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Old May 24, 2012, 01:57 PM
Grumpy old git.. Who me?
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Originally Posted by Aeroplayin View Post
If there is anything I can do to lighten the financial load on you from here, let me know.
Thanks for the offer Ken was kind enough to explain the repair process and it's not difficult so I'm going to have a go at sorting it myself. I'm almost tempted to have a go at re-winding to lower KV too but all things considered I'll probably just leave it with the factory wind and run a 13" prop.

When my Loctite black arrives I'll post some pictures of the repair process that may possibly come in useful for others (providing I dont mess it up of course)

Steve
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Old May 24, 2012, 02:05 PM
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Good luck Steve
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Old May 24, 2012, 02:17 PM
Grumpy old git.. Who me?
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Originally Posted by -= GYRO =- View Post
Good luck Steve
thanks

As far as motors goes i need some luck I got to thinking at if the 750kv Motrolfly was struggling to cope with the 14" APC then possibly my similar kv hand wound Scorpion might also be feeling the strain.

So I openned it up to check and sure enough there was the tell tale rub marks on the stator and magnets

Is it just me

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Old May 24, 2012, 02:57 PM
Not as Good as The Kid
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Originally Posted by JetPlaneFlyer View Post
Thanks for the offer Ken was kind enough to explain the repair process and it's not difficult so I'm going to have a go at sorting it myself. I'm almost tempted to have a go at re-winding to lower KV too but all things considered I'll probably just leave it with the factory wind and run a 13" prop.

When my Loctite black arrives I'll post some pictures of the repair process that may possibly come in useful for others (providing I dont mess it up of course)

Steve
Yes, good luck. I'm sure you can handle the repair considering what you have done with motors recently.

I think it is evident that 14 inch props is just not an option at the higher RPMs on this size motor, in the same way that the 13 is not good on 4S with the Torque 820Kv and the 16 is not good on the 500Kv. The limitations are somewhere for every motor and I think we found it for the Motrolfly.
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Old May 24, 2012, 03:58 PM
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Looks like its tine to adjust some endpoints on my 14" plane. A thought also crossed my mind.


These failures may have occurred at lower rpm than 8500. Steve probably didn't take off and then immediately go into a high speed tumble, the damage could have occurred later. Its a correlation vs causation issue, since batteries flatten out after the initial peak we may see damage at lower rpms. Have you pulled your bells yet Jim?
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Old May 24, 2012, 04:47 PM
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Originally Posted by stgdz View Post
Looks like its tine to adjust some endpoints on my 14" plane. A thought also crossed my mind.


These failures may have occurred at lower rpm than 8500. Steve probably didn't take off and then immediately go into a high speed tumble, the damage could have occurred later. Its a correlation vs causation issue, since batteries flatten out after the initial peak we may see damage at lower rpms. Have you pulled your bells yet Jim?
No, and I may not for a while. The 638 will have been flying with the 14 Xoar on 4S for a year next month and Tom's will be flying with the 14 APC on 4S for a year in July (or something like that). I'm also going to guess that most motors will have some degree of rubbing no matter what, and considering the punishment we put these motors and airframe through, I'm actually surprised they both haven't fallen apart by now.

Even if I saw some rubbing, I would not change anything because I'm happy with the performance and the durability. Considering what we put these motors through, and the 4315 during all the testing with big and non-electric props with zero failures, I may be just a little overconfident in how bulletproof they are if you don't over-extend them, and I think we found the tipping point now.

I'm going to guess that the only reason I got it right was purely by chance because I started low and went up from there until I found an acceptable level of performance, and was lucky that the 683 was the stock wind at the time.
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Old May 24, 2012, 05:14 PM
Grumpy old git.. Who me?
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Jim.
The rubbing between stator and magnets on the Motrolfly is not trivial. The heat caused by the friction has already burned away some of the enamel on the stator and I think that given time if I hadn't picked it up it might well have burned through the insulation on the winding wire, and the motor would be toast (maybe the ESC too?)
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Old May 24, 2012, 05:39 PM
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Originally Posted by JetPlaneFlyer View Post
Jim.
The rubbing between stator and magnets on the Motrolfly is not trivial. The heat caused by the friction has already burned away some of the enamel on the stator and I think that given time if I hadn't picked it up it might well have burned through the insulation on the winding wire, and the motor would be toast (maybe the ESC too?)
Thanks. Didn't know it was that bad. Are you saying there may be signs on the enamel on the outside of the motor?
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Old May 25, 2012, 02:03 AM
Grumpy old git.. Who me?
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It's the green insulating enamel on the iron stator core that had burned, just very locally to where the rubbing occurred but it shows how much heat must have been generated. I've touched it up with some green enamel paint

here's a pic showing where the enamel had burned off:

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Old May 25, 2012, 05:47 AM
Grumpy old git.. Who me?
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I've been testing the motors i got recently from Hobbyking. The dirt cheap Turnigy L3040A 480kv is an interesting little motor. At 195g probably a bit too heavy for the 48" EXP's and too light for the 60" but it's of interest non the less.

It's rated for 6s which makes it similar on RPM as a 720kv motor on 4s, but much less amps.

Here's some peak numbers I've just recorded:

1060W
45.4A
23.2V
8239RPM
calculated thrust: 4kg (8.8lb)

My set up:
2 x Gens Ace 3s, 2200mAh, 30c in series
Caste Ice 50 ESC
Turnigy L3040A-480kv
Xoar 15x7

The motor was a little warm after bench testing but not too bad. Efficiency based on mechanical power at the prop vs electrical input power looks very good (80+%). Batteries were cool and showed very little sag (due to modest amps pull).

This motor was $17.34

Steve
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Old May 25, 2012, 07:39 AM
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Originally Posted by JetPlaneFlyer View Post
Here's some peak numbers I've just recorded:

1060W
45.4A
23.2V
8239RPM
calculated thrust: 4kg (8.8lb)

My set up:
2 x Gens Ace 3s, 2200mAh, 30c in series
Caste Ice 50 ESC
Turnigy L3040A-480kv
Xoar 15x7

The motor was a little warm after bench testing but not too bad. Efficiency based on mechanical power at the prop vs electrical input power looks very good (80+%). Batteries were cool and showed very little sag (due to modest amps pull). This motor was $17.34

Steve
That's and interesting find. It's almost cheap enough to act like a disposable motor. You can replace it every 3 months regardless of its condition and still come out ahead compared to $80 motors that last a year under 3D abuse.

I will add that my Watts-out formula shows that on 23.2v, the motor is operating at a Kv efficiency of 73.985% at 8239 RPMs, and 8239 RPMs with a 15x7 prop with a pK of between 1.03 and 1.06 is 816 to 839 Watts-out. So there is still a 220W Watts consumption and the typical 9 to 10 Amps being used to generate the unused power, and this is probably where the heat is coming from.

I just shot the inside lining of my desk lamp with a 60W bulb with my Cen-Tech and got 40.55 C. Nothing scientific here, just an analogy that 60W is generating what I need, plus a whole lot of what I don't need here in Florida, just like some motors.

In comparison, my 683Kv has an 87W margin with the 14x7e, and the 600Kv on 5S has an 81W margin with the 13x6.5e.

Using the same air density/temp constant I used for the chart I posted a few days ago, this motor with the 15x7 shows a full Kg increase in static thrust over top-rated 750Kv with the 14x7 on 4S and an airspeed of 1 mph less (54.6) than the 683Kv with the 14x7 on 4S. The Watts-out is only 33W more than the 750Kv with the 14x7 too.

With this said, this still seems like an acceptable trade-off to swing a 15x7 on the right plane under 4 pounds. There may be some good 51 to 55 inch airframes offered by HK that you guys in the UK and AUS can get delivered inexpensively. I think HK recently came out with something to go head-to-head with the 51 Slick 540 and this may be a good power plant for that airframe.
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Old May 25, 2012, 07:50 AM
Grumpy old git.. Who me?
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Jim,

I think Pk of 1.03 is much too low for a Xoar.. Didnt you get a figure of about 1.13 from your own testing?.. that would give efficiency around 85%

Anyway.. you read my mind.. Using this motor in the Hobbyking Slick is exactly what i have in mind. If it self destructs then as you say; no great loss

Steve
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Old May 25, 2012, 08:08 AM
Grumpy old git.. Who me?
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I trawled back through the thread.. both of our memories were playing tricks on us From your own testing you got a average Pk value of 1.083 for the xoar.. That calculates out at about 860W out and 81.5% efficient. ....that's pretty good

http://www.rcgroups.com/forums/showp...&postcount=342
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Old May 25, 2012, 08:10 AM
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Originally Posted by JetPlaneFlyer View Post
Jim,

I think Pk of 1.03 is much too low for a Xoar.. Didnt you get a figure of about 1.13 from your own testing?.. that would give efficiency around 85%

Anyway.. you read my mind.. Using this motor in the Hobbyking Slick is exactly what i have in mind. If it self destructs then as you say; no great loss

Steve
We did find that pK seems to vary as lenght and/or mass increases, and 1.13 was the relative pK for the Xoar against the constant of 1. The problem is that I don't have enough data to create a relative constant between sizes, so I use the lower range of published constants for e props, which seems to be between 1.03 and 1.06.

Another point is that I have never used a constant less than 1, and I have never seen one used in the physical formulas I have used, but I did see that it is possible. Moving the baseline from 1 to zero would move the Xoar pK to 1.03, but trying to hit a moving target without sound data to work from caused me to use the same pK for composite props and the same pK for Beechwood props and let the Kv efficiency dictate the rest. Besides, unless I can state for a fact the the pitch is perfect on all props tested against each other, a few 100th of a tick will not change anything as significantly. If the prop loads the motor more than another prop, it will show up in the RPMs.

The bottom line is that I use the same two constants for all props of the same material, and therefore the data is somewhat relative between materials, and certainly within the same population. I think that if you put an APC on that motor, we should be able to agree on the output.

I saw that Slick a few weeks ago but I would have the same issues you have getting planes from here... the cost of transport, quality, and customer service EF provides make the HK products less attractive and only marginally less expensive. But it seems like a no-brainer for you guys... let me know how it works because it looks like a good setup at a really good price.
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Old May 25, 2012, 08:25 AM
Not as Good as The Kid
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Originally Posted by JetPlaneFlyer View Post
I trawled back through the thread.. both of our memories were playing tricks on us From your own testing you got a average Pk value of 1.083 for the xoar.. That calculates out at about 860W out and 81.5% efficient. ....that's pretty good

http://www.rcgroups.com/forums/showp...&postcount=342
Changing the pK will only change the Watts-out, not the Kv efficiency. A 1.083 pK makes the output 858 Watts-out but doesn't change the fact that you recorded 8239 RPMs. My KvE value is calculated by the difference between how many RPMs 23.2v should turn a 480Kv motor with no resistance, and what it is actually turning.

8239 = (23.20v * 480Kv) * 0.73958

This shows me the work the motor is capable of versus the actual work, and all motors handle the load differently. Watts-out is just a product of (Diameter^4), ((RPMs/1000)^3), pK, and pitch.

Changing the pK to 1.083 reduces the Watts margin by 18W to 202W from 220W.
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