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Old Jan 25, 2014, 06:34 PM
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Smaller prop gave me higher amps??

I had a 9x6 prop on my Predator Drone with a Turnigy motor that stated 9x6 was the best prop to use. The motor specs are 17.5 amps and 180 watts. I tested the 9x6 with my power meter and got 210 watts and 21 amps. I picked up an 8x6 prop and my watts went up to 300 and amps to about 30. The 8x6 is a much thinner Master Airscrew prop. The 9x6 is a thicker APC - See links.
Shouldn't my power usage hab\ve gone down about 15-20% with the smaller prop?
I realize I am going to need to go to an 8x4, but this test stymied me.

8x6: http://secure.hobbyzone.com/MAS0860G3.html
9x6: http://secure.hobbyzone.com/APC0906P.html

motor: http://hobbyking.com/hobbyking/store...dProduct=36717
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Last edited by bbells; Jan 25, 2014 at 06:37 PM. Reason: add email notification
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Old Jan 25, 2014, 07:08 PM
No bounce, No play.
davidmc36's Avatar
Canada, ON, Ottawa
Joined Oct 2010
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I believe there is a large difference in efficiency of the two style of props. You may be just "spinning your wheels" with the MAS. What is the RPM with the two different props on? The MAS may spin at a higher RPM but it may take a lot more power to get it there due to weight/shape/etc.
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Old Jan 25, 2014, 07:45 PM
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That must be it.
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Old Jan 26, 2014, 12:40 AM
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Georgia
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One of the props referenced is a "pusher prop" ???
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Old Jan 26, 2014, 03:07 AM
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Yes. Couldn't find a pusher with the right twist and length so I needed to change the rotation of the motor and use a regular prop. Now I will be cutting down the pusher.
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Old Jan 26, 2014, 03:19 AM
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Australia, ACT, Kambah
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Quote:
Originally Posted by bbells View Post
Yes. Couldn't find a pusher with the right twist and length so I needed to change the rotation of the motor and use a regular prop. Now I will be cutting down the pusher.
Being able to reverse rotation with just switching wires means that 'pusher' props are not really necessary - you've already solved that problem. They ARE necessary for glow engines which have a specific rotation direction.
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Old Jan 26, 2014, 10:22 AM
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Dr Kiwi's Avatar
Chattanooga, Tennessee, United States
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As David said, "MAS-Electric Only" props are only good for stirring paint.
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Old Jan 26, 2014, 04:20 PM
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Nashville, NC, USA
Joined Mar 1999
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His link is to a MAS nitro prop.
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Old Jan 26, 2014, 04:22 PM
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davecee's Avatar
Nashville, NC, USA
Joined Mar 1999
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Quote:
Originally Posted by davidmc36 View Post
I believe there is a large difference in efficiency of the two style of props. You may be just "spinning your wheels" with the MAS. What is the RPM with the two different props on? The MAS may spin at a higher RPM but it may take a lot more power to get it there due to weight/shape/etc.
If the MAS is pulling MORE amps then it's turning LESS rpm, assuming same motor and battery.
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Old Jan 26, 2014, 04:49 PM
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William A's Avatar
Tacoma,WA USA
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Quote:
Originally Posted by davecee View Post
His link is to a MAS nitro prop.
Ditto. Heavy emphasis on "nitro".
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Old Jan 26, 2014, 06:04 PM
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Toronto Canada
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Nitro props usually have thicker hubs. Because most of the inner portion is stalled statically, that means that they will appear less efficient statically.

That is not necessarily so at Pitch Speed where the full lengths of the blades of the prop are at their maximum lift-drag ratio.
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Old Jan 28, 2014, 04:48 PM
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Thanks for the help.
OK, I changed props and used an APC 9x6 sf and an APC 8x3.8 sf and pretty much the same thing happened. The watts and amps were slightly higher with the smaller prop!
I made a point of trying props that are basically the same design, except one is shorter with less pitch. If I had any hair I would be pulling it out right now.
There is nothing rubbing on the motor. The motor is about 1500kv with a maximum 180 watts and 17.5 amps. I am getting 230 +/- watts and high 20's in amps.
Could this be a caused by my 2 foot battery lead going back to the motor? I'm starting to think I should just limit my throttle so my amps don't go over spec and fly the damn thing with the smaller prop until the motor gives out. Would I get better results if I put my watt meter closer to the motor rather than closer to the battery?
Any help would be appreciated.
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Old Jan 28, 2014, 05:19 PM
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Fourdan's Avatar
Antony (France)
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Bbells

Something is curious in your results
Try Scorpion Calc freeware (menu propellers)
http://www.rcgroups.com/forums/showthread.php?t=736782
Good for an order of magnitude.

prop #67 APC 8x3.8 SF @ 9000 rpm corresponds to 68W (power at shaft)
prop #121 APC 9x6 SF @ same speed 9000 rpm corresponds to 178W
of course the motor efficiency is different and loading is lowering the rpm

I suggest you to check rpm (tachometer)
Note that power is roughly kp(rpm^3) for a given prop
Louis
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Old Jan 28, 2014, 05:29 PM
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Ok, I am going to borrow another watt meter. I will post the results.
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Old Jan 28, 2014, 06:14 PM
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Who knows. Maybe my new motor is shorted or something inside.
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