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converting gp j3 cub 20 size

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Old 02-22-2004, 11:59 PM
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Spadhawk
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Default converting gp j3 cub 20 size

Hi, I have a 90% built gp J3 Cub 20 size that I would like to convert to electric.
Can anyone give me some advice on what motor, speed controller, batteries etc.
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Old 02-24-2004, 09:29 AM
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Matt Kirsch
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Default RE: converting gp j3 cub 20 size

Reverse your shopping list, and you'll already be halfway there. You need to figure out how much power you want/need first, and the motor is not where the power comes from.

I'm going to make a rough guess that this plane will weight about 3lbs ready-to-fly. It's a slow flyer and has a lot of wing area, so it doesn't need an excessive amount of power. For land operations, I would start with 75 Watts per pound. If you're talking about floats, then 100 Watts per pound is what I'd use (the floats will add weight later, bringing the Watts-per-pound ratio back down a bit). So, we're talking about 225 Watts for land operations or 300 Watts for water.

You can use any combination of cell count and Amps to get 225 Watts. For example, 10 cells and 22.5 Amps, 8 cells and 28.125 Amps, or 6 cells and 37.5 Amps. The lower the Amps, though, the better. Lower Amps allows you to carry lighter batteries, or get more flying time out of heavier batteries. Personally, I would run 8 cells at about 30 Amps for land-based operations. That gives you an effective 240 Watts, which is close enough, erring on the side of R-R-R

Now on to the motor choice. I like to keep it simple, because I haven't gotten a handle on gearboxes yet. A quick scout around Hobby Lobby's website yields a few motors that can handle 8 cells and ~30 Amps:

Jeti 15-4 with 8.5x5 prop (~29 Amps)
AXi 2820/10 with 11x7 prop (~29 Amps)

Don't worry about "Watts to prop" figures. The Watts-per-pound rule of thumb I used takes inefficiency into account. Actually, it was originally designed for cobalt motors that are only about 60% efficient, and we're using brushless motors that are 80-90% efficient, so we'll get more effective power and better performance.
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