Achenbach, a German, designed an aircraft with a single rotor to ensure lift and also featuring a transverse airscrew to counteract torque.

P.Lambermont "Helicopters and Autogyros of the World", 1958


This model by the German Achenbach was the first to use a tail rotor to counteract the torque from the main rotor.


G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984