I have the YS 91 AC in a 56" span scale acro sport bipe near similar spec to the Skybolt. I originally flew it with the OS 91 FS in and experimenting prop wise with the YS led me to larger finer pitch ie 15x8 and 16x6 especially as nitro wing suggested... 16x6 was a great combo for this size bipe. I found aileron flutter became a worry at the higher speeds with smaller higher pitched props. Slow it down and have fun with the great vertical and prop hanging on offer with bigger prop ;o)