A ram air system for an aircraft fluid system, the ram air system comprising: a ram duct having: an inlet for receiving ambient airflow; and an outlet for discharging the ambient airflow overboard; a diffuser positioned within the ram duct to define a fan duct and a bypass duct; a fan positioned within the fan duct; and a modulating valve positioned in the bypass duct, the modulating valve comprising: a valve body positioned in the bypass duct to control flow of bypass air to an outlet of the bypass duct; and a valve drive connected to the valve body to change position of the valve body; characterised in that the valve body comprises an annular piston between the diffuser and the ram duct ^; and the diffuser comprises: an annular body having: an inlet end; and an outlet end; and wherein the fan is positioned at the inlet end; and the ram duct comprises: a circinate body surrounding the annular body, the circinate body comprising: a first end concentric about the inlet end; a second end concentric about the outlet end; an inlet positioned on the circinate body between the first end and the second end; a turnabout configured to direct airflow from the first end to the inlet end; and an outlet section having a window that defines the outlet of the bypass duct; wherein the annular piston is positioned between the outlet end and the second end adjacent the outlet section. a ram duct having: an inlet for receiving ambient airflow; and an outlet for discharging the ambient airflow overboard; an inlet for receiving ambient airflow; and an outlet for discharging the ambient airflow overboard; a diffuser positioned within the ram duct to define a fan duct and a bypass duct; a fan positioned within the fan duct; and a modulating valve positioned in the bypass duct, the modulating valve comprising: a valve body positioned in the bypass duct to control flow of bypass air to an outlet of the bypass duct; and a valve drive connected to the valve body to change position of the valve body; a valve body positioned in the bypass duct to control flow of bypass air to an outlet of the bypass duct; and a valve drive connected to the valve body to change position of the valve body; characterised in that the valve body comprises an annular piston between the diffuser and the ram duct ^; and the diffuser comprises: an annular body having: an inlet end; and an outlet end; and wherein the fan is positioned at the inlet end; and the ram duct comprises: a circinate body surrounding the annular body, the circinate body comprising: a first end concentric about the inlet end; a second end concentric about the outlet end; an inlet positioned on the circinate body between the first end and the second end; a turnabout configured to direct airflow from the first end to the inlet end; and an outlet section having a window that defines the outlet of the bypass duct; wherein the annular piston is positioned between the outlet end and the second end adjacent the outlet section. an annular body having: an inlet end; and an outlet end; and an inlet end; and an outlet end; and wherein the fan is positioned at the inlet end; and the ram duct comprises: a circinate body surrounding the annular body, the circinate body comprising: a first end concentric about the inlet end; a second end concentric about the outlet end; an inlet positioned on the circinate body between the first end and the second end; a turnabout configured to direct airflow from the first end to the inlet end; and an outlet section having a window that defines the outlet of the bypass duct; wherein the annular piston is positioned between the outlet end and the second end adjacent the outlet section. a circinate body surrounding the annular body, the circinate body comprising: a first end concentric about the inlet end; a second end concentric about the outlet end; an inlet positioned on the circinate body between the first end and the second end; a turnabout configured to direct airflow from the first end to the inlet end; and an outlet section having a window that defines the outlet of the bypass duct; wherein the annular piston is positioned between the outlet end and the second end adjacent the outlet section. a first end concentric about the inlet end; a second end concentric about the outlet end; an inlet positioned on the circinate body between the first end and the second end; a turnabout configured to direct airflow from the first end to the inlet end; and an outlet section having a window that defines the outlet of the bypass duct; wherein the annular piston is positioned between the outlet end and the second end adjacent the outlet section.