A propulsion assembly having a heat exchanger and a system for supplying cold air including an air inlet in a stream of air, an air duct connecting the air inlet fluidly to the exchanger, and an air flow valve with a variable flow rate inside the duct, the valve including a hub having blades projecting radially from the hub forming a helix, each blade having a root mounted rotatably on the hub, the valve comprising an electric motor to drive the hub by a motor shaft, and structure for varying pitch angle of the blades, the extremity of each blade being flush with a wall of the duct, the valve controllable to a closed configuration where pitch angle of the blades is 0° and the valve prevents passage of air, an open configuration where pitch angle is 90°, and/or a charge configuration where pitch angle is between 0° and 90°.