sentences:  To use this wasted effort, the placement of a second rotor behind the first takes advantage of the disturbed airflow, and can gain up to 40% more energy from a given swept area as compared with a single rotor.Other advantages of contra-rotation include no gear boxes and auto-centering on the wind (no yaw motors/mechanism required).A patent application dated 1992 exists based on work done with the Trimblemill.