Abstract: A fluid proportioning device which operates in a linear fashion and has a constant-volume output as a function of damper position. A sheet of flexible material is moved against a plate having two groups of apertures which are in respective fluid connection with two fluid streams, the sheet being operative when in a selected position to cover a selected percentage of each group of apertures to apportion the fluid flowing therefrom. The sheet is a flexible foil sheet which is wrapped around two spaced cylindrical members and stretched therebetween, with one portion of the sheet maintained flush against the aperture plate. When the cylindrical members are moved along the plate direction the foil rolls around the members to the desired position.