control. The vehicle used collective pitch control. Built by the US Air Service, it made its first flight in October 1922. About 100 flights were made by the end of 1923. The highest it ever reached was about . Although demonstrating feasibility, it was underpowered, unresponsive, mechanically complex and susceptible to reliability problems. Pilot workload was too high during hover to attempt lateral motion. Postwar era The Convertawings Model A Quadrotor was intended to be the prototype for a line of much larger civil and military helicopters. The design featured two engines driving four rotors through a system of v belts. No tail rotor was needed and control was obtained by varying the thrust between rotors. Flown many times from 1956, this helicopter proved the quadrotor design and it was also the first four-rotor helicopter to demonstrate successful forward flight. Due to a lack of orders for commercial or military versions however, the project was terminated. Convertawings proposed a Model E that would have a maximum weight of with a payload of over 300 miles and at up to . The Hanson Elastic Articulated (EA) bearingless rotor grew out of work done in the early 1960s at Lockheed California by Thomas F. Hanson, who had previously worked at Convertawings on the quadrotor's rotor design and control system. The Gloster Crop Sprayer project of 1960 was an early example of a quadcopter drone. To be powered by a 105 hp Potez 4E air-cooled flat four-cylinder engine, its 20 gal payload was discharged through a 22 ft spray boom. Two operators carried homing beacons at opposite ends of the spray run, so that the quadcopter would always home in on a beacon and not overshoot. However, despite the much simplified design and operational requirements compared to a piloted machine, the parent company board refused to develop it and it remained a paper project. The Curtiss-Wright VZ-7 of 1958 was a VTOL aircraft designed by Curtiss-Wright in competition for the US Army Transport and Research Command "flying jeep". The VZ-7 was controlled by changing the thrust of each of the four ducted fan rotors. The Piasecki PA-97 was a proposal for a large hybrid aircraft in which four helicopter fuselages were combined with a lighter-than-air airship in the 1980s. Current developments The Bell Boeing Quad TiltRotor concept takes the fixed quadcopter concept further by combining it with the tilt rotor concept for a