

LI, New York, USA
Joined Mar 2003
25,441 Posts

Flieslikeabeagle
I just took delivery of that very motor for use in a 2M eglider, a Cox Dust Devil. http://www.coxmodels.com/prodinfo.asp?number=006103 I am setting it up with an 11X8 prop on 4S lipo. Should finish up around 45 ounces, I think. Did not run it through any calc programs, but I think it will work well. Targeted at about 230 watts, or about 80 watts/per pound. What does your program say about that combo? One note on the "trainer" How much ground clearence will there be when the tail comes up. Can it swing a 12" prop? Might be too big. 




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I think a 12x6 folder would yield a little better speed match to the airframe and a little more thrust, for very little increase in current draw. All this is assuming Hobby King's 750 rpm/V spec is correct, of course. I've measured some of their motors very close to the advertised Kv, and others have been off by a huge amount  one motor advertised at 800 rpm/V actually measured right around 700 rpm/V, a full 25% lower! Quote:
IMHO one of the best improvements you can make to an electric models powertrain efficiency is to use a bigger propeller. If that requires bigger wheels and/or modified landing gear, its still worth doing. The difference in acceleration, responsiveness, reduced prop noise, and increased prop efficiency is usually well worth the effort. Flieslikeabeagle 



LI, New York, USA
Joined Mar 2003
25,441 Posts

Thanks for running the numbers. Now, let's just look over the options and how one might use the info provided by Webocalc.
11X8 is at the lower end of the MFG recommended props so that was what I chose. However on a 4 cell lip it might not be the best choice. What could I do with this new information? First, I may be exceeding my total watts. The motor is rated at 262 watts. 24 amps at 13 volts ( 4 cell under load ) would yield 312 watts. So I may have a problem with heat. I will only be running 20 seconds so the motor might be OK with it. And I don't have to run full throttle, but I have to remember not to run full throttle for extended periods or I could overhat things if I keep this setup. I could always drop it to a 3 cell lipo for lower volts and lower current draw and lower total watts. Could probably save a couple of ounces in the weight of the plane. I could stay with this battery and go to an 11X6 prop which would also bring me down more into the range for this motor. Maybe we can run those in webocalc and see what they yield. I want to be around 20 amps. Does Webocalc predict static readings or predicted readings in the air, when the prop has a chance to unload? Typically readings in the air are lower. I added this commentary so people could see some of the back and forth of selecting a power system, some of the design tradeoffs and some of the considerations. Thanks for making WeboCalc available to everyone and for running my numbers. Do you have any other thoughts? This is all for the benefit of the readers of the thread so comment away on my setup or the one for the trianer. 



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I find the "total watts" numbers are quite misleading  it's not the total watts into the motor that heats it, it's the difference between the total watts in and the total watts out. When you go to a higher battery voltage (as you chose to with your 4S combo), it is very likely that you can increase the total watts into the motor without much additional heat, sometimes with none at all. Let's run through some numbers to make this clearer. Here is some of the data Hobby King publishes for this motor: Code:
TGY AerodriveXp SK Series 2836 750Kv / 262W Resistance: 130mOhm Idle Current: 0.4A Rated Power: 262W First, let's consider running this motor at 10 V and 25 A, well within the 262 W suggested rating of the manufacturer. With 10 V from the battery and an idle current of 0.4 amp, the heat generated in the motor due to the idle current is given by (heat = current x voltage), or 10V x 0.4 A. This yields 4 watts of heat. That's not all, though, because the additional 24.6 A of current (25 amps total motor current less 0.4 A idle current) is flowing through the motor resistance of 130 milliohms, or 0.13 ohms. The formula for heat generated in a resistor is current x current x resistance, so this gives us (24.6 A x 24.6 A x 0.130 Ohm), which works out to 78.67 watts of heat. So when we run this motor at 10V and 25 A, the total heat generated in the motor is 4 W + 78.67 W, or roughly 83 W total. That's a lot of heat, by the way! With 250 W of power going into the motor and 83 W wasted as heat, only 167 W is making it to the propeller. The motor efficiency is therefore 167W/250W, which is only 67%. This is not exactly jawdropping efficiency  a carefully used cheap ferritemagnet Speed 500 brushed motor is equally efficent, though at lower power and greater weight. Unfortunately, this low efficiency is fairly typical of generic small outrunners pushed hard and run on relatively low voltage. Now, let's consider running the same motor on a 4 cell lipo pack, as Ed is planning to do. At about 3.5 volts/cell under load, we can expect to have about 14 V from the battery. If the idle current were still 0.4 A the heating due to the idle current is now 0.4 x 14, or 5.6 watts. In practice the idle current goes up a little with increasing battery voltage  let's guess it goes up to 0.5 A. This would push the heating due to idle current up to 0.5 x 14, or 7 W. Let's try to run the motor at the same 250 W input power as we did earlier on the 10 V battery. With 14 volts on tap, the current draw for 250 W is given by (current = power/voltage). This works out to 250/14, or 17.86 amps. Okay, now let's calculate heating in the motor resistance. As before, the heating is given by (current x current x resistance). With 17.86 amps total current and 0.5 A of that being idle current, the remaining current is 17.36 amps. So the heating is 17.36 A x 17.36 A x 0.13 ohms, which works out to 39.16 W, or roughly 39 W. Total heat generated in the motor is now 7 W from the idle current, plus the 39 W from the rest of the current, for a total of 46 W of heat. Remember, we still have 250 W input power, the same as we did when running the motor on 10 V. Did you notice that this is a lot less heat than before? By running the motor on a 4S lipo pack instead of at 10 V, we are able to put the same 250 W into the motor, and lose less of it as heat. This means the motor runs cooler, AND we get more power into the propeller. A very nice case of win, win! Let's calculate the motor efficiency again. 250 W in, 46 W heat, therefore 204 W out to the propeller. Efficiency is 204/250 or 81.6%. This is a whole lot better than our previous 67%. The efficiency is now only a hair lower than some much more expensive namebrand motors. Running motors at higher voltage and lower current when possible is a good way to squeeze better performance from any motor! All this should make my point clear: specifying the total watts into the motor is almost meaningless as a guide to using the motor within its limits. What actually matters is keeping the total watts turned into heat in the motor to a manageable amount. Finally, let's do a third calculation. Or first example of 10 V and 25 A put 250 W into the motor, well within Hobby King's 262 W specification, and we found that this combination generated 83 W of heat. Now let's find out how much power we can put into the motor at 14 V (4S lipo), and still generate the same 83 W of heat in the motor as before. As before, with 14V and 0.5 A (guess) idle current, we have 7 W heating from the idle current. With 83 W total heating as our target, that leaves 76 W for heating in the 130 milliohm motor resistance. That lets us figure how much current is allowed. Since power equals (current x current x resistance), we can rearrange that formula and get [current = square root of (power/resistance)]. With 76 W of heating and 0.13 ohm, that works out to square root(76/0.13), or 24.18 amps. Now we have to add back our 0.5 A idle current, and so we get 24.68 amps of motor current. (Notice that this is only very slightly less than our previous 25 A current draw when we ran the motor at 10 V). Okay, we have 24.68 amps into the motor, a 14 V battery, and 83 W total heating in the motor. Power into the motor is now (24.68 x 14), or a whopping 345.5 watts, and yet the motor is no hotter than it was when we were running it at a mere 250 W earlier! Power wasted as heat is 83 W, so the power to the propeller is 262.5 watts. Efficiency is 262.5/345.5, or 76%. Not spectacular, but much better than our original 67%  a whole 9% better, in fact. So, since the motor heating is the same in both cases, do we really have a 250 W motor, or a 345 W motor? The short answer is that it depends on the wisdom of the user! If you run it on 3S lipo, you're stuck at around 250 W and poor efficiency. Run the same motor on 4S, and you can safely pump 345 W into your "250 W" motor, with no more motor heating than before. Quote:
However, if this combination still produced enough performance for your tastes, it would be a viable choice. The slight reduction in total weight might perhaps reduce sink rate and make for better thermalling performance on calm days, too, depending on how the wing reacts to the slightly lower Reynolds numbers at the lower flight speeds. If you're reading between the lines, you can pick up the fact that I'm not enthusiastic about dropping down to 3S. It costs a lot in efficiency and power without much benefit, so the engineering side of my brain doesn't like the idea. However, if the emotional side of my brain was still happy with the resulting flight characteristics, there is no harm in going this route. Quote:
3 W more heating on top of 83 W is only about 3.5% more heating, a completely negligible figure. For all practical purposes, you can ignore it, and the motor is still good for 25 A, whether you run it on a 3S pack or a 4S pack. As a result of this and the increased motor efficiency at the higher current, the motor is capable of putting out about 33% more power on the 4S pack compared to the 3S pack, for about the same motor heating. So, if you believe the numbers (and they are based on sound electrical calculations, so there is no reason not to believe them), you are not exceeding the motors ratings on 4S lipo. AND you're going to get much better performance. It may be worth mentioning at this point that I began using brushless motors in 2005. I'm not sure how many of them I've used since then, but surely more than a dozen. I love aerobatics, so my models see full power on every flight, though not for the entire time, only for flying vertical uplines, the upward half of loops, knife edge segments, etc. Despite this sort of frequent fullthrottle usage, in these three years of flying with brushless motors, I have yet to burn up or damage even one single motor! So whatever I'm doing with the math is working  the motors live long and happy lives, while in many cases churning out much more power than the manufacturer says they can. Quote:
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Would it be worth it? For fun flying, probably not; convenience and expense both say otherwise, since the outrunner will certainly provide "good enough" performance. But when you're really trying to squeeze the best/lightest performance from an electric power system, remember: the key is high motor rpm, low propeller rpm, as big a propeller as you can use, and high battery voltage. This combination almost invariably boils down to having to use a geared motor. That was a very long post, so I'm going to go away and let my finger tips recover from pounding on this keyboard! Flieslikeabeagle 



Boston, MA
Joined Jan 2007
437 Posts

Small list of proven Slow Stick power systems compiled by me
http://spreadsheets.google.com/pub?k...t3tDMaEA&hl=en Fill out the form below to add your contribution to the list above: http://spreadsheets.google.com/embed...jA6qRWt3tDMaEA 


LI, New York, USA
Joined Mar 2003
25,441 Posts

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Ed 




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Ed, Chris, thanks! It's always nice to get more for our money than we thought we were getting, isn't it? That's part of the reason it's fun to get more power out of a motor than the manufacturer says you can.
Back in 2004 or so RCG user Vintage1 started a thread titled "can motors on 3S lipo" which is where I first learned about this idea  running motors on higherthanusual voltage lets you increase motor efficiency and get more power out of the motor. That thread is the germ of the math I presented in my previous post, so all credit goes to Vintage1. Flieslikeabeagle 


Winnipeg, Canada
Joined Apr 2007
1,507 Posts

Excellent info here Flieslikeabeagle! Now I feel like going to the bench and start playing with my motors
The Hobbycity motor you used in your example does not specify maximum voltage, I checked some of the ones I am using (also Turnigy from HC) do specify voltage range (7.4V~11.1V). Like this one for example: http://www.hobbycity.com/hobbycity/s...idProduct=3883 Does what you just explained still apply top this motor? Thanks in advance! E 



Earaoz, usually it comes down to the maximum rpm the motor can handle without slinging magnets  as the voltage goes up, so does the motor rpm. Some manufacturers do specify a maximum rpm, and then we're all set. Max voltage = (max rpm/motor Kv).
Unfortunately, unless the manufacturer specifies the maximum rpm, we're left to grope in the dark and guess. The other thing to keep in mind is that if we're using directdrive, as the voltage goes up, the prop size comes down. Small props are inefficient and should only be used if absolutely necessary, so if the motor Kv is already fairly high, it may not be an option to use it on higher voltage. Sometimes Hobby City will carry two motors that are identical except for Kv  let's pretend we have two "Axlotl Rumblecogger 3548" motors, one with a Kv of 1200 rpm/V, the other with a Kv of 800 rpm/V. Hobby City also says both motors are for 2S and 3S lipo use (8.4V and 12.6V maximum, respectively). In this case, we can figure that the 1200 rpm/V motor is rated to survive a maximum rpm of at least 1200 rpm/V x 12.6 V or 15120 rpm. Since both motors are identical except for the wind, we can safely expect the 800 rpm/V motor will also tolerate at least the same rpm. So now we can figure how much voltage the 800 rpm/V motor can handle: 15120 rpm/800 rpm/V, or 18.9 V. So in this case we can be confident that our 800 rpm/V motor can quite safely be run on a 4S pack, even if Hobby City/ Hobby King/ United Hobbies/ Name of the month says otherwise. Flieslikeabeagle 



OH MY GOD!! SO MUCH INFORMATION. MY BRAIN IS GETTING FRIED!! lol
Wow, you guys have done an excellent job with this. I didn't know there was a thread like this. I wish I would have found it sooner to stop myself from posting twice asking for a place like this with no responses. Anyway, thanks everyone. 


Joined Jan 2009
7 Posts

HI guys im new to this site and RC both...i have the Piper cub j3 by Exceed RC, i want to upgrade its motor to a brushless, right now it has 360, what do you recommend? i want something that i can later take out and use in other planes (most of them)
thanks 



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http://www.rcgroups.com/forums/showp...&postcount=125 You're not going to find one power system that will work in most RC planes  there is simply too much variety, you will never find a motor that will work in a 15" wingspan 20 gram indoor model as well as in a big 96" wingspan 15lb glow conversion electric model. But you can find one power system that will work in many small models of about the size and weight of your Exceed Piper Cub and with about the same wing area. The parts list I put together for you is is one example of a combination that will work in many similar models  for instance, it will nicely power most of the original GWS foamies that were originally designed to use the GWS 300 and 400 geared brushed motors. Flieslikeabeagle 

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