Photos: A century of helicopters

While mainstream helicopters held to one of the two basic designs--the single large rotor of the R-4, or the tandem rotors of the flying banana--aeronautical research also branched off into other designs for vertical takeoff and landing. This is the Vertol VZ-2 (Model 76) in 1960; test flights of the VZ-2 ran from about 1957 to 1964. As with other VTOL aircraft, such as today's Osprey (more on that shortly), propellers on the wings also function as helicopter rotors when the wings are rotated skyward. Vertol Aircraft was started by Frank Piasecki after he left Piasecki Helicopter; it later was taken over by Boeing.