An impeller is usable in a variable diameter bioreactor having multiple vessel sections of successively increasing or decreasing volume. The impeller includes an impeller blade extending along an impeller blade axis between first and second axial ends and having opposed impeller blade faces, an impeller blade leading edge, and an impeller blade trailing edge. Each of the leading and trailing edges defines a helix or spiral between the axial ends of the impeller blade. In certain arrangements, the impeller blade is one of at least two impeller blades joined together along an impeller shaft extending axially along the impeller blade axis mentioned, and the helix or spiral has a pitch approximately equal to one half of a length between the first and second axial ends.