When center of gravity is too far back, the plane is very unstable, unpredictable and tends to stall very easy. If the center of gravity is too far forward, the plane is stable but very sluggish,
and may not want to climb. If the C.G. is too far off in either direction,the plane becomes a rock!
The C.G. of most conventional type aircraft is roughly 25%-30% of wing chord from the leading edge root.
Good luck!
Bud