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Author Topic: picking electric motor size based upon weight  (Read 1117 times)

Offline Fred Cronenwett

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picking electric motor size based upon weight
« on: March 26, 2023, 02:23:08 PM »
This blog discusses how to size the right electric motor for your model. They discuss Watts per Lb, I have used this same idea to pick the right electric motor for my models.

https://innov8tivedesigns.com/resources/lucien-s-blog.html

I have no idea how many watts per lb is ideal for CL stunt, I use 120-140 watts per lb for the CL scale models. That way I have emergency power if needed at full power.
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Offline Tim Wescott

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Re: picking electric motor size based upon weight
« Reply #1 on: March 26, 2023, 04:25:08 PM »
Based on a long-ago study I did going through the "list your setup" thread:

For a typical sized stunter, peak usage is 11 watts/ounce (so -- 180 watts/lb) if you're using a constant-speed setup.  7 watts/ounce (110 watts/lb) average (but that's for sizing the battery).

Little "1/2-A" sized ships seem to use more.

I know that an active timer like the Burger or Fioretti timer uses more, but I don't know how much more.
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Brent Williams

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Re: picking electric motor size based upon weight
« Reply #2 on: March 27, 2023, 03:21:07 AM »
In the RC world, 150 watts/Lb is suggested for aggressive aerobatics, 200+ watts/Lb is suggested for 3D flying.  More is more gooder, they say.

150-200 watt/LB is the general range that has been mentioned over the years as effective for stunt use. = 9.5 watt/oz to 12.5 watt/oz.
Tim's 175w/lb (11watts/oz) number is safe to use and given his EE background, it is probably a more informed recommendation than most can give, including any of my comments.

As an example, in a 53-54oz plane, I have used both the Cobra 2820/12 970kv (rated 590 watt) and the larger Cobra 2826/10 930kv (rated 660 watt).  Same prop was used with both motors, a cut down 12x6 prop to 11".  I swapped the motors back to back to evaluate nose weight changes rather than solve any performance needs.  Performance and power seemed basically the equal between both as far as I could tell. 

53oz @ 150watt/Lb = 496 watts required
53oz @ 175watt/Lb = 583 watts required
53oz @ 200watt/Lb = 662 watts required

In my ~64oz stunt ship, the BadAss 3515/580kv on 6s is rated for 1070 Watts.  Using the 200watt/lb figure, my plane's requirement is still only 800 watts.  The plane has excellent vertical and overhead performance.  Cool running system. No complaints!
Laser-cut, "Ted Fancher Precision-Pro" Hard Point Handle Kits are available again.  PM for info.
https://stunthanger.com/smf/brent-williams'-fancher-handles-and-cl-parts/ted-fancher's-precision-pro-handle-kit-by-brent-williams-information/

Online Dwayne Donnelly

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Re: picking electric motor size based upon weight
« Reply #3 on: March 27, 2023, 07:05:27 AM »
I was taught 150 watts per pound for a stunt plane.
My purpose in life is to serve as a warning to others.

Offline spare_parts

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Re: picking electric motor size based upon weight
« Reply #4 on: March 27, 2023, 07:28:45 AM »
As an example, in a 53-54oz plane, I have used both the Cobra 2820/12 970kv (rated 590 watt) and the larger Cobra 2826/10 930kv (rated 660 watt).  Same prop was used with both motors, a cut down 12x6 prop to 11".  I swapped the motors back to back to evaluate nose

Performance would be equal because the Kv is about equal if the prop and battery is the same. The difference in efficiency between similar size motors will be negligible.

We used W/lb and pitch speed in the 90's for RC, back when 100/W/Lb was tough to attain and got 3 minutes total flight (not at full power) on NiCd batteries.
Greg

Offline Tim Wescott

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Re: picking electric motor size based upon weight
« Reply #5 on: March 28, 2023, 02:08:36 PM »
150-200 watt/LB is the general range that has been mentioned over the years as effective for stunt use. = 9.5 watt/oz to 12.5 watt/oz.
Tim's 175w/lb (11watts/oz) number is safe to use and given his EE background, it is probably a more informed recommendation than most can give, including any of my comments.

Oh, all I did was go through the "list your setup" thread and fit numbers to the data.  Don't go blaming my EE background on my cowboy data mining.

I had forgotten the other recommendation that I've heard that seems to work -- which is that if a motor manufacturer lists a weight range for 3D, and your plane matches that motor, it's a good bet it'll work.
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline John Rist

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Re: picking electric motor size based upon weight
« Reply #6 on: April 01, 2023, 07:59:59 AM »
I look at the prop charts for a motor.  If it will turn my desired prop size at the RPMs I want it will probably work for me.   D>K

PS you need head room for your governor to work.
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Online Dennis Toth

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Re: picking electric motor size based upon weight
« Reply #7 on: April 01, 2023, 10:53:43 AM »
For a new model size, I'm a bit lazy so I look through the list your setup section. Many people have experimented with different setups and shared their results so we all can benefit. I like the BadAss series of motors so I then see what Kv matches up with the battery pack I intend to use. Try to play the min pack weigh vs current draw to get the lightest power package I think will work. Then tune the prop to meet the load.

Best,    DennisT


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