Mar 11, 2012, 09:34 PM  

To clarify, that’s 20 Wattsout.

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Mar 11, 2012, 09:54 PM  

The question is, will a 13x6.5 on a true 750Kv turn 9000 RPMs? If the answer is yes, then 750 Wattout is great and the Kv efficiency is only down to 78%. But if you are not getting 9000 RPMs, then I would question the efficiency of the setup. A 13x6 should give you 8980 at 78% of 750Kv and 700W which is also great.
Try to get an RPM reading tomorrow with your current setup first. Volts would be nice too. 
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Mar 11, 2012, 10:09 PM  

Yes... BTW  Tom is getting 8430 RPMs, 670W and 43A at 15.64v on his 683Kv 2820750 with a 14x7 Xoar and you can witness the power in his videos (flying his plane, not mine).
If you are getting additional torque because of the 700Kv wind with the 14x7 prop, then I'm guessing you are getting at least the same efficiency, which would bring your Watts up to about 722, which could be the reason why you are reporting torque that Tom doesn't feel at all. Let's see what your RPMs are tomorrow. His are 8430 and yours will have to be in the 8640 range to get the additional 50 Wattsout with the same 80.4% Kv efficiency. 
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Mar 11, 2012, 10:47 PM  

If you are using a 14x7 on a 700Kv with 4S and there is no voltage change and no Kv efficiency change, then to get the same Wattsout from a 13x7, you will need to raise the Kv of the motor to 780Kv. So at 80% Kv efficiency, the 700Kv will swing the 14x7 8640 RPMs which needs 722 Wattsout, and the 780Kv will swing the 13x7 an additional 1000 RPMs (approximately) and 725 Wattsout.
Just remember that the main variable in the gyroscopic formula is RPMs, not the diameter of the rotating body. This smaller prop rotating 1000 more RPMs many not solve your problem if gyroscopic effect is really the cause. 
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Mar 12, 2012, 01:48 PM  

Right. As I said, 8640 RPMs on 15.35v is 722 Wattsout at 80% Kv efficiency. Considering the same Kv efficiency, you can be registering 3.82v per cell or 15.28v to get 8600 RPMs. If you are sagging to 8300, then the voltage can be dropping to 14.8v, which means 647 Wattsout. This means the 750Kv is the right Wattsout matchup for your 13x8 prop.

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Mar 12, 2012, 02:16 PM  

14x7 with the 700Kv @ 14.8v @80% Kv efficiency
RPMs = (700Kv x 14.8v) x 0.8 RPMs = 8288 Watts = 1.03 x 0.58 x (8.29^3) x (1.17^4) Watts = 637 13x8 with the 750Kv @ 14.8v RPMs = (750Kv x 14.8v) x 0.8 RPMs = 8880 Watts = 1.03 x 0.67 x (8.88^3) x (1.08^4) Watts = 657 Pretty close. 
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Mar 12, 2012, 03:01 PM  

Another reminder that Ken said the new stock DM2820750 are really wound to about 720Kv.

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