First reported flight of a coaxial rotor helicopter
Thursday, April 30, 1908
This was a machine designed by John Newton Williams for Emile Berliner.
Note: Only the year of this milestone is known although it is known that it occurred pre May 1908.
Source: "The American Helicopter: An Overview of Helicopter Developments in America 1908-1999", Michael J. Hirschberg, July 2000, Page 4