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Old Oct 24, 2008, 08:51 AM
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Flieslikeabeagle

I just took delivery of that very motor for use in a 2M e-glider, 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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Old Oct 24, 2008, 01:02 PM
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Quote:
Originally Posted by aeajr
I just took delivery of that very motor for use in a 2M e-glider, a Cox Dust Devil.
http://www.coxmodels.com/prodinfo.asp?number=006103

I am setting it up with an 11X8 prop on 4S lipo.
WebOCalc thinks that is a very fast combo (for a big floaty glider, that is). The pitch speed is predicted to be up around 60 mph. WebOCalc thinks the thrust will be right around 48 oz (but WebOCalc is a bit conservative in thrust predictions), for a nearly 1:1 static thrust:weight ratio. Current draw is predicted to be around 24 A.

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:
Originally Posted by aeajr
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.
Good point. If it's a traditional overbuilt wooden ARF trainer, then it is also going to weigh a whole lot more than my guess of 35 oz, and will need a whole lot more power as a result.

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
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Old Oct 24, 2008, 02:28 PM
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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 set-up.

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 trade-offs 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 set-up or the one for the trianer.
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Old Oct 24, 2008, 04:22 PM
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Quote:
Originally Posted by aeajr
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.
Ed, my thoughts are along the same lines as yours. I do not anticipate any problems with this motor at this power level, especially for the use you're planning - an LMR glider, not a hovering 3D monster. Gliders use full throttle rarely and for short bursts, unlike a 3D machine flying "on the prop" and not "on the wing".

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 28-36 750Kv / 262W
Resistance: 130m-Ohm
Idle Current: 0.4A
Rated Power: 262W
There are two important things to focus on: the idle current of 0.4 amp, and the internal resistance of 130 milliohms. Using some basic electrical formulae and these two motor parameter, we can calculate the heat generated in the motor for any given power supply voltage and motor current.

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 jaw-dropping efficiency - a carefully used cheap ferrite-magnet 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 name-brand 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:
Originally Posted by aeajr
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.
You certainly could. As discussed above, your motor would run considerably less efficiently and put out considerably less power without much reduction in motor heating.

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:
Originally Posted by aeajr
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.
If the motor can take 25 A from a 3S lipo pack (which is roughly what it takes to come up with that 262 W figure), then the motor can take the same 25 A from your 4S lipo pack with negligible additional heating - only 3W more in the calculation I did above. (The 3W is the difference between the 4 W of heating due to the idle current when running at 10V, and the 7W of heating due to idle current when running at 14 V.)

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 full-throttle 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:
Originally Posted by aeajr
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.
It predicts static readings. At this point, I don't know how to predict the reading in the air (meaning that I don't know of a suitably accurate theoretical way to predict and calculate it in a way that matches reality).
Quote:
Originally Posted by aeajr
I added this commentary so people could see some of the back and forth of selecting a power system, some of the design trade-offs and some of the considerations.
Excellent idea, as you can see, I took some time to compose a reply that I hope will contain a good deal of useful information and food for thought.
Quote:
Originally Posted by aeajr
Thanks for making WeboCalc available to everyone and for running my numbers. Do you have any other thoughts?
Since you ask...with a 76" wingspan, this model would really benefit from a much bigger propeller. A 16" prop, for instance, would produce significantly quieter and more efficient performance. I fully realize that this is not the "usual way things are done", that a 16" folding prop is expensive, and that you will not find an off-the-shelf outrunner at a reasonable weight and size that can spin such a big prop off a 4S lipo pack at the current draw you want (20 - 25 A). But the fact remains that such a combination would produce better performance, though it would probably require a motor with a planetary gearbox to get a Kv low enough.

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
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Last edited by flieslikeabeagle; Nov 03, 2008 at 02:47 AM. Reason: Fix a small math error (forgot to add idle current back in the 3rd example).
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Old Oct 26, 2008, 01:08 PM
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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
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Old Oct 26, 2008, 08:12 PM
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delete

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Old Oct 26, 2008, 10:06 PM
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Quote:
Originally Posted by flieslikeabeagle
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
A fine piece of work my friend. Well worth the read and much appreciated.

Ed
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Old Oct 26, 2008, 11:17 PM
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Quote:
Originally Posted by flieslikeabeagle
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
Thanks Flieslikeabeagle for the time and effort you put into your reply, I also learned a lot and found it very interesting.
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Old Oct 27, 2008, 01:22 PM
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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 higher-than-usual 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
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Old Oct 27, 2008, 01:42 PM
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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
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Old Oct 27, 2008, 03:00 PM
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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 direct-drive, 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
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Old Nov 16, 2008, 11:19 AM
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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.
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Old Nov 16, 2008, 11:39 AM
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merlin, thanks for pointing me to this thread. Definitely lots of information and explains pretty much all I needed to know about electric power, motors, etc.
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Old Jan 23, 2009, 10:41 PM
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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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Old Jan 29, 2009, 05:55 PM
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Quote:
Originally Posted by matt90
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
Matt, perhaps you missed it, I gave you one answer to your question where you first asked it, in the Mini Super Cub RTF thread:
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 15-lb 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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