Could use some sage advice on electric motors... - RCCanada - Canada Radio Controlled Hobby Forum
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Old 02-15-2006, 05:44 PM   #1
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Could use some sage advice on electric motors...

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Hi, one & all.

I could really use some sage advice. I've been struggling to make sense of the virtual Niagara of dope on the subject of electric motors. There seems to be no common set of standards to which suppliers, merchants & admen adhere when it comes to describing their stuff. To a lot of people this must result in complete confusion, black depression and ultimately, suicide or worse.

I have cobbled up a "Wattage" Piper Cub 400, WS = 46.5" / WA = 330 sq" / Wgt.RTF = 1.4lb (22.4oz) and now I'm trying to find just the right motor. The paperwork that came with the kit calls for a "direct drive 400 type motor." Do you think I can find any common component with which to identify a "400 type" motor? Well, no. Even the kitmaker adds to the confusion by suggesting on the side of the box a "direct drive 380 motor!" What!! If either one will do, does that mean they are the same? If so, what sense could be made of that? Every question has two or more answers.

All I want is a direct drive motor, brushless I'm thinking, which, when hooked up to a 2 or 3 cell LiPo will provide enough muscle to fly this plane off water (floats) and snow (skis) and have enough for entertaining performance, and decent endurance. I specify direct drive because I'd rather have a high speed 6-7" prop doing the pulling than a geared unit swinging a large one - if that is practical. I think models tend to look more toylike with an oversized prop on the nose.

If anyone would be kind enough to suggest a motor, a place to start, or a simple formula I can use to make an informed decision when trying to choose between the hundreds of possibilities in an increasing and chaotic supply, I would be eternally grateful.

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Old 02-15-2006, 05:59 PM   #2
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Yes, i attribute my hair loss to trying to figuring the jibberish.

When you start looking at all the numbers/codes for the motors available it is mind boggling. Someone could do some good by putting together a cross reference sheet that covered all of the motors available to us.

For engines we can generally compare by the displacement of the motor. When it comes to the electric motor model numbers I am lost. I know a bit about AXI - but I cannot tell you how AXI xxxx compares to E-Flight xxxx or Mega xxxx or Hacker xxxxx or Astro xxxx. None of them use the same codes.

A numbering standard from the manufacturers would alleviate our problems. In the meantime some electric guru can jump in here and help us. Ideally if such a cross reference sheet were made it would be great if it were posted as a reference document here on RCC on the Home Page.
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Old 02-15-2006, 08:08 PM   #3
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What really matters are the motor constants and usage limits such as max rpm, maximum current and maximum power output. Some of the latter vary depending on the cooling available.

If you don't want to crunch numbers or use a program, many manufacturers publish performance data with prop, power, (sometimes static thrust), efficiency and rpm for a given input voltage. Studying these charts and watching your pitch speed requirement can give you a pretty good idea how a setup will work.

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Old 02-21-2006, 10:27 PM   #4
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Most electric motor manufacturers will recommend a prop range for the direct drive motors. What you really need to know is performance vs watts per pound.

100 watts/lbs = outstanding
75 watts/lbs = good performance

so you want a motor that delivers about 150 watts (1.4 lbs plane) or 110 watts (still good for what you want to do). At that point the prop size and pitch is exactly the same as glow... ie diameter = thrust and pitch = speed.
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Old 02-21-2006, 11:44 PM   #5
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I'll take a shot
E-flite Park 400 outruner
3 cell 2100 mah thunder power lipo
e-flite 20 amp speed control

prop 9x7 to 11 x7
good to go
should give you 1.7 lbs thrust on a 1.4 lbs plane so water takeoff should be no problem. I would also guess flight times in the 20 - 30 minute range of scale like flying
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Old 02-22-2006, 07:10 AM   #6
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There is another post here containing information on how to use the "free for 30 days" MotoCalc" software. This software is specifically designed to help in analysing motors for electrics. You may want to read through the thread:

Hope this helps.


Wayne Miller
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Old 02-23-2006, 02:56 PM   #7
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Thanks to everyone who replied. A lot of useful information there.
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