From Appendix 7: Horace Pentecost was the designer of the small personal Hoppicopter helicopters.

-The simplest was the first one, from the year 1945, and was known as “Strap-On”. It was fitted to the body by means of a harness or strap with a rotor above the pilot's head.

"Pentecost Strap-On from 1945" (PiP)

-The engine was an air-cooled two-cylinder, see photos, at head height, behind it. It was an adapted motorcycle engine.

"Two views of a version with landing gear"