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Choice of power?

Old 06-26-2003, 09:55 PM
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Default Choice of power?

Hello;

I have now completed enough of my Gee Bee Z that it is ready to take on motor/esc/batteries and prop. All the airframe needs now is the cowling and wheel pants - I think it can fly without those for now...

Can you help me with my numbers here? I'm fairly new to e-flight, but I have a few different proposals here and I'm wondering which are realistic.

First, the airframe: It's a low-wing model with about 48" X 8.5" wings (408 in sq.). The dry airframe weighs 1130g. To get the CG right, I'm going to need about 850g of "power", all hung on the firewall somewhere, bringing the total weight to (deep breath) 4.3 pounds, or about 70 oz., making the wing loading 24.6 oz/sqft . Should be a pretty sporty fly, don't you think? The build came out a little heavier than I had hoped...

Anyway, I was doubting for a bit that it was going to take to the air at all, but I started shopping around and checking with MotoCalc and the results look fairly encouraging.

MotoCalc gave me some reassuring numbers, and seems to be suggesting:

Hacker B40 11S , 4.5:1, with 11x10 prop, on 7 cells
Kontronic Fun 400-42 , 3.5:1, with 10x7 on 7 cells
Hacker B40 12S , 4.1, with 11x10 on 7 cells

In all cases, it used SanyoRC300's, a 2750mAh NiMH cell.

That's great, but I don't really know that I have a source for this equipment. I do most of my shopping at Hobby-Lobby and it looks like they carry a different line of motors. So, flipping through their catalog, and looking for about 260W (60 per pound for ROG), I'm considering:

Strontium 150 on 10 cells, 2:1, with 11.5x7, produces (or consumes?) 309W (big prop, pulls 54 amps, yuck!)

Groing brushless:
AXI 2808/16 on 8 cells with 9.5x5, 280W, draws 28A (better)
AXI 2808/20 on 10 cells, 9.5x5, 319W, draws 26A (even better)
AXI 2820/12 on 14 cells, 9x6, ???W, draws 25A (sounds great, but I'd need a new charger - not sure I can fit 14 cells in plane!)

Back to cheap cans:
Speed 600 6V on 10 cells, 2:1 with 10x6, 294W at 24A (sounds good, and cheap)
Speed 700 BBTurbo 8.4V on 10 cells, 2:1 with 14x7 399W at 32A (plenty of power, big prop though)

I'm leaning toward the AXI2808/20 . But I'd like to appeal to the seasoned veterans for guidance. Can you tell me what you use perhaps, in a similar plane, and where you get it?

Thanks!
Dan.
Old 06-30-2003, 03:24 PM
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Default Choice of power?

You need more than 60W/lb on a scale plane with that high a wing loading. I wouldn't settle for less than 80W/lb, brushless. Methinks you'll be a might disappointed with performance otherwise.

Trying to put that much power through those smaller AXi motors (the 2808s) is a recipe for trouble. The 2820/12 is closer, but I think the 2820/10 would be a much better fit. You can use fewer cells.
Old 07-10-2003, 05:28 PM
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Default Choice of power?

Matt;

Thanks for the input - I have a AXI 2820/10 with an 11x6 on the way (I may move up to an 11x7 later).

I'm shopping for cells now, and was about to get the Sanyo HR-CP2600's (a NiMH battery), but will they handle discharge to 40 Amps? The internal resistance is 4 - 4.5 milliOhms, if that helps...

If not, does anyone know of a NiMH ten-pack, around 20 oz, that will provide a 40A current?

Thanks!
Dan.
Old 07-11-2003, 11:14 AM
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Default Choice of power?

The small motors just don't last By the time you buy 3 of them at
4 long flights ea the brushless is paid for. Like BykrDan said
trouble. CU

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