RE: OS 91 FX what prop to use?
I use a 15x8 Menz on my 91FX mounted in a 8 lb Decathlon.
I get 88-8900 rpm peak and the engine seems to like that rpm.
APC props will run a little more rpm than a Menz.
If I wanted to go a little faster I would try a 14x10 APC.
Good Luck
John