Mil V-12
The Mil V-12 is the largest helicopter ever built. Its first flight was an unsuccessful attempt in June 1967. It had a successful flight in July 1968, only to be cancelled. Design limitations forced Mil to adopt a twin rotor system but design studies of a tandem layout, similar to the Boeing CH-47 Chinook, revealed major problems. The single rotor layouts also studied proved to be non-viable, leading to the transverse layout chosen for the finished article.