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Old Oct 27, 2006, 10:30 AM
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rule of thumb for kv of motor needed for wing?

Is there a 'rule of thumb' to determine kv of the motor needed for a wing?

I've been a kit builder, foamie from scratch builder, pusher jet builder , etc for a while. But I'm about to build a zagnuts variant (cuz I don't have a flying wing yet). I have a good number of motors lying around, but really need to know some 'rules of thumb' to determine what should be used. (e.g. given wingloading and size, what would be reasonable kv?)

Since most of my previous planes have been slow fliers, this is a new world, so I have to make certain I have a motor appropriate for the task (and prop). I have a number of ARC-130's lying around, but I believe I don't have a gear ratio 'small' enough to be useful. Most of my BM motors would be geared too slow as well. I do have one or two BM motors in the 1500-1700-ish kv range that may be appropriate if I slap a 7 inch prop on them.


(Assume 3S, if you have useful info 2S, feel free to provide that as well)
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Old Oct 27, 2006, 12:19 PM
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It depends a lot on how many amps you want to pull - 10A is a lot different from 30A-50A!

An ARC-130 (4600Kv) is not suited to DD - even a 4.2x4 pulls 17A @ 6.5v!

An ARC-110 (3100Kv) is not too bad. 4.2x4 pulls 14.50A @ 10.9v and gets you 11.62oz @ 102mph! On 2s you could use a 4.5x4.1 (9.45A @ 6.9v), 4.75x4.75 (12A @ 6.9v), 5x5 (13.2A @ 6.7v) or 5.5x4.5 (14.3A @ 6.6v).

If you need a 7" prop you need less than 2000Kv for sure. A BP 21 (1750Kv) could run a 7x5 APC E, perhaps even a 7x6 APC SF at around its 12-13A limit.

If you want to go crazy get a Medusa 28mm or 36mm and at the right Kv with a 7" prop, you can suck 30-50A!
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Old Oct 27, 2006, 01:05 PM
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I'm thinking of using the BP21-ish (likely 1750 kv range, as they're never quite the same one batch to the next) with the 7 inch prop. Just because I can't find any use for those series of motors anymore, since I started using the ARC-130 with gearboxes. Since I started with decent gearboxes, I haven't used my 'cheap outrunners' any more. I was looking for an excuse to use them. Looks like the BP-21 will be the candidate.

And, I wasn't clear, I'm looking for something 18 amps (preferably much less), as Phoenix Thunderbirds will be the controller, or one of the chinese variants [since this is going to be a 'fun toy' play].

The prop size isn't fixed, I'm just trying to find out what would be appropriate.

But, given that I have motors lying around in the sub 1000 kv range (which would be out from what you suggest), and motors in the 4000 kv range (given available gearbox gearing, those would be out as well), and 'high resistance' 1500-1750' kv motors. Looks like the left over outrunners are going to be the motor to use.
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Old Oct 27, 2006, 01:40 PM
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Get yourself a Todds Models TM2000 gearbox - I'm guessing here - but at 3:1 your ARC 130 might be good with a 7x5/7x6.
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Old Oct 27, 2006, 02:02 PM
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Quote:
Originally Posted by Dr Kiwi
Get yourself a Todds Models TM2000 gearbox - I'm guessing here - but at 3:1 your ARC 130 might be good with a 7x5/7x6.
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Old Oct 27, 2006, 02:18 PM
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Heck, that is a wonderful gearbox - I guess you'll have to get a Cobri 20.
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Old Oct 27, 2006, 02:21 PM
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Quote:
Originally Posted by Dr Kiwi
Heck, that is a wonderful gearbox - I guess you'll have to get a Cobri 20.
Best ratio that can give is 3.4:1 (effective 1294 kv, versus 1466 with the 3:1)

ARC-130 has a 2 mm shaft. (68 tooth spur, and 20T pinion is the best available for a 2 mm shaft).
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