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Old Apr 16, 2012, 10:45 AM
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Joined Apr 2012
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Motor, Batteries, and ESC selection

Ok so this might sound a little odd, and is perhaps really simple for most of you out there, but I don't know that much about rc aircraft or their basic construction. I am in the process of building a new prototype machine, and am trying to determine the best setup to purchase for my power supply system. Currently my design requirements are as follows.

RPM requirement: 25,251.73
Torque requirement: 0.37-1 N m
Power Output of Motor: 2644 W
Rotational Direction: Clockwise and Counter Clockwise (No difference in power between direction of rotation)

I need to find 2 identical motors that can rotate in different directions with the same torque and rpm. The motors I am looking at are the Tacon Big Foot 160 Brushless Out Runner Motor for Airplane (245KV), as they appear to meet these requirements. The problem is that I do not know if they can rotate in opposite directions, I don't know how to choose an ESC, Batteries, Controls, so on and so forth. I also do not know how much cooling they will need as they will not have props attached to move air. If anyone has any advice or can point me towards some place that could give me a little guidance it would be greatly appreciated.

Thank you very much
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Old Apr 16, 2012, 11:01 AM
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Doug Bartley's Avatar
Canada, ON, Owen Sound
Joined Oct 2008
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Motor direction can be either CW or CCW, , changing any two wires from the ESC to motor does that.
You will require TWO esc's to control each motor individually, they should match each other.
Without airflow, I doubt your motors would survive long at 2400watts, you may have to devize a means of blowing air over both motors and their escs.
The motor at 245 kv, even on 12S (most practical peak voltage readily availble) will only reach10,780rpm (no load). You would need a motor with approx 450kv, that would put about 300A requirement on the esc and huge batteries to deliver it. FWIW Doug B
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Old Apr 16, 2012, 11:09 AM
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Thank you for the information. I figured the motors would be able to change direction fairly easily, and that I would need to incorporate some form of active cooling on the motors. How to I go about selecting the right ESC and batteries? And should I assume that if I need 2 ESCs that I will also need 2 Battery sets as well as an addition ESC and Battery for cooling fans? I apologize for my complete lack of knowledge. Conceptual physics is more my strong suit, not so much with the practical application you might say...
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Old Apr 16, 2012, 02:58 PM
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Blackpool, Great Britain (UK)
Joined Dec 2003
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2644watts shaft output, at [say] 80% motor efficiency, requires 3305 watts input from the battery.

On 12S [aprox 42volts], the current will be 79 amps.

80% efficiency implies a %age no-load speed of about 0.85. So, for an operating speed of 25,251 rpm, the required no-load speed is 29,700 rpm. At 42volts, this indicates a Kv of about 700 rpm per volt.
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Old Apr 16, 2012, 03:25 PM
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Thank you for the input. I should have mentioned in my original post that the requirements I put up were maximum requirements. I would like to get as close to them as humanly possible. None of the motors I have found operate with a Kv of 700 and an Output power of 2600 watts or so. I can come down quite a bit on these requirements if I must, but in an ideal world these specifications would be really great. Thank you for the help.
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Old Apr 16, 2012, 03:42 PM
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I should rephrase my original question because while your replies are extremely helpful I find I am confused about what it is I need. What do you think would be the ideal battery and ESC for the Tacon Big Foot 160 Brushless Out Runner Motor for Airplane (245KV)? I realize now that the rpm's will be far below what I need them to be but I can gear it to the correct rpm and still get the right power. All of my previously mentioned design requirements include adjustments for ineffeciencies and losses, and as such they are already compensated for. If my actual rpm, torque, and power are a little lower than desired it will be fine.
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Old Apr 16, 2012, 03:53 PM
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Thank you both very much you have definitely opened my eyes and I am going to quit bothering you until I have selected appropriate engine, based on what I have learned here. Thank you very very much .
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Old Apr 17, 2012, 09:59 AM
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Tucson
Joined Nov 2009
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Whoa, Charlie!

Those rpms will probably require the prop tips to go over the speed of sound! rpms are probably too high. Go with a bigger bite prop to get speed, but slow down the rpms!!!

Bigger electric motors typically run in the 9,000 - 10,000 rpm range. The bigger (electric) props are more efficient than the smaller glow props. Think about 16 - 18 inch props.
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Old Apr 17, 2012, 10:36 AM
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You're absolutely right. What I am turning is definitely going to go faster than the speed of sound. Fortunately I am not using it to turn a prop so it's not really a concern or ducted fan so it's not really that important. But yeah it is definitely going to be moving like a bat out of hell. lol
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