PROP size 2-3 or 4 blade - RCCanada - Canada Radio Controlled Hobby Forum
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Old 02-23-2009, 12:40 PM   #1
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PROP size 2-3 or 4 blade

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I am going to building a P-47 and for a little look of scale i was thinking of putting a 4 blade prop on it , my engine a thunder tiger .91 can turn a 13x6
can i just put a 3 or 4 blade 13x6 on it or do i have to brake out the slide ruler and divide by 3.1415 or some crazy thing like that ?
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Old 02-23-2009, 01:23 PM   #2
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Re: PROP size 2-3 or 4 blade

props: generally from 2 to 3, drop an inch and maintain pitch.

and I found this elsewhere (as your 13x6 is not shown in the table):

There is a very complicated formula for determining the load factor of a propeller, but in it's most simplistic form, for a 2-bladed prop, the load that a prop places on a motor is equal to the diameter cubed times the pitch or D x D x D x P. For a 2 bladed 10x6 prop, the load factor would be 10 x 10 x 10 x 6 or 6,000. For a 12x8 prop it would be 12 x 12 x 12 x 8 or 13,824.

The more complete formula, which takes the number of prop blades into account is D x D x D x P x Square root (N-1), where N = the number of prop blades. For a 2 bladed prop, the square root of (2-1) is the square root of 1 which is 1, so the term just drops out of the equation.

For a 3-bladed prop, the correction factor is the square root of (3-1) or the square root of 2, which is 1.414.

For a 4 bladed prop, the correction factor is the square root of 3, which is equal to 1.732

So if you have a 3-bladed 9x7 prop, then the load factor is 9 x 9 x 9 x 7 x 1.414, which is 7,216, and this would be roughly wquivilent to a 2-bladed 10x7 prop, which has a load factor of 7,000.

If you had a 4-bladed 12x7 prop, then the load factor would be 12 x 12 x 12 x 7 x 1.732 or 20,950 This would be roughly equivelent to a 2-bladed 14x8 prop, which has a load factor of 21,952.

In the end, if the load factor of 2 props is the same, you will get similar RPMs from the two props, and similar performance.

Last edited by voyager_663rd; 02-23-2009 at 01:29 PM.
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