Abstract: An impelling mechanism that can be operated as a single-stage compression or expansion positive-displacement rotary engine or compressor. The impelling mechanism has a housing defined by an upper casing and a lower casing. An annular shaped swept volume chamber is formed in the upper casing. A plurality of blade members are pivotally supported on a disc-shaped rotor that is keyed to a shaft passing through the housing. These blade members travel within the swept volume chamber and have their attitude maintained at the same orientation throughout the 360 degrees of revolution of the rotor. The blade members each have a crankshaft attached to their bottom ends which have a first crankshaft throw and a second crankshaft throw. These crankshaft throws are journaled in the finger portions of their respective blade control yokes. These blade control yokes have their axis of rotation laterally offset from the axis of the drive shaft and their own axes are 90 degrees offset from each other.