What is "hoppi-copter"

The Hoppi-Copter was a functional backpack helicopter developed by Hoppi-Copters Inc. founded by Horace T. Pentecost in the 1940s. The original Hoppi-Copter consisted of two contra-rotating rotors on a pole attached to a motorized backpack. Although it was capable of flight, it was extremely hard to control.

Later prototypes of the Hoppi-Copter included versions with the pilot in a sitting position, and were in effect miniature one-man helicopters of a more conventional design, though retaining the contra-rotating rotors and thus obviating a tail rotor. None were adopted commercially.