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N00B question: Low Cell Count & Higher KV vs High Cell Count & Lower KV

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N00B question: Low Cell Count & Higher KV vs High Cell Count & Lower KV

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Old 02-26-2014, 01:24 PM
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JKEpps
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Default N00B question: Low Cell Count & Higher KV vs High Cell Count & Lower KV

Hello all,

I'm new to electrics, and have converted an Alpha 40 to electric using an outrunner motor and A123 batteries. In doing my homework and research, I'm wondering about an item. I read the "Everything You Wanted To Know about Electrics" eBook posted here, and I'm sure the answer is in there, but it's not jumping out and grabbing me.

My question is, generally speaking, when using an outrunner motor, for general sport flying, what are the pros/cons of using a lower cell count with a high KV motor, vs adding cells and a lower KV motor?

Thanks,

-Jorden
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Old 02-26-2014, 02:24 PM
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Dr Kiwi
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To get the same amount of "power" (watts = V x I) you will need higher amp draw with the lower cell count. Losses through heat are going to be proportional to I squared... so the higher voltage, lower current combo is probably better.

... but, a lower Kv motor will have more turns and thus more winding wire... thus more resistance... so you lose on the roundabouts some of what you gain on the swings!

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Old 02-28-2014, 12:13 PM
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Generally you go for lower amps and higher voltage.

In general, for typical electrics in the 1 to 6 pound range amperage between 10 and 50 amps is a good working range. You can run higher amps but typically I would say, as you approach 40 amps you should consider going to a higher voltage.

As you go to a higher voltage you will spin the motor faster. So a motor of 1000 kV on a 3 cell lipo, 11.1 volts is going to try to spin at about 11,100 rpm. If you put a 4 cell lipo on it it is going to try to spin at 14,400 rpms.

If you use the same prop on both packs the amperage draw on 4 cells will jump a LOT and perhaps higher than your ESC or motor can handle.

So, motor kV, prop size and battery voltage is a balancing act.

If that is still confusing then consider the gears on a car or a bicycle. The higher the kV, the higher the gear. Now, if you want to spin a small prop you can run in a higher gear, a higher kV. If you want to spin a larger prop you run in a lower gear, a lower kV. The car's motor is the same, or your leg strength is the same, you just use it differently depending on what gear you are in.

In "Everything you wanted to know about Electric Flight", look at these three chapters but don't look at them as stand alone discussions. Look at them as three aspects of the same thing, how to balance kV, battery voltage and prop size.

3 ............Sizing Power Systems
4.............Props vs. Amps
45 ...........What Do Motor KV Ratings Mean?

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Old 03-03-2014, 03:53 PM
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Originally Posted by aeajr View Post
Generally you go for lower amps and higher voltage.

In general, for typical electrics in the 1 to 6 pound range amperage between 10 and 50 amps is a good working range. You can run higher amps but typically I would say, as you approach 40 amps you should consider going to a higher voltage.

As you go to a higher voltage you will spin the motor faster. So a motor of 1000 kV on a 3 cell lipo, 11.1 volts is going to try to spin at about 11,100 rpm. If you put a 4 cell lipo on it it is going to try to spin at 14,400 rpms.

If you use the same prop on both packs the amperage draw on 4 cells will jump a LOT and perhaps higher than your ESC or motor can handle.

So, motor kV, prop size and battery voltage is a balancing act.

If that is still confusing then consider the gears on a car or a bicycle. The higher the kV, the higher the gear. Now, if you want to spin a small prop you can run in a higher gear, a higher kV. If you want to spin a larger prop you run in a lower gear, a lower kV. The car's motor is the same, or your leg strength is the same, you just use it differently depending on what gear you are in.

In "Everything you wanted to know about Electric Flight", look at these three chapters but don't look at them as stand alone discussions. Look at them as three aspects of the same thing, how to balance kV, battery voltage and prop size.

3 ............Sizing Power Systems
4.............Props vs. Amps
45 ...........What Do Motor KV Ratings Mean?
Awesome, Thanks for the info.

-Jorden
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Old 03-29-2014, 10:33 AM
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Originally Posted by JKEpps View Post
Hello all,

I'm new to electrics, and have converted an Alpha 40 to electric using an outrunner motor and A123 batteries. In doing my homework and research, I'm wondering about an item. I read the "Everything You Wanted To Know about Electrics" eBook posted here, and I'm sure the answer is in there, but it's not jumping out and grabbing me.

My question is, generally speaking, when using an outrunner motor, for general sport flying, what are the pros/cons of using a lower cell count with a high KV motor, vs adding cells and a lower KV motor?

Thanks,

-Jorden
The Alpha 40 I have was designed to accept an engine or motor
The recommended motor was an E-Flite 25. (The lower KV version and a 4S 3300mah Lipo is what I used with a 60A ESC)
The higher KV version would use a 3S lipo.
Obviously, higher voltage means less current, and slightly less IR loss.
The 4S Lipos I used weigh about 14 Oz.
One problem/concern was that the Alpha 40 I have ended up being tail heavy.
Rather than add a bunch of weight, I ended up relocating the wing about 1 inch
back from the designed position.
Knowing that, I'd recommend using an E-Flite 32, mainly for the added weight.
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Old 03-29-2014, 12:16 PM
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Originally Posted by chuckk2 View Post
The Alpha 40 I have was designed to accept an engine or motor
The recommended motor was an E-Flite 25. (The lower KV version and a 4S 3300mah Lipo is what I used with a 60A ESC)
The higher KV version would use a 3S lipo.
Obviously, higher voltage means less current, and slightly less IR loss.
The 4S Lipos I used weigh about 14 Oz.
One problem/concern was that the Alpha 40 I have ended up being tail heavy.
Rather than add a bunch of weight, I ended up relocating the wing about 1 inch
back from the designed position.

Knowing that, I'd recommend using an E-Flite 32, mainly for the added weight.
How much weight did you avoid?
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Old 04-05-2014, 07:56 PM
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Enough to refill my weight bin! (several ounces) Actually, I had no way of measuring at the time.
Had I known that the problem was going to occur, I'd of likely just used an E-Flite power 32and a
bigger battery in the 4000-4400mah range.
Moving the wing back on the Alpha 40 was fairly easy. Add a bit of reinforcement, move the
Rubber band dowels, add a spacer to the top front of the cutout, and away we go!
I also had the opposite problem with an Advance 25 low wing trainer. Since there was no easy way to relocate the wing,
I had to add tail weight. This was interesting, since different production runs of both models had reputations
of needing weight just the opposite of what I ended up doing!

With that experience in mind, when HF had a sale on digital scales, I bought one!
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