Okay, so all this calculating and measuring and messing around with drills, pots and motors ultimately leads to one thing – the final installation of the Speed 480 motor into the Super Cub. A motor that boasts a whopping 3893kV over the 2600kV for the stock motor. A motor that’s capable of delivering twice as much power as the stock 480 motor at a premium price of about $37. Not much more than that will get you a much better brushless motor, but I had the motors already from some previous projects.
My prediction is that the SC on this motor is going to rock, for like 5 minutes, and then either the motor is going to overheat and die, the ESC is going to cook, or those retarded small Tamiya plugs are going to melt – or all three at the exact same time. But I still have to at least try it out. So today I picked up the rest of the parts I needed to do the installation, which required driving to both hobby shops in town. I picked up a set of Deans (for when those Tamiyas melt), a 12T 32P pinion (for the 3.2mm shaft on the 480), some 2.5mm x 8 screws and washers, and a new gearbox assembly, just in case. I almost bought a prop tachometer, I really want to see the difference between two motors at the prop, because that’s what really counts. Knowing the kV and free-running RPM doesn’t tell you much when it comes down to achieving a known thrust figure.
The motor installation went as expected, the gear mesh was a little tight, so I put 4-5 pieces of tape on the inside bottom of the gearbox and then ran some thin steal wire around the back of the motor and pulled it down against the shaft-part of the gearbox. The spur and pinion mesh nicely with just the right amount of play. I tested out the new motor with my ICE charger and went to the motor break-in mode. I set 1.5V and started it up, and it spun and sounded silent and smooth. So I slowly cranked up the voltage till I hit the max it will do, 8.0V. The current draw was about 1.85A, no prop. But man, it was spinning fast! I could tell, it already sounded faster than the stock motor.
Then I installed the prop and did the same thing, started out at 1.5V and went up slowly from there. I only got to 4V before I got scared and shut it off. The prop was spinning at INSANE speeds and the thrust it was creating made it difficult to hold the motor in my hand. So I didn’t dare try any more voltage till I got the thing installed onto my plane. But at 4V, it felt like the stock motor with the stock 7-cell pack at full throttle, no kidding.
With the motor assembly now mounted on the plane I felt more comfortable about giving it a bit more throttle and seeing how much air it could push. It was already after dark, so I knew I wasn’t flying it today. I flipped on the Tx and started up the throttle nice and slowly. About 1/4 way up the motor kicked in and just started humming, it sounded good. I got to half throttle and it sounded and felt about like the stock motor and battery. I hit 3/4 throttle and the thing starts sounding like an engine, you could just hear the air accelerating behind the prop, and by the time I hit full throttle, the plane litterally wanted to jump out of my hands. It pulled so hard it was unbelievable. Next thing I want to do is make a thrust meter so I can measure how much thrust different planes/motors/props have. But that’s a project for another day.
All isn’t without worry though. The rudder and elevator were bouncing out of control the whole time. Obvious radio interference from the new motor. It’s got some 102 caps on it (.001uF), however the stock motor has 104 caps (.1uF), so I pulled the caps off the stock motor and soldered them in parallel with the 102 caps already on the Graupner motor (making a .101uF) and that seems to have done the trick. There is only a little bit of interference when idling at a slow RMP, less than 1/4 throttle, everything else seemed to run fine with no glitching. The .1uF caps should have a lower cutoff frequency than the .001uF caps, in fact I don’t think I’ve ever seen .001uF caps on any motor, I think its just too small a cap to actually do anything. A 1uF cap might be even better, in addition to the other two. The other thing I’m now worrying about is the motor felt warm after only 10 seconds or so of full throttle running, even the Tamiya plugs felt warm. For now I’m going to leave the Tamiyas on there, they can act as my fuse, so hopefully they go before my ESC does or my motor does. They should act as a current limiter to some extent as well, since they have a ton of internal resistance, they will help tone down the current that motor with that massive prop on it are going to try and pull. Hopefull the ESC can handle it, I expect some 20A out of this thing. I couldn’t measure it tonight, because my current meter tops out at 10A, and its unfused, so if you exceed it, which I think I will, then you fry the precision dropping resistor in the DMM and then I’m out a DMM that can measure current. Anyway enough talk, here’s some pics.
Here’s a real short video clip running the new motor
