If you are the same guys asking about flutter over in the RV world, you have probably gotten the impression that this is somewhat of a black art. Flight test programs spend a lot of time proving that the airplane is flutter free in the expected operating envelope of the aircraft, and a little more. The challenge is that creating a mathematical model that accurately describes all of the mechanical characteristics of a wing structure is time consuming and expensive. So the engineers are conservative with their design safety margins.
I will note that when a control surface is in trail and its actuating linkage is disconnected, it has much less leverage to put force into the structure that it is connected to. That is, and aileron is connectect to the wing by the hinges and moves about the hinge line. The mechanical mechanism by way it puts force into the wing structure includes the control linkages, the elasticity of those control linkages, and the points in the wing that anchor them. So in an airplane that uses control cables, those cables must sized and be tensioned do that they do not function as springs that help the aileron resonate against the airflow. The distance between the aileron hinge and the attachment of the cable or control arm form a lever arm that translates the forces into the control linkage and thence into the wing structure. The stiffness of each of those items contribute to the movement, spring response, and possible resonant frequencies of the whole assembly.
There is a lot more to the topic than can be covered in a web post.
Hope this makes sense,
Wes
N78PS