Broken 3- and 4-Blade Propellers

You have a sport or scale airplane with a 3 or 4-blade propeller…and you break one, and don’t have a spare, or they are no longer available.

What to do?  Go to https://www.spiritsofstl.com/interesting-things/ and scroll down to Prop Load Calculator, and use it to help modify a 2-blade prop…problem is solved!  You can download it.  The 3 vertical dots next to file name has the option. (You need MS Excel or equivalent)

Background:

There is a very complicated formula for determining the load factor of a propeller. In its 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 10 x 6 prop, the load factor would be 10 x 10 x 10 x 6 or 6,000. For a 12 x 8 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 9 x 7 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 equivalent to a 2-bladed 10 X 8 prop cut down to 9.6 x 8 prop, which has a load factor of 7,078.

If you had a 4-bladed 12 x 7 prop, then the load factor would be 12 x 12 x 12 x 7 x 1.732 or 20,950 This would be roughly equivalent to a 14 X 8 2-bladed prop cut down to 13.5 x 8 prop, which has a load factor of 19683.

In the end, if the load factor of props is the same, you will get similar RPMs from the two props, and greater speed.  ALWAYS KEEP THE 2-BLADE LOAD FACTOR BELOW THE ORIGINAL 3 OR 4-BLADE LOAD FACTOR!

What to do:

The purpose of the charts that follow, using the Prop Load Calculator, is to show that a stock 3 or 4-blade prop can be replaced with a 2-blade prop with the diameter cut down to achieve a similar load factor.  Thus, not overloading the motor and ESC.  It also allows the use of a cheaper propeller that is normally available at the local hobby shop.  Speed will increase significantly when using a two-blade prop that is sized to not exceed the stock 3 or 4-blade load factor and not harm the motor or ESC.

When going to a 2-blade propeller setup from a 3 or 4-blade SPINNER you may have to enlarge notches directly across from each other on the stock 4-blade spinner.  On a three-blade stock spinner you will have to create a notch directly across from one of the existing notches.  OR, you can buy a same color 2-blade spinner that is the same diameter as the original spinner. 

Example photos…