ORIGINAL: LouW
....I think what is confusing is that there are two independent factors. First the surface must be mass balanced such that the natural frequency of the surface is not in the range of forcing frequency at the speeds at which the aircraft is designed to operate. This doesn’t necessarily mean on the hinge line. Many small aircraft that operate at lower speeds have surfaces without additional mass balance and the cg is well aft of the hinge line. Flutter only occurs when the natural (resonant) frequency of the surface and the forcing frequency at a particular speed coincide......
This is the key to the whole mass balancing issue. All surfaces will flutter under the right conditions. Adding mass balancing is just a way to move the overall mass distribution and balance to a point were this flutter resonace is outside the aircraft's normal operating range.
Everyone is saying that mass balancing is intended to move the balance point to the hingeline or in front of it. That would be a perfect case but I seriously doubt that many achieve that.
Years ago I used horn type mass balancing on a little 1/2A speed sport model. In my case the strip ailerons were made from light wood that was just TOO flexible and this made the surfaces flutter like a clarinet reed in a dive. I added a couple of little wire arms and wound some solder onto the arms flared in with epoxy and that fixed the flutter probem. It wasn't enough to totally balance the ailerons but it did move the balance forward enough and change the resonant frequency enough that I could dive vertically from great height without any flutter from that point on.