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RIP, my good man! My fellow aviators miss your amazing talents!
I would NOT put that on an RV.....
Per Tony B. publishing, a rule of thumb has it that when you decrease the propeller diameter by one inch, or decrease the pitch by one inch, the rpm will increase by about 50 to 100 rpm . . . say, on the average, 80 rpm.
What you need to do is not try and calculate what you want for a diameter pitch value, but contact someone you want to have make you a prop and talk with them about what you want to focus on performance wise and then let them handle the rest.
Exactly what Rvbuilder2002 said!
Diameter is determined by engine power available and ground clearance. A given power can only turn a prop with x amount of surface area and y amount of pitch. Other factors are blade design- chord width, thickness of blade at given stations, is it dihedrial or anhedrial.
What i found out is prop pitch between manufacturers for a fixed pitch seems irrelevante. I have a Catto on my RV9a. Got fthe one recommended by Catto. Whent I took it back to osh last summer i recorded rpm and speede and fuel flow. I met up with the great VAfer Dan Horten and he did the math and we realized the prop was over pitched by almost 200 rpm. I was leaving 30 hp on the table. Catto repitched it in the usual why by reshaping the trailing edge. Wow what a difference. Higher rpm during climb and great cruise at an awesome efficient 22nmpg. So the inches of pitch didnt matter, but after first long cross countey and generating the numbers, the real tweak needed to make it a great prop became obvious to the experts at catto. Catto gives a free prop repitch i think for this reason. The prop is a perfect compromise for me now. But took a tweak to make it the best.jMHO
Was it really a repitch?
Yes, it is. When you remove material from the "face" of the prop toward the trailing edge, it actually reduces the pitch and chord of the prop.
There was a recent article in kitplanes that explains how they repitch a prop by taking a little off the trailing edge. They explained to me this is what was done and the prop pitch number was updated on data plate. I think i went 1 to 2” lower pitch as marked on data plate the dia did not change but i did notice a slightly fifferent airfoil profile pulling plane out of hangar
Interesting. I understand that when re-pitching a metal prop you actually change its "twist". No other reasonable way to adjust its load on the engine.
As my math teacher used to say about algebra, one is not to wonder why, just invert and multiply. I dont second guess the experts they asked me how much more rpm i wanted and Catto nailed it perfectly.
