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Convert a 2 blade prop size to a 3 blade prop?

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Convert a 2 blade prop size to a 3 blade prop?

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Old 02-25-2006, 07:09 PM
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SU29
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Default Convert a 2 blade prop size to a 3 blade prop?

What is the rule of thumb for determining what size three blade prop to use, for instance, if normally I use a 12 X 8 prop on a particular engine and I want to try a 3 blade prop for more clearance.

thanks
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Old 02-25-2006, 08:43 PM
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Default RE: Convert a 2 blade prop size to a 3 blade prop?

Drop an inch in diameter to a 11-8, and maybe even to 11-7. General rule for going from two to three blades is drop one inch in diameter OR drop an inch in pitch at same diameter. You want more clearance, so change diameter.
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Old 02-26-2006, 11:02 AM
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Default RE: Convert a 2 blade prop size to a 3 blade prop?

I partially agree with Khodges (1/2 agree ). Drop 1" in diameter, but keep the same pitch (agree with Khodges) -- or, keep the same diameter & drop the pitch 2".

In your example of a 12-8 2B, go to either an 11-8 3B, or a 12-6 3B. That is assuming that the prop is made by the same manufacturer, of the same material, & has the same general blade shape & aspect ratio -- otherwise, all bets are off.
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Old 02-26-2006, 11:21 AM
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Default RE: Convert a 2 blade prop size to a 3 blade prop?

ORIGINAL: SU29

What is the rule of thumb for determining what size three blade prop to use, for instance, if normally I use a 12 X 8 prop on a particular engine and I want to try a 3 blade prop for more clearance.

thanks
Playing with ThrustHP:
12x8 2 blade load 165888
3 blade comparison
11x8 load 175692
11x7 load 153731
11x7.5 load 164711

So, the 11x8 will probably turn a couple of hundred rpm less and maybe have slightly lower thrust than your current prop
the 11x7 should turn a couple of hundred more and maybe have slightly more thrust than your current prop
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Old 02-26-2006, 12:59 PM
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Default RE: Convert a 2 blade prop size to a 3 blade prop?

Thanks for the feedback guys, I think I get the picture. Carrell, where or how did you come up with the load numbers?

Jeff
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Old 02-26-2006, 09:17 PM
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Default RE: Convert a 2 blade prop size to a 3 blade prop?

ThrustHP is a free program
http://freespace.virgin.net/barry.ho...e/goodies.html
It does a calculation for static thrust and horsepower. From what I've read the thrust numbers are somewhat overstated.
I put the two props side by side in the calculator and arbitrarily plugged in 10000 rpm. Then I adjusted the 3 blade rpm up or down until the horsepower was equal to the 2 blade number. Whether the horsepower or thrust calculation is perfect doesn't matter to me. It gives a basis for comparison to get you close. After that you buy some props and see how they work for you [since your plane/engine/etc will work slighlty different than mine].
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