Three-Blade (or more) Propeller Conversion
by: Marl Bell
I wanted to change the propeller on my Phoenix Models Edge 540 from a 2-blade to a 3-blade propeller. I have heard a number of "rules-of-thumb" in calculating this. Some say that you just need to drop the diameter of the 2-blade prop by one inch while keeping the pitch the same. But I wanted to see if there was a formula that would calculate this. While searching RC Groups, I found a couple of methods that seem to work pretty well.
I put two of the formulas that I found on the RC Groups website into an Excel spreadsheet which you can download HERE. Input cells have an orange background with black numbers and calculation cells have a gray background with orange numbers. I prefer "Method #1: Match Load factor." You can see how close the 3-blade propeller matches the load of the 2-bladed prop by looking under the heading "% Difference." You can experiment with different number of blades and blade diameters to try and get a perfect load match.
I had originally planned to use a 13" x 6" APC on my Edge 540 which uses an O.S. .55 for power. But since I had an aluminum, 3-blade spinner, I thought it would look cool to have a 3-blade prop- plus I'd have more ground clearance! The calculation yielded an 11" x 7", 3-blade propeller. At the time of this writing, I have four flights using this prop. It seems to load the engine just right and has plenty of power!
I put two of the formulas that I found on the RC Groups website into an Excel spreadsheet which you can download HERE. Input cells have an orange background with black numbers and calculation cells have a gray background with orange numbers. I prefer "Method #1: Match Load factor." You can see how close the 3-blade propeller matches the load of the 2-bladed prop by looking under the heading "% Difference." You can experiment with different number of blades and blade diameters to try and get a perfect load match.
I had originally planned to use a 13" x 6" APC on my Edge 540 which uses an O.S. .55 for power. But since I had an aluminum, 3-blade spinner, I thought it would look cool to have a 3-blade prop- plus I'd have more ground clearance! The calculation yielded an 11" x 7", 3-blade propeller. At the time of this writing, I have four flights using this prop. It seems to load the engine just right and has plenty of power!