A fan section of a gas turbine engine, comprising: a nacelle (100; 158) and a fan; the nacelle including a nacelle inlet and a flutter damper, the flutter damper comprising: a circumferential array of chambers (118, 162) disposed about the nacelle inlet, wherein a desired damping volume includes a volume of the circumferential array, each of the chambers including: a first cavity having a radially inner side in fluid communication with a flow path, and a second cavity being radially outside of the first cavity and having a radially inner side in fluid communication with a radially outer side of the first cavity, wherein the first cavity has a smaller volume than the second cavity, the first cavity and the flow path fluidly communicate through a first perforated surface, and the first cavity and the second cavity fluidly communicate through a second perforated surface; wherein the flutter damper has an impedance characteristic at one or more target frequencies defined as: ƒ target = ƒ S, ND + Ω ⋅ ND wherein f S,ND is a resonance frequency corresponding to a structural mode of a fan blade of the fan, ND is a nodal diameter count of the structural mode, and Ω is a rotational speed of the fan blade; wherein the flutter damper has the following impedance characteristic at the one or more targeted frequencies: R ≥ 2 ρc − 3 ρc ≤ X ≤ − 0.6 ρc wherein R is the real part of the impedance characteristic, X is the imaginary part of the impedance characteristic, ρ is air density, and c is speed of sound; and characterised in that the targeted frequencies include: f S, ND = frequency of first or second bending mode of fan with ND nodal diameters 1 ≤ ND ≤ 3 Ω Mreltip = 0.85 ≤ Ω ≤ Ω Mreltip = 1.2 wherein Mreltip is a relative Mach number for a radial outertip of the fan blade, and the bending mode is a vibrational mode of the fan blade. a nacelle (100; 158) and a fan; the nacelle including a nacelle inlet and a flutter damper, the flutter damper comprising: a circumferential array of chambers (118, 162) disposed about the nacelle inlet, wherein a desired damping volume includes a volume of the circumferential array, each of the chambers including: a first cavity having a radially inner side in fluid communication with a flow path, and a second cavity being radially outside of the first cavity and having a radially inner side in fluid communication with a radially outer side of the first cavity, wherein the first cavity has a smaller volume than the second cavity, the first cavity and the flow path fluidly communicate through a first perforated surface, and the first cavity and the second cavity fluidly communicate through a second perforated surface; a first cavity having a radially inner side in fluid communication with a flow path, and a second cavity being radially outside of the first cavity and having a radially inner side in fluid communication with a radially outer side of the first cavity, wherein the first cavity has a smaller volume than the second cavity, the first cavity and the flow path fluidly communicate through a first perforated surface, and the first cavity and the second cavity fluidly communicate through a second perforated surface; wherein the flutter damper has an impedance characteristic at one or more target frequencies defined as: ƒ target = ƒ S, ND + Ω ⋅ ND wherein f S,ND is a resonance frequency corresponding to a structural mode of a fan blade of the fan, ND is a nodal diameter count of the structural mode, and Ω is a rotational speed of the fan blade; wherein the flutter damper has the following impedance characteristic at the one or more targeted frequencies: R ≥ 2 ρc − 3 ρc ≤ X ≤ − 0.6 ρc wherein R is the real part of the impedance characteristic, X is the imaginary part of the impedance characteristic, ρ is air density, and c is speed of sound; and characterised in that the targeted frequencies include: f S, ND = frequency of first or second bending mode of fan with ND nodal diameters 1 ≤ ND ≤ 3 Ω Mreltip = 0.85 ≤ Ω ≤ Ω Mreltip = 1.2 wherein Mreltip is a relative Mach number for a radial outertip of the fan blade, and the bending mode is a vibrational mode of the fan blade.