Glow to electric conversion

I have been searching and reading all I can about how to choose motor size for a plane that has been specified for a certain size glow
engine. Although the information I have been able to find is both consistent and informative, there is one thing that does not add up:
Let's look at a .40 size trainer as an example. It weighs around 5-6 pounds. From what I have read, a power loading of 75-100 W/pound is recommended for trainers. This means I would need 375-600 Watts of power for this plane.
It is standard practice to put a .46 on .40 size plane, even for a trainer. The OS .46 LA is rated at 1.18HP, or about 880 Watts. The OS .46 AX is rated at 1.63 HP, or over 1200 Watts.
Why does an electric only need half (or even a third) of the power you'd have used with a glow engine?
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On Saturday, May 25, 2013 5:51:23 AM UTC-5, H wrote:

I think you are right. I can't imagine having a less power for a trainer. You need the power to get out of rookie trouble. Though, a .46 AX on a trainer will make it snap roll nicely. I say go bigger on the electric. mk
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First off, a glow .46 is overkill for a .40 size trainer. It's just that there aren't too many .40 engines available anymore.
secondly, you're talking about max power output on the glow engine. One doesn't run the engine wide open all the time. Usually it's about 1/2 throttle.
Anyway you're quoting "rules of thumb." Rules of thumb vary along with thumb sizes. ;>)
CR
On 5/25/2013 5:51 AM, H wrote:

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On 2013-05-30 03:34:27 +0000, Charley38 said:

Could the increase in efficiency with the electric system be part of the reason for needing less power ?
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"H" wrote in message
I have been searching and reading all I can about how to choose motor size for a plane that has been specified for a certain size glow engine. Although the information I have been able to find is both consistent and informative, there is one thing that does not add up:
Let's look at a .40 size trainer as an example. It weighs around 5-6 pounds. From what I have read, a power loading of 75-100 W/pound is recommended for trainers. This means I would need 375-600 Watts of power for this plane.
It is standard practice to put a .46 on .40 size plane, even for a trainer. The OS .46 LA is rated at 1.18HP, or about 880 Watts. The OS .46 AX is rated at 1.63 HP, or over 1200 Watts.
Why does an electric only need half (or even a third) of the power you'd have used with a glow engine?
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Try here. Lots of good guys.
http://www.rcgroups.com/aircraft-electric-airplanes-4/
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The HP ratings of glow engines are based on RPMs that are not typically see n in RC flying. So the numbers you quote in HP is an over inflated marketin g number.
Generally trying to equate glow engines with electric motors is a waste of time. Using the watts/pound approach is much more effective.
I find most glow pilots are going to want at least 100 watts/pound for thei r planes. If you are the "too much is about right" type than make 150 watt s/pound your baseline.
But you only need about 75 watts/pound to fly the plane in most cases, but we tend to over power our models. A full scale J3 cub has about 55 watts/ pound equivalent power and it flies just fine. But most RC pilots would wa nt more than J3 cub scale type flight performance.
Here is a FREE on-line e-book that you may find useful.
EVERYTHING YOU WANTED TO KNOW ABOUT ELECTRIC POWERED FLIGHT www.wattflyer.com/forums/showthread.php?t1368 http://www.rcuniverse.com/forum/m_7100376/tm.htm
On Saturday, May 25, 2013 6:51:23 AM UTC-4, H wrote:

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