Flutter is only possible if you have similar structural and aerodynamic frequencies. One without the other would produce much lower amplitudes.

Look at a mass-spring system suspended on an eccentric tappet which sits on the edge of a small rotating wheel. When the wheel turns, it raises and lowers the top of the spring, and the mass on the bottom will barely move. Now change the rotation frequency to something close to the resonance frequency of the mass-spring system, and the mass will produce wild oscillations.

Now use a beam (which also has its eigenfrequencies in bending and torsion), and attach to it an airflow which will periodically separate, as in a Karman vortex street. Normally, nothing happens. Now change wind speed such that the separation frequency is close to one of the structural eigenfrequencies of the beam, and you will get wild excitations again. It gets worse if the deformation will induce flow changes, because now the resonance will be self-propelling over a wider frequency range. This is flutter.

The deformation stores energy, and if the aerodynamic forces change such that they support the elastic motion, they will add a little energy with each cycle, such that the eventual amplitude will become immense, right to the point of failure.

Flutter and structural resonance are inseparable. One is part of the other.

Flutter is only possible if you have similar structural and aerodynamic frequencies. One without the other would produce much lower amplitudes.

Look at a mass-spring system suspended on an eccentric tappet which sits on the edge of a small rotating wheel. When the wheel turns, it raises and lowers the top of the spring, and the mass on the bottom will barely move. Now change the rotation frequency to something close to the resonance frequency of the mass-spring system, and the mass will produce wild oscillations.

Now use a beam (which also has its eigenfrequencies in bending and torsion), and attach to it an airflow which will periodically separate, as in a Karman vortex street. Normally, nothing happens. Now change wind speed such that the separation frequency is close to one of the structural eigenfrequencies of the beam, and you will get wild excitations again. It gets worse if the deformation will induce flow changes, because now the resonance will be self-propelling over a wider frequency range. This is flutter.

The deformation stores elastic energy, and if the aerodynamic forces change such that they support the elastic motion, they will add a little energy with each cycle, such that the eventual amplitude will become immense, right to the point of failure.

Flutter and structural resonance are inseparable. One is part of the other.