Hi. For those still curious about flutter, I've put together a bit more information.
I found a youtube posting by an amateur rocket oriented person discussing fin flutter on rockets. He ran some computer simulations using NASTRAN.
Look at the various ways the simplistic fin he modeled can vibrate. This has application to wings and control surface flutter, although the aircraft situation is more complex. Each vibration mode is unique and has its own frequency. As you go to the higher order modes, the
frequency of vibration is increasing. Just energizing one of the oscillation modes at its characteristic frequency is sufficient to cause a flutter problem.
Here is the flick:
The Wikipedia article on Aeroelasticity has some good info, look at the section on flutter.
http://en.wikipedia.org
/wiki/Aeroelasticity
When something flutters, the energy of the airstream around the surface is extracted by the flutter process to drive the oscillation.
So how does this energy couple to the surface? I'm not a specialist in aerodynamics, but I believe a process akin to 'Karman Vortex Street' generation is at the root of this coupling. See this Wiki article for yet more info:
Kármán vortex street - Wikipedia, the free encyclopedia
As usual, I'd love to hear from someone who has better understanding of the subject than myself. My formal background is in physics and not aerodynamics.
Sid