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Power Systems Talk about motors, ESC speed controllers, gear drives, propellers, power system simulators and all power system related topics

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Old 12-30-2009, 06:21 PM   #1
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Default lower kv bigger prop, or higher kv smaller prop?

Guys,

I am trying to figure out if it is better to power my new Christmas plane (E-Flite AT-6 ) with a lower Kv or higher Kv motor.

I am trying to understand why it would be better to go with a lower Kv motor when I can get more watts of pull out of a higher Kv motor. With my charger set up, I am limited to using 3 cell lipo batteries. Based upon review of some prop data it looks like I can get a lot more watt power out of a higher Kv motor when using the same size prop. I don't want to go any bigger than 12" for a prop, or larger than a 3s battery. So is there any down side to using a higher Kv motor over a lower Kv motor when powering a plane? Do you use up more battery power to spin a higher Kv motor? ie. lower flight times?

E-flite recommends a Power 25 870kv motor for the plane, but I don't believe I can get enough watts out of that motor running 3 cell batteries to feel comfortable. There are a number of higher Kv Scorpion motors that are of similar weight and dimensions that will pull many more watts of power with the 12" prop. Should I go for a higher Kv rated motor?

Thanks very much for your input! I am sorry if this subject has been beaten to deathe

Forge

ps. Happy New Year to all!
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Old 12-30-2009, 07:47 PM   #2
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Forge - i do not see the issue with the suggested motor. Per P-calc, that motor/prop/batt combo will give 80oz of thrust at 58mph pitchspeed while consuming 410+W. at the heaviest suggested flight weight, that is still a 1.11:1 thrust to weight ratio. personally, i think its perfect.
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Old 12-30-2009, 07:51 PM   #3
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You don't get something for nothing in this game. With the same voltage battery (3S = 11.1V) and the same size prop a higher Kv motor will turn the prop faster and use more power (watts).

But to do that it needs more amps (watts = volts x amps and the volts can't change). Because it's using more amps it's using the battery up faster and you'll get shorter flights.

If you think you need more power than the recommendation then go for it but make sure the motor can use the prop you want without burning up and your battery can safely deliver all the extra current you need and be prepared for relatively short flights.

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Old 12-30-2009, 09:16 PM   #4
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Guys,

Thanks for the feedback. Isn't the advantage of the higher Kv motor that I can run at lower throttle settings if I don't need the power and conserve my battery, but I can power up quickly and have more power reserve if I make a low altitude mistake? The scorpion motors that I'm looking at can handle the watts I need and I always check watts and amp draw on all my motor prop combos before I fly.

Wingnut,

I would like to get around 110 to 120 watts of power per pound and at 4.5 pounds that would put me at around 500 to 540 watts. 410 seems like the lower end of the spectrum for my flying skills.

Thanks!

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Old 12-31-2009, 09:13 AM   #5
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Originally Posted by Forge View Post
Guys,

... Isn't the advantage of the higher Kv motor that I can run at lower throttle settings if I don't need the power and conserve my battery, but I can power up quickly and have more power reserve if I make a low altitude mistake? ...
That kind of performance is best provided by a lower Kv motor turning a larger, more efficient prop. By going to 4S, you would have even more emergency power available with the ability to run quite efficiently at lower throttle settings. Higher Kv motors are better suited to high speed applications where an immediate, powerful thrust is less important.

~Tim
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Old 12-31-2009, 12:33 PM   #6
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Forge - I hear you when you when it comes to 'emergency' power. I myself like a 1.5:1 or higher thrust to weight ratio. Honestly, i stopped using the Watts/lb method a while ago since when it comes to flying a plane thrust and pitch speed is what is really important. If you have reliable test data for the motor/batt/esc combos you are looking at, then that is the best way to determine your required flight setup. You can search for Dr Kiwi's test data on just about any motor available. His data is thorough and reliable.

But more to your question, for a given amount of input Watts a lower kV motor will spin a larger, hence more efficient prop for better thrust and a higher kV motor will spin a smaller prop faster for more speed. What it comes down to is how fast do you want to fly and how much over-thrust do you need. That will give you the parameters to look for in a power system.
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Old 01-15-2010, 07:20 PM   #7
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Default motor kv rating

I have been swapping over from glow to electric for the past two years. I have bought and installed several motors on planes and did not REALLY understand the significance of the kv rating. Sure, I knew what it stood for - rpm per volt. However, not until recently did it all come together when I installed a BIG 480 kv motor on my new plane. The motor is a Turnigy 50-52b with a 480 kv rating. I used this motor in my Tucano with a 6S battery setup and got nearly 1000 watts of power in. I will be using only a 4S battery in my new plane and thought this motor would kick out somewhere near the same power. After testing with several props, I could only crank out a maximum of about 350 watts! I was starting to think the ESC was bad or the motor had gone south. Then I reviewed the Emeter readings and notice I was only turning around 6500 rpm no matter what prop I installed. That's when the light came on about the kv rating.

For this 480 kv motor running on a 4S battery (14.5 volts) I was only going to turn a maximum of 6960 rpm (100% efficient). So my 6500 rpm was correct. Needless to say, spinning a typical prop (11 x 6) at 6500 rpm is not going to generate a lot of thrust or draw enough power for my 6 pound plane.

So, no matter what prop I use, the 4S battery (14 volts) is going to limit the resulting power. With the 6S battery I was looking at a prop speed of near 10,000 rpm - much more thrust and power. So my little pea brain has now figured out that the kv rating is important. It determines the prop speed for the battery (voltage) being used and therefore the thrust generated AND the power. So I will have to step up to a 6S battery OR use a higher kv motor if I stay with the 4S battery to get more thrust/power..

I hope this is all correct or I have self educated myself into the wrong conclusion. Anyone care to comment?

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Old 01-15-2010, 07:31 PM   #8
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I'd say you've pretty much got it.

Of course you could run a really huge prop which would use up more power. But it could only do that by drawing huge currents which may be too much for any or all of the motor, ESC or battery.

Power = voltage x current

So if you've lowered the voltage (6S/22.2V down to 4S/14.8V) you can only get the power back by INCREASING the current. And the way you cause the motor to draw a lot more current is by using a much larger prop.

Steve
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Old 01-15-2010, 07:54 PM   #9
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Vic... one other thing to remember about Kv is that once you put any prop on, your motor won't get you 100% of Kv x V. With a tiny prop you might get 90% of Kv x V... over-load it with too big a prop and you might be <60% of Kv x V.....you are best to prop for about 75% of Kv x V for the best useful "power-out".
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Old 01-15-2010, 09:14 PM   #10
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Thanks folks for the info and renewing my confidence in figuring this electric stuff out.

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Old 01-16-2010, 01:59 AM   #11
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Hey guys,

Thanks so much for the input. I am still learning and always trying to expand my knowledge base. This stuff is pretty complex, at least for me, but I think I have it under control for now... at least until I go airborne, that is.

Anyway thanks for the insights. Have a great weekend, I hope you all get some good flying weather.

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Old 01-16-2010, 03:10 AM   #12
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Try Drivecalc, It's pretty good about getting the info very close...

http://www.drivecalc.de/

Here's a couple of that 25 motor...

SK


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Old 01-16-2010, 05:00 PM   #13
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Hey ministeve2003,

WOW! Thanks for the recommended software. Awesome calculator that I have not seen before.

As long it doesn't crap out my mac, I am really impressed... and for free too. WOW, very cool.

Thanks!

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Old 01-16-2010, 05:40 PM   #14
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Default Drive Calculator

Yep, that is a very nice tool for Freeware. I used it to compare my readings with similar motors. It is very close.

It brings up another motor/ESC feature - timing. I was surfing around finding out more info on power systems and ran across a few topics on setting the timing of the system using the ESC. I went tot he shop and cranked my timing up from 1 degree to 15 degrees and saw about a 500 rpm increase. I then cranked up the timing to 30 degrees and got an amazing 7650 rpm (6500 rpm at 1 degree).

I read that these big motors like advanced timing so I will leave it at 30 degrees. There was no hint of locking up or getting out of sync so this motor might just do fine at the higher setting.

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Old 01-16-2010, 05:49 PM   #15
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Question power equivilents

I have bullet connectors on the ESC and battery for this motor. However, my Emeter has the Deans "T" connector. Without going through the hazzle of making some adapters, I thought I would ask you folks about power equivilents.

The question is, if another modeler uses the same prop as I use and turns the same rpm, would we both get about the same power readings (neglecting motor efficiencies)? In other words, if someone uses a 13 x 8E APC prop and turns 7500 rpm with a resulting 600 watts, shouldn't I get approximately the same power from my setup if I use the same prop and turn 7500 rpm? My thoughts are that it takes the same amount of power to turn that prop at 7500 rpm no matter what motor or battery is being used (P = I X E).

Any thoughts from the guru's?

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Old 01-16-2010, 07:03 PM   #16
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The power OUTPUT being used by the prop will obviously be similar. But since it's power input to the motor that you're able to measure it's a bit pointless saying "neglecting motor efficiencies".

E.g. one motor might be running at 70% efficiency and another at more like 85%. If the output power at the prop is 500W the input will be roughly 720W for one and 590W for the other. That's quite enough difference to be fine in one setup but destroy the motor, ESC and/or battery in the other .

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