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Electric motor questions

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Electric motor questions

Post  Cox International on Wed May 20, 2015 8:54 am

What is typical RPM for electric model motors?

Why can't one use propellers designed for nitro engines on electric motors?

Assumption being that shaft size on electric motors is smaller and therefore the props can't be centered?

Assumption being that electric props have a bigger surface area because electric motor RPM is lower and nitro props would not provide enough thrust?

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Re: Electric motor questions

Post  JasonB on Wed May 20, 2015 9:09 am

Props designed for electric motors are typically more lightly built, because there's no pulsation of power like in an IC engine, so the blades flex less and are subject to less fatigue (each revolution of an IC engine, the prop blades absorb the power stroke, then give a good bit back to power the next compression - They basically accelerate and decelerate every revolution) . This allows them to be built slightly more aerodynamically efficiently than can IC engine props. An electric prop on an IC engine, however, will quickly fail and throw a blade. Their lighter weight also lends less flywheel to the engine.

You absolutely CAN use IC props on electric motors. In my experience, electric props often use just as large (or larger) a shaft hole than to IC engine props. Smaller electric props especially, because they're held on by a collet rather than a simple screw.

I've seen electric motors for different 1/2a scale electric planes that turned anywhere from under 10Krpm to one turning a 4x4 prop at a good bit over 40,000. Its props looked very much like IC props, but were still more lightly built. Slower, larger props are more efficient, so non-speed focused RC planes do tend to turn lower RPM's.

J
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Re: Electric motor questions

Post  duke.johnson on Wed May 20, 2015 10:35 am

If you are trying to figure out the RPM's for a given motor, a good start is, (Kv*volts)* 0.8=RPM's.
So, if you have a 1800Kv motor on 3 cells. 1800 X 11.1= 19980 X 0.8=15984 RPM

As for props, I usually run E-props, they are way more efficient. They will increase run time and lower wattage.
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Re: Electric motor questions

Post  Cox International on Wed May 20, 2015 10:44 am

Thanks for the replies.
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