Abstract: A journaled rotor is provided for rotation about an axis to be disposed transverse to a fluid flow path and the rotor includes a plurality of mounting portions spaced radially outwardly from and circumferentially about the axis having a plurality of vane assemblies rotatably mounted therefrom for angular displacement about axis generally paralleling the axis of rotation of the rotor. Vane assembly displacement control structure is operatively connected to the vane assemblies for angularly displacing the vane assemblies relative to the corresponding mounting portions 180 degrees each 360 degrees of angular displacement of the rotor and in directions opposite the direction of angular displacement of the rotor upon rotation of the latter.