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Old Jun 16, 2012, 10:48 AM
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Why does my smaller motor give more thrust ?

Recently I have bought a new Dualsky XM2812CA-27 20 gram motor as an upgrade to my Dualsky XM2215MA-17 16.5 gram motor which I use on a lightweight 125 gram indoor 3D foamie. However, when attaching this new motor to my plane I noticed that there was no difference in performance. Actually I would say that the motor felt a bit weaker than my previous one I had installed. I thought that there might be something wrong with the ESC settings so I set it to many different settings and different timings and still the motor was not performing better. Thinking that I might need a bigger speed controller I went and bought a Dualsky 10amp ESC and tried the motor with that but still no change.

Now, to be sure that the motor had the same or less thrust , I hooked them up to my digital scale. I then used a fresh 2cell 25C 370 mah Lipo to test each motor and a GWS 8X4.3 prop. I tested each motor several times but there was a clear difference in pulling power. My old 16.5 gram , 62W , 1620rpm/v motor gave a reading of -102 grams each time while my new 20 gram, 80W , 1780rpm/v motor gave a reading of -92 grams each time. I tested these motors with different ESCs but still the old 16.5 gram motor came out tops. (And yes, I zeroed the scale with each motor)

So my question is, why on earth am I getting more thrust with a smaller motor with lower specs ? Surely a larger motor with more than 1000 more total rpm and 18W more power should be giving more thrust right ? Just to be a bit more sure the motor is not faulty, I got hold of a Turnigy 2204 motor that many people say is a really good motor. I hooked it up and it also gave me more or less a reading of -92 grams. I put it on my plane and it performed the same as my 20g Dualsky motor.

So yea, I dont know what is going on. Any thoughts ?
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Old Jun 16, 2012, 11:01 AM
Currently flyin Crack Yak
United States, AZ, Gilbert
Joined Jul 2006
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It's not the size of the motor it's the amount of thrust it gives lol
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Old Jun 16, 2012, 11:52 AM
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ernie b's Avatar
United States, IL, West Chicago
Joined Feb 2008
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Dan

Download the drive calculator from Dualsky, it will let you know the corect prop for the new motor.

http://www.dualsky.com/product.asp?sid=119&nid=231

I think you are trying to use too large of a prop for this motor, Higher KV usually needs a smaller prop.

Ernie
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Old Jun 16, 2012, 12:25 PM
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Originally Posted by ernie b View Post
Dan

Download the drive calculator from Dualsky, it will let you know the corect prop for the new motor.

http://www.dualsky.com/product.asp?sid=119&nid=231

I think you are trying to use too large of a prop for this motor, Higher KV usually needs a smaller prop.

Ernie
The motor manual recommends a 8X4.3 prop, exactly the one I am using. Problem is since I am competitive with indoor 3D, going for a small prop is going to decrease the performance of my plane. So even if this motor does need a smaller prop, I wouldnt use it. Also, ive heard of many people successfully using a 8X4.3 prop on similar sized motors that have sometimes more than 2000rpm/v.
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Old Jun 16, 2012, 12:29 PM
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Originally Posted by ryanotown22 View Post
It's not the size of the motor it's the amount of thrust it gives lol
Yes I know larger size doesnt necessarily mean more thrust. What I was trying to say was that the larger motor with better specs still had less thrust than the smaller motor with lower specs.
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Old Jun 16, 2012, 12:33 PM
certified crash instructor
Scottsdale Arizona
Joined Dec 2009
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You get more thrust with the smaller motor due to less rpm. The larger motor is meant for speed on a high pitched prop. Best way I can explain it, take 2 identical cars and race them, one catch though, leave one car in first gear and the 2nd in 2nd gear, the first has more torque and the 2nd has more speed. Even if the 2nd car was more powerful the first one would have more torque but less speed. Your small motor is like the first car in first gear. If you had a 3000Kv motor it would be like starting off in third gear.
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Old Jun 16, 2012, 12:53 PM
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Originally Posted by slo-fly View Post
You get more thrust with the smaller motor due to less rpm. The larger motor is meant for speed on a high pitched prop.
So then why is the thrust almost the same when I tested the 20gram, Turnigy 2204 , 1450rpm/v motor ? Not just that but the manual and website recommends a 8X4.3 prop with 2 cells. If you look at similar motors such as the Axi Goldline race motor or the Hacker A10-9l (which both have a higher rpm by the way) they also recommend a 8X4.3 prop.
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Old Jun 16, 2012, 01:01 PM
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United States, IL, West Chicago
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Hi Dan

I can't explain why, but when I run the calculator for that motor, I get 55% efficiency with the 8x4.3 so almost half of your power is being dissapated as heat by the motor. The sad thing is that motor never does generate a lot of thrust at high efficiency in any of the charts.

P.S. I hope that someone that is running that exact motor can help.

Ernie
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Old Jun 16, 2012, 01:02 PM
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Originally Posted by slo-fly View Post
You get more thrust with the smaller motor due to less rpm. The larger motor is meant for speed on a high pitched prop. Best way I can explain it, take 2 identical cars and race them, one catch though, leave one car in first gear and the 2nd in 2nd gear, the first has more torque and the 2nd has more speed. Even if the 2nd car was more powerful the first one would have more torque but less speed. Your small motor is like the first car in first gear. If you had a 3000Kv motor it would be like starting off in third gear.
Ok well then I dont really understand how this whole thing works. I would think that getting a motor with more power and higher rpm will result in more pulling force due to the fact that more air is being moved per minute.
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Old Jun 16, 2012, 01:10 PM
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For 3D foamies you typically over prop since you spend so little time at full power.
Efficiency of 50% simply means that you are at maximum power for the whole system. Good for power/weight ratio which is what you are after.
(Dr Kiwi regularly says we run too much prop on these motors.)

But this does not explain why your large/ higher Kv motor is making less rpm/thrust.

It might be the wrong Kv. Any way you can measure no load rpms of the two motors?


Pat MacKenzie
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Old Jun 16, 2012, 01:34 PM
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United States, IL, West Chicago
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Dan

I went to the Dualsky website again, and looked closely at the specs for your old and new motor. The older motor specifies a 8x4.3SF prop and the newer motor specifies an 8x4.3.

Are you running the slowfly prop or an E-Prop?

Ernie
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Old Jun 16, 2012, 06:22 PM
Lost but making good time
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Dan,

Have you ever checked how many amps you were pulling on those motors versus the amp rating for the individual motor...

When I put the battery specs, esc specs, prop specs, and Kv specs into this program, it shows both motors are being over amped with both types of 8" props but the 1780 kv motor is supplying 310 grams of thrust versus 306 grams for the 1620 kv motor. They are supplying these values with max amps plus with the 8" APC E props. If you use the 8" APC SF props , they are really going over max amps. Check it out.

http://www.adamone.rchomepage.com/calc_motor.htm

Regards,

Hankg
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Old Jun 17, 2012, 04:19 AM
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Originally Posted by ernie b View Post
Dan

I went to the Dualsky website again, and looked closely at the specs for your old and new motor. The older motor specifies a 8x4.3SF prop and the newer motor specifies an 8x4.3.

Are you running the slowfly prop or an E-Prop?

Ernie
Im running a slowfly GWS prop, I find this prop best for indoor 3D just like the majority of indoor 3D pilots.
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Old Jun 17, 2012, 12:50 PM
Lost but making good time
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Dan,

Substituting a GWS 8x4 prop, you are pulling 341 grams with the 1780 motor and 330 grams of thrust with the 1620 Kv motor. Your instincts are correct...according to the program, you are pulling more thrust with the heavier motor although not a great difference...11 grams. You are operating in the "amp for 15 sec" area though, for both motors.

Pmackenzie seems to be correct when according to Dr. Kiwi, "we over prop these little motors". In order to get the watts the manufacturer says they will produce, the program points out you will be running them in the high amp limit.

Regards,

Hankg
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