Clarence L. Johnson, one of America's foremost aircraft designers, died Friday at St. Joseph's Medical Center Pavillion in Burbank, Calif. He was 80 and lived in Encino, Calif.

The cause was not disclosed, but a Lockheed spokesman said Johnson, who developed advanced civilian and military planes, including the U-2, for Lockheed, had been in failing health for several years.Johnson played a leading role in the design of more than 40 aircraft, including the YF-12 and the SR-71 Blackbird, both able to make long-duration flights at speeds of more than 2,000 mph; the U-2 high-altitude reconnaisance plane; the F-104 Starfighter, and the P-38 Lightning of World War II.

He had a private pilot's license, but he said he had flown only as a passenger on virtually ever plane he designed.

Johnson, known as Kelly, was affiliated with Lockheed for more than 50 years. He started as a tool designer in 1933 and retired in 1975 as a senior vice president.

For 30 years, Johnson was head of Lockheed's advanced development projects, known as the "Skunk Works."

It was a small unit of technical and production specialists Johnson put together in 1943 to build the first jet fighter, the F-80 Shooting Star, which was designed and produced in 143 days.

Secrecy of the unit is so great that even the origin of the name is shrouded in mystery, although it apparently was derived from Al Capp's cartoon strip, "Li'l Abner," which featured a moonshine distillery called the Skonk Works.

Johnson received some of the most prestigious awards in aviation, including two Collier Trophies, two Sylvanus Albert Reed awards, the Wright Brothers Memorial Trophy and the Daniel Guggenheim Medal.

He also received three presidential citations, including the Medal of Freedom, presented by President Lyndon B. Johnson in 1964.

He is survived by his third wife,, the former Nancy Powers Horrigan; a brother, Clifford, of East Tawas, Mich., and Pinellas Park, Fla.; and a sister, Alice Roseveare of Saratoga, Calif.