The usual way of designing large fans is with wide and stiff blades rigidly connected to the hub. This way leads, often, to high time-dependent load variations causing high vibration amplitude and high stress. Both stress and vibrations are strictly related to the blade's dynamic response to the aerodynamic force's pulses. A proper choice of the blades' mechanical and physical characteristics (stiffness and mass distribution, aerodynamic features, energy dissipation capability) can resolve in a substantial reduction of the blades' dynamic response. Cofimco, established in 1975, has since then continuously studied blades' mechanics, with regard to both fiberglass and aluminum blades.