Mike Cross used to fly a Goldberg Ultimate (54 inch around 8 pounds) and hover it on a YS 91. I think... On paper a 16x4 @ 10,000 rpm is about 17 pounds of thrust. Reailty is about a 9 to 10 pound hover. That's one of those hypothetical, sea level, everything perfect type of estimates by the way. I personally would try to keep it under 8 pounds. Full throttle hovers anywhere near Mother Earth are scarry. You want some juice left for bail outs. My $0.02.