Excellent example of the autogiro, a STOL intermediate step to the true vertical takeoff and landing (VTOL) helicopter. Lift is derived from the unpowered rotor spinning as a result of forward airflow. Kellett built seven of these for the US Army - the autogiro's "low and slow" flying abilities combined with the excellent view from the cockpit seemingly made it well suited for the observation mission. However, the advent of the first practical helicopters during WWII spelled the end for the autogiro. Much of the technology developed for the aircraft such as the XO-60 found its way into the helicopter. (Smithsonian caption)
The XO-60 was last autogiro produced by Kellett. Army had purchased and tested 3 earlier models-- YG-1, YG-1A, and YG-1B. (One of the latter later modified as XR-2 and a second as XR-3.) Single-seat version of YO-60, KD-1B, operated by Eastern Airlines in airmail shuttle service between Camden Airport, Philadelphia, and roof of main Philadelphia Post Office. Before YO-60's first flight, Army contracted with Sikorsky Aircraft to build 2-place observation helicopter, XR-4, powered by 160 hp Warner engine. Apparently, Kellett XO-60 was hedge against the failure of XR-4. Autogiro's ability to take off and land almost vertically and fly "low and slow" made it well suited to observation and liaison missions. YO-60 featured 2-place tandem seating with pilot in front and observer in rear seat which could swivel in any direction. Cockpits enclosed in transparent sheeting which overhung fuselage sides, thus allowing good downward visibility further enhanced by windows in fuselage floor. Wingless YO-60 powered by 300 hp engine which engaged rotor blades prior to takeoff but disengaged during flight. Several design changes made in earlier models to improve stability and performance. Most noticeable were 2 vertical fins installed below horizontal stabilizer; improved directional control over earlier single-fin arrangement. Modified rotor head provided cleaner design and eliminated need for interblade cables. Rotor head pylon shortened and externally braced to fuselage. Prompted by near-fatal accident after hard landing when entire overhead assembly broke loose causing blades to strike crew. Newer blades thicker and had constant taper to improve efficiency and add strength. Change in landing gear strut length also aided near-vertical takeoffs by changing angle of attack for ground takeoff roll. None of 7 aircraft assigned to operational reconnaissance units. Sophisticated control systems developed by Cierva and Pitcairn for autogiros were used in early Sikorsky designs under license from Autogiro Company of America. Other helicopter manufacturers, under umbrella of military procurement expediency, "excused" from licensing requirements; Pitcairn sued. After 26 yrs of litigation, Pitcairn won over $31 million from government after courts found 11 autogiro patents had been infringed; he died 17 years earlier so award went to estate. Although autogiro never gained popular acceptance, technologies provided significant basis for development of helicopter rotor designs.