Back to the Future of Rotorcraft? A Bi-Rotor Configuration Whose Virtues Went Unexamined for Decades

The aim of this paper is to explore the history of the lateral bi-rotor in relation to the present state of the art. It examines the relevance of some early developments in the light of current knowledge, and traces the route by which this technology survived in model scale aircraft, then examines its application to current full scale machines. Don Juan de la Cierva was the first to employ rotor-head designs in his Autogiros that then made the helicopter rotor possible - this irony is further examined with regard to the pioneering Berliner bi-rotor helicopter. Another irony is that the American-built Berliner bi-rotor efforts presaged the Focke-Achgelis bi-rotor designs flown so convincingly in Hitler's Germany. The merits of the lateral bi-rotor autogyro (BRAG) are examined in the light of subsequent developments, and conclusions are drawn that suggest that this has long been an overlooked technology.