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Old Aug 08, 2014, 04:10 PM
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Motor/Battery/Prop Combination Selection

We are developing a motor/battery/propeller combination selection tool and want to make sure it's layout and content is really what you guys need when you're building a fixed-wing aircraft.

I accidentally posted the wrong image of the mock-up here. Please scroll down a few posts to see the correct image.
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Last edited by blucraft; Aug 09, 2014 at 01:55 PM. Reason: Attached wrong image in discussion
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Old Aug 09, 2014, 02:54 AM
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You might do better posting in the Multirotor section e.g. http://www.rcgroups.com/multirotor-power-systems-846/

In this Aircraft section many of us mainly use 1 motor plus wings for the lift .

Steve
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Old Aug 09, 2014, 01:51 PM
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Thanks slipstick. I didn't realize I was in the wrong group.

I'll edit the original post, because we'd like to do the same thing for fixed-wing builders.
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Old Aug 09, 2014, 01:58 PM
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I've attached an image of the front-end of this tool for selecting a motor/battery/prop combination.

Would this tool be helpful to you guys?

Any feedback is appreciated!
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Old Aug 09, 2014, 05:36 PM
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I'm not sure why you'd start with cruise L/D, when that information just isn't widely available, let alone even discussed, or its effect on power system design understood.

You'd need to be more transparent about what motors are going into the database. Calculators that spit out specific motors that people either don't know, can't get, or aren't willing to pay for are of limited use, even though it's not too hard to look for another motor of similar Kv and weight.
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Old Aug 09, 2014, 07:50 PM
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Good points. Yes, the entire database would be available through the drop-down menu under motors, so you could see any of the motors in the database. And - good suggestion. It would be nice to have a list of similar motors and their price so you can quickly see what other options are out there. We will only offer motors that are widely available on the market.

As for L/D, it seems that most people cruise at an L/D of about 10 to 15. I thought that would be an easy way to figure out how much thrust the user needs. Is there a better way? Generally, the user doesn't know how much thrust they need. They have an estimate on structure+payload weight. What methods do you use to choose a power plant from that starting point?
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Old Aug 09, 2014, 10:11 PM
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For most RC models, cruise power and thus cruise L/D is largely irrelevant. Folks are much more concerned (even if indirectly) with excess thrust, which more directly translates into short takeoffs, 'get out of trouble' margin, strong vertical lines for aerobatics, and for some, 3D capability , ie hover and punch out performance. And that is fairly simply catered for by flying style and watts/pound rules of thumb. That tends to get distilled when it is so cheap and easy to provide huge excess thrust to a no-brainer default of 100W/lb, which on reasonable size props gives very strong thrust margins with typically close to 1:1 static thrust:weight.

Unless ultimate system efficiency is an important design goal, it doesn't need to get much more complex than that. And BTW, I'd be a little surprised if most sports models could achieve better than L/D=10.

Also, on that basis there is already an excellent tool that distils airframe characteristics prop size limits and power supply constraints and desired flying style into a generic motor and prop choosing tool - that's WebOCalc http://flbeagle.rchomepage.com/
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Old Aug 19, 2014, 03:36 PM
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Thanks for the feedback! Just curious - what keeps everyone from using the tools that are currently out there? It seems that one of the most common topics in the RC world is how to choose a motor/battery/prop combination. Why is this so difficult if the tools already exist to help you do that?
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Old Aug 19, 2014, 03:53 PM
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Quote:
Originally Posted by scirocco View Post
Also, on that basis there is already an excellent tool that distils airframe characteristics prop size limits and power supply constraints and desired flying style into a generic motor and prop choosing tool - that's WebOCalc http://flbeagle.rchomepage.com/
I like that one very much as well. I have read criticisms that it is not specific enough in that it models only generic components (although it is specific to APC props). It offers less information than others but it gives me everything I am looking for. And the systems I have bench tested have all performed pretty close to webocalc projections.
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Old Aug 19, 2014, 04:11 PM
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Yoda- What if that information were formatted like what I've proposed above? Would that me more/less useful? I'm looking to help people see how to change their design to get better flight times, performance, etc.
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Old Aug 20, 2014, 03:02 AM
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Quote:
Originally Posted by blucraft View Post
Thanks for the feedback! Just curious - what keeps everyone from using the tools that are currently out there? It seems that one of the most common topics in the RC world is how to choose a motor/battery/prop combination. Why is this so difficult if the tools already exist to help you do that?
It's not that it's difficult it's mainly that many people would rather trust the real world experiences of other people rather than believing numbers on a computer screen, particularly as we all know that if you get the input to a computer program a bit wrong the result can be total garbage.

Of course the advice you get from anonymous strangers could be rubbish too but by the time several people have told you the same thing it gets easier to believe. That's the joy of forums .

Steve
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Old Aug 20, 2014, 03:18 AM
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Antony (France)
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Hi
Interesting project
Particularly the comparison graphs
Good courage
Louis
author of Scorpion Calc freeware
(http://www.rcgroups.com/forums/showthread.php?t=736782)
see menu "propellers"
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Old Aug 20, 2014, 10:47 AM
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France, RA, Cormaranche-en-Bugey
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Hi,

I hope the picture you have posted is not really what your program computes.

30 g motor with kv of 5930, spinning a 3 g APC 12x8 propeller (foam made ?) on a 30 g 3S lipo...

Yes, sure you can do better for endurance !!

This is a tool for FPV right ?

I believe there is 2 ways for such optimisation.

- real life tests,
- deep computation and then... real life tests.

The computation is only here so that you spend less time trying various setups.

But there is no possible final answer, though it is possible to get very very close.

Nice project indeed. Good luck.

One thing why I wouldn't use it : there is no access to the formula used and the data source.

But I'm a nag.
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