The concept of the rigid rotor coupled to a gyroscope system was developed by an Advanced Concepts Group led by Irven Culver to seek significant improvements in performance, cost, reliability, and handling characteristics of helicopters. Following testing of a small radio-controlled vehicle - with a 1.52m diameter, two blade, hingeless rotor driven by a McCoy 60 model aeroplane engine - the small design team undertook in July 1959 to design and build an experimental two-seat helicopter for full-scale demonstration of the new concept.
Designated CL-475, this research helicopter had a steel and aluminium tubing covered frame with fabric and a Plexiglass cabin with side-by-side seats. Its 140hp Lycoming VO-360-AIA four-cylinder air-cooled engine initially drove a two-blade wooden rotor, with gyroscopic control being provided by a double metal 'lollipop' attached to the blades and connected to the swashplate by springs. In this form the CL-475, which was registered N6940C, was completed in autumn 1959 and was trucked to Rosamond Lake in the Mojave Desert where initial trials could be made without attracting undue attention.
Excessive vibration was encountered during the first flight on 2 November, 1959, and forced Irv Culver's team to experiment during the next six months with a variety of three- and four-blade wooden rotor designs. The vibration problem, however, was brought within reasonable limits only after the adoption of a three-blade metal rotor and the installation of a new gyroscopic ring attached directly to the swashplate. The CL-475, which in mid-1960 had been moved to the Lockheed facility at Rye Canyon, was then evaluated by FAA, NASA and military pilots and proved to be easy to fly. In fact, a pilot without previous helicopter experience was once able to ferry it alone. Pleased with the results, Lockheed incorporated the rigid-rotor concept in its entry for the US Army light observation helicopter (LOH) competition in 1961. Although the Army did not select this Lockheed design, it had sufficient confidence in the new concept to order jointly with the Navy two Lockheed XH-51 research helicopters.
At the end of its trial programme, the CL-475 was put in storage until 1975, when it was donated by Lockheed to the National Air and Space Museum, Smithsonian Institution. It has now been loaned to the US Army Aviation Museum at Fort Rucker, Alabama.
Lockheed began developing its rigid rotor concept with the CL-475 helicopter design in 1959 and the performance of the CL-475 encouraged Lockheed to continue development. Lockheed submitted the CL-475 to the Army as a candidate to replace the Bell OH-13 Sioux and Hiller OH-23 Raven observation helicopters. Lockheed also tested the commercial market waters without success. However, in February 1962, Lockheed's Model 186, a new design based on the CL-475 rigid rotor, was selected as the winner for a joint Army-Navy program to evaluate the rigid rotor for high-speed flight capability.
Lockheed CL-475 prototype
Engine: 1 x 140 hp Lycoming VO-360-AIA
Main rotor: three-blade
Main rotor diameter: 32 ft
Weight: approx 2,000 lb
Seats: two side-by-side