My Pretty Big (Giant?) Nutball

[Foamforce]

I built the PB Nutball from 1/2” XPS insulation. It’s about 60” diameter. To make it easy to transport, the wing tips are hinged on the bottom so that they fold underneath for transport. Rubber bands keep them in place during flight. The vertical stabilizer slides out of the slot too, so the entire plane flat packs like an IKEA bookshelf except for the power pod. To stiffen the wing, I ran a 3/32” strip of pine along the back, in front of the elevator. In the front I cut the front 12” off and glued in the same pine strip there. That worked slick.

Today was the maiden and it went great! It taxis really well, and it floats off the ground. I had good control. There are two problems that I need to address. The first is that something is fluttering, I think probably the vertical stabilizer, so I’ll reinforce that. The second is that the landing gear is too flimsy, so I need to reinforce that, probably similar to how the landing gear on the Storch works.

 

[Foamforce]

After reinforcing the landing gear and vertical stabilizer, I took it out again. It worked well for about five minutes and then the motor died. Fortunately, it goes nicely and I landed without damage.

I can’t figure out why the motor burned out though. The motor was very hot when it came down and there is bubbling (though not dark) on the windings. Also, it takes effort to turn it. The ESC tests out fine with other motors.

The motor itself is a 4250 560kv. I was using a 13x6 electric prop on it, which is the recommended size. I was using 2x 5000mah 3s batteries running in series for 6s, which is the recommended voltage with that prop size. It seems that the maximum wattage is 1760w, which would be ~70a at 6s. I was using an 80a ESC with it. In my bench testing, it pulled about 40a at full throttle.

I pulled it apart to check the bearings and shaft because I suspected those. The bearings seem pretty smooth and the shaft looks straight. I did find a nick in the wire where it goes onto the housing.

So my guesses are:

• During my bench testing, my initial run failed when the motor and prop pulled the screws out of the firewall and the motor flew across the room. I didn’t see any damage, but maybe it caused some invisible damage.
• It was a hot day and I ran it hard, so it just got too hot…
• Maybe I misread the specs?
• Possible that one input wire shorted against the body, causing the windings to melt somehow?
• Maybe the shaft and c-clip was rubbing against the firewall, although I didn’t notice that when I spun it by hand.
• Maybe I needed to adjust the ESC timing for such a low KV motor?
• Or maybe I got what I paid for, which wasn’t much. 🙂

Any thoughts?

 

[Foamforce]

Here’s the stator. It was a bear to get off! You can see the melted coils there.

Does this look particularly poorly wrapped, or is this normal?

 

[Piotrsko]

Looks typical for handwrapped by the average employee who doesn't care. If it was a super bargain, well there is the first clue. Didn't do a motor run on the ground pre flying to see if the motor got hot?

Modern ESC's determine motor timing by sensing what is occurring in the motor which is why there are no shaft position sensors needed any more.

 

[Foamforce]

Looks typical for handwrapped by the average employee who doesn't care. If it was a super bargain, well there is the first clue. Didn't do a motor run on the ground pre flying to see if the motor got hot?

Thanks for replying @Piotrsko !

Yep, it’s a cheap motor, so I didn’t expect a ton. My testing was on the bench, with the same prop, and it maxed out at ~40a without getting hot. On the first two flights, the motor got warm, but not hot. On the flight where it burned up, I may have pushed it a little harder because it was more windy that day, but probably mostly around 1/2 to 5/8 throttle. When it came down though, it was extremely hot.

The differences the day it burned up were…

• It was a sunnier day. Maybe 75 and sunny vs 65 and cloudy.
• It was a little windier, so I was using a little more throttle.
• I had added a some reinforcement to the landing gear and vertical stabilizer, so slightly more weight and drag.

The more I think about it, the more I’m thinking it was probably just either a defective motor or maybe since of the winding insulation was damaged when it went flying across my basement. Do you have any other thoughts?

Now that I have the stator off, I’m going to try to rewind it. I’ve watched a few videos and it doesn’t look hard, so my next step is to figure out which wire to buy. This one uses the braided winding. Any idea what average the braided has over a single thicker wire?

 

[Foamforce]

I’m reading now that the advantage of stranded welding wire is that it’s easier to wind and you can possibly fit a little more in there. However, I haven’t found any for sale yet.

I feel like I’m getting into RC archaeology, reading and watching all this stuff from 2005-2010 on the subject. 🙂

 

[Tench745]

Yep, it’s a cheap motor, so I didn’t expect a ton. My testing was on the bench, with the same prop, and it maxed out at ~40a without getting hot. On the first two flights, the motor got warm, but not hot. On the flight where it burned up, I may have pushed it a little harder because it was more windy that day, but probably mostly around 1/2 to 5/8 throttle. When it came down though, it was extremely hot.

The differences the day it burned up were…

• It was a sunnier day. Maybe 75 and sunny vs 65 and cloudy.
• It was a little windier, so I was using a little more throttle.
• I had added a some reinforcement to the landing gear and vertical stabilizer, so slightly more weight and drag.

The more I think about it, the more I’m thinking it was probably just either a defective motor or maybe since of the winding insulation was damaged when it went flying across my basement. Do you have any other thoughts?

Now that I have the stator off, I’m going to try to rewind it. I’ve watched a few videos and it doesn’t look hard, so my next step is to figure out which wire to buy. This one uses the braided winding. Any idea what average the braided has over a single thicker wire?

Is it possible your ESC wasn't calibrated correctly in your bench test, and you were actually only getting partial throttle?
I quickly ran your numbers through eCalc with a similar size and Kv motor. (Not sure the manufacturer on yours). That motor had a max power of 877 watts.
At full throttle on 6s with a 13x6 prop it would be drawing almost 50amps, for a total of 1032 watts. If the wattage limit of the motor in eCalc is anything like yours, it sounds like ya cooked it.

I’m reading now that the advantage of stranded welding wire is that it’s easier to wind and you can possibly fit a little more in there. However, I haven’t found any for sale yet.

I feel like I’m getting into RC archaeology, reading and watching all this stuff from 2005-2010 on the subject. 🙂

Very few people wind their own motors anymore. It's something I've wanted to explore, but never had the opportunity.

 

[Foamforce]

Is it possible your ESC wasn't calibrated correctly in your bench test, and you were actually only getting partial throttle?
I quickly ran your numbers through eCalc with a similar size and Kv motor. (Not sure the manufacturer on yours). That motor had a max power of 877 watts.
At full throttle on 6s with a 13x6 prop it would be drawing almost 50amps, for a total of 1032 watts. If the wattage limit of the motor in eCalc is anything like yours, it sounds like ya cooked it.
View attachment 244110


Very few people wind their own motors anymore. It's something I've wanted to explore, but never had the opportunity.

I believe I calibrated on the bench, but it’s possible! I’ll re-do the bench test with the new motor that’s coming.

Interestingly, the vague specs indicate that it’s capable of 1760w and 80a, so I thought I was well within the specs, but the specs might be wrong. I really suspect the 80a figure in particular.

Here’s the one I bought:

https://a.aliexpress.com/_mLf8Zu2

It looks the same as a few others sold on various sites. They usually list 13x6 as the prop size.

I removed the windings yesterday and counted 8 winds, not 9 as the specs state, so maybe mine was 630kv instead is the advertised 560kv, needing a slightly smaller prop.

At this point I’m going to try to figure out what size magnet wire to get to rewind this one, just for fun. The old one used 5 stands of .36mm wire. I’d like to do single strand if I can figure out how to convert the size. Meanwhile, a new replacement motor is coming, and I’m going to do a a lot of short test flights with that one to see is it’s getting hot. Then I’ll switch to a smaller prop if I need to.

Anybody here able to help with winding calculations?

Here’s a glamour shot of my unwrapped stator. It came apart pretty cleanly. The green is nicked in just a couple places.

 

[Piotrsko]

Winding your own leads down the dark addictive diy motor modification tunnel. First thing you need to fix the nicks with some sort of high temp paint after getting the burnt gunk off

My motor experience is large industrial, but theres stuff that translates.
They use multi strand because it carries more current with less heat in a smaller diameter. Caveat here, not much less heat. Power is watts consumed times efficiency factor. Want lots of power, you're needing lots of watts and will make lots of heat

There are published tables for amp turns or motor turns per wire size. You already know how many turns. You'll also need motor varnish, which comes in temprature grades, but there are substitutes.

Watch motor rewinding stuff on the web for how to, but don't believe everything they say. DUH!!

 

[Tench745]

I believe I calibrated on the bench, but it’s possible! I’ll re-do the bench test with the new motor that’s coming.

Interestingly, the vague specs indicate that it’s capable of 1760w and 80a, so I thought I was well within the specs, but the specs might be wrong. I really suspect the 80a figure in particular.

Here’s the one I bought:

https://a.aliexpress.com/_mLf8Zu2

It looks the same as a few others sold on various sites. They usually list 13x6 as the prop size.

The description in that link says " Maximim power 560W" which I think is a typo. It says voltage is 11.1V-36V and a max amperage of 80A. If we assume maximum amp draw is only possible at the lowest voltage, then that's actually 888W max power, which is close to the 800W-900W range I've seen on other motors of similar size and Kv rating I looked at. (Spektrum, FMS, no-name stuff, etc.)
Digging into the "Aeolian" brand motors, I do see some listings that say max power around 1100W and one that agrees with the 1760W you saw.
I think you were flirting with the upper end of what that motor can realistically do and some slight damage from it's trip across the room might have been enough to tip it into charcoal territory.
I'm excited to hear how the rewind project goes!

 

[Foamforce]

The description in that link says " Maximim power 560W" which I think is a typo. It says voltage is 11.1V-36V and a max amperage of 80A. If we assume maximum amp draw is only possible at the lowest voltage, then that's actually 888W max power, which is close to the 800W-900W range I've seen on other motors of similar size and Kv rating I looked at. (Spektrum, FMS, no-name stuff, etc.)
Digging into the "Aeolian" brand motors, I do see some listings that say max power around 1100W and one that agrees with the 1760W you saw.
I think you were flirting with the upper end of what that motor can realistically do and some slight damage from it's trip across the room might have been enough to tip it into charcoal territory.
I'm excited to hear how the rewind project goes!

Holy smokes, I think you’re right, I forgot to take into account that the 80 amps was probably at 11.1v.

I also got some 16” props for 3s. It seemed underpowered like that, but it took off with such low throttle on the 13“ props and 6s, that I’ll bet it could handle it.

 

[Foamforce]

Holy smokes, I think you’re right, I forgot to take into account that the 80 amps was probably at 11.1v.

I also got some 16” props for 3s. It seemed underpowered like that, but it took off with such low throttle on the 13“ props and 6s, that I’ll bet it could handle it.

Oh, and having 8 winds instead of the 9 that was stated in the specs would also increase the amp draw, so that probably pushed it further over the line. When I rewind, I’ll do 9 winds. Just gotta figure out how to use the wind calculator now.