From Wikipedia, the free encyclopediaKellett XR-8 "synchropter" http://en.wikipedia.org/wiki/Kellett_XR-8The Kellett XR-8 (later redesignated XH-8) was a helicopter built in the United States during World War II. It was a two-seat machine intended to demonstrate the feasibility of a twin-rotor system, and while it accomplished this, it also demonstrated a number of problems that prevented further development of this particular design.Bell Model 30 http://en.wikipedia.org/wiki/Bell_30The Bell Model 30 was the prototype for the first commercial helicopter, and the first helicopter built by Bell Aircraft Company. Designed by Arthur M. Young, the type served as a demonstration test bed for the successful Model 47.Click to subscribe! http://bit.ly/subAIRBOYD The most viewed aviation channel on YouTube.