Wikipedia
Petróczy-Kármán-Žurovec
Petróczy, Kármán and Žurovec were Hungarian and Czech engineers who worked on helicopter development immediately before and during World War I. Between them they produced two experimental prototypes, the PKZ-1 and PKZ-2, intended to replace the dangerous hydrogen-filled observation balloons then in use. As such, these craft were tethered on long cables and were not intended to fly freely. After the war other engineers, notably Oszkár von Asboth, further developed the design.