What are the structural requirements to prevent flutter?

Flutter prevention structural requirements include: Adequate torsional stiffness (GJ) to separate bending and torsion frequencies - rule of thumb is torsion frequency > 1.3 times bending; Mass balance of control surfaces to prevent coupling with surface flutter modes; Stiffness requirements for control surface actuation (to prevent control surface flutter); Structural damping contribution; and Clearance between critical flutter speeds and dive speed (Mach) with appropriate margins per FAR 25.629. Structural design must provide required stiffness while minimizing weight. FEM modal analysis feeds aeroelastic analysis. Ground vibration testing validates structural dynamic model; flight flutter testing clears envelope incrementally.