I'd suspect most heli engines are ringed because of all the time they spend sitting or hovering close to the ground kicking up dust. Ringed engines are far more dirt tolerant than ABx. I'd guess your engine would spin an 8x6 or maybe 9x6 prop at 20K, not much point though because that would be way above the max HP figure. With a flywheel you'd probably go to 30K+ for a short while