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Guide to motor selection - suitable power?


Tony Harrison 2
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I've come across suggestions that as a basic guide in choosing a brushless motor, calculate around 75W per pound weight of aircraft. Does this still hold? I have in mind a fairly hefty high-wing model which should weigh all-up between five and six pounds - which suggests any motor from 450W upwards. So allowing some leeway, paerhaps a 500W? I'm still very much a beginner, have no interest in aerobatics, just want to fly safely & securely. All advice very welcome.

rgds Tony

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75W per pound is a good figure for basic flying, and sounds ideal for your proposed use.

If you wanted a more aerobatic performance aim for around 100W per pound, or at least 150W per pound if you want a decent 'vertical climb' type performance.

 

Note - Motor ratings tell you how much power the motor can handle, not how much it produces with any given prop or battery. For the above figures to apply you need to consider what power your motor will produce with your chosen prop and battery.

 

Dick

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1 hour ago, Dickw said:

Note - Motor ratings tell you how much power the motor can handle, not how much it produces with any given prop or battery. For the above figures to apply you need to consider what power your motor will produce with your chosen prop and battery.

Thanks Dick. Sounds like 500W should be fine for my purposes. I know nothing about props, but initially I'd be using a 4S Lipo rated at 3700 ma/hr.

rgds Tony

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1 hour ago, Tony Harrison 2 said:

Thanks Dick. Sounds like 500W should be fine for my purposes. I know nothing about props, but initially I'd be using a 4S Lipo rated at 3700 ma/hr.

rgds Tony

 

Tony, there are some good threads started by Tim Mackey in the Electric flight for beginners section - sadly they don't seem to be "pinned" to the top of that forum any more, but here are most of the links...

 

https://www.modelflying.co.uk/forums/index.php?/search/&q="for beginners"&type=forums_topic&nodes=57&updated_after=any&sortby=relevancy&search_and_or=and&search_in=titles

 

Sstart there, it will really help your understanding of electric powertrains. This thead is also a good starting point....

 

 

 

Edited by MattyB
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55 minutes ago, MattyB said:

Tony, there are some good threads started by Tim Mackey in the Electric flight for beginners section - sadly they don't seem to be "pinned" to the top of that forum any more, but here are most of the links...

Thanks Matty, but reading all that stuff about kilovolts reminds me of school physics lessons, and makes my head hurt! So long as I end up with a motor of adequate (or a bit more than) power for my plane, I'll be happy to leave the techy background to others. It seems like 500W will do for starters, and I could always upgrade to something with more poke subsequently if I felt it necessary. I'm still very much a beginner: I like scale models, and high-wing planes that are sort of semi-scale, and I can barely fly as yet; I wouldn't wish to move onto fast aerobatic planes in any case, not my thing. I'm in the process of moving from a very light high-wing trainer, to a different high-wing job that's bigger, a fair bit heavier with greater momentum, more solidity in the air - I've handled (under supervision) such aircraft and found them far more "solid" in the air, more confidence inspiring, less blown all over the place by the slightest gust of wind of thermal. I have the plane, plus some bits & pieces to adapt it with (it was originally i/c engined), and needed some reassurance about motor selection. Seems to be sorted now, thanks to the advice from you and others.

rgds Tony

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7 hours ago, Tony Harrison 2 said:

I've come across suggestions that as a basic guide in choosing a brushless motor, calculate around 75W per pound weight of aircraft. Does this still hold? I have in mind a fairly hefty high-wing model which should weigh all-up between five and six pounds - which suggests any motor from 450W upwards. So allowing some leeway, paerhaps a 500W? I'm still very much a beginner, have no interest in aerobatics, just want to fly safely & securely. All advice very welcome.

rgds Tony

Tony, having been down the same road, I would recommend the following;

 

Err on the side of more power than less........nothing worse than a gutless model!.

 

Download "e.calc" and pay $6 for the full version......I would have been totally lost without it.

 

Get a "Wattmeter" essential bit of kit to make sure you do not cook the esc or motor.

 

If you can afford it only buy "electric" rated props.

 

Steer clear of really "cheep" motors, you will end up with a vibrating horror which runs the bearings for a pastime!.

 

During lockdown, I sold everything IC and converted at least eighteen models, best thing I ever did.

 

Its like starting again...............but well worth the effort.

 

Good luck.

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I agree, err on the side of excess power, you can always throttle down, you never know when you might need a quick burst of power.

My Riot flies in a spirited manner on 140 W/lb, my hummer is great fun on 212 W/lb.

If in doubt give George Worley a call at 4-Max, purveyor of high quality brushless motors.

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Tony. I've bought several motors from Airtek, for each motor on their website they have a data sheet giving power output, recommend prop sizes and IC equivalent making it easy to choose the right motor. They also give a dimensional drawing so you can make sure the motor will fit.

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11 hours ago, Tony Harrison 2 said:

Thanks Matty, but reading all that stuff about kilovolts reminds me of school physics lessons, and makes my head hurt! So long as I end up with a motor of adequate (or a bit more than) power for my plane, I'll be happy to leave the techy background to others. It seems like 500W will do for starters, and I could always upgrade to something with more poke subsequently if I felt it necessary.

 

Sorry, but if you are going to do electric flight to any significant degree (especially if you are speccing your own powertrains) you are going to need to roughly understand how motor Kv, pack voltage and propeller interact to give a given power level. Unfortunately you cannot just choose a 500W motor if you want 500W of power - that motor could generate 5W, 1000W (if only once!) or anything in between depending on the pack used and prop attached. It is not that hard, but a bit of reading will be instructive - the motors thread is probably the one to choose...

 

 

This article also provides a nice worked example of how you might choose a powertrain as well based on motor and model specs.

 

If you really don't feel you want to read up then that is your prerogative, but expect to burn up some expensive equipment (and possibly the models they are in if you get it really wrong - lipo batteries do need to be treated with care) along the way. Knowledge is power - ask this guy, who probably wishes he'd thought a bit more carefully about the powertrain and intallation for his 3D printed EDF airliner... 😉

 

 

Edited by MattyB
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He expected two pretty large EDF units to run off a single 4Ah battery, looks like a 4s too. Asking for trouble, they must have been pulling around 50 amps each at WOT no wonder the battery and wiring had a meltdown, Two batteries, two ESCs and a separate receiver battery would have been more appropriate. Must have taken weeks to print all the components.

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Looks like the ESC wire insulation melted due to the high current draw resulting in a short which caused the battery to fail (that's the polite way to put it!). The connectors look like XT60s, which would indicate that the power system is seriously underspecced.

Edited by Shaun Walsh
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12 hours ago, SIMON CRAGG said:

Tony, having been down the same road, I would recommend the following;

 

Err on the side of more power than less........nothing worse than a gutless model!.

 

Download "e.calc" and pay $6 for the full version......I would have been totally lost without it.

 

Get a "Wattmeter" essential bit of kit to make sure you do not cook the esc or motor.

 

If you can afford it only buy "electric" rated props.

 

Steer clear of really "cheep" motors, you will end up with a vibrating horror which runs the bearings for a pastime!.

 

During lockdown, I sold everything IC and converted at least eighteen models, best thing I ever did.

 

Its like starting again...............but well worth the effort.

 

Good luck.

Thanks Simon, handy advice. Appreciate the hint about power. I had planned to use the prop that comes with the plane, originally i/c engined - perhaps this is too heavy and a dedicated electric prop is better. Initially using a 500W motor provided by a thoroughly experienced and very knowledgeable friend, with batteries to match, 4S/3700m/amp, and a 60amp ESC. I wonder if you're from Devon: only ever heard "for a pastime" used by a couple of Devonian native friends...

rgds Tony

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5 hours ago, MattyB said:

If you really don't feel you want to read up then that is your prerogative, but expect to burn up some expensive equipment (and possibly the models they are in if you get it really wrong - lipo batteries do need to be treated with care) along the way. Knowledge is power - ask this guy, who probably wishes he'd thought a bit more carefully about the powertrain and intallation for his 3D printed EDF airliner...

 

Take your [point Matty, but I've acquired some experience with LiPos and motors, after building two traditional kits, outfitting them with motors etc using advice from friends and a good dealer; plus I've kitted out another ARTF job. I am technically quite competent, just get lost with too much electrical deatil & theory! I'll be installing a 500W motor provided by a friend (very knowledgeable & experienced), with the 4S/3700m/amp LiPos originally used with it. I think this will work, and I have the option subsequently of installing a bigger motor.

rgds Tony

 

 

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27 minutes ago, Tony Harrison 2 said:

Thanks Simon, handy advice. Appreciate the hint about power. I had planned to use the prop that comes with the plane, originally i/c engined - perhaps this is too heavy and a dedicated electric prop is better. Initially using a 500W motor provided by a thoroughly experienced and very knowledgeable friend, with batteries to match, 4S/3700m/amp, and a 60amp ESC. I wonder if you're from Devon: only ever heard "for a pastime" used by a couple of Devonian native friends...

rgds Tony

A wattmeter is your best friend, enables you to check that you're not overstressing the power train and enables you to compare the power requirements of different diameter/pitch props. Lastly you can estimate how long you can fly for on a given battery capacity.

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1 hour ago, Shaun Walsh said:

A wattmeter is your best friend, enables you to check that you're not overstressing the power train and enables you to compare the power requirements of different diameter/pitch props. Lastly you can estimate how long you can fly for on a given battery capacity.

Would either the little "Smart Guard" LiPo battery checker, or my multimeter, do the job? I'd prefer not to have to buy yet another gadget.

rgds Tony

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1 minute ago, Tony Harrison 2 said:

Would either the little "Smart Guard" LiPo battery checker, or my multimeter, do the job? I'd prefer not to have to buy yet another gadget.

rgds Tony


No, they are not rated to measure the under load currents we are talking about here. A wattmeter is absolutely a crucial bit of kit for anyone entering electric flight, mine has paid for itself many times over in terms of avoided equipment burn outs and accidents whilst I was learning about electric power trains. They are very cheap these days too - less than just one burnt out ESC…!

 

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PS...

 

2 hours ago, Tony Harrison 2 said:

Thanks Simon, handy advice. Appreciate the hint about power. I had planned to use the prop that comes with the plane, originally i/c engined - perhaps this is too heavy and a dedicated electric prop is better. Initially using a 500W motor provided by a thoroughly experienced and very knowledgeable friend, with batteries to match, 4S/3700m/amp, and a 60amp ESC.

 

You can't just fit a prop intended for the IC version of your model and expect it to generate the same level of power. This is not because it is too heavy (though that won't give any advantages, and IC props are generally a smidge less efficient too) , but because it's the combination of prop, pack voltage and motor Kv that (more or less) define the load on the system, current drawn and thrust produced.

 

Electric motors are more flexible than IC engines in terms of prop choice, and you can achieve a given power rating in multiple ways with the same motor by varying the prop size and cell count. For instance I might be able to get 300W out of the same motor using a large prop on 2S, or a smaller/finer pitched prop on 3S or 4S. The former will draw more current and therefore reqire a bigger ESC, and probably be a bit less efficient too. Because of these factors if you don't do rough calcs upfront you can easily create a setup that quickly overloads your motor, ESC, wiring and/or battery (lipos can only deliver so much current before the voltage collapses, and that ability to provide current is normally denoted as a C-rating - the max theoretical current that can be delivered is essentially the capacity multiplied by the C rating).

 

In summary, to know your powertrain will power your model sufficiently without any magic smoke escaping(!) you need to know the rough AUW, motor Kv, motor mass (outside of EDFs a rule of thumb with most common motors in sport flying applications is don't try and generate more than 3W/g), capacity and C-rating of your pack (halve the marked C-rating for calculations as there is a lot of "sticker engineering" in mainstream lipos!), motor Kv and max current (power ratings are IMO not to be trusted), and ESC rating. 

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41 minutes ago, MattyB said:

you need to know the rough AUW, motor Kv, motor mass (outside of EDFs a rule of thumb with most common motors in sport flying applications is don't try and generate more than 3W/g), capacity and C-rating of your pack (halve the marked C-rating for calculations as there is a lot of "sticker engineering" in mainstream lipos!), motor Kv and max current (power ratings are IMO not to be trusted), and ESC rating. 

Thanks again Matty. I think I have most of that info to hand, provided by an expert friend (who suggests perhaps a 12x6 prop) and I'll proceed carefully.

rgds Tony

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Tony, I baulked at getting a wattmeter a few years ago, but it is a MUST HAVE to check your system. Worse case if goes to rats in the air you could end up hitting somebody!.

 

Would you like me to check its ok on e.calc? I am very familiar with it, so could quite easily tell you if your set up is going to be ok, and what to expect.

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3 minutes ago, SIMON CRAGG said:

Tony, I baulked at getting a wattmeter a few years ago, but it is a MUST HAVE to check your system. Worse case if goes to rats in the air you could end up hitting somebody!.

 

Would you like me to check its ok on e.calc? I am very familiar with it, so could quite easily tell you if your set up is going to be ok, and what to expect.

Yes please Simon, but I'll have to get back to you next week with the details.

rgds Tony

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Hi Tony,


Another way of trying to find a path through this electrickity maze might be with the help of a tachometer. I’m reluctant about the wattmeter, in my view this is just telling me how many volts and amps are being converted into heat due to the resistance of the power train whereas a tacho will tell me how fast the prop is turning and therefore is an indication of the flying performance of the model. Also by comparing the prop’s speed against the unloaded revs I would get a fair idea as to what the current flow could be.

 

Might be worth a little discussion if this is of any interest at all.


PB

 

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