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Old Oct 14, 2012, 07:16 PM
jackerbes is offline
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USA, ME, Ellsworth
Joined May 2008
18,097 Posts
Originally Posted by flydiver View Post
I won't even pretend to dabble in the efficiency realm. I have been following the latest LRK discussion with interest as I have a bare stator looking for a wind and it seemed interesting....and easy.

But the efficiency discussion made me think of the beginning of this thread and how it was found a rewind by one of the better winders on the groups didn't make a whole lot of difference. Jack has done a lot of work on this motor since then and seems to have possibly found otherwise with some winds.
This is all very puzzling.
Yes it is!

I'm not sure that I am one of the better re-winders here, especially with regard to understanding motors and motor efficiency. Maybe I'm just good at getting wire on them and little else.

But when I look at the RPM I am getting out of the slow fly props I am getting those props well up into their useful RPM range.

The maximum safe RPM for APC slow fly props is 65,000 / diameter in inches. I use the same value for the Great Planes props because GP does not publish a limit and I have found the 65.000 limit to be safe in my testing and use. It is just my opinion that GP props are safe to that limit.

So if you look at the RPM I am getting from those props and the safe maximum RPM numbers, the motor looks like it is working well:

Great Planes SF 11 X 4.5 (65,000 / 11 = 5909 RPM max)
3S test RPM = 5245
4S test RPM = 5958

Great Planes SF 10 X 7 (65,000 / 10 = 6500 RPM max)
3S test RPM = 4543
4S test RPM = 5251

APC SF 10 X 4.7 (65,000 / 10 = 6500 RPM max)
3S test RPM = 5724
4S test RPM = 6563

So I get confused when I find this is what appears to be a useful motor but it is found to be inefficient by the efficiency numbers.

The testing with the APC Thin Electric 13 x 8 does not make it appear as useful. The RPM limit for the APC TE props is 145,000 / diameter in inches. So the tested RPM on that prop is:

APC TE 13 X 8 (145,000 / 13 = 11,154 RPM max)
3S test RPM = 4473 RPM
4S test RPM = 5022 RPM

The motor is attaining only 45% of the maximum allowable RPM with that prop. But then, when I look at the prop database at and see that the APC TE 13 x 8 trust to RPM curve shows the prop to have a useful range that runs from 3900 to 6600 RPM I get confused again. The LRK wind on 4S is attaining 76% of the useful range now and, with a turn or two less on it, could probably attain all of it.

I'm not trying to argue with motor experts, just come to where I can understand it all...

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