I don't have a 91FX in my UCD, but it does have a ST 90 if that will help. I tried a 15x6 Zinger, 14x6 Zinger, 14x6 Rev-Up, 14x6 APC, 13x8 APC, and 15x4 APC. I liked the quicker spool up times of the wood props.....but overall the UCD flew best with the APC 15x4. For a 91FX, I'd start with the APC 15x4 and experiment from there.