The first mass produced helicopter

Sikorsky R-4 was designed by Igor Sikorsky. R-4 has three-bladed main rotor and a single engine. R-4 was a huge improvement on its predecessor VS-300 which was one of the first free flight helicopters. It began its mass production in 1942 for the United States Air Forces and the Navy. R-4 was also used by the British who called the helicopter "Hoverfly". The R-4 was famous for being difficult to fly but despite that this helicopter saved countless soldiers during the Second World War.