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Old Jun 06, 2005, 08:51 PM
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Joined May 2005
825 Posts
MotoCalc confusion

I am very confused about the results from MotoCalc as well as the online motor calculator at gobrushless.com. I have purchased a BP21 (although on E-bay, not balsapr). I am pretty sure that it is 21t, delta-wired. Because I was unable to learn anything about prop sizes, amp draw, etc from the calculators I have just tried things empirically. Right now I have found that one of the best props in terms of thrust and amps is a direct-drive GWS 1060. I can get about 10 minutes of flight or more out of my 1500 mAh LiPoly battery, comparable to the old EPS-350 with a B gear. I don't know what the thrust is, but it is better than the stock system by about 30%. Maybe 12-14 oz of thrust. I dunno. According to my inductive ammeter, the motor is drawing just under 10 amps at full static load. I have been unable to replicate these numbers with this prop on either gobrushless's calculator or motocalc. I've been told that the bp21 is similar to the Hacker 40/21L parameters in motocalc, so I've tried those. According to motocalc, the 1060 should be drawing 22 amps and putting out over 20 oz of thrust. What am I doing wrong here? I'd love to use these tools to plan my own motor designs but so far I have a huge disconnect between reality and how to plug numbers into the similator. Can anyone explain what I'm missing? I've been reading a lot of forum entries on the subject, but I must be missing it. Feel feel to point me at the relavent threads.
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Old Jun 06, 2005, 10:36 PM
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Adam Neat's Avatar
Winchester; Ohio
Joined Feb 2001
876 Posts
Try here; http://www.rcgroups.com/forums/showthread.php?t=338451

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Old Jun 07, 2005, 03:15 PM
flying low & slow
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Joined Mar 2003
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Interesting static motor test data, but it seems that the Power Out formula used needs some sort of prop constant as it results in efficiencies above 100% (up to 139%) in several cases...
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Old Jun 07, 2005, 07:14 PM
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Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
5,190 Posts
Using Gene Bond's constants with a 2S lipoly and EP1060, Motocalc predicts a miserable 37% efficiency! I got a better match to Gene Bond's data using Kv=1550 rpm/V, Rm=0.275 Ohms, and Io=0.7 Amps.
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Old Jun 08, 2005, 04:16 AM
flying low & slow
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Joined Mar 2003
365 Posts
Also wonder why the no load current Io is used as a constant in motor calculations despite the Io value is dependent on the voltage applied to the motor… ?
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