In general, flutter is when an outside force stimulates a structure at its natural frequency. Any and all surfaces will flutter if excited at the proper frequency. When that happens, a small input can produce a large output, enough to tear the structure apart. Keeping contol rods ridged, stiff with no slop or spring back prevents the initial initialization. Building stiff and ridgid (but light) surfaces also helps prevent flutter from ever starting (this raises the natural frequency above the exciting frequency). The biggest cause for flutter initiation in models is sloppy hinges, soft control systems or flimsy structure. Hinge gap is seldom a cause of flutter although it does cause surface deflection to be less effective.