Earlier quoted context omitted.
1954
How do you know that?
Sources:
https://abpic.co.uk/pictures/view/1140732
> It is the only surviving HZ-1. The prototype, designated DH-4 ...
https://transportation.army.mil/museum/coldwar/index.html
> The HZ-1 was tested at Fort Eustis in 1956.
https://archive.org/stream/ArmedForcesFilms1956/ArmedForcesF...
> IESM-543 . Released July 1956 ... The Aerocycle is an experimental one man flying machine.
http://www.aviastar.org/helicopters_eng/lockner_helicovector...
> DH-4 first flew in January 1955.
https://asc.army.mil/docs/magazine2/armyalt-oct-dec-15.pdf
> In 1954, he proposed that if the rotors of a helicopter were on the bottom, a pilot could use his own weight to steer the vehicle using kinesthetic control, similar to riding a bicycle.
https://en.wikipedia.org/wiki/De_Lackner_HZ-1_Aerocycle
> First flight 22 November 1954
https://www.youtube.com/watch?v=VyBHM1vZLzc
> The De Lackner DH-4 "Aerocycle", later renamed the HZ-1 Aerocycle