The XH-44 was the first helicopter with coaxial rotors to fly successfully in the United States. The XH-44 was also the first helicopter to fly successfully with all-metal blades and a rigid rotor. It was designed in 1944 by Stanley Hiller, Jr. who was 19 at the time.
Photo taken at Udvar Hazy Center, National Air and Space Museum, Chantilly VA, USA
Image donated to AHS International (Image provided under Creative CommonsAttribution-ShareAlike 4.0 International/CC BY-SA 4.0 license)