New helicopter design has flapping rotors

[Carlo]

Though some may appreciate this short read with an animated gif of the idea.

www.abc.net.au/science/news/stories/s142728.htm

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You might try
http://www.ScienceOxygen.com/aviation204.html

It does not provide the answer directly .
It is with a collection of links on this field for helicopter design.
You might as well just start from there...