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Sep 17, 2010, 11:38 PM
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Why would a 12X10 show more Amps than 12X12? Looking for right prop selection...


I have a Sundowner, that I am trying to squeeze every bit of speed out of that I can with my current setup. I have a Turnigy Aerodrive 50-55 580KV motor http://hobbyking.com/hobbyking/store...idProduct=7071 and a Hobbyking 90-100A ESC http://hobbyking.com/hobbyking/store...es_90-100A_ESC I have a Rhino 6s 4900mah 20-30C lipo. I flew the plane using a wood "vintage" Top Flite Power Prop 11X12 once. Everything came down cool. It was suggested to try a larger diameter to get a little more thrust. So I purchased a APC Thin Electric 12X10, and 12X12...and a Master Airscrew 12.5X8.5 Formula Prop. I made a shunt (from tips here) for my voltmeter and my readings made sense except for one. The 11X12 came in at high 70's low 80's depending on how fast I applied power. The 12X12 was 91A, and the 12X10 was 95/96A. Why would the 12X10 show more? I then tried the 12.5X8.5 and it was right around 63A. The two 12" props are probably pushing/exceeding the limits of the motor so I may not fly with them. Either way at 90ish amps, the lipo is close to its C rating. During the tests, which lasted only a couple minutes it got warm, sort of soft, and dropped from 4.21 to 3.73V. So it would be a short flight anyway....I'm fine with that as long as I don't kill the motor/ puff the lipo. The 12.8 Formula prop "sounded" the coolest...was loud So back to the question....why would a 12X10 show more than a 12X12? I tried mutiple times came back the same evertime.
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Sep 18, 2010, 12:01 AM
TMO
TMO
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Were these static tests?

Static testing of highly pitched props is often thrown off by the prop being stalled at higher rpm. Essentially the prop is cavitating and is not meeting the same air resistance as a lower pitched prop. A far better measure of performance in this case would be to measure actual in flight draws. I often put my Watts Up in line and fly with it. It's not perfect, but it will measure the peak. I have some slippery warbirds that only draw 2/3rds of static draws at WOT in the air. I wish I had one of those fancy loggers!

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Originally Posted by planebldr
why would a 12X10 show more than a 12X12? I tried mutiple times came back the same evertime.
Sep 18, 2010, 12:42 AM
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Yeah these were static tests in my basement. This was all just to get a general idea...so I don't go too high on the Amps and find out during flight that I was too high and fry everything up. They only got as high as they did when at full. At 2/3 they were high 60's-mid 70's. Then it just spiked at the 90's. So most of the flight wouldn't be bad. Just the "punch it and go" it would spike. A good meter is on the "must get soon" list. Just wanted a general idea to get me going.
Sep 18, 2010, 10:05 AM
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Quote:
Originally Posted by planebldr
why would a 12X10 show more than a 12X12? I tried mutiple times came back the same evertime.
Power dissipation is proportional to the area of the propeller.

If the props were different aspect ratios, the 12X10 could draw more statically than the 12X12.
Sep 18, 2010, 10:27 AM
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Either way though....I probably shouldn't use either for flying. Oh well.


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