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Old Jan 11, 2013, 02:21 AM
Jim in the Desert
United States, NM, Las Cruces
Joined Aug 2007
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Confused - prop size vs motor size vs thrust

I have been looking at threads on picking a power system, etc., and am confused.

If you use a smaller prop, say a 9x4.7 on a slow stick, you get more RPM. If you switch to a larger prop, say 11x4.7, you get less RPM. But the prop pulls more air, being bigger. So which one is going to move you through the air better, the bigger prop at lower RPM, or the small prop at more RPM? And which will put more straing on the electrical system and motor?

I know if the motor has high Kv you can burn it up with too big a prop. But that doesn't make sense to me. Why doesn't the higher RPM put just as much strain and heat on the motor?

Thanks
Jim
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Old Jan 11, 2013, 02:42 AM
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Hi Jim!

A running electric motor works also as a generator, generating a voltage opposed to the voltage of the battery. If you run the motor without a prop, it spins fairly fast, creating a voltage that is almost as high as the battery's. The resulting voltage differenc is only small, resulting in very little (idle) current through the motor.
If you reduce the RPM by mounting a propeller, the generated voltage is smaller, the voltage difference to the battery higher, the resulting current bigger.
If the motor is blocked, it does not generate any voltage, and the resulting current is the highest. Almost a short circuit, if it would not be for the little resistance the motor windings have.

More current always means more heat.

This is why a bigger prop translates to more current with everything else being equal.

Generally, a bigger prop has a better efficiency than a small one. But a small propeller can keep it's efficiency in a wider speed range.
If you put a big prop on your plane, the acceleration will be better, but you will not be able to fly very fast. When your airspeed approaches the flow speed of the propeller, the efficiency declines.
If you put the same power in a smaller propeller, it will spin faster, allowing for higher speeds because of the higher flow speed. But you will have less acceleration, less "pull".

Think of it like the gears in a car. 1st gear is fairly good for acceleration, but you can't drive very fast with it. 2nd gear is better in this regard, but it won't allow for quick acceleration.

Thus, it makes alway sense to choose the propeller according to the desired speed range of your plane.

It does not makes things simpler that electric motors typically are more efficient at higher RPMs.
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Old Jan 11, 2013, 02:55 AM
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United States, AK, Fairbanks
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You've managed to bottle up a whole lot of big questions in that one little post

Let's start with the props... For a given motor, you're working with a relatively small band of its RPM range (about the top 25% of Kv*Vin). The reduced diameter of the 9" prop means it requires less power to spin but also produces substantially less thrust for a given RPM. Since the motor's maximum RPM is a finite number and its "minimum RPM" is going to be ~75% of Kv*Vin (where the motor will generally be drawing its max current), you'll end up simply using less power and getting less thrust overall. This is less demanding for the power system, but obviously now you have less thrust on hand.

As for the motors burning up, that one tends to trip folks up who aren't used to how this stuff works. The motor is drawing the lowest current when there's no load on the shaft because it can spin just about as fast as it wants to (approximately RPM = Kv * Vin). This means that the back voltage is almost dead equal with the input voltage, meaning that the motor draws very little current. It's happy. NOW... We place a prop on the shaft and that motor is forced to slow down by some amount, which means that the back voltage drops while the input stays about the same. This makes the motor draw more current. It's working harder and using more energy to keep that prop spinning, and some of the energy it takes in is wasted as pure heat. That heat warms the motor up, and if there's too much of it then the motor fries.

I apologize if none of this makes sense, by the way. It's been a long day and I've been trying to program pitch curves on my old DX6 for the past two hours. My brain's got a bit of a sputter in its ticktock... Someone else will probably be along soon to explain it better
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Old Jan 11, 2013, 02:56 AM
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Oh, goody. Looks like Julez has you covered
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Old Jan 11, 2013, 09:26 AM
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Jim,

For the same input power, if you want maximum thrust go for large diameter, low pitch... if you want speed go for smaller diameter, higher pitch.

Cheers, Phil
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Old Jan 11, 2013, 01:52 PM
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Jim, as a suggestion you could tell us what motor and airframe you are trying to fit. A high wing trainer will have a different set up than a pylon racer or a glider.
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Old Jan 11, 2013, 02:52 PM
Jim in the Desert
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Thanks Julez, C4H10 and Dr. Kiwi.

Jim, I am looking at this motor on a GWS slow stick, built lightly, 800-1500mah 3S but wondering also about 2S.

http://www.headsuprc.com/servlet/the....8oz%2C/Detail

However, I want to understand the principles in general because I have several planes I plan to get to.


Say, I notice you live in Las Cruces. Wonder if we've met at the field? I stopped flying when I got my motorcycles 5 years ago but am getting back in. Forgive me if I'm making a gaffe, at least it's in very slow motion :-).

Jim
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Old Jan 11, 2013, 03:46 PM
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It's all good. I am hard to fly with on any schedule due ro work and family. I do not fly at NMSU at this time and I rarely get out to the club field. I applaud your knowledge quest and will be happy to explore it with you. PM me and we can exchang contact info.
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Old Jan 11, 2013, 06:11 PM
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Jim, I fly my SlowStick with an AXI 2212/34 (~740Kv) on 3s and an 11x7 GWS HD.. draws about 10A... so your FirePower 400 will have just a bit more get up and go.

Cheers, Phil
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Old Jan 11, 2013, 07:31 PM
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Quote:
Originally Posted by Dr Kiwi View Post
Jim, I fly my SlowStick with an AXI 2212/34 (~740Kv) on 3s and an 11x7 GWS HD.. draws about 10A... so your FirePower 400 will have just a bit more get up and go.

Cheers, Phil
Why so overpowered? I fly mine with an AXI 2212/34 on 3s and an 11x5.5 APCe. Draws about 7 amps at 5000 ft altitude. Flies just great even at the lower atmospheric pressure of my field altitude.

Larry
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Old Jan 11, 2013, 08:38 PM
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I'm bringing my driver to your place to see how far I can hit a golf ball!
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Old Jan 11, 2013, 11:24 PM
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Do that. We have a great golf course. Of course, right now it is under six inches of snow.
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Old Jan 12, 2013, 12:56 AM
Jim in the Desert
United States, NM, Las Cruces
Joined Aug 2007
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Phil and Larry, well I would have gone with that motor had I known about it, but I just called Jesse at Heads Up and asked for recommendations. I want to fly as slow as possible.

May I ask why you use props with 7 and 5.5 pitch? I bought 11x4.5 and 11x3.8, thinking it would let me fly slower. But maybe the bigger bite slows the RPMs more so you fly slower with your props than I would. I need to get my Dr. Kiwi engine stand and Medusa gear set up so I can explore this. This is one of the funnest conundrums in RC, and why I bought Phil's lovely stand to begin with.

BTW I am at 4000 to 5000 feet, depending on where I fly around here.

Thanks
Jim
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Old Jan 12, 2013, 10:37 AM
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Larry, I'll bring some yellow or orange golf balls (sorry, I don't do pink).
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Old Jan 12, 2013, 05:43 PM
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C9, minimum flight speed isn't related to the power system. Regardless of which prop you run, the plane's minimum flight speed will remain the same because it's a function of wing loading and other characteristics of the plane itself, so reducing pitch won't "let the plane fly slower". It will only force you to fly slower at full throttle.

Higher pitch means higher top speed (at the cost of more power), so you have a broader speed range available. Depending on the setup, you may be able to cruise at the same speed using less power because you don't need the RPMs to be as high.
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