Igor Sikorsky's first attempt at building a helicopter. The airframe was a wire-braced wooden cage, enclosing a 25hp Anzani engine. Co-axial steel shafts carried the two contra-rotating rotors, each with two blades comprising wooden ribs with linen covering. Two months of testing in the summer of 1909, including at least one narrow escape when the helicopter almost capsized on him while at maximum rpm, convinced the young designer that it was too heavy to become airborne with such an inadequate power plant.
John W.R. Taylor "Images of America - Sikorsky", 1998
The S-1 was the first attempt by I.I.Sikorsky to build a flying machine. It was a helicopter with two coaxial two-bladed propellers (upper 4.6m diameter, lower - 5.0m).
Tested, but failed to take off - falling on one side.