A 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; and 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; characterized in that the flutter damper further comprises: at least one stiffening structure (132; 134) 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, wherein the at least one stiffing structure includes first and second stiffening structures , the stiffening structures being a substantially "C" shape when viewing into a side surface of the chamber ; the stiffening structures protrude outwardly from the top surface , a front surface and a back surface of the chamber and extend from the front surface to the back surface of the chamber; and the stiffening structures divide the top surface of the chamber into substantially equal portions in the width direction. 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; and 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; characterized in that the flutter damper further comprises: at least one stiffening structure (132; 134) 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, wherein the at least one stiffing structure includes first and second stiffening structures , the stiffening structures being a substantially "C" shape when viewing into a side surface of the chamber ; the stiffening structures protrude outwardly from the top surface , a front surface and a back surface of the chamber and extend from the front surface to the back surface of the chamber; and the stiffening structures divide the top surface of the chamber into substantially equal portions in the width direction. at least one stiffening structure (132; 134) 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, wherein the at least one stiffing structure includes first and second stiffening structures , the stiffening structures being a substantially "C" shape when viewing into a side surface of the chamber ; the stiffening structures protrude outwardly from the top surface , a front surface and a back surface of the chamber and extend from the front surface to the back surface of the chamber; and the stiffening structures divide the top surface of the chamber into substantially equal portions in the width direction. <EOS>