Why is flutter a function of true, not indicated, airspeed? I am learning about gliders in preparation for an exam. It says in the textbook I am reading that for a glider, you use indicated airspeed for most relevant speeds (stall speed $V_s$, best glide speed $V_{BG}$ etc). However, $V_{NE}$ should never be exceeded in true airspeed. As far as I can tell, this is because flutter depends on true airspeed rather than indicated airspeed. (Let me know if this assumption is wrong, because that would make the title wrong.)

Why is this the case? Is flutter somehow related to the speed of the air molecules, rather than to the energy provided by them?
Flutter happens when structural and aerodynamic eigenfrequencies converge. Aerodynamic eigenfrequencies depend on the time the air molecules take to transverse an aerodynamic surface.