I saw the Rossi 53 review at Just Engines, and they say the 53 can swing 12x8 at over 12000 rpm w/ the mini-pipe. So using a 13x4 APC (the narrow, not the wide), I could see it turning at 13000 rpm, which is real good for the Flip. That's more thrust than a YS 63 would ever pull w/ a 14x4W @ 10500. Weight is 440g + 90g for minipipe = 18.65 oz, which is just a hair heavier than YS 63's 17.9 oz (including muffler?) I would still go with the YS 63 for the fuel delivery system, but theoretically, the Rossi 53 could give more thrust.

http://www.justengines.unseen.org/Rossi53r.htm

PROP PERFORMANCE ROSSI MINI PIPE

10x6 RAM 17.500 RPM

10x7 RAM 16.800 RPM

11x6 RAM 14.300 RPM

11x6 APC 14.100 RPM

11x7 RAM 13.900 RPM

11x8 APC 13.500 RPM

12x6 RAM 12.900 RPM

12x8 APC 12.100 RPM

13x8 APC 11.100 RPM

Idle 2.800 RPM

Average noise test: 96 DB at 7 meters