Unfortunately, mass balancing has little or no effect on eliminating flutter, in fact in some rare cases, it may even cause it. Flutter is best solved by changing the rigidity (stiffness) of the aileron itself and, in some cases by where the horn is attached to the aileron. You want the horn to be at about 1/3 or 2/5 ths of the span of the aileron. Above all, have all slop out of the hinges, control horns (no oversized holes or wallowed out Zbends) and pushrods and make sure the pushrods are very stiff. If the ailerons are driven by a single servo with wire control rods to each half, any spring or give in these can contribute to the onset of flutter. The only cure for that is to increase the diameter of the wire so that there is no spring back if the surfaces are pushed on.