At one time had a 10 pound plane (GPSpacewalker) with a OS.91FX that I wanted to learn to hover with. I tried a lot of different props listening to advice from local experts and could not find anything that had the pull to hover.
Then I found a .91FX engine test from Engine guru Clarence Lee who tached this engine w/various props: 13x6 12,500\13x8 10,400\14x6 10,900 \15x6 10,200 \15x8 8,100\16x6 8,300. Using this prop/rpm information, I plugged these numbers into a thrust formula published in a magazine. The clear thrust winner on paper was the 15x6 @ 10,200 so I tried it and I had a winner. It had very little pull out, but enough for up high practice. I also noticed that my fuel usage noticeably decreased.
I then picked up a high volume Ultra-thrust muffler (tuned for lower RPM—I think the tower .75 muffler is lower volume & is tuned for higher RPM). My .91FX was turning the lower 10’s with an APC 15x6 stock muffler, baffle removed. When I put the high volume ultra thrust on, it jumped to the high 11’s!! This was a dramatic increase in thrust; pull from hover was very good—enough so that I felt comfortable hovering on the deck. I can say though, that with the way I was using the left stick, my fuel consumption went way up(I was using HP).
The .91FX with high volume Ultra-thrust is now on a 8.75 pound plane (4*60) and it easily wins a vertical drag race with a 10.5 pound/1.6FX plane .