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Speed 400 to brushless conversion?

Old 03-15-2007, 03:07 PM
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Default Speed 400 to brushless conversion?

Hello Sal,

I've built a Foamflyer So 10 type plane. It is designed to use a speed 400 6V with an 8 cell 2/3 AA 1100 Nimh pack and a 6-3 or 6-4 prop. weight range 16-20 oz ws 36" wing area 252 sq-in. Clark Y 15%.

I have a Towerpro 2408-21 with the same battery pack and a AUW of 20 oz. What prop would be appropriate for use in this plane as a trainer?

Thanks!
Old 03-16-2007, 11:08 AM
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Default RE: Speed 400 to brushless conversion?

Towerpro is a chinese motor. I am not familiar with this type. Please give me the Kv, Io and internal resistance of this motor and I can help you further. This very low cost motor is an outrunner and looks like it might be a little too small for this use, but since it suggests an 8 x 4 prop for use with 3 cells that is what I would use. I tried to find specs on the internets but could not find anything except the Kv (1750) and the amp draw at the suggested prop size. You should get 21 ounces of thrut at 13 amps on a 3-cell battery. This seems a little optimistic to me but it should power your airplane.
Old 03-16-2007, 11:53 AM
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Default RE: Speed 400 to brushless conversion?

Sal I have to use the battery I have.

I got 1.4 amps without prop at 15185 RPM and 10.06 volts measured at the battery while spinning.

I don't know the internal resistance and was wondering how you measure it -- do you measure across any 2 of the 3 motor wires, or do you adjust that figure because there are three actual coils?

Thanks!
Old 03-16-2007, 12:41 PM
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Default RE: Speed 400 to brushless conversion?

Also, sorry to ask additional possibly dumb questions being new to this, but wouldn't 21 ounces of thrust on a 20 ounce AUW provide vertical performance?

I'm actually looking to fly this as a trainer -- I'm also a little confused by that thrust figure and the part where you say the motor may be too small.
Old 03-16-2007, 12:50 PM
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Default RE: Speed 400 to brushless conversion?

Thinking a little further about this electrically, if the coils are delta connected the resistance across any two wires will be the sum of two coils in series on one branch paralleled with one coil on the other branch.

If star connected the resistance measured across any two wires will be the resistance of two coils in series out of the three.

So how is this done -- what is meant by the internal resistance of the motor?

Added today 3/17/07:

In a thread I found people talk a little about this -- here they seem to say you double the measured resistance on a star, and imply you triple the resistance on a delta in order to make the data valid for moto-calc.

Is this true?
Old 03-17-2007, 10:28 AM
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Default RE: Speed 400 to brushless conversion?

That sounds correct to me. I think the motor will work fine and using an 8 x 4 prop will give you more thurst than weight so you have plenty. Now its just throittle management.

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