ORIGINAL: rajul
Another factor in flutter is the cg of the control surface itself. In full scale aircraft, weights are sometimes added to the control surface to bring its cg closer to the trailing edge of the wing/elevator/vert fin
Exactly. The mass and mass distibution of the control surface plays a huge role. This explains why sealing the gap could cause flutter. There are many factors involved in the causes of flutter. The flag explanation is typical of this. Sealing the gaps is not the cure-all for flutter, it just helps with control authority when all other negative factors are eliminated. All scientific fact is a mere approximation under ideal conditions.
Safe Flying!