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OS91-4 to eflite Power 110

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Old 12-26-2018, 08:40 AM
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MarkoV
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Question OS91-4 to eflite Power 110

Hi All,
My 12lbs Norseman with floats was powered with an OS91-4 (13-9 prop) by the previous owner. I have an eflite Power 110 drive with 100amp controller and 9S 5000 batteries. Max power is required when taking off from the water at about 25mph. How can I compare available power from that 91 at 25mph?
eCalc says the Power 110 has available thrust at 25mph of 221.8 oz.

Dave
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Old 12-31-2018, 03:53 AM
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Better not do glow equivalencies, e-motors are more flexible and can run at lower rpm (prop more effictive). Different beasts. Also, (2-stroke) glow max. power is specced at useless high rpm.
.Vriendelijke groeten Ron

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Old 12-31-2018, 03:54 AM
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Remember, Kv is not a rating, not a figure of merit. And it is no indicator for a motor's max. power, max.current, max.torque.

An excellent quote from
brushless motors Kv?.
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. On the other hand, 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, e.g. limited prop diameter, if it's a pusher configuration, or if you already have a bunch of 3S packs and don't want to buy more, and so on.

Minor lay-out changes by RvS
Vriendelijke groeten Ron
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Old 01-02-2019, 03:46 PM
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Thanks for that comprehensive list of information Ron. Unfortunately there is just to much information overload for me to take advantage of much of it.

A Power 110 drive claims to be the equivalent of a 1.1 cc nitro and if that is in the ballpark than I should be ok using it to replace the OS91/4. Once I can take it to the field, the wattmeter will have to tell me if the conclusions reached with eCalc will get me off the ground with the eCalc calculated prop. From what I've read the electric drive will give me more power at the slower take off speed than the nitro does and right now that's what my biggest concern is.
I'll also need to be concerned about cooling when flying off the water; Not sure how to prevent water from getting into the components when providing big openings for cooling.
Dave
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