Question for Leonard or anyone that has an answer

Since you have modified several of the OS 35 FP engines, have you done any experimenting with three blade props on the ones you modified? I am not speaking of the unmodified ones. What three blade prop(s) worked? I don't have a problem with ground clearance so that is not a consideration - I just think I would prefer a 3 blade on this plane just for the looks.

Well, although I always wanted to do a little experimenting along this line, I have never used a 3-blade on the modified 35 FP. We used a stock 2-blade 12.5 x 5.5 Bolly on the engine with very good results, however (best prop we ever tried). "Rule of thumb" with props of this size is when going from 2-blade to 3-blade, go down about 1 inch in diameter. Therefore an 11-11.5 inch Bolly (or equivilent) set at about 5.5 pitch should do the trick. Now, of course, the "perfect", off-the-shelf prop in this size doesn't exist, but Bolly does make a 11.25 x 6.5 3-blade prop and this could easily be repitched to 5.5. Alternately there are 12 inch props in 4.25 and 6.5 inch pitches that could be cut down and repitched.

Incidently, while we were doing some prop testing a while back, there are existing theories being passed around that all you need to do is change the last inch or inch and a half of a prop. To test the theory, we tried that with both of these props, repitching the outer tips only to 5.5 and running them on the 53. There was a tremendous difference in the performance between these two props. We then repitched the entire blade to 5.5 on each prop and found them then to be nearly identical. If you are going to repitch props, make sure you do the entire blade.

Several years back saw a formula for switching from 2 blade to 3 or 4 blade props

2 blade to 3 blade use 90% of the 2 blade diameter.
2 blade to 4 blade use 81% (90% squared) of the 2 blade diameter.

For a 12.5" two blade this would give an 11.25" three blade and a 10.125" four blade.

I used an 11.25 x 5 three blade on mine. Worked very well.