I have an OS 26 FS that is a really nice runner. I use 10x4 and 10x5 props. I got mine used and it had a 9x4 when I bought it; RPMs were way too high. It worked, but I wouldn't expect it to last. I think a 9x5 would also allow it to rev quite high and might not be best for longevity. If I wanted more speed on a clean air frame I would go to a 9x6 (or a Bolly 9.5x5, but you can't get them anymore). Jim