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Measuring electric output

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Old 02-24-2013, 11:35 PM
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howardh
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Default Measuring electric output

I'm not sure I fully understand what I'm looking for to get the best performance out of a brushless motor and prop combo. I'm putting together a Hangar 9 Toledo Special. I'm going electric with it and I purchased the recommended sizes of electric power equipment. I have a Turnigy 60 amp speed controller, a Turnigy 670KV 46 size brushless motor and I'm using 3700 mah 4S 35C lipos. I've also purchased a range (3) of electric props of recommended sizes. My question is measuring the draw (?) and what I need to look for with the various props. I have an Astroflight Wattmeter to test it with. Can someone tell me what to look for and what the readings will mean when one prop is higher or lower in draw. Or, can you direct me to a simple explanation. I've found some writings on the web but I get more confused the more I read. Especially when it doesn't relate to RC motors and the application. Thanks for any assistance.
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Old 02-25-2013, 06:03 AM
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Walt Thyng
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Default RE: Measuring electric output

First thing, you've got a great shop in the Orlando area, Bob's. Go there and ask questions.

Okay, as to your specific questions: higher amp draw ALWAYS means less duration. It MAY mean more thrust. If your props are all by the same maker the higher raw probably means more thrust. However, in my experience, An APC prop will usually draw fewer amps than a MA and still produce more thrust. It has to do with prop design and efficiency.

Now it gets a little dicey. In general a larger diameter and lower pitch will produce "tractor-like" power while a smaller diameter and greater pitch will produce "race car" power.

Finally, the Toledo Special is neither a hot rod nor a 3D bird. It is a little draggy. so you need between 80 and 100 watts per pound to fly it as intended. That's where your Whattmeter comes into play; it should tell you the max amp draw and how many watts-in you get with any given set-up. For most of us that's good enough, but if you want to get really techy you need know your motors efficiency in % and take that percentage of the "watts-in" to determine the actual "whatts-out" of your set-up.

Oops, one more thing, download the free Drivecalc program from the net and take time to learn it. It will provide you with a ton of information.

Hope all this helps. Keep asking questions, because the only really dumb question is the one that;s not asked.
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Old 02-25-2013, 12:48 PM
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Default RE: Measuring electric output

Walt,
Thanks for the info. Very educational. I am familiar with and a customer of Bob's for many years, but due to the distance I don't go to often. But it is my choice of LHS.
According to H9 the flying weight is 6.25 lbs. The 3 props I bought are 12x8, 13x8.5 and 14x7. According to what you've explained then there is no exact number for efficiency but a variation depending on what type of characteristics I'm looking for. I'm not that "techy" (evidently) but just looking to get decent performance but not burn anything up with too much draw/heat. I will try for the best cooling/air flow through the cowl and fuse. I know throttle control will be like a rheostat for heat. But I don't want to exceed whatever it is I shouldn't exceed. ??!!?? The motor is rated at 670KV and the speed control is 60 amp. What range would be safe to operate in?
Thanks again,
Howard
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Old 02-25-2013, 08:28 PM
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Walt Thyng
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Default RE: Measuring electric output

Given what info I have, I'd probably go for the 14/7 with the Toledo.

There's an old rule of thumb that you should never exceed 80% of any parameter. With 35C packs that's not a problem for the battery.. For the ESC you'd be safe up to 50amps continuous. Motor, ESc and battery should not be too hot to touch.

Remember your ESC is simply a very fast on/off switch, so your motor sees full voltage whenever it is on.
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Old 03-14-2013, 05:57 PM
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Default RE: Measuring electric output

13x6.5 APC Electric prop using a 4c 3300mah battery and a 60A (or more) ESC
568Watts full throttle ~5+minutes flight time
thrust weight .87:1
This is a bit short of power to me. I prefer at least a 1:1 thrust weight ratio
A larger prop may be more efficient, but, you would need to change to a 3C battery to avoid
motor operation above the rated current.
A 13"x6.5 prop is more efficient than a 12"x8, but you indicated that you had a 12"x8

E Calc is your friend!
And it has the motor you mentioned in the motor spec choices.

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Old 03-25-2013, 04:44 AM
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Default RE: Measuring electric output

I found this in an HK review:

Hi, I used this motor, with a Turnigy Plush 60A ESC, two 5000mah 2s 40c Turnigy Lipos hooked in series (4s total), and a 13x8 prop. Wow, way cool. Loads of power for my Carl Goldberg Eagle 2. Pulls 54 amps total with this set-up. Flight time is around 10-14 minutes depending on throttle. I hope that this helps!
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Old 03-25-2013, 05:59 AM
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Default RE: Measuring electric output


ORIGINAL: Dr Kiwi

I found this in an HK review:

Hi, I used this motor, with a Turnigy Plush 60A ESC, two 5000mah 2s 40c Turnigy Lipos hooked in series (4s total), and a 13x8 prop. Wow, way cool. Loads of power for my Carl Goldberg Eagle 2. Pulls 54 amps total with this set-up. Flight time is around 10-14 minutes depending on throttle. I hope that this helps!
Thank you.
Very helpful!
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