The reason you need to CG balance blades on a heli is because of the inertia caused by rotating mass. When a mass is rotating about a fixed point, the inertia is calculated by multiplying the mass by the moment arm distance to the spin center. If the moment arm on one side of the spin center is longer than the other, you will have a larger moment of inertia on the blade where the CG is located farther from the spin center. This will cause the system to oscillate while spinning.

It is sort of like taking a very thin rod with a weight at the end of it and comparing that to a very short but heavy rod. Experience says that you may be able to statically balance them, but the CG is not in the same place for each side. So if you stuck that on your heli, what would happen? You would have a serious vibration problem. The static balance says they are fine, but the CG is off and that is the root cause of the problem.

Brian