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Old 08-12-2017, 02:33 AM   #1
Mad Max
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Default What Motor for 5 or 5.5 prop?

I'm scratch building a Britten-Norman Islander from plans for a 60" wing span with two 10 size glow engines. For the electric conversion, I have reduce the size to 36" wing span. I'm projecting the total weight to be about 30 ounces. My question is what two motors can be used given that the maximum prop diameter is only 5.5"? For the life of me I can't seem to find any appropriate motors that can turn this size prop without getting into very high KV ratings that seem to be for quads. Any help is more than welcome!
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Old 08-12-2017, 03:36 PM   #2
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What rpm do you want that prop to turn at?
What battery voltage?

Note that the Kv motor parameter (in rpm/volt)is not a rating, not a figure of merit.

It's all about what the motor wants to do, Kv, versus what the motor can do, i.e. max.current and max.power ...
  1. Kv and voltage determine how fast motor wants to run.
    rpm_noload = voltage × Kv
  2. Rpm and prop determine torque, torque in turn determines current.
    current = torque × Kv (in SI units!)
  3. Max.current and max.power determine whether that voltage/motor/rpm/prop combo can run without going up in smoke
  4. Current is proportional to proppitch¹, voltage², Kv³ and propdiameter⁴.

Originally Posted by scirocco View Post
While an absolutely critical part of the system ...
Kv is actually the item one should choose last.
  1. Decide your peak power requirement based on the weight of the model and how you want to fly it.
  2. Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
  3. Pick a prop that will a) fit on the model and b) fly the model how you want - often as big as will fit is a good choice, but if high speed is the goal, a smaller diameter higher pitch prop will be more appropriate.
  4. Look for a size class of motors that will handle the peak power - a very conservative guide is to allow 1 gram motor weight for every 3 watts peak power.
  5. Then, look for a motor in that weight range that has the Kv to achieve the power desired with the props you can use - a calculator such as Ecalc allows very quick trial and error zooming in on a decent choice. For a desired power and prop, you'd need higher Kv if using a 3 cell pack compared to a 4 cell pack. Or for a desired power and cell count, you'd need higher Kv if driving a smaller diameter high speed prop compared to a larger prop for a slow model.
The reason I suggest picking Kv last is that prop choices have bounds - the diameter that will physically fit and the minimum size that can absorb the power you want. OTOH, combinations of voltage and Kv are much less constrained - at least before you purchase the components.

So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, eg limited prop diameter if it's a pusher, or you already have a bunch of 3S packs and don't want to buy more, and so on.

Minor lay-out changes by RvS

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Old 08-12-2017, 06:00 PM   #3
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Thanks for the reply. I plan on using a 3 cell LiPo, 2200 mAh battery. I don't know what the prop rpm should be. All I know with my limited knowledge is that smaller props usually go with higher Kv motors and larger props run with smaller Kv motors. Beyond that and Ohm's Law I'm a glow guy trying to get into electrics. I'm just trying to find 2 motors that can run with 5 to 5.5 inch props. One option that was suggested to me was to get, say, 1800 Kv motors and some 5", 5.5" and 6" props (the 6" cut to 5.5") and a watt meter and try out various combinations. Not too scientific, but . . . maybe a good first step??
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