Pheonix model Sbach .46 prop help

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02-06-2022, 07:38 PM

Angler-Hi

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Pheonix model Sbach .46 prop help

I'm currently building the Pheonix Model Sbach .46....I'm using the recommended Rimfire .46 motor. I'm having a bit of an issue deciding on the prop size. The Rimfire manual for this motor says to use "appropriate" sized prop. That's it. When I look online it suggests a range of 10x5 to 11x5. Maybe I'm feeding too much into it, but a 10x5 seems a little small for this plane. Especially with it being a sport plane. Anyone else ever built this particular model or use this motor than can give me some insight? Thanks.

02-06-2022, 08:31 PM

ron_van_sommeren

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Was the prop suggested by the kit manufacturer, or by the motor manufacturer? Always high propload with lowest voltage, to keep current in bounds.
What is motor velocity konstant Kv?
What voltage do you want to use?
And since prop effectivity increase with diameter: what is the max. propdiameter you could use w.r.t. ground clearance?

Intermission

Current drawn is proportional to pitch

proportional to voltage²

proportional to Kv³

and proportional to diameter⁴

.
Small changes, massive effects.
Some silly, realistic, expensive and hopefully useful examples.

Below an excellent quote about motor selection and powersystem misconceptions, from
brushless motors Kv?.

Quote:

Originally Posted by scirocco

While an absolutely critical part of the system ...
... Kv is actually the item one should choose last.

Decide your peak power requirement based on the weight of the model and how you want to fly it:
Magic numbers for modellers.
Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
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.
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.
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

Last edited by ron_van_sommeren; 02-06-2022 at 11:55 PM.