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Old 04-20-2011, 11:46 PM   #1
krtf4u_corsair
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Default Watt per Pound

While I was doing some studying and learning of electric RC I came across a question. When using the Watt per pound rule, does that take into account the 70% efficiency of a typical brushless motor?

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Old 04-21-2011, 12:21 AM   #2
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Originally Posted by krtf4u_corsair View Post
While I was doing some studying and learning of electric RC I came across a question. When using the Watt per pound rule, does that take into account the 70% efficiency of a typical brushless motor?
Yes it does, but the higher quality brushless motors are getting close to 90% efficiency, such as the larger Hacker motors. (Some of those real inexpensive motors are running under 60% efficiency. If you see a motor that is running over 100 watts per ounce of motor weight, use caution, that's likely to be really pushing the motor windings. One popular motor is rated by the mfg at 2000 watts, the motor weighs in at 12.45 ounces. Thats 160 watts per ounce of motor weight.)

If you've not run across it before, check out www.motocalc.com, free for 30 days, then $$$. You can plug in your models wing span, wing area, bare weight, the motor brand and model, prop, batteries, and this program will let you know if it will fly, and how well it will fly. This program warns the modeler that the 2000 watt motor above will be running with over 400 degrees F on its windings at full throttle. And at full throttle, motor efficiency drops below 60% at 2000 watts)

Modern electric power models are running perhaps 100 watts per pound of model weight and higher. With the proper propeller setup, most any model with proper Center of Gravity, and reasonable wing loading will fly.

My current model is an Extra 330, 78 inch wingspan, 1200 square inches, Hacker A60-16M motor, 12S2P A123 cells, 19x12 APC-E prop that turns 6600 RPM. The model has 140 watts per pound of weight.

This power system hauls that 17 pound model up at 60 degrees and keeps on going.

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Old 04-21-2011, 02:30 AM   #3
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Thanks for the watt per motor ounce rule! I didn't even consider this as an issue. Gee and I thought I was ready to find a power system . Well I guess that why I need motoclac. Thanks a lot!

Oh and sounds like you have you a nice bird there. You 3D with it?

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Old 04-21-2011, 02:47 AM   #4
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Originally Posted by krtf4u_corsair View Post
Thanks for the watt per motor ounce rule! I didn't even consider this as an issue. Gee and I thought I was ready to find a power system . Well I guess that why I need motoclac. Thanks a lot!

Oh and sounds like you have you a nice bird there. You 3D with it?
Hi
Heres a very good free program
http://www.brantuas.com/ezcalc/dma1.asp
Take care dear friend
Yours Hank

"When wild the head-wind beat,Thy sovereign Will commanding, Bring them who dare to fly, To a safe landing."
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Old 04-21-2011, 03:17 AM   #5
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[QUOTE=krtf4u_corsair;802681]Thanks for the watt per motor ounce rule! I didn't even consider this as an issue. Gee and I thought I was ready to find a power system . Well I guess that why I need motoclac. Thanks a lot!

Oh and sounds like you have you a nice bird there. You 3D with it?[/QUOTE]


Nope, with those 12S2P A123 cells and their four pounds, it's marginal in hovering. But, if I wanted a helicopter, would have built one

Take a look:
http://www.wattflyer.com/forums/showthread.php?t=59273

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Old 04-21-2011, 03:37 AM   #6
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Originally Posted by kyleservicetech View Post
Modern electric power models are running perhaps 100 watts per pound of model weight and higher. With the proper propeller setup, most any model with proper Center of Gravity, and reasonable wing loading will fly.

Wing loading-sming loading! At 200 watts per lb wings arent required.

Shoot a an F-16 had double the wing loading of a man hole cover and flew even better than a yard dart!

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Old 04-21-2011, 03:57 AM   #7
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Originally Posted by hillbillynamedpossum View Post
Wing loading-sming loading! At 200 watts per lb wings arent required.
Wrong Wrong Wrong!

With no wings, what handles torque of the motor?
LOL

I agree though, once you get above perhaps 100 or 150 watts per pound, you no longer have an airplane! My Hacker A60-16M motor hauls my 17 pound airplane up at 60 degrees, and keeps on going. Not many full scale prop planes can do that.

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Old 04-21-2011, 03:59 AM   #8
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Im just pulling your chain!

aint you ever seen Larry's sock plane?

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Old 04-21-2011, 04:00 AM   #9
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Originally Posted by hillbillynamedpossum View Post
Im just pulling your chain!

aint you ever seen Larry's sock plane?

Possum
LOL We've got to have fun!

Larry's sock plane???
What's that???

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Old 04-21-2011, 04:24 AM   #10
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Larry took a motor, esc, rx, and a batt pack and tied a pair of socks to it....Theres a vid of it somewhere....it kinda flew.

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