Changing Your Aircraft Propeller Blade Count

For the sake of sharing the information n for my own notes, i quote this related infos on chosing your propeller.

Dr Kiwi - Rcgroups

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.

Here is another way of looking at it:

I've always used this formula: 2/x to the .25 power. 2 = number of blades of two bladed prop. This is a constant. X = number of blades you want (usually 3 or 4) The answer you get is a decimal. Multiply it by the diameter of the two bladed prop and your answer is the new prop.Use the same pitch.

To simplify; for a three blade, 2/3 raised to the .25 power = .903602 This is the two to three blade factor.

To go from a two to four blade, 2/4 raised to the .25 power = .8408964 This is the four blade factor.

2/5 to the .25 power = .7952707 5 blade

2/6 to the .25 power = .7598357 6 blade

So, if you have a 10-7 two blade and want a three blade: 10 x .903602 = 9.036 diameter. Round it off to 9, use same pitch, 9-7 three blade.

If you start with three blades and want to go to two, divide the three blade diameter by the same .903 factor to get the two blade diameter.

"Propeller design theory says:

2 to 3 blades, reduce diameter by 10%,

2 to 4 blades reduce diameter by 15%,

pitch stays the same in both cases.