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Model Forum / Radio Controlled / Air Models / October 2005how to calculate thrust from motor-prop combo?
Hi all, If I am using a 4200 KV motor with a 6.6:1 gearing ratio and a 12x6 prop, how can I calculate the thrust generated by this system? Also, I am using a 25A ESC and a 11.1V 3-cell LiPo battery. Thanks! >Hi all, > [quoted text clipped - 4 lines] > >Thanks! www.motocalc.com Download and use 30 days free, nominal (~35 bucks) cost to keep it. Abel I would like to know how to do the calculations myself...and not have a computer program do them for me. Thanks > I would like to know how to do the calculations myself...and not have a > computer program do them for me. > > Thanks ---------------------------------------------------------- My web page: http://www.geocities.com/Yosemite/Geyser/2126/flyinggadgets.html has the equation and offers a spreadsheet that performs the calculation. Brian | | If I am using a 4200 KV motor with a 6.6:1 gearing ratio and a 12x6 | | prop, how can I calculate the thrust generated by this system? | | Also, I am using a 25A ESC and a 11.1V 3-cell LiPo battery. | > I would like to know how to do the calculations myself...and not have a | > computer program do them for me. ... | My web page: | http://www.geocities.com/Yosemite/Geyser/2126/flyinggadgets.html | has the equation and offers a spreadsheet that performs the calculation. Not exactly. Unless your web site has more formulas than I see here, it doesn't answer the question. Your formula takes RPM as an input -- but RPM is not one of the known variables. I don't know if the actual formulas are documented anywhere, but Motocalc does do a good job at coming close to the actual results. There's also a few calculators out there on the web. If any of them are done in pure javascript, you should be able to pull the formulas used out of those. Also, note that when you say `thrust', I'm assuming that it's `static thrust' that you're interested in. The thrust can and does change with differences in speed.  SignatureDoug McLaren, dougmc@frenzy.com The dandelion swayed in the gentle breeze like an oscillating electric fan set on medium. > | | If I am using a 4200 KV motor with a 6.6:1 gearing ratio and a 12x6 > | | prop, how can I calculate the thrust generated by this system? [quoted text clipped - 23 lines] > thrust' that you're interested in. The thrust can and does change > with differences in speed. ------------------------------------------------------ My equation requires rpm. If you can measure rpm you can solve for static thrust. If you can't, it is of no use to you. Brian | > | | If I am using a 4200 KV motor with a 6.6:1 gearing ratio and a 12x6 | > | | prop, how can I calculate the thrust generated by this system? | > | | Also, I am using a 25A ESC and a 11.1V 3-cell LiPo battery. ... | > | My web page: | > | http://www.geocities.com/Yosemite/Geyser/2126/flyinggadgets.html | > | has the equation and offers a spreadsheet that performs the calculation. ... | My equation requires rpm. If you can measure rpm you can solve for static | thrust. If you can't, it is of no use to you. If you actually have the setup in front of you where you can actually measure the RPM, you might as well just measure the thrust directly. All it takes is a reasonably accurate scale and perhaps a little cleverness. The Kv isn't the only variable involved in a motor. Ultimately there's three variables -- RPM/V, Idle current and resistance, often shortened to Kv, Io and Ri. If you know these three variables, you know all the electrical characteristics of a motor. http://www.csd.net/~cgadd/eflight/calcs_motortest.htm includes a calculator that's in javascript, so you've got all the formulas right there. (So, look at that page, view the source, and there's your formulas, along with some constants for certain motors and batteries.) Ultimately, we almost have enough information to calculate the static thrust of such a setup, without actually having to set anything up. (I'd assumed that that was the original point, right?) We can guess at the internal resistances of the battery, motor and ESC (or we could look them up if we knew the exact models, or could just ignore them, knowing that our results would be too high) and so that just leaves the Io value that we need. Io isn't hard to measure, but if you know the exact motor involved, you can just look it up. SignatureDoug McLaren, dougmc@frenzy.com If you receive email saying "Send this to everyone you know," PLEASE pretend you don't know me. >I would like to know how to do the calculations myself...and not have a >computer program do them for me. > >Thanks That being the case, go to the Maxx Products web page and find the appropriate table entry for your HG20254266 motor/gearbox. How hard do you want to make it? Abel Ted shuffled out of his cave and grunted these great (and sometimes not so great) words of knowledge: > Hi all, > [quoted text clipped - 4 lines] > > Thanks! The program MAY be a better bet. You can play "What If" with it for various props/pitches, gearbox ratios, and batteries, etc. > Hi all, > [quoted text clipped - 4 lines] > > Thanks! ---------------------------------------------- If you measure the rpm while using a zinger or APC prop I can tell you what your thrust is. Brian |
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