I too have faithfully used the 15/6 on past UCDs. Obtaining RPMs in the specified range, but found in a hover it was at the top end of the throttle range. It would pull out but needed more. In my goal to hover off the deck, I felt there wasn't enough there. When I changed I went too far with the 16/4. It was locally available, but apparently too much for this engine choice. Maybe 30% heli is required. With the 4 pitch it now hovers at half throttle, leaving a wider margin for escape in poor attitudes. Also it now torques in hover with the 16" diameter prop which was an effort with the 15/6. With the 4 pitch it now jumps off the deck on take off. It seems the 15/4 should have been the next step from a 15/6. I will get it on today and see if the weather co-operates and will report back on RPMs and performance. Joe