RE: Cermark F-16 First Flight
put more prop on the engine. The 12x8 is pretty light, and although the OS will turn decent rpm..... its got better power around 10000 - 12000 rpm. With a stock muffler, it is not going to seriously unload in the air. What you see on the ground is about what you get.
Try a 12x10 or 12x12 on it for starters. Should help with the speed. Experiment a bit.
Crazy4Flight can tell you a bit about his GP F-4E Phantom with the Jett 120 on it. Lots of power, turned lots of prop, but its just not going to unload. The OS108 runs similar.