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Emax 1200kv motor/prop

Old 07-25-2014, 06:14 PM
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Theplumber
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Default Emax 1200kv motor/prop

I just received a emax 1200kv motor, which on it says 9-10" prop. What will happen if i run a smaller prop? say 5-8"
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Old 07-25-2014, 07:26 PM
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kyleservicetech
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Originally Posted by Theplumber View Post
I just received a emax 1200kv motor, which on it says 9-10" prop. What will happen if i run a smaller prop? say 5-8"
Putting a smaller prop on the motor will not hurt anything. On the other hand it will really reduce the power output. Maybe to the point where your airplane won't fly!
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Old 07-25-2014, 07:28 PM
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Thanks
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Old 07-25-2014, 07:46 PM
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There are HUGE differences in props. An 8" slow fly (fat) may draw more power than a 10" skinny prop.
But, in general, amp draw goes down with prop size.
Too small, harmless to the motor, but it may be underpowered
Too big, may overheat and burn up the motor. Over propping is common in this sport with people trying to get increased performance for nothing.....they think.
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Old 07-25-2014, 07:52 PM
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all my props are skinny, and sized for all my motor's or smaller, so i guess im ok, as long as it make's enough power, thanks
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Old 07-26-2014, 01:25 PM
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ron_van_sommeren
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This is the Brushless Motor Construction subforum. You'll get more viewers/answers in the Power Systems subforum.
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Old 07-26-2014, 01:31 PM
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Old 07-29-2014, 10:40 PM
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Yep flew my 24" wing today with the 1200kv emax and 5x5e" prop. It flew but needed full power and was very slow, i will try again with 8x4 tomorrow. (That prop was way to small for it.)
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Old 07-30-2014, 12:02 PM
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Originally Posted by Theplumber View Post
I just received a emax 1200kv motor ...
Keep in mind that ...

Last edited by ron_van_sommeren; 07-30-2014 at 03:24 PM.
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Old 07-30-2014, 10:37 PM
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Flew again today with 8x4 prop, had to modify ailerons to fit prop in. Flew great very fast, prop made all the difference in the world.
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Old 07-31-2014, 01:52 AM
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Originally Posted by Theplumber View Post
Flew again today with 8x4 prop, had to modify ailerons to fit prop in. Flew great very fast, prop made all the difference in the world.
For what it's worth, the formula for input horsepower to a propeller is per below:

HP=P*D^4*RPM*3/1.4E17.

So if your prop has a pitch of 4, a diameter of 8 and an RPM of 9600, horsepower is:

HP is 4*8^4*9600^3/1.4E17.
HP=0.103

Watts is HP*746 or 0.103*746 or 77 Watts.

Note that "^" is the "Y to the x" power on your scientific calculator.
So 3^4 is 3*3*3*3 or 81.

Note that "E" is the exponent, or 1.4E17 is 1.4 with 17 zeros behind it.
(140000000000000000)

If you go to a 10 Inch diameter prop, assuming same RPM you get:
HP is 4*10^4*RPM^3/1.4E17
HP is 0.25 HP or 188 Watts.

If you go to a 5X5 prop, again assuming the same RPM you get:
HP is 5*5^4*9600*3/1.4E17 or 0.019 Hp. That's 14.7 Watts.

That's why a wattmeter is so important when trying different propellers on your power system. What seems to be a fairly small change (ie 8 inch to 10 inch diameter prop) more than doubled the watts input to your motor.

You do need a scientific calculator to figure this out. I've got the
Realcalc APP" Scientific Calculator APP on my cellphone that works very well.

https://www.quixey.com/app/21074804/...13282bb126418e
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Old 07-31-2014, 02:50 AM
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Old 07-31-2014, 12:31 PM
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Originally Posted by kyleservicetech View Post
For what it's worth, the formula for input horsepower to a propeller is per below:
HP=P*D^4*RPM*3/1.4E17
Typo Denny? Should be
HP = P * D^4 * RPM^3 / 1.4E17
using superscript
HP = P * D^4 * RPM3 / 1.4E17
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