Which motor is more powerful ?

It's anybody's guess, the specs on the HP site don't make any sense. The motor you bought has fewer turns, so the Kv (rpm/volt) should be higher. Also fewer turns generally means bigger wire, ergo higher current handling capability. The web site says 970 Kv for the 31/21, and 1750 for the 31/12, which seems okay, but the current range you cited as printed on the boxes seems bass ackwards. The recommended prop size for both (from the web page) is also the same, and that makes no sense either. The 31/21 motor with it's lower Kv should swing larger props, at a lower rpm. The 31/12 is considerable heavier than the 31/21 by the specs, which suggests a longer stator and magnets/can, but dimensions on the web ste are the same. It's getting clear that the ad writer just plain screwed up. Not much useful info provided by the distributor, but at least the answer to your 'which is more powerful' question is easy. With any given prop and battery pack, the motor that draws more current is the more powerful, which should be the 31/12 unless KMS has come up with their own convention for motor designation. OTOH, the motor with more turns should spin a much larger prop and produce more thrust, even though max current (hence power) may be lower. What do you want, thrust or speed?

Abel

Beats the hell out of me ... just wanted the most powerful of the two. It was a one day sale and they are closed now anyway, so it's academic. The plane is a Pitts weighing in at around 30 oz. I've had WACOs with OS 90 4 strokes that had thrust and torque and were a sight to see and hear ... and have had other gas powered planes that honked ... just now getting into electric and finding the comparisons of power plants a little nebulous and not as cut and dried as with the gas engines. As with everything, glean a little knowledge here and a little there and sooner or later the Penney drops. As for thrust or speed, it being an aerobatic biplane, I would guess that I would want thrust, What would be a good prop for that using the Tannic LI-PO 3 cell 1550mAH, 11.1 v.? .

If the advertised Kv of 970 is anywhere near real, it should be larger than the recomended 8/4 or 9/4. I'd start with an APC 10/4.7 and check that current is within the max rated 13 A. You'll want all the power you can get from a motor that size (about 120-140 watt) in a 30 oz model. At that output, you'll need to provide for plenty of cooling, too.

Abel

Abel Pranger wrote in news: snipped-for-privacy@4ax.com:

Yeah. I'm flying that motor now; I first tried it with a 9x4.7 and it was rather anemic. It perked right up with the 9x6, though. My flights are

With a 30 oz plane on 300 sq in, you need to keep the speed up, so I'd be careful with the 10x4.7. Not saying I wouldn't try it, just that I'd be careful. I'd try it on mine but I don't have the ground clearance.

Mark-

Good point. If it needs more speed, a 9X6 like you are using would be the logical next step.

Abel

They are identical in performance once the input voltage is adjusted for the the turns.

Turns make no difference to power, only to what pack you have to use to get the power.

The fact that they list the same pack for both probably means that is the pack they stock, not that they actually understand antything more about eletric motors than they have wires on and occasionallty spin round.

Hi..

A have biplane ultimate from Phase3 with a Kms 31/21 engine and with kms speed controller (18A) and a 3 elements lipo 11.1V 1050mAh. Th recomendation of the brand on the engine is to use a 8x4, 9x propeller.. but with these propeller sizes the plane haven´t enough power.. I only feel some power with a 11x4.7 slowflyer propeller.. The problem is when in full acceleration, in 1 minute or less if yo put the fingers on the engine you'll get burn 'couse its very very ho and the speed controller its very hot to... and to finish the pac became hot to..

What would be the problem? the propeller to large and pushing to much amps? dont understand, because this configuration its the same tha hobbypeople.net is selling on a ultimate brushless combo.. includin the propeller size..

thks in advance

Tiago Silva Portuga

-- tiagovsilv

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Yes, the motor is pulling too many amps with that prop. Try a 10 x 4.7 prop! That's probably just right.