Hello OS Manufacturing Representative,
I have one of your 55AX engines in an Ultra-Stick 60 airplane and a 12 X 6 APC propeller pulls it nicely. I've decided to use the same model engine in a 58" Wingspan Corsair kitted by Phoenix Model, however, for the Corsair I would like to use the largest 3-Bladed propeller that would be the equivalent of the 12 X 6 that I use on the Ultra-Stick. I would think I would go down in pitch when I go up in prop diameter, but not sure. Can you recommend a 3-Bladed propeller size and pitch that I can use on the 55AX Engine?
Thank you.
From Mike D'Amico
West Melbourne, Florida

General rule for going from two to three blades is drop one inch in diameter OR drop an inch in pitch at same diameter. 11X6 three blade works.

My way of selecting a 3-bladed prop for an engine is as follows: Assuming a proper sized 2-bladed prop, plus its RPM is known for the engine, enter those data in any decent propcalculator, and note down the power generated.

Now search that same propcalculator for a 2-bladed prop that absorbs as close as possible to 2/3rd of that power at the same RPM. That is the size of 3-bladed prop you are looking for.

If APC is the brand to go for, indeed an 11 x 6 would be very close to an APC 2-bladed 12 x 6. Slightly heavier.
However, a Master Airscrew for example, would need to be 11 x 7 in order to be equivalent to a 2-bladed APC 12 x 6.
When going from 2-bladed to 3 bladed, it is always advisable to select a prop with a bit more pitch, the higher velocity of the propwash compensates a bit for the narrower diameter of that propwash, and flight performance will be more similar. At least, that is my experience.

If I understand the OP correctly, he wants to go UP in diameter. I'm not sure he can go up but if he stays the same diameter, he should use a 12x4 or 12x5 3-bladed prop. I had a similar issue with a 43 inch Corsair. The reason the Corsair had the inverted gull wing was to have the largest diameter prop for that huge engine. I used an OS 32SX which had very high RPM. It would swing a 10x6 2-bladed prop at around 18K RPM. So I put a 10x4 3-bladed. It swung that at 18 as well. I would like to have had a bigger prop but couldn't find one.