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Tutorial on electric power systems needed

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Old 03-17-2011, 03:43 PM
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combatman
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Default Tutorial on electric power systems needed

Hello everyone,

I am going to be making a decision regarding what my first glider purchase should be. While your suggestions are appreciated, I have a pretty good idea it will be an Oly II from Skybench.

Anyway, here is my dilemma. I'd like to power it with a brushless motor, and have read a lot of threads on the motors that people choose to power their 2m and larger sailplanes. But the motors have those crazy number designations, none of which I understand except for the Electrifly motors. I've seen Axi, Himax, etc, but don't understand how the particular motors were chosen.

I know that Electrifly has a motor calculator on the site, but there is no "glider" option. So, I just choose "trainer" and get a small propeller recommendation, smaller than the ones I see listed in many of these threads. For example, Electrifly recommends something with a 10" or 11" prop, whereas I have seen people use up to 16" prop on their 100" span sailplanes.

So, here are my questions:

1. How do you figure out which motor you will use? I am sure you take into account the glider weight, etc, but I don't know any good sites to look at other than Electrifly.
2. How do you figure out what prop size and pitch to use? As I said, the Electrifly calculator lists small prop sizes.
3. Where do you get your folding props?? At Hobby Lobby they are out of stock, Tower has just a few to choose from, etc.
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Old 03-17-2011, 07:11 PM
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Default RE: Tutorial on electric power systems needed

Some good reading:

http://homepage.mac.com/kmyersefo/framesefo.html
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Old 03-17-2011, 08:11 PM
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Default RE: Tutorial on electric power systems needed

Here's a bit of a write up I did some time back for another thread with the same sort of question.

_________________________
Motors these days come with ratings for max continuous current, peak current for X seconds, continuous watts and a Kv or RPM/volt rating.

Watts makes the model climb. 30 to 40 watts per lb of model weight gives an "uphill glide" to shallow climb. 60 to 70 watts per lb model weight give a sporty fast and roughly 45'ish degree climb. 100 watts/lb is going to be nearly straight up. And at 120 watts/lb and higher you are going straight up anywhere from slow to darn fast depending on how much you over power it.

The max amps for so many seconds is only an issue if you'll be using it in some form of competition where the motor run is restricted. For sport flying you should not ever push the motor that hard unless you're willing to live with the time limit on full power at that sort of current.

Watts is voltage times current. So a 3S or 3 series pack with the higher voltage will not need as much current pulled from it to produce the watts needed. A 2S pack will need to supply more current to reach the total wattage.

You get the watts from loading the motor down with the correct size prop. A 3 series Lipo pack is 11.2 volts and the motor will use the smaller suggested sizes to draw the amount of current to reach the total watts you need. If you run with a 7.4volt 2S pack you'll need a bigger prop to load down the motor to make it to draw the amount of current to reach the desired total watts you want.

The motor needs to be rated to withstand the max current and max continuous watts of power. So if you want 210 watts of power for a model to meet your power to weight needs then go for a 250 to 300 maximum watts motor. Over specifying the motor this way means that it's less likely to overheat and try to burn up on you. And these days the weight penalty for using a slightly oversize motor is very little.

All else being equal bigger props with high pitch pull better for gliders. But for a fast flying model such as a hotliner sailplane or a sport model or a racer you can go too far on that front. For them a smaller option turning faster will let the model fly faster. So you can see that how many watts you run affects which size range of prop you want to go with and to some extent how many cells you want to run in the pack.

The Kv value is sort of like the "gear ratio" of the motor. Kv stands for "RPM per volt". So a Kv=1000 motor will try to turn the prop at 7400 rpm on a 2S pack and at 11,000 rpm on a 3S pack. Which is another way of realizing why the 3S pack motor will try to draw a lot more current because it is having to work harder if you should mistakenly try to run a big prop for that motor on a 3S pack where it should be running a 2S pack. A low Kv motor will turn big high pitch props well but not work with small props. A high Kv motor will only turn small props or in the higher values is intended to be used with an external gearbox reduction setup. Nice motors for 2 meter sailplanes seem to have Kv values of around 900 to 1500. This is outrunner territory and you'd be lucky to find an inrunner with that sort of Kv value. So don't try. Just go outrunner and call it a day. For speedy models or gearboxes you want to go with higher Kv values in any given frame size.

For a two meter model try to pick a setup that uses about a 10 to 13 inch prop setup. Folders are easily found in that size range so that's another reason for using a slightly lower to mid Kv motor. For hotliners or sport/racer designs where you want high level flying speeds you want smaller and higher pitch props that are a match up for the higher Kv value motors.

Once you know the max watts you want to run with and if you are using a 2S or 3S pack you can use the Watts = Volts x Amps rearranged to Amps = Watts / Volts . With that figure known multiply it by 1.5 and that is the minimum current rating ESC you should get. ESC's need to be under loaded like this to avoid overheating due to the way we pack them into the fuselage where they can't cool easily. If you could mount it so there's air flowing by then you could get away with a lower power ESC. But then again no one ever had issues with bigger rather than smaller.

I can't help on the prop issue but hopefully this will at least give you some sense of what is needed for the motor and battery pack.
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Old 03-22-2011, 10:20 AM
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Default RE: Tutorial on electric power systems needed

Yes, BMatthews, that does help. Thank you. My biggest challenge right now is finding a properly-sized folding prop and all that is needed for it (hub, blades, etc).
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Old 04-01-2011, 06:24 PM
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Default RE: Tutorial on electric power systems needed

EVERYTHING YOU WANTED TO KNOW ABOUT ELECTRIC FLIGHT
http://www.rcuniverse.com/forum/m_7100376/tm.htm
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