Lockheed CL-475

The Lockheed CL-475 was a two-seat, single-engine light helicopter that Lockheed built to explore rigid rotor technology, and only one was made. It came from an idea Irv Culver presented to Lockheed management in 1958 for attaching the rotor blades rigidly to the hub and controlling the large gyroscopic moments with a feedback device he called the compliance factor. The CL-475 had a fabric-covered steel and aluminium structure, side-by-side seating under a glazed cockpit, a tricycle undercarriage and a 140 horsepower Lycoming piston engine. It first flew on 2 November 1959. Early vibration was cured after six months by moving to three metal blades and adding a gyroscopic control ring, and the stability that followed informed the Lockheed XH-51 and the AH-56 Cheyenne. Lockheed gave the helicopter to the National Air and Space Museum in 1975, and it went on display in 2018.