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Old Jun 30, 2011, 10:31 AM
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cannondale1974's Avatar
Lexington, KY
Joined Oct 2009
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Help me (not) fry this motor: What size prop would you use?

Not looking for maximum speed but good "thrust/vertical." Here's the motor in question: http://www.hobbypartz.com/75m55-opti...-1800kv-2.html

Motor is going on a Stryker so I'm limited in diameter size of the prop. I put a 6x4 on it and the wattmeter showed a measly 17 amps and 215 watts. Props I have are:
6x4
6x5.5
7x4
7x5
7x6

ESC is 36 amps, how big would you go and still keep things safe?

Thanks guys.
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Old Jun 30, 2011, 12:42 PM
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whitecrest's Avatar
Orleans, MA
Joined Feb 2007
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Given your requirements, I'd go with the 7x4. That should provide excellent thrust while not overloading the motor or ESC.
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Old Jun 30, 2011, 12:48 PM
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efflandt's Avatar
United States, IL, Elgin
Joined Mar 2011
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The chart listed there does not seem to match up with that specific motor (which 450 1800 kV is it), but both 450 1800's list ratings with 7x6 prop on 3S. What S battery are you using if you are pumping 215 watts into a 165 watt motor at 17A (ie, over the 12.6V max of 3S)? 28A on 4S might let the magic smoke out.
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Old Jun 30, 2011, 02:23 PM
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cannondale1974's Avatar
Lexington, KY
Joined Oct 2009
691 Posts
Thanks guys, it's the 75M55 motor. On a 7x4, wattmeter showed 261 watts and 24 amps. On 7x6 it showed 290 watts and 25 amps.

What's a bit confusing is on the case of the motor it states "165 watts" but in the chart it's showing 326 watts on a 7x6. Wasn't sure which number to use as a guideline. I'll only be flying on 3 cell and most my batteries are 25 and 30C.

Thanks again!
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Old Jun 30, 2011, 03:18 PM
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Orleans, MA
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I couldn't find a straight spec list for the motor listing a sustained current rating. However, one review claimed to be running an 8x4 on 3S with good results. I wouldn't exceed 7x6 on 3S until you're sure it doesn't get too hot. Motor specs are frequently hyped. If your motor weighs about 2.5 oz, you should be Ok on the 7x4.
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