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are all watt numbers equal?

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Old 01-20-2013, 11:26 AM
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Bobt2
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Default are all watt numbers equal?

heres where i,m lost. ok, watts per lb.???just a general number???. 2 motors. motor e has a 12x6 on 4 cells. i get 627 watts/41 amps on 4 cells. thinking of changing to motor h. rated at 615 watts/43 amps on 4 cells. HOWEVER it is rated with a 14x7 prop. 2 inches bigger. so, is it going to be a big improvement?if it matters ,it is a 52" 3-d airframe. thanks......
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Old 01-20-2013, 12:56 PM
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Walt Thyng
 
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Default RE: are all watt numbers equal?

motors can be wound to do different jobs like go fast or have gobs of power. I suspect that motor "H" has a lower Kv My guess is that you wouldn't be happy with a 12/6 on "H"

To answer your basic question: a watt is a watt is a watt; just like a horsepower is a horsepower. But how you deliver that power makes considerable difference in how it performs. A 500 hp corvette is not going to pull the same load as a 500 hp turbo diesel. On the other hand, the Vette's gonna get there first.

Kv (rpm/volt) is what makes the difference on an electric motor: the lower the Kv the more torque and vice versa. (you don't see any 250 Kv EDF motors)
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Old 01-20-2013, 01:13 PM
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Bobt2
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Default RE: are all watt numbers equal?

ok, yes. i think understand. motor e is 820 and motor h is 700. so h is going to turn a bigger prop, slower. probably just what i want.
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Old 01-20-2013, 04:05 PM
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Default RE: are all watt numbers equal?

BRAND H is also known to under rate their motors ...
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Old 01-26-2013, 07:39 AM
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Default RE: are all watt numbers equal?

One useful "rule of thumb" is that motors can cope with an input about 3W/g of motor weight.

Obviously it is horses for courses when choosing the correct Kv for your motor... for the same input power you can get more thrust/less speed with a lower Kv motor running a larger diameter/lower pitched prop..... but if you need speed, then choose a higher Kv motor, with a smaller diameter prop with higher pitch.
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Old 02-18-2013, 09:30 PM
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Default RE: are all watt numbers equal?

One method of getting a rough idea of the correct prop size with an electric motor involves using a desired RPM.
I usually use 10-12k RPM.  Max prop length is usually determined by ground clearance.  The Motor ratings as to watts, voltage, and current
along with kv are also factored in with programs such as E Calc.

10-12k was chosen based upon the motors and propellers I generally use.  A smaller lighter plane might use a higher RPM with a shorter prop.
My common propellers sizes are between 12" and 14", with pitch and number of blades dependent on the motor ratings.
6 to 8 pitch is also common.  Each propeller size and type has a maximum safe RPM set by the MFR.
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Old 02-19-2013, 03:58 AM
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Default RE: are all watt numbers equal?

Note that the actual rpm of a motor will not be the theoretical based on the motor KV and battery voltage. It will be lower. How much lower depends on the motor used and the motor load. Typically the higher quality motors drop less rpm than the cheaper ones.
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