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I did a static run of the new motor with the new ESC and within 3 minutes of running the motor began smoking and the RPM’s began to drastically drop. Another minute or so later and the battery was dead. It went through a newly charged pack in under 5 minutes and by the end the motor was smoking hot, and hot and smoking. I got out my IR thermometer and measured 190ºF on the motor, just a tad on the high side of things. The heatsink on the new FET measured about 100ºF and the batteries were about 125ºF and so were the Tamiya plugs. The only thing that was within a safe operating temp was the newly outfitted FET in the speed controller! Figures, the weakest link in any electrical/mechanical system is always the first to go. First it was the ESC, so we fixed that, and now it’s the motor. I suppose a veteran airplane guy would say, well duh the prop’s too big and the pitch is too steep. Unfortunately the 10×8 prop is the only one that fits on the stock gearbox without mods. Maybe I need to mod that and try out some different props?

So in the end, the Graupner 480 BB Race motor isn’t the best choice of motors for the Super Cub, given the same prop, gearbox, batteries, radio and receiver/ESC unit. The size of the prop is too big and/or 3:1 gear ratio is too small. Getting the motor to work I think would take a new radio (to stop the glitching), a high-current li-po battery pack (to take the high current draw), and new ESC that can also handle the current, a gear box with a 4:1 or 5:1 gear ratio and props with less pitch and/or props that are smaller. I’m not sure what it would take to get the exact optimum amount of usable thrust out of this motor for a plane this big. I’m thinking that this motor may actually be better suited towards smaller, lighter planes.

So I’m putting back in the stock motor. But all is not lost, at least now I’ve got a super beefed-up speed controller that can easily handle a 3S li-po pack with its 11.1V or possibly even a 4S pack for a whopping 14.8V. And I learned a thing or two about putting a new motor into my Super Cub. For now the stock motor should last for a while, and when it finally dies maybe I’ll start looking again for an ideal brushed 480 replacement. Or finally go to a brushless out-runner and be done messing with the power plant in this plane. It might be time to try doing a custom aileron modification. The plane already has plenty of power with the stock motor and the 8-cell pack, I’m sure it’s even better with the 3S li-po. I was able to fly it later tonight, once it stopped raining, and I can still say that this plane is so much fun to fly. I think it flies way better with the stock motor anyway. But it was a bit windy so I did a few loops and brought it back in. It’s pretty cool flying into the wind at 1/4 throttle and just watching the plane hover in one place. Anyway, back to the drawing board.

For anyone interested, I ran a simulation of this setup in MotoCalc thought it was pretty funny when they predicted the motor temperature to reach an outrageous 334ºF during full throttle! They weren’t far off, my motor was well on its way to trying to achieve that temperature. I remember seeing this long before I attempted the motor, but I thought, eh, it’s a simulation, it could be wrong? Well it wasn’t. There’s a pretty cool option called Opinion that spells out in “plane” English (lame pun, I know) how your proposed setup is going to perform (the 100+ graphs they give you just don’t mean much to someone who doesn’t know what he’s looking at, ooh, the pretty blue line goes up and the red one goes down, so is that good or bad?)

Download the MotoCalc Simulation of the Super Cub with the 480 BB Race Motor.

Worth noting however is that if we drop the prop from a 10×8 to a 10×6 it appears as though we wouldn’t have the overheating issue and the plane wouldn’t suffer a huge hit on overall performance, although it states that it will be harder to fly and would be more suitable for more experienced pilots. So is that me, yet?

My parts from Digikey arrived yesterday (after only 2 days) so last night I spent an hour soldering up everything into the receiver unit and it works great! Of the 5 FETs I bought, I tried out the IRL3713 FET first and didn’t have any problems fitting it in (other than my usual sucky soldering skills) and from some initial tests it appears to work just the same as the stock FET, but with 260A current carrying capacity, 330W power dissipation, and an on-resistance of only 0.0033 ohms. I had to cut the plastic case on the receiver unit to fit the heat sink, and I had to remove the connector for the drop module but hadn’t intended on ever using it anyway. The FET may have been fine without the heatsink, but the heatsink definitely won’t hurt, it was just harder to install and now it doesn’t look stock anymore. (As it turns out after subsequent runs, the heatsink is definitely NOT required, even when running a 3S lipo pack, so for simplicity sake, I would avoid using the heatsink. I’ve since removed mine and it works fine, barely warm to the touch). I used a dab of Arctic Silver 5 heatsink compound to ensure good thermal conductivity to the heatsink. Computer builder geeks love this stuff because it has an incredibly low thermal resistance, probably the lowest available in the industry for PC use, but who says it you can only use it on a CPU? I had some left over from the last couple of PC’s I built so I thought, why not? The stuff works great, but is a mess to clean up.

I still had some radio glitching, even after adding (3) .1uF caps to the motor. So I decided to add (3) .1uF caps straight to the PCB in about the same fashion as you do on the motor. I also changed out the tiny stock diode for a beefier 600V 1N4005 diode which should work a bit better at suppressing any voltage spikes caused by the motor. Now there’s just a small amount of rudder and elevator glitching and only at really low throttle, so it should be fine. A Futaba 6CH radio is sounding really good to me about now, and Tower has $25 off any order over $149. I need to pick up a better radio, soon.

I haven’t gotten to fly the Cub with the newly modified speed controller yet since the weather here has been super windy and cold this past week and today it’s raining. But I have just run the motor full blast for several minutes and the heatsink just barely gets warm, so I think this mod should last through just about any sized brushed motor I can throw at it.

The stock receiver module, IRL3713 FET, 1N4005 Diodes and some solder

Diode and FET installed, the gray band on the diode connects to the (+) red side

0.1uF caps installed across the motor and diode

Top view housing cutaway with new FET

Adding the Arctic Silver 5 heatsink compound

A thin layer is all that’s necessary, this is probably too much

The finished product but with way more current capability than before

Installed in the Super Cub

Since I fried the speed controller portion of the receiver module on my Super Cub, I figured I would see if I could fix it first before I just buy a whole new unit. There’s a single TO-220 package FET on the board which is marked with the numbers 07N03L. Doing a Google search on that number reveals a part made by a company called Infinion for which the datasheet is readily available. You can download it here. So let’s just see what this little IC is made of, and maybe we can figure out why we cooked it and what we can replace it with.

Vds = 30V: Drain-to-source voltage. This is good, it means a lipo 11.1V pack alone isn’t going to burn up the chip.
Rds = 6.6mohms: Drain-to-source resistance is 0.0066 ohms. This is a decent number, but for real high-current applications you like to see one more zero in there. It’s still lower than those mini Tamiya plugs. =)
Id = 50A: Drain current is 50 amps. So by design the chip can carry 50A. That may seem good, but it doesn’t mean it can drive a motor that draws 50A, because there’s still power dissipation and heat to worry about. The manufacturer assumes the user is going to allow for adeqaute power dissipation by heat sinking the case and thereby maintaing a reliable operating temperature. So it’s up to the end user how many amps the device can ultimately carry. In the case of the Hobbyzone unit, there is no heat sink at all. And there is no airflow. These two factors alone are enough to drop the actual current rating to at most 25% of the rated current. Given that derating factor, we might expect to get a usable 12-13 amps out of the ESC. I’ve seen other data sheets claim their parts can operate at 50% their rating current without heat sinking, but it’s still pushing the thermal limits of the device, which will ultimately lead to failure.

International Rectifier make a whole line of HEXFET Power MOSFETs which are designed to be used in motor-control applications (in automobiles for example) requiring extremely high current and/or voltage capabilities. In a TO-220AB package they make a part that can carry a whopping 260A. That’s the first part I’ll be picking up. There’s an ongoing thread over at RCUniverse by a guy named khalsans who is extremely knowledgable with respect to electronics, specifically he’s done a lot of investigation into the workings of these receiver/ESC units. He replaced this very same FET in his plane with a IRL3803 MOSFET from International Recitifier. He popped a heat sink on there and says the unit works much better and is able to handle the load of the bigger motors they’re using. That part is rated for 140A continous and Digikey has it for $3.90. But, they have several more units with even higher current-carrying capability but otherwise whose specs are nearly identical that I’d like to try out. One of them being the IRL3713 which is a 30V N-Channel MOSFET with a 260A Id rating. I should be able to get away without using a heat sink and not reaching the thermal limits of the part. It’s most likely overkill, which is why I’m going to buy a few others as well that range from 140A to 210A. The on-resistance is only 0.003 ohms too, which is good, it means more voltage at the motor compared with the original FET which was twice as high.

While I had my receiver/ESC apart I thought I’d better measure the gate voltage from the control chip on the PCB to see if it fits within the specs on the new Power MOSFET I was picking out. I measured a analog increase from 0V to 4.4V while throttling up on the transmitter. Half throttle measured 2.50V. 1/4 throttle measured about .7V. Of course this signal is pulsed, so it’s not pure DC except at full throttle. While the DMM reads 2.2V for example, it just means we’re looking at a square wave with 4.4V peaks and about 50% duty cycle. The stock FET was rated at min=1.2V and max 2.0V. The IRL3713 is rated at 1V and 2.5V for min and max. It looks like it should be a good replacement, since it has the low turn-on threshold that we need. And it only costs $2.70 and I only have to buy 1. Thanks Digikey! But no thanks on the $5 handling fee for orders under $25. That always happens to me, and I can never find enough parts to buy at once to not just pay the $5. Of course a week later I find something I need and by then I just end up paying the $5, again. But it’s better than having to buy 100 parts just to meet the minimum buy, right?

A few hours later – Order has been placed with Digikey. I bought 5 different FETs including two made by Infineon that appear to be exact replacements to the original one, but with twice the current and power dissipation. The 100A one was only $1.86, so what could it hurt to try out? Also at the recommendation of khalsans I picked up some 1N4005 diodes to replace the shunt diode across the motor. And lastly I bought 10 0.1uF caps for 19 cents each. A couple more of those on the motor might clean up my radio glitching problems. Now I just need to find a good 3S lipo for cheap…

I just got back from flying the Super Cub with its newly outfitted Speed 480 BB Race motor and for the first 4 minutes it was amazing. The plane flew through the air with authority and power. I could just let it climb at about a 50 degree angle and it just kept going up and up and up without losing speed. Doing big loops was no problem at all. But, within a few minutes of takeoff, the motor began sputtering and glitching, so I thought I’d give it a rest and cut the throttle and glide a bit, but when I cut the throttle, nothing happened. The motor kept purring away full tilt. I still had control of the plane, but I couldn’t stop the motor. Then 5 seconds later the motor dies, completely. I was about a hundred feet up and a 50 yards away or so, so I glided back for a gentle landing. Upon inspecting the plane I immediately noticed the smell of burned IC’s. That really gross, stays-in-your-nose for hours kind of burnt electronics smell. The motor didn’t respond to any throttle inputs, but the rudder and elevator controls were still in tact, obviously since I successfully landed the plane, but it looked as though I had smoked the ESC.

The motor felt warm, but not unlike the stock motor. It was within tolerable limits for a few minutes of full running. The batteries were also warm, but not hot. Those darn Tamya plugs didn’t melt though, too bad, but they were also warm. However what was hot was the receiver/ESC unit. I dragged the plane inside and pulled out the receiver unit, now just a bit warm, and that burnt silicon smell was strong. Upon inspection of the circuit board I noticed quite a few things gone wrong. I’ll let the pictures tell the story, but the one FET that basically is heart of the ESC, was completely cooked. It must have already been in close proximity to the jumper for the voltage cutoff because it melted right into that little jumper. The FET looked black and burned on the one side and the PCB looked charred around the 3 leads. It wasn’t until I turned it over that I realized how hot it must have got, it actually re-flowed the solder on the IC leads! One of the leads isn’t even connected to the trace on the board anymore, it’s just a lead in a hole with a big blob of solder further down the exposed trace. Instead of nice shiny solder joints, there were cold solder joints on the other leads, and the rest of the solder around the area looked cold too. Assuming the solder Hobbyzone uses is a standard 63/37, tin/lead alloy solder, the chip must have reached 183ºC (or 364ºF) to melt the solder around the leads of the part like it appears to have done. Now that’s pretty hot. As it turns out though, these power FETs are designed to operate up to 175ºC, so that’s all it took to melt the solder on the board.

I’m going to try and just replace the one FET, maybe stack two of them on there, or get one that can take the extra current, since it should only be a couple of bucks to fix. I think the rest of the receiver is still working fine, since it it still responds to controls from the radio. Download the data sheet for this power FET here. That exact same part should get my cub flying again but I’d have to I re-install the stock motor, which I might do. But they make FETs that can handle a lot more current also, so I may look into that as well. I’d like to try out the 480 motor a bit more.

In the end, even after less than 5 minutes of run time, I think the Graupner Speed 480 BB Race motor is one possible upgrade to the stock motor, however, the stock electronics need to be replaced with an aftermarket radio and at least a 35A brushed ESC. I still had a little bit of interference but a new radio would most likely clean that up. Other things I didn’t get to try was cutting a 1/2″ off either side of the prop, thus making it a 9×8 instead of a 10×8, possibly allowing a few more RPMs to squeak out and maybe a bit more thrust. Also, I think an 11T pinion would fit and still mesh with the 36T spur gear. That would provide a gear reduction of 3.27:1 instead of 3:1. This would reduce some of the stress on the motor, reducing the current draw, and lighten the load on the ESC and batteries. And lastly, I didn’t get to try out a 3S lipo, which ultimately would be the final piece of the Speed conversion for achieving ultimate flight performance. But alas, this will all have to come another day. Until then, check out these pics of one fried speed controller.

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

I thought of another way to more accurately measure the true RPMs of the drill I was using to measure the kV of the stock motor on the Super Cub. Instead of measuring another motor whose kV I assumed to know, I ran the drill into a 10k potentiometer and measured how many times it rotated with a frequency counter. I tore the back off of a 10k pot so it would spin indefinitely, then put 5VDC on one leg while running ground and the other leg to my DMM. A 1k ohm resistor across the the DMM to creates a voltage divider network that isn’t dependent on the high impedance input of the DMM for proper operation. The rotating pot creates a sawtooth waveform at the rate of 1 Hz per revolution which the DMM (frequency counter) can easily read. Take that number and multiply by 60 and you get RPM of the whatever device was used to spin the pot, in this case, my cordless drill. (Since the pot spins with a certain amount of friction, the drill has to have plenty of torque to overcome the friction and not affect the measurement. This method propably wouldn’t work for directly measuring the free-running RPMs of just a motor because the friction of the rotating pot would adversly affect the measurements. If the motor were geared high enoough though, it could still work).

At this point we’ve removed just about all the variables and we’re left with only the accuracy of the DMM, which according to the data sheet is about +/- 1Hz. The reading I got toggled between 20 and 21 Hz, so I took the average and figured 20.5Hz. Take that number x 60 and you get 1230. So the final measured RPMs of my drill came out to be 1230 RPMs. So the kV of the stock motor on the Super Cub turns out to be 1230 / (1.420 / 3) = 2599 RPMs/Volt. Since the DMM is only accurate to 1Hz, we still have about a 5% error in that number, but it’s about as close as I can measure with the test equipment I have at home. An o-scope on the other side of the potentiometer would give us a much more accurate reading, but I’ll save that test for another day.

So when determining a motor upgrade for the Super Cub, anything that provides about 2600 RPMs/V should do the trick, granted the motor has the power as well. Since the cub is geared, there is a 3:1 reduction on that number bringing the prop RPM to 866. Of course this is unloaded, but the absolute highest free running RPMs at the shaft is around 866 RPMs. So if you’re looking to change the Cub from an in-runner to an out-runner and connecting a prop directly to say a brushless motor, anything that has a kV rating of at least 866 should perform as well or better than the stock motor, again granted that it’s a big enough motor and can provide the torque and power necessary. Makes sense that the Park 450 motor is a popular upgrade, since it has a kV of 890, and can probably provide a lot more power, especially at 11.1V from a 3S lipo. Putting a motor with a kV at or over 1000 in a direct-drive app would just cause the SC to absolutely scream. Or a kV of 3000 or more would be great for keeping the stock gear box with the 3:1 gear ratio. My Graupner Speed 480BB has a kV 3890 which is significantly more than the stock motor and a lot more the the popular brushless setup. Oh no, could be bad news, because nothing great doesn’t come at the cost of something else. I suspect my runtimes will be 6-8 minutes tops and there may be smoke and fire, maybe. Tomorrow we get the motor into the gearbox and finish out the tests. If it draws too much current, I simply won’t run it. But I am anxious to see how much more thrust I can generate with the new motor, should be fun if nothing else.