Rotary damper comprising: a housing having at least one fluid chamber filled with a viscous fluid; the fluid chamber being defined by a bottom wall, an upper wall, a curved cylindrical and/or conic side wall, and optionally a shaft wall; a wing shaft provided in the fluid chamber for rotation relative to the housing; a wing provided on the wing shaft and extending in an axial direction thereof, the wing projecting in a radial direction of the wing shaft toward the curved cylindrical and/or conic side wall of the fluid chamber and comprising: a first side and a second side opposite to said first side, a top side at or near a tip end of the wing facing the curved cylindrical and/or conic side wall of the fluid chamber, a bottom side facing the bottom wall of the fluid chamber and an upper side opposite to said bottom side facing the upper wall of the fluid chamber, optionally a back side; a fluid passage between the wing and/or the wing shaft and a wall (25, 26, 10) of the fluid chamber for allowing the viscous fluid to flow between the first side and the second side of the wing; at least one groove (16.1, 16.3, 16.5, 16.7, 16.9, 16.11, 16.12) is provided in the fluid passage, the groove being provided in a side (13, 27, 28, 31) of the wing and/or in a wall (10, 25, 26) of the fluid chamber and/or in the wing shaft; the at least one groove (16.1, 16.3, 16.5, 16.7, 16.9, 16.11, 16.12) is entirely filled with the viscous fluid, the viscous fluid in the at least one groove (16.1, 16.3, 16.5, 16.7, 16.9, 16.11, 16.12) being in fluid communication with the viscous fluid of the fluid passage allowing the viscous fluid of the fluid passage to flow through the at least one groove (16.1, 16.3, 16.5, 16.7, 16.9, 16.11, 16.12); characterized, in that the at least one groove (16.1, 16.3, 16.5, 16.7, 16.9, 16.11, 16.12) forms a widened portion of the fluid passage perpendicular to the direction of flow of the viscous fluid. a housing having at least one fluid chamber filled with a viscous fluid; the fluid chamber being defined by a bottom wall, an upper wall, a curved cylindrical and/or conic side wall, and optionally a shaft wall; a wing shaft provided in the fluid chamber for rotation relative to the housing; a wing provided on the wing shaft and extending in an axial direction thereof, the wing projecting in a radial direction of the wing shaft toward the curved cylindrical and/or conic side wall of the fluid chamber and comprising: a first side and a second side opposite to said first side, a top side at or near a tip end of the wing facing the curved cylindrical and/or conic side wall of the fluid chamber, a bottom side facing the bottom wall of the fluid chamber and an upper side opposite to said bottom side facing the upper wall of the fluid chamber, optionally a back side; a first side and a second side opposite to said first side, a top side at or near a tip end of the wing facing the curved cylindrical and/or conic side wall of the fluid chamber, a bottom side facing the bottom wall of the fluid chamber and an upper side opposite to said bottom side facing the upper wall of the fluid chamber, optionally a back side; a fluid passage between the wing and/or the wing shaft and a wall (25, 26, 10) of the fluid chamber for allowing the viscous fluid to flow between the first side and the second side of the wing; at least one groove (16.1, 16.3, 16.5, 16.7, 16.9, 16.11, 16.12) is provided in the fluid passage, the groove being provided in a side (13, 27, 28, 31) of the wing and/or in a wall (10, 25, 26) of the fluid chamber and/or in the wing shaft; the at least one groove (16.1, 16.3, 16.5, 16.7, 16.9, 16.11, 16.12) is entirely filled with the viscous fluid, the viscous fluid in the at least one groove (16.1, 16.3, 16.5, 16.7, 16.9, 16.11, 16.12) being in fluid communication with the viscous fluid of the fluid passage allowing the viscous fluid of the fluid passage to flow through the at least one groove (16.1, 16.3, 16.5, 16.7, 16.9, 16.11, 16.12); characterized, in that the at least one groove (16.1, 16.3, 16.5, 16.7, 16.9, 16.11, 16.12) forms a widened portion of the fluid passage perpendicular to the direction of flow of the viscous fluid.