


To clarify, that’s 20 Wattsout.






Thanks for the info AP.
I did put a smaller (Xoar 13 x 6.5) on it. It did tumble better. I think it is due to the large prop screwing things up. I really notice some strange behavior like precession and prop torque with this larger prop. I have an overkill JR in the elevator and some nice Airtronics digitals for ailerons and rudder so I am pretty sure that is not the issue... 





What would happen if I put a 13 x 8 on there AND changed to a 2820 with higher KV?
Or just rewound this 2815 to higher KV? 





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. 





Volts at WOT?






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. 





Will do. Don't forget I am using the 2815 which should produce less power than the 2820 right? Also, I am not complaining at all about the amount of power/ thrust I am getting out of the current 14 x 7 setup. I just feel like the large prop is causing some unwanted effects in certain aspects of the flight envelope.... I am hoping to get the same useable thrust with a prop that is 1" less in diameter if that is possible. But it is looking like I will have to change something other than just the prop to do so....






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. 





OK,
I broke my last 14 x 7 xoar, so I used an APC. At the very first time I went WOT I got 8600 RPMs very briefly. After that it would jump to 8300 RPM briefly and then sag. I will have to set up my spc on my Optima RX to check volts at a later time. 





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.






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. 





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






Aero,
thanks for the analysis. I think where we are differing in our approaches is that you are trying to make the data fit an assumed 'normal' kv x volts / RPM ratio of approx 0.82 (I don't like the term efficiency for this ratio) and then assuming the data must be incorrect if it doesn't fit. Also you assume that this RPM ratio is the same as the actual output/input efficiency of the motor when in fact it usually is not. The relationship of actual RPM vs that calculated by kv x voltage is not a constant. It is a decreasing curve which starts at a ratio of 1.0 on no load and decreases in theory to 0 when the motor stalls (though it would burn out before you ever got there). Yes a good motor with a well selected prop may well end up with a ratio of around 0.8 but it's incorrect to assume all motors with any prop will be anywhere close to this figure, you could very easily be 10% out either way. Bear in mind that mechanical power is proportional to the cube of prop RPM so a 10% error in RPM produces a 33% error in power. Even as a rule of thumb these 'efficiency' assumptions are at best 'shaky'. Attached is a plot from Motrolfly calc. Note the following points:
Steve 

Last edited by JetPlaneFlyer; Mar 12, 2012 at 04:34 PM.




Thanks AP!



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