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Old Jan 28, 2012, 01:58 PM
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Is the RCTiger MT3506 motor too good to be true?

I'm hoping there's enough experience on these forums to settle this. I've been searching high and low for the best motor to maximize quadcopter flight time. I ran across some data from RCTiger on their MT3506 motor:

http://www.rctigermotor.com/uploadfi...0080440884.jpg

These numbers are off the hook. If they are true, it means with 3s8000, and a 1.5kg AUW I could fly for over an HOUR.

However, the RCgroups plot they show next to their data, doesn't even come close to the same g/W static thrust efficiency. For example, at 500 g thrust, the user data says 9.1 g/W, pretty ordinary. Their data for a 13x4.5 prop says about 16 g/W @ 500 g thrust. That's a big difference. The distributor said their values came from actual test data. I smell a fish.
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Old Jan 29, 2012, 07:45 AM
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A similar motor, 5010 from SmartDrones, with 15.7" carbon props, has actually the record for longest flight, 51 min, 4*2200mAh, computed efficiency of 14.7g/W, so could be true...
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Old Jan 29, 2012, 07:54 AM
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Not sure about the data however, I have had a gut feeling for long time that large props spinning slow is the key. I have tried getting APC to make a 16" Slow Fly prop now for more than a year, was told it is on their agenda to do but it is not a high priority.
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Old Jan 29, 2012, 08:43 AM
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For same prop manufacturer, i.e. same airfoil and material and mould, theory is clear: always bigger is more efficient.
However, there are props and props... EPP1045 the original from maxxprod is a dog compared to the newer gemfan, which are almost as good as apc.
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Old Jan 29, 2012, 08:51 AM
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Do you happen to know if gemfan makes a 16" or larger in both CW and CCW Slow fly?
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Old Jan 29, 2012, 08:54 AM
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No, 12" max so far.
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Old Jan 29, 2012, 10:14 AM
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Originally Posted by RENATOA View Post
A similar motor, 5010 from SmartDrones, with 15.7" carbon props, has actually the record for longest flight, 51 min, 4*2200mAh, computed efficiency of 14.7g/W, so could be true...
He said his actual consumption was 9276 mAhr. (Not sure how he pulled that much from an 8800 mAhr pack). So with that his average power was 121 W. The AUW was 1500g, so his efficiency there was about 12.6 g/W.

If the manuf. MT3506 data are true, one can get >16 g/W at 500 g thrust. Meaning a new record is possible.
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Old Jan 29, 2012, 10:22 AM
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Originally Posted by rimshotcopter View Post
Not sure about the data however, I have had a gut feeling for long time that large props spinning slow is the key. I have tried getting APC to make a 16" Slow Fly prop now for more than a year, was told it is on their agenda to do but it is not a high priority.
Your gut feeling is correct. It can be proven mathematically. If,

T=kt*p*d^3*w^2
where
T=thrust generated by prop
kt=a constant
p=prop pitch
d=prop diameter
w=prop rpm

P=kp*p*d^4*w^3
where
P=power consumed by prop
kp=a constant


then one can solve for static thrust efficiency (g/W) by dividing T by P, which reduces to :

T/P (g/W) = sqrt [(kt^3*p*d)/(kp^2*T)]

This shows that as d increases, so does g/W. Also shows that as T decreases, g/W increases. (So you want to make your craft as light as possible, to require the least thrust for hover). It also shows that g/W increases at higher prop pitch, but I think this is just an artifact of the simplification of the Abbott equations.

Thus, to maximize flight time, you want to have lightest craft (there are now 2 reasons for this. One is so that you require the least power, the other is that the prop is more efficient!) You also want to use the largest prop possible to generate this required thrust.
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Old Jan 29, 2012, 10:23 AM
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So I have to wonder, why is there such a large discrepancy between the manuf data and that performed by someone here on RC groups? The rcgroups data says it's only 9-10 g/W @ 500g thrust, which is pretty ordinary.
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Old Jan 29, 2012, 10:24 AM
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... Isn't it obvious? Same reason people put a "400Hz" sticker on a ESC and sell it for 2x the price
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Old Jan 29, 2012, 10:32 AM
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... Isn't it obvious? Same reason people put a "400Hz" sticker on a ESC and sell it for 2x the price
So you're saying the manuf data is BS? I wish I could see their test setup. Hasn't someone on this forum used these motors? I know warthox has.
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Old Jan 29, 2012, 03:59 PM
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edit - n/m
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Last edited by rogue277; Jan 30, 2012 at 12:29 PM.
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Old Jan 29, 2012, 04:13 PM
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BTW, thank you rogue277 for that information.
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Old Jan 29, 2012, 10:59 PM
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I bought 5 of these, and got them a couple of days ago. I also bought some APC 15X4E props to test them with. Haven't touched them yet. Took one of the motors out to look at it. I've never had a motor that looked quite like that. Solid looking ... kind of heavy.
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Old Jan 30, 2012, 01:18 AM
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Originally Posted by midiman007 View Post
I bought 5 of these, and got them a couple of days ago. I also bought some APC 15X4E props to test them with. Haven't touched them yet. Took one of the motors out to look at it. I've never had a motor that looked quite like that. Solid looking ... kind of heavy.
Sorry to threadjack. Where did you find apc 15 X 4 thin E props? I am looking all over and couldn't find them.
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