Depending on ur flying abilities and prefferences,for most all 91 class FOUR STROKES accept the YS brand,you'll find they run the best with any prop between 12/11 to 14/6 size and pitch. On my OS,Magnums,Thunder Tigers,Satio's,Enya's and even my ASP 80,the best all round props i've found are the Master Airscrew 13/6,13/8,or 14/6 props. As to the YS 91,nothing smaller than a 14/8 Gruptner prop. If you really need ground clearance,you can always use a 3 bladed prop on your 91 in the 12/8,13/6,and 14/4 configurations.