Saito 100 is best for this plane, not so much because of power, but because of its light weight, which enables CG to be moved back to 6". It has enough power to do what the plane is designed for, but not like a rocket. I have Saito 100 on my UCD as well. With the 15x6 prop, it hovers at exactly half throttle stick as well, but its rpm is more like 3/4 of peak. When I go to full throttle, it would pull out, but not jump out.

Pugs, what prop do you like most with your Saito 100 on UCD? I had APC 15x6, and it has pretty good power w/ 9500 peak (10% nitro), but throttle response is not instantaneous. I switch to Pro Zinger 15x6 to improve throttle response due to its light weight, but rpm dropped to 9300 peak (15% nitro). The engine seems to be struggling sometimes during pull-out and inverted flat spin. It seemed like with the Pro Zinger, it would lean out in the air, although I richened it up on the ground by 200 rpm. I haven't tried the 15x4W yet. Hopefully I will get over 10k peak with it, and decent throttle response. I tried the 16x4W, and it was just too heavy: it would not idle, and spool up was like 1/2 second.

I wonder if the OS 91 Surpass's throttle response is better than the Saito 100. But to get enough thrust with Surpass 91, 15x4W is probably the only choice.

People have complained about using 15" prop w/ the 91 2 strokers as been unresponsive. 14" props is not quite enough for this plane, so 91+ 4-stroke is a good start. I wonder how well the MVVS 91 does w/ 15" props? It has the power of 91 FX. If it turns 15" prop w/ good response, it might just be the perfect engine for this plane. It can be turned into rear exhaust. So if its stock Pitts muffler is used when it's configured as rear exhaust, the exhaust can go into the exhaust channel at the bottom, and keep most of the plane clean.

Any engine that's heavier than 25 oz including muffler, you will need to put the battery/weight at the tail to get good flying balance.