Abstract: An angular momentum mass flowmeter for measuring the mass flowrate of a fluid stream includes a flowmeter housing having an inlet or upstream end for receiving the fluid stream and an outlet or downstream end for discharging the fluid stream. A rotatable impeller is provided for measuring the mass flowrate. A swirl cap is disposed downstream of the impeller for imparting angular momentum to the fluid stream passing thereover and for causing the angular momentum to rotate the impeller. A control valve, disposed intermediate the impeller and the swirl cap, modifies the flow area of the fluid stream passing over the swirl cap to regulate the angular momentum of the fluid stream.