Wood,

I have the YS91FZ on my UCD3D and I love it. I have used a 15x6, 16x4w, and currently use a 16x6 (all APC) on the engine and all have slightly different characteristics when flying. Ground clearance with the 16" prop is not a problem. You want to prop this plane for thrust and throttle response but not for speed (although I can fly at full throttle with the 16x6 and have not gotten flutter - I don't do it much though.)

I just use the YS91 stock muffler coming out the side. Three of them in our club are configured this way. Futaba 3004's work fine for me on the elevators. I have Hitec 525's on the ailerons and a 605 on the rudder.

I know its a long discussion, but all of the common mods are posted here. It's good reading.

Chris