Flutter dampening acoustic liner for a turbofan engine

A gas turbine propulsion system comprising:

• a nacelle;
• and
• a flutter damper secured to the nacelle, the flutter damper comprising:
• an acoustic liner having a perforated radial inner face sheet and a radial outer back sheet, the acoustic liner being configured for peak acoustical energy absorption at a frequency range that is greater than a frequency range associated with fan flutter;
• a chamber secured to the radial outer back sheet, the chamber being in fluid communication with the acoustic liner, and the chamber being configured for peak acoustical energy absorption at a frequency range that is associated with one or more fan flutter modes;
• and
• at least one stiffening structure connected to a top surface of the chamber that tunes the top surface out of the frequency range associated with one or more fan flutter modes.

A method of mitigating fan flutter in a gas turbine propulsion system, comprising:

• securing at least one flutter damper to of the gas turbine propulsion system, wherein the flutter damper comprises:
• an acoustic liner having a perforated radial inner face sheet and a radial outer back sheet, the acoustic liner being configured for peak acoustical energy absorption at a frequency range that is greater than a frequency range associated with fan flutter;
• a chamber secured to the radial outer back sheet, the chamber being in fluid communication with the acoustic liner, and the chamber being configured for peak acoustical energy absorption at a flutter frequency range that is associated with one or more fan flutter modes;
• and
• at least one stiffening structure connected to a top surface of the chamber that tunes the top surface out of the frequency range associated with one or more fan flutter modes.