The flutter phenomenon is a kind of aero-elastic instability that occurs when the effective damping (structural + aerodynamic) becomes negative. The aero-elasticity is the structural instability due to the interaction between the aerodynamic forces and the structural motion. Roughly speaking the flutter is controlled by three modes namely lateral, heaving and pitching motion due to the three component of the aerodynamic forces component (drag, lift, moment). However is it was demonstrated that the effect of the lateral component (drag) was negligible, hence this can be disregarded [Theodorsen, T. 1934 ]. As a results, only two motions (heaving and pitching) can be considered for flutter evaluation. Amid various approaches, flutter speed can be improved by redefining the body-shape at the earlier design stage using an appropriate shape optimization method,....etc

Hope this will be helpful!!

Well said Dr. Severin Tinmitonde... The mode in terms of torsional and bending mode must be considered due to the interaction of the three forces of elastic, inertia and aerodynamic.

You may find dominent modes (usually the first mode) using centre manifold theories (nonlinear theories)

In practice, flutter of control surfaces is prevented by mass balance (zero moment) about the hinge line.