RE: 50CC Extra 300 snap
David, the weight seems pretty reasonable depending on wing area. I have seen other 50cc airplanes at over 18 lbs perform well. Your CG tests are throwing me a little. The inverted test sounds about right but most aerobatic airplanes I have set up have a slight tuck to the gear while in knife edge unless the CG is on the forward side. A forward CG will contribute to an elevator snap. Running a lot of throw will cause the snap as well especially with alot of expo. Anything 50% or more on expo will give you that soft feel up to a certain point all of the sudden you have a ton of throw. What I would do at this point if it were my airplane is to back the expo to 35% and max throw to 20 degrees. This should cover all flight envelope except for 3D. If the elevator feels too soft at that point then you may look into CG. A couple more questions while we are here, have you measured the airplane to see where the wing, stabs and thrust angles are at? Do you have some up elevator trimmed in? Does the elevator trim change from half to full throttle?