|Launoy & Bienvenu|
A year after the first successful ascent by a hot-air balloon, Launoy, a naturalist, helped by a mechanic named Bienvenu, constructed a small device which seemed to have been inspired by the "Chinese top" principle.
This scientific curiosity, consisting of two small propellers at the end of a shaft and with motive power coming from the tension of a bow bent through twisting its string around the shaft, was shown to the astonished members of the French Academy of Sciences in 1784.
P.Lambermont "Helicopters and Autogyros of the World", 1958
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The first firm historical evidence of a helicopter being built dates from 1784, when two French artisans, Launoy and Bienvenu, devised an ingenious toy consisting of two propellers made of birds' feathers fixed to the tips of a shaft, around which two strings were twisted, tensioning a spring in a crossbow arrangement. As it straightened out, the spring caused the propellers to rotate for a few seconds, sufficient to send the toy spinning a few meters.
G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984