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…

Prop Load Examples

In the Calculator Excel file plug in the prop diameter and pitch in the row with the number of blades you have.  You will get a “Load Factor”.  Experiment by plugging in different diameters and pitches to get the same or less load factor of the prop you started with.

FMS P-51 Example

FMS P-51 family uses a stock 10.5 X 8 Four-Blade Prop (Voodoo, Blue Thunder, Miss America, Dago Red and P-39). Cost at Motion RC $8.99

APC 12 X 12 with the diameter cut down to 11.00″ gets almost the same load factor. Cost at Mark Twain Hobby $4.99

DiameterPitchNo. of BladesLoad Factor
10.508.00416,040
11.0012.00215,972
11.1012.00216,412
11.257.00314,093

FMS Tony Example

FMS “Tony’s”, A6M5’s, P-40’s, “Typhoon” use a stock 10.5 X 7 Three Blade Props.  Cost at FMS Models $13.99

A stock APC 10 X 10 stock 2-blade prop gets almost the same load factor.  Cost at Mark Twain Hobby $3.99

DiameterPitchNo. of BladesLoad Factor
10.507.00311458
10.0010.00210000
10.5512.00214091
10.508.00416040

Finally, you can even take any 2-blade prop and find a prop with more pitch and keep lowering the diameter incrementally (i.e., 9.75, or 9.6, etc.) using the prop load calculator to a smaller number that doesn’t exceed the load of the original prop.  For example, you could put in a 10 X 6 propeller in one of the two rows that show 2-blades under the column heading “No. of Blades”. The load factor would be 6,000. Then put in a 9 X 8 in the other row with 2-blades under the column heading “No. of Blades”.  Result will be 5,832 which is just below 6,000. The 9 X 8 prop has 2 more inches of pitch than the 10 X 6 and will obviously go faster without loading the motor or ESC.  It just so happens that the stock props used in example above have always worked. This will also work on nitro props too.

Try it…!

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