Atlantic-Fokker CO-4A

The Atlantic-Fokker CO-4 was a two-seat corps-observation biplane evaluated by the U.S. Army Air Service in the mid-1920s. Derived from Fokker's Dutch-designed C.IV and built in America by the Atlantic Aircraft Corporation, it was powered by a 400-plus horsepower Liberty 12 engine and was notable as one of the first U.S. Army aircraft to use a metal propeller. After the prototype was passed over in favor of the Douglas O-2, the design was revised as the CO-4A, with an improved Liberty engine, a redesigned nose, and side-mounted radiators, and a small number were purchased for service evaluation.

The CO-4A served only briefly in limited numbers and did not enter large-scale production, but it represented part of the Army's effort during the 1920s to modernize its observation fleet using foreign-derived designs.