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How to Find the Right Size Electric Motor ?

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Old 06-15-2004, 11:19 PM
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Air-Wolf-4U
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Default How to Find the Right Size Electric Motor ?

Hi All!
Being new to the electric field I was wondering if there is a "Formula" to choose the right electric motor for a plane? If we were to keep the weight the same and just focused on the power plant would you please offer some suggestions and formula's.....

Lets say we have a plane that asks for a Four Stroke 91 as its power plant. I know that this four stroke turns a 15*6 Prop at 12,000 RPM.

Now if I look at some electric motor's; I see one that turns a 14*10 Prop at 8000 RPM (50 Amps at 24 Cells)...How do you figure out how close this is compared to the glow one? Slower RPM and a bit shorter prop but a lot more pitch.

Would it be better to go to a larger electric motor with the same cells? There is a similar one that turns a 16*8 Prop at 7000 RPM (28 Amps at 24 Cells). This setup has a more manageable Amp rating but it is a slower RPM but a larger prop. How does this compare to the electric motor I mentioned above? For the same weight in batteries, I am turning a larger prop but slower, and at a much lower amp hour rating...is this better?

Any help would be greatly appreciated! Thanks again!

Dwayne
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Old 06-16-2004, 08:15 AM
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Matt Kirsch
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Default RE: How to Find the Right Size Electric Motor ?

You can't really compare a glow engine to an electric power system. As much as half the power put out by a glow engine is wasted whipping the air around. Because an electric motor can easily be geared to spin a larger, higher pitched prop at a much lower RPM, more energy is put into moving the airplane and much less into beating the snot out of air molecules for no reason

What you need to do is fit the electric power system to the plane itself. Looking at complete power systems with performance specs, like you are in this case, is really the ONLY way to fly. The most important factor is power, Watts. Cell count times Amps gives you Watts. Divide Watts by the anticipated all-up weight of the plane to get an idea of the power loading of the airplane. For glow-like performance on a typical sport plane, there is no sense in going with any less than 100 Watts per pound. More is always better.

In your first example, 50 Amps on 24 cells, you have 1200 Watts. That will comfortably fly an airplane that weighs 12 pounds.

In the second, 28 Amps on 24 cells, you have 784 Watts, enough to fly about 7.5 pounds of airplane.

That's a HUGE difference. These two power systems don't even come close to being comparable on the same plane.

Another thing to consider is weight. 24 cells is a bit more than 3 pounds. While the 28 Amp/24 cell combination is easier on the Amps, it's quite heavy for the power it puts out. Such a power system is only good on a plane that has a large wing area, a slow flyer that depends on lift, not power, to fly.
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Old 06-17-2004, 01:30 AM
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Default RE: How to Find the Right Size Electric Motor ?

Hi Matt!
Thanks for the info...I appreciate it!

You had mentioned that the two examples that I gave with electric motor's do not even come close. Would you be able to explain this to me as I do not fully understand.....

If the one engine spins a larger prop but works at a lower amperage, does this give you way less power? Is it all based on Wattage and very little to do with the size of the prop?

I see one motor that turns a 14*10 Prop at 8000 RPM (50 Amps at 24 Cells).

Another Motor that turns a 16*8 Prop at 7000 RPM (28 Amps at 24 Cells).

These are both the same type of motor's; just the one that turns the larger prop can take 30 cells so it will have way more power if I add them. Am I thinking about this all wrong?

Thanks for your help!

Dwayne
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Old 06-17-2004, 08:03 AM
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Matt Kirsch
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Default RE: How to Find the Right Size Electric Motor ?

You got it. Even though the prop is a larger diameter, it has 20% less pitch and is spinning at a 12.5% lower RPM.

More diameter takes more power.
Less pitch takes less power.
Less RPM takes less power.

Overall, a 16x8 at 7000 would take about half as much power to spin than a 14x10 at 8000 in your example. Aerodynamic relationships are not linear; everything works in at least two dimensions. Double the speed, get four times the drag.

The reason we concentrate on Watts is so we know that there is enough power to turn the "right" prop for the plane, whatever that prop may be.
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Old 06-18-2004, 10:37 PM
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Default RE: How to Find the Right Size Electric Motor ?

My head is spining along with the electrons here. I'm currently building a Martin Mariner 84" span, and finished, but without power plants, glo or electric, should weigh in at under 8 lbs.

I had envisaged just fitting two of everything used in the Hangar 9 Corsair conversion, except UBEC, and thought that would be OK. The problem that arose was that I couldn't fit a 15 inch prop because of clearance, both from the fuselage and water, so had to go to somethng below 14".

The following is advice received from a guy here in Japan who specializes in electric powered models. Is this good information?

"Brushless motors have a wide usable voltage range, but when running direct drive the prop choice will pretty much dictate what voltage is appropriate. Volatge and prop size are inversely related - higher voltage means you have to fit a smaller prop in order to hit our target amperage of about 40A in a sport model (up to maybe 50A for a very aerobatic model with AXI 4130) For example, the 4130 really can't use "3S" at all. You would have to fit a REALLY big prop at this voltage, and rpm would be low (rpm is a function of voltage) so airspeed would be inadequate ... Also the total power is V*A = W, so you'd be limed to 11V*40A = 440W with 3S, which isn't enough for a large airplane....

So with the AXI 4130, you really want 6S. Our Polyquest 2600-3S can handle 20A with ease (high voltage, capacity, many lifecycles at 20A), so you need to parallel two of these together to get 40A (3S2P). Then by taking two of these sets and wiring them is series, you double the voltage to "6S". So you need a total of 4 packs per motor. The designation would be: PolyQuest 2600 6S2P (so 5200mAh total, 22.2V nominal)"

The total cost of all components for this setup is around US$1500 which is a lot of loot if it's not going to work for me. That doesn't include a charger.

Additionally, could I draw from one bank of batteries, rather than having a set for each motor (same number of batteries and cost, but a single power source rather than two). What are the advantages and disadvantages??

Sorry if any of these appear dumb quetions, but this is my first foray into electrics, and it's a steep learning curve.

I'd appreciate any advice the experts can offer.
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Old 07-11-2004, 09:43 PM
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Default RE: How to Find the Right Size Electric Motor ?

It is not so difficult. The engines and motors are easy to compare.

Four stroke 90 size – 1.7 HP – 1.7X 750 = 1275 W
We need motor that has the same power.

That means, we need 1275W/ 0.75 = 1700W (0.75 – motor efficiency).
This number defines our battery (battery has almost 100% efficiency).
We start with it.

We can have 1700W on different batteries. Battery with 10 A and 170 V gives you the same power as battery with 100 A and 17 V = 1700W.
Available batteries limit your choices of current and voltage. There are few of batteries only we can use on your plane. Lipos have more voltage and less current, NiMHs are opposite. When you know the battery you are going to use you can choose motor according to the battery’s max current and voltage. Your props diameter (and RPM also) depends on your battery and motor and trying different props you can tune your current and flight duration. Usually known motors have recommended props and RPMs.

Propeller has efficiency itself. Usually, it is 0.5 (for our models). It means, that only half of 1.7 HP is used to drive your plane, glow or electric. Common believe that bigger props are more efficient (0.6 and more), smaller – less (0.4).
Reduction gear adds into the count its own efficiency too. You gain some efficiency with bigger prop and you loose with redactor.
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Old 07-12-2004, 05:23 AM
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Default RE: How to Find the Right Size Electric Motor ?

ORIGINAL: ukrconsul

It is not so difficult. The engines and motors are easy to compare.

Four stroke 90 size – 1.7 HP – 1.7X 750 = 1275 W
We need motor that has the same power.
read Matts first reply... it it more difficult than that...

give me 1700watts and I´ll return 300oz of thrust and 55mph to you... a 91 can not do that. I choose to spin a 20x10 instead of a puny 14x7, which makes the electric setup way more efficiant.

I found it more to be like you need 1.5times the glow hp to equal an "electric horsepower", (I know what a horse power is but you get the picture...)

so 0.67*1250 ~ 850watts, put 850 watts through a 8.8oz Hacker B50 and you get 160oz thrust and 50mph, more like the .91 4stroke...

again, this is only a rule of thumb it does not apply to all setups, but for properly geard motors it is a hint to the right direction
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Old 07-12-2004, 09:54 PM
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Default RE: How to Find the Right Size Electric Motor ?

If you check out Hobby Lobby's most recent R/C magazine ads, they have a pretty good explanation there.
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Old 07-12-2004, 10:13 PM
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Default RE: How to Find the Right Size Electric Motor ?

Use that Motorcalc program, and just enter all your parameters. Seems to be a good start anyway. You can download a free trial program [link=http://www.motocalc.com/motodown.htm]Here[/link]
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