ORIGINAL: rtn9105
When I give full elevator input the fuse will get to about a 45deg angle and do a nice spin but it won't flatten out any more than that. It does the same thing inverted as up right. On my first UCD I could get the plane to this same point, then start to slowly cross the aileron input and it would flatten right out and do a nice tight flat spin but this one won't flatten out no matter what input I give.
That is exactly what I expected you to say and it sounds like one of 3 things.
1. Not enough power
2. Not enough elevator throw
3. CG is not far enough back.
It takes power to do a flat spin as well as having the right throws setup. If you dont have the power to pickup the nose, regardless fo how much elevator throw you have it will not flatten out.
Since you are stuck with whatever engine is on it, and power may not be the problem, increase your elevator throw 5 degrees at a time and test it. Once you get the spin setup and you are centering the ailerons increase power until it flattens out. That might be as much as 3/4 to full throttle. If it flips over when you increase throttle, just dont add as much throttle on the next try. If you get to 45-50 degrees of down elevator throw and it still will not flatten out, you might double check you cg and if you have room move it back a little at a time. On a UCD you should not need to go back more than about 5.5"+/-