The present teachings generally include a rotor assembly having a plurality of rotor sheets or layers mounted to a shaft, and methods of construction for a rotor assembly. Each rotor sheet or layer in the assembly may be provided with a central opening extending between the first and second sides through which the shaft extends. In one aspect, the rotor sheets or layers can be provided with a plurality of lobes extending away from the central opening, wherein each of the lobes can have a lobe opening extending through the thickness of the sheets or layers. In one example, the rotor sheets or layers can be rotationally stacked to form a helical rotor. In one example, the rotor sheets are formed from a pre-cured composite material and are bonded together with an adhesive.