
The first thing to consider before starting a brushless upgrade is whether it will actually be beneficial. You will most likely end up with a significant boost in power and a sizeable reduction in weight. Both of those things are typically very desirable in any kind of airplane…but not always.
A recent upgrade exception for me was my Great Planes Wright Flyer. Like the Skimmer 400, it also was designed for brushed motors and NiCad batteries. I considered a brushless makeover for this model as well. I was unsure, however, whether the airplane's flexible plastic framework could handle more powerful motors. I ultimately decided to play it safe and stick with the stock brushed motors. I did, however, upgrade to a modern LiPo battery for MUCH longer flight times.
Another reason to forego a power upgrade is simply because the system you already have is working well. "If it ain't broke…" applies here. Brushless power is not a prerequisite for fun. In fact, some of my best RC memories include models with modest brushed motors. So don't feel like an upgrade is always mandatory. In the case of the Skimmer 400, it did not have a motor installed when I bought it. I thought the airframe felt robust enough to handle substantial power. The brushless upgrade was an easy decision.
Where's the Beef
A brushless motor the same size as a Speed 400 would easily handle more power…a lot more. I have some brushless motors of that size that routinely produce more than 400 watts. Such power would be ridiculous in the Skimmer. It would rip the wings right off! A more sensible choice is to step down to a smaller motor with less of a power boost. I've found that the small brushless motors used on many 250-class racing drones will often work really well in Speed 400 designs.
I chose the Emax MT2204-2300 motor with an Emax 12-amp ESC for the Skimmer. This brushless motor has the same 28mm (1.1") diameter as the Speed 400, but it is only 18mm (.7") long. As a bonus, the spacing of the mounting holes on the Emax motor matches that of the Speed 400. So it could be bolted right onto the airplane's firewall without modification.
The 2204 brushless motor weighs only 25 grams (.9 oz). That's 1.4 ounces less than the Speed 400. This weight savings is obviously a good thing overall, but it does introduce some challenges with balancing the airplane. Achieving the proper center of gravity (CG) is critical for the model to fly well. With a traditional airplane like this, the CG is typically located at about 25-30% of the wing's chord (front to back width).
The motor and battery are the two most significant masses affecting the balance of an RC airplane. The Skimmer 400 was designed to balance properly with the speed 400 in the nose and the NiCad battery under the wing. Swapping to the lighter brushless motor made the airplane extremely tail-heavy, so additional steps were taken to balance things out.
The first thing I did was to mount the Emax motor to the front side of the firewall rather than within the nose of the fuselage. This gave the featherweight motor a slightly longer moment arm and offset some of the weight difference. It may not be sleek with the motor perched out front, but the model was already ugly. Plus, the motor is exposed to plenty of cooling air.
My next step was to move the battery forward. I also had to upgrade to a newer battery. Technology left the 600mAh NiCads in the dust long ago. Even 1600mAh high-discharge NiMH cells of the same size are now relics. Upgrading to LiPo cells was the only practical choice. The Emax motor works well with 3-cell batteries and a 6-inch propeller, so I culled a few 3-cell candidates from my battery collection.
My goal was to identify the smallest (i.e. lightest) 3-cell battery that would provide the proper CG. By repositioning the radio receiver to the former battery location beneath the wing, I opened up a sizeable mounting space in the fuselage between the wing and the motor. I eventually settled on a 3S-1800mAh battery. The Skimmer balances well with this battery all the way forward. The LiPo battery is the same weight (144 grams/ 5oz) as the older NiCad. But it has higher nominal voltage (11.1v versus 8.4v) and three times the capacity (1800mAh versus 600mAh).
Another consideration for brushless conversions is the volume of the space where the motor will be mounted. As with the Skimmer, you can often get by with using a smaller motor when converting to brushless. That helps. However, most brushless motors used in RC airplanes are the "outrunner" variety, where the outer case of the motor spins. It is vital to ensure that the space around the spinning parts is clear of any stray wires or other potential protrusions that could ensnarl the motor.
Mounting the Skimmer's motor to the outside of the firewall effectively negated any concerns about fouling the outrunner motor. Had I chosen to mount the motor within the fuselage, however, it would have been a much different story. It is a tight fit. I would have been forced to route the three wires from the motor to the ESC very precisely and somehow lock them in place well clear of the motor. In several my other airplanes, I have secured threatening wires by gluing or taping them down.
I never intended to fully restore the Skimmer 400 to like-new condition. Sometimes, the true beauty of a $5 airplane is that it is a $5 airplane. You can take it out and enjoy it without any concern about what happens. Sure, the electronics add to the overall investment. But those parts almost always survive a crash and can be transplanted into the next model. So, once I had the Skimmer's power system squared away, I took a minimalist approach to getting it airworthy.
You can sometimes revive an old, wrinkled covering job with a heat gun and iron. But that was not the case with the Skimmer. Its brittle covering refused to flatten out in most places. That's okay. Wrinkles add character, or something like that. Right? I patched up the holes in the covering with packing tape and called it done.
The Skimmer 400 is a three-channel model (rudder, elevator, and throttle). I installed two E-flite micro servos in the stock locations below the wing to actuate the rudder and elevator. An old, but pristine Spektrum AR6110e receiver is a good match as well. I linked the model to my Spektrum iX12 transmitter.
With a 6x4.5 propeller attached to the Emax motor, the power system pulls 13.8 amps and produces 160 watts of power. That is significantly more power than a Speed 400 could ever hope to crank out, and a lot of grunt for a 538 gram (19 ounce) airplane such as the Skimmer. I was excited to try it out.
I wasn't quite sure what to expect when I launched the Skimmer for the first time. With the motor set to about half-throttle, I gave the airplane a firm, level toss. I was happy to see it fly away very gently. I love it when a plan comes together!
The Skimmer 400 is designed to be a powered sailplane. The point is to climb it up and then shut down the motor to glide for as long as you can. While I've never flown a Skimmer 400 with the stock, brushed power system, I am certain that my model has MUCH better climb performance. I can stick the nose of the airplane nearly straight up and keep climbing if I want to. And yes…I want to!
The Skimmer is not really intended for aerobatics, but the motor upgrade does open up that possibility. The airplane is capable of basic maneuvers like loops and inverted flight (briefly). With that said, this model is really at its best with a gentle touch on the sticks. It also prefers calm days over windy ones.
When I get tired of soaring and aerobatics, I like to get down close to the ground and carve graceful tracks through the sky at low throttle settings. Even ignoring the $5 price tag, this is a relaxing, stress-free airplane. I'm sure that the Skimmer 400 is fun to fly with the stock power system. I am equally sure that the brushless upgrade makes it a considerably more versatile model.
Not all brushless upgrade projects are a simple plug-and-play affair. The power, weight, and volume considerations introduced by going to brushless power can complicate matters. As we've seen with the Skimmer 400, you sometimes have to improvise to make everything work together. That's okay because the reward is worth the trouble!
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.