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Old Jun 16, 2012, 03:14 PM
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Help--2-blade to 3-blade folding prop comparison

I'm in the process of electrifying a 6 meter, 32 lb. KA8 glider. I will be nose-mounting an e-flight 160 series outrunner. It produces 22 lbs. of static thrust @ 2400 watts, with 10 cell lipo and a 20-10 prop . The glider ground clearance is insufficient for anything more than around 15 inches in diameter, so I'd like to use a 3-blade folding prop and yoke. I need help figuring out the 2 - 3-blade conversion. I think a 15-10 3-blade might be close to the 20-10. I have an accurate test setup to verify the motor and output, but didn't want to buy a bunch of props I can't use. Any Ideas??? Thanks. Bob
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Old Jun 16, 2012, 04:16 PM
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The equation for converting blade numbers while maintaining the same power level is D2 = D1 * (B1 / B2) ^ 1/4. So a 20 inch 2 blade prop would use the same power as a 20 * (2 / 3) ^ 1/4 = 18.08 inch 3 blade prop. Going to a 4 blade prop would be 20 * (2 / 4) ^ 1/4 = 16.82 inches. So even with a 3 or 4 blade prop, at 15 inches you would have to increase the prop speed and/or pitch to achieve your desired power level. Either that, or live with the reduced power.

Larry
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Old Jun 16, 2012, 04:53 PM
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The equation for converting blade numbers while maintaining the same power level is D2 = D1 * (B1 / B2) ^ 1/4. So a 20 inch 2 blade prop would use the same power as a 20 * (2 / 3) ^ 1/4 = 18.08 inch 3 blade prop. Going to a 4 blade prop would be 20 * (2 / 4) ^ 1/4 = 16.82 inches. So even with a 3 or 4 blade prop, at 15 inches you would have to increase the prop speed and/or pitch to achieve your desired power level. Either that, or live with the reduced power.

Larry
Thanks, So since I need the same thrust as the 20-10, and can only use 15" dia., 3-blade, my question is what pitch would the 15" 3-blade need to be? Sounds like the 15-10 wouldn't be enough pitch to maintain the power. Maybe a 15-12 would do it? I could go with a 16 if I have to, so maybe a 16-12? Thanks.
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Old Jun 16, 2012, 09:27 PM
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It's difficult to say what effect pitch changes will have other than power tends to change linearly with pitch change. The equation I supplied is just like motor calculators, it isn't an exact science, but it should get you in the ballpark as far as far as prop diameter is concerned. Any of these pediction formulas are just estimates. The final results have to be verified by actual measurements and any required adjustments made from there.

Larry
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Old Jun 16, 2012, 10:18 PM
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It's difficult to say what effect pitch changes will have other than power tends to change linearly with pitch change. The equation I supplied is just like motor calculators, it isn't an exact science, but it should get you in the ballpark as far as far as prop diameter is concerned. Any of these pediction formulas are just estimates. The final results have to be verified by actual measurements and any required adjustments made from there.

Larry
Thanks Larry, I will set up the 160 motor with the triple yoke and 15-10 folding blades, which I have. I'll get an amp, watt and thrust reading, and go from there. The next size folding prop (aeronaut) is 15-13, so that will be the next step. I'll reply with the results as the testing goes, and we'll have that info in our data banks. In my previous tests with folding props in the 12-15" range, I've found generally that each 2" of diameter = about 1 pitch, so the 13 pitch probably is going to be close. Thanks again, Bob.
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Old Jun 17, 2012, 03:33 PM
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I've found generally that each 2" of diameter = about 1 pitch, so the 13 pitch probably is going to be close. Thanks again, Bob.
It's all relative. Power changes with the fourth root of a change in diameter so that generalization would be valid for only a small range of diameters and pitches. For example, with a 2 inch change going from a 6" to a 6" prop power would would decrease to a whopping 20% of original. Whereas, going from a 20" to an 18" prop power would decrease to only 66% of original.

So going from an 18" three blade prop to a 15" three blade prop you can expect to see power drop to approximately 50% of original. To regain that power with a pitch change would mean doubling the pitch from 10" to 20". Going from 10" pitch to 13" pitch would increase power by only about 25% of the reduced power level. That still leaves you at approximately 63% of your original power level with the 18" three blade propeller.

The best way to regain the lost power would be to increase the speed of the propeller. That is because power changes with the cube of the change in speed. So it takes a lot smaller speed change than it would a pitch change.

Larry
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Old Jul 13, 2012, 09:32 PM
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20x10 2-blade = 18x10 3-blade?

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Originally Posted by Lnagel View Post
The equation for converting blade numbers while maintaining the same power level is D2 = D1 * (B1 / B2) ^ 1/4. So a 20 inch 2 blade prop would use the same power as a 20 * (2 / 3) ^ 1/4 = 18.08 inch 3 blade prop. Going to a 4 blade prop would be 20 * (2 / 4) ^ 1/4 = 16.82 inches. So even with a 3 or 4 blade prop, at 15 inches you would have to increase the prop speed and/or pitch to achieve your desired power level. Either that, or live with the reduced power.

Larry
Hi Larry, Thanks for the explanation. Here's my question. The 20x10 2-blade fixed APC prop was the highest thrust, but no one makes that size in the folding one, so if I understand your above statement, a 20x10 2-blade = pretty close to an 18x10 3-blade? I can get the 18x10 in a folding prop. I then wonder if the folding prop blades are as efficient as the solid ones. If they are not, I can also get the 20x11 folding blades for the 3-blade setup. What do you think? Your help and opinion is greatly appreciated. Thanks, Bob.
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Old Nov 01, 2012, 08:32 PM
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Propellers Balde convertion

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Old Nov 01, 2012, 08:32 PM
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Propellers Blade convertion

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Old Nov 01, 2012, 08:42 PM
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Propellers Blade convertion

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