Hi Ehab,

Do any random search on the words "flutter" and "balance".

I just did and a random hit from

http://www.iasa.com.au/folders/Safet...lent_vibes.htm

<<snip>>

.......if the aerofoil moves slightly upward, the mass of the control surface causes the control to lag behind and rotate with its trailing edge down. Because of structural flexibility this tends to happen even if the control system is held firmly. The deflected configuration momentarily increases lift on the aerofoil and acts to increase its initial displacement. Eventually, structural stiffness overcomes the aerodynamic forces and the aerofoil starts to return to its normal position. As the aerofoil moves down, the control surface again lags behind, but now rotates trailing edge up and so again accentuates the displacement, this time downward.

Above a certain speed, there is sufficient aerodynamic energy for successive vibrations like this to build up progressively. This is flutter.

This simple type of flutter can be suppressed by adding balance weights to the control surface ahead of its hinge line. Doing this eliminates the tendency for the control surface to lag behind the vertical flexure of wing or tail and consequently the two motions no longer interact.........<snip>

There are a ton of references out there.....

Check with Bob Parks, he can explain it as well