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2 blade to 3 blade

Old 06-17-2015, 12:39 AM
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340cudaman
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Default 2 blade to 3 blade

I have the fun cub. I wish to use a 3 blade instead of 2. I like the extra ground clearance vs a 2 blade. I have a 10-7 3 blade master airscrew, phoenix 50 edge 50a 34v esc, himax brushless outrunner HC3516-1130 and will be using a flightPower li-po 3s 11.1v 25c 24w battery.
Will the 10-7 work with this set-up or will I nedd to use a different prop? I purchased the plane in kit form and is almost completed, so I haven't tested any props yet. Thanks in advance.
Old 06-17-2015, 05:00 AM
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jester_s1
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MAS should have the amp draw data for you to see if your power system can handle it and so you can calculate your wattage to see if you'll have enough power. All you need to know is your motor Kv to do the calculations.
Old 06-18-2015, 12:25 AM
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340cudaman
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jester_s1..Thank you for responding. The kv for my Himax Outrunner is HC3516-1130kv, Rm=.030, lo=1.8 (this is the info on "Electrical Specifications" on the instruction sheet). Can you help me with MAS? (I can't seem to find it on the internet). I wanted the 3-blade for the ground clearance as most of the fields I'm able to fly on are not cut short. I'm fairly new to electric flight, all my other aircraft were either AFR or RTF. I had always gone with the prop size that came with the aircraft.
Thank You in advance for your help. 340cudaman
Old 06-18-2015, 06:00 AM
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It looks like speaking from memory has me at a loss again. I couldn't find a chart from Windsor on the MAS props. But we're not out of luck yet. Some simple arithmetic will allow you to compare props of the same style for load factor, which is the amount of torque at any given RPM it take to turn them. The formula is D x D x D x Px Square root (N-1). D is diameter, P is pitch, and N is the number of blades. So for your application, D cubed is 1000, times the pitch of 7 to get to 7,000. Since N is 2 and N-1 is 1 and the square root of 1 is 1, it doesn't modify that number at all. So the load factor of your 10x7 prop is 7,000. It's not an absolute number as different 10x7 props will load the motor differently, but it's useful for comparing within the same brand. If you are happy with the amp draw and power you currently have, you'll want to keep the same load factor. So then you play around with the numbers to get it right. If you go down to a 9x7 3 blade, that gives you 9x9x9x7x1.414 (the square root of 2). That comes out to 7,216, which is right at a 1% change. So you should see similar amp draws between those two props on the plane in flight.

Of course, nothing is quite that simple. There are other factors to the way those two props will perform. You'll have some efficiency loss by going to a 3 blade prop since the blades' turbulence will interfere with each other more. You'll also have a greater percentage of the blades' length blocked by your spinner and fuselage. So you might find that the new prop has a different feel in the air than the 2 blade one did, or you might not notice it at all.
Old 06-19-2015, 12:13 AM
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340cudaman
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Thank you jester_s1 for you quick response. I'm not nor was I ever very good with algebra. It was very informative. I guess my question really is will the 10-7 3-blade prop be a good starting point for the fun cub with the motor I purchased? Thank you in advance.
Old 06-19-2015, 06:53 AM
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What is the stock prop? It's a safe bet to go one inch down with the same pitch when switching from a 2 blade to a 3 blade.

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