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Old Mar 09, 2006, 08:57 PM
jcosta is offline
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Lisboa, Portugal
Joined Jan 2004
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Originally Posted by elfwreck
Beagle claims to fly often, and I believe him, he also gets a kick out of doing the math (something I'm not so good at). As long as you're having fun with the hobby/sport why worry about how others get their kicks?
My only issue was that the math is not giving him good information.

This happens too often in E-flying. If there is not a recomended or stock power system, I go for a watts/kilo formula (that changes depending on the type of plane and type of flight desired - 100-150 watts/Kg for a trainer type plane, 250-400watts/Kg for an LMR type sailplane. Then I select a power system - motor, battery, eventualy gearbox - use a calc program to predict a prop diameter and then I measure, as the calc program will be wrong. Usually I go 1 inch down in diameter from what the calc program tells me. In the case of the Axi2820/10 with 8 cells in my Magister I actually ended up with a prop 2" smaller than the calc program predicted. Curiously enough I see lots of people recommending this motor with the too big prop.
Moving to real life, everybody and 2 calc programs suggest a 12x7 prop and I found out an 11x7 prop measured too close to the current limits both for motor and ESC. I tried a 10x7 and I actually measured the current I "should" be reading for a 12x7...Same with RPMs and volts. I didn't measure static thrust as I still haven't bought the necessary fisherman dynamometer.
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