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Feb 06, 2020, 08:45 PM
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83" KMP B-25 Electric, need help with prop size.

As title states, been putting this plane together over the past couple weeks and am getting to the point of mounting the motors. I am using two G-Force G60 400kv motors. They are likely overkill I know, but I already had one so figured I would try to use them. Based on the weight others have ended up with these planes, I expect weight will be around 16-18 lbs. Based on others, I bought a 14x7 prop (what the LHS had), hooked it up and put my newly acquired wattmeter to it. With a 6s battery it would only pull about 24 amps, and didn't really feel like it had a bunch of thrust. I had wanted to run counter rotating props, but selection is slim. Heck, selection is slim for three-blade props period. I can run a 15" prop, but will only have about 1.5" of ground clearance which I worry might not be enough, heck, 2" with a 14" prop doesn't seem like much either, but I guess I may just have to live with it.

So, opinions on prop size? I tried the motor with a 16x8 3-blade and it pulled about 50 amps. I don't know how hard it pulled because it was mounted in a vise. I found some 15x10 wood three-blade props from china on ebay, I hate to spend $40 each on props, and they also don't have a reverse pitch, so I can't do counter rotating props.

Any help appreciated.
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Feb 06, 2020, 10:07 PM
Rampage's Avatar
To be honest on a trike gear you probably won't even be able to tell the difference with or without counter-rotating props. I run CCW props on my Royal B-25s (and my 60" Twin Otter and my 80" Twin Otter and..) and they fly just fine.

With the 16" 3-blade how many watts were you producing?

At 83" your B-25 is 1/10th scale (rounding up just a hair) so the 15" prop is actually the closest in scale size to the real one. Real B-25 had 12'7" (or 151") so a 1/10th scale prop would be 15.1". So the 15" is the scale size.

It looks like the manufacturer calls for a 16x10x2 on 6s for that motor, so a 15x10x3 would probably work pretty well.
Feb 06, 2020, 11:23 PM
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With the 16x8x3 it was over 900 watts at WOT.

I went ahead and ordered two 15x10 3-blades from ZYhobby. $53 shipped for the pair off their website. Hoping for smooth landings and short grass at the field haha.
Feb 06, 2020, 11:26 PM
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And I did weigh everything (separately) with two 6s batteries (5000's) it was about 17 lbs. I think it will need lots of weight up front to balance, so I plan on putting both the batteries above the nose gear. Once I get the other motor installed I will be able to better figure out where the batteries need to get the CG right.
Feb 08, 2020, 12:16 PM
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Katniss's Avatar
Master Airscrew also makes a 14x9x3. Gives you the 2" ground clearance and better pitch then the 14x7.

A different option would be the Mas 13x12x3.....does come as Cw and ccw versions.
The higher pitch will load the motors up properly.

I do have a 95" ESM me-110 flying on power 60s spinning MAS 16x10x3s. Two 6s 5000 batteries in the nose......combo works really well.

Have a vq p38 with power 60s and the 13x12x3 props....same size as your B25
Feb 23, 2020, 10:38 AM
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It is almost ready for a maiden flight. Still waiting on the 15x10 props to show up. Weighs 19lbs 6oz with two 6s 5000mah batteries in the nose. Balances out just a touch in front of where COG is supposed to be.

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