Hey Vince,
I have, but haven't run it yet - sorry, not much help there!! When I was researching the OS 110 here on RCU I did find a comment (somewhere) that a fellow flier replaced the
YS 110 in his aerobatic (may have been a 3D airplane) and felt the power was almost the same as the YS. As for the 120AX, I ran one in an Explorer VR-90 about 3 years ago (for 200 flights) and was very impressed with the engine. Ran an APC 15x10 from the beginning and had plenty of power and pretty good throttle response (not as good as a 4-stroke). Also, good fuel economy and I only the changed the original plug once after 140 flights (though the idle had been getting crappy for about 10 flights - I'm just cheap). The downsides: no pump, and the muffler bolt failed after about 150 flights. I also 'strength' tested the engine when I made a vertical landing [X(] with a Chip Hyde Dream after just 3 flights [:@] - I forgot to put the screw back in the elevator servo output arm (I knew it as soon as I lost elevator control - killed the power and watched the 'event'). Fortunately, the ground was pretty soft and the engine didn't suffer any damage.
Lastly, the tank placement issue for an unpumped engine (also know as the cave-man set-up as it was referred to in another thread here about the OS 160FX I think). First, the CG shift may not be noticable as the tank size isn't that big. It also depends on where the CG is located (I did not have any trim changes with the Explorer) - the more rearward the cg (within the range) the more sensitive the airplane becomes to CG changes. You can also use the 2 tank approach - 1 main tank on the CG and one, small 'header' tank (no more than 2 ounces) at the firewall. I used this set-up on an Arch Nemesis powered with an unpumped ST 2300 and it worked very well. I'm not sure if it would work with a 4-stroke.
Just a few things to think about!
-Will B.