Along with getting a new lipo battery pack for my SC, I’d like to swap out the stock 480 motor for something that can handle the 11.1V from a 3-cell lipo pack and provide a little bit more thrust for more daring tricks and the ability to get out of sticky situations a little better. The problem is, where do you start? With no information readily available about the stock 480 motor, you’re kinda in the dark as to what would be a good replacement. A prop RMP meter would be a good tool to have, Tower sells one for $20 that I may pick up soon. But until then I decided to try out a technique I read about somewhere on the Internet for measuring the a motors kV, otherwise know as a motor’s constant – if you put in 1V into a motor and get out 1000RPMs, that means the motor has a kV of 1000. As it turns out if you put in 1000RPMs into the same motor, you will also get out 1V. So with a cordless drill and a voltmeter, we can measure the effective kV of the stock Hobbyzone motor. With that info, we can better gauge picking a new motor, ideally one with a higher kV, hoping that it will yield more RPM’s at the prop, and therefore provide more thrust for our plane.
The trick is that you have to know the RPMs of the drill being used to spin the motor, and it has to be exact. Well, on the side of the drill it’s printed 1300RPMs in High mode, so for starters we can use that. I didn’t feel like pulling the motor out of the gearbox, so I popped the drill on the shaft where the prop would go, and fired away. The gearbox on the Cub is 3:1, so for every single rotation of the drill I got 3 rotations of the motor and the result was a solid 1.420V on the voltmeter. Since we were geared we need to calculate 1.420 / 3 = 0.473V per 1300 RPMs. To get kV we just divide the known RPM of the drill by the voltage that said RPM generated. 1300 / 0.473 = 2746. Voila, the kV of the stock motor for the SC is 2746, or somewhere close to that. As I said, this method is entirely dependent on knowing the RPM of the drill being used to turn the motor. 1300 is what the drill says, it may not be what the drill really is. So as a sanity check, I popped on a Graupner Speed 480 BB Race motor, of which I know has a kV of 3893. If I spin the motor at the supposed 1300 RPMs from my drill I should read about 0.333V. If the drill is actually spinning faster, then that number will be higher, if it’s spinning slower, then that number will be lower. I measured 2 different 480 motors and took the average of 3 readings: 0.308V, 0.312V, 0.296V. There is some error in saying that I know for fact the constant of both those motors is 3893, but I have to start with something that I think I know. Now in order to achieve the intended kV of these motors, I can now calculate the RPM of the drill. As it turns out, the drill is probably only spinning about about 1200 RPMs, not the 1300RPMs as claimed by the drill. Now taking that into consideration for calculating the kV of the stock SC motor, we actually get kV = 1200 / 0.473 = 2536. This is still not exactly right most likely, however it’s probably within +/- 5%. Which for the most part is accurate enough for what I’m trying to calculate.
Some other useful things I measured were no-load and loaded current draw of the stock motor. No load current (no prop) with a newly charged 9.6V battery pack was 0.97A. The loaded current (prop attached) was 9.4A. Of course this is with 100% drag seeing as how I was holding onto the motor/gearbox assembly while the thrust created by the prop was fighting me. During flight the current will drop as the plain approaches cruising speed, and then go up again as you fight through the wind or climb into an impressive loop. But 9.4A is a good amount of current. That’s over 90 watts of power, not bad for a RTF plane. However, the no load current of the 480 BB Race motor is 2.44A, therefore I might can expect the loaded current to be similar proportion as the stock motor or 2.44 * 9 = a whopping 22A, or 211 watts. This is where things get tricky, because there’s a chance that 22A is too much for the ESC built into the one-piece receiver unit in the SC. But then again it might be fine. The other thing is even though the Graupner motor spins 53% faster than the stock motor, it may not actually generate more thrust, since the extra RPMs most undoubtedly come at the expense of torque, the torque required to turn that massive 10×8 prop at several thousand RPMs. There’s a good chance the motor will simply over heat and die, all the while not proving any performance gains over the stock motor. It’s a gamble any way I look at it. I think one of the other Speed 480 motors would work better, one with a lower kV, therefore drawing less current and providing more torque. The other thing to do is just try it, which I might just do. All I need is a 12T, 32-pitch pinion and I should be set. As it turns out the gears in the gearbox for the SC are standard 32P, which I would have thought they’d be metric. (A a metric 0.8 module gear is nearly the exact same size as 32P, so it still could be a metric pitch, however a 32P standard gear should mesh without problems since they are so close). I determined this by measuring the diameter of the spur gear somewhere in the middle of the teeth at each end. Since this gear has 36 teeth, a 32P pinion would result in a diameter of 36 / 32 = 1.125 inches. Whatdaya know, I measured exactly that. So $3.25 should get me a new gear, and we can see what the SC does with a 480 BB Race motor. Hall of fame or hall of shame? We will see…
Measuring the kV of the Stock Motor – (1.420 / 3) * 1200 = 2536 RPMs per Volt
Double-checking the RPM of the Drill Using a Motor with a Known kV
Measuring the Diameter of the Gear – 36 / 1.125″ = 32P
Measuring the No-load Current of the Stock 480 Motor – 0.97A on 9.6V Pack
Measuring the Loaded Current with the 10×8 Prop – 9.43A Continuous
