Jack211...
I've used both a 16x4W prop and a 16.5x5W prop (APC). The 16x4W gave over 10000RPM and only about 11.5 lbs thrust. The 16.5x5W gave me 13lbs 0ozs thrust at about 8800 RPM. I liked that prop but felt it loaded the engine a little too much at my elevation. I think at lower elevation it might be fine. For me the 16x6 APC seems to net the most thrust and allows mid 9000 RPM which is perfect for the YS110. I also used a 17x4W which neted 12lbs 2ozs thrust at about 9700 RPM. The 17x4W is my 2nd choice. It works fine and is actually easier to hover even though overall thrust is less. It really slows a plane down tough and other maneuvers are not as nice.

gjeffers...
What kind of economy did you get? Are you using the stock UCD fuel tank? How many minutes flying time?

Thanks
Barry