CO-1 to CO-8
Fokker, Cox-Klemin, Atlantic
However, that would be a hazard in a crash landing, and was not used on the two CO-4s delivered in 1923 with a rounded radiator, larger tail, and other changes to meet Air Service standards. Five CO-4A two-seaters were delivered in 1924 with a streamlined nose and side radiators that could be extended as the temperature required. Three of these planes were service tested at Langley Field, armed with one fixed .30-caliber Browning and the usual two Lewis guns.
The last Fokker observation biplane tested by the Army was the Atlantic AO-l built by the New Jersey factory affiliated with Fokker. Essentially it was a CO-4 refinement with the circular radiator and was tested October 7, 1924.
Another foreign design tried in 1924 was the Cox-Klemin CO, a modified Heinkel HD 17 powered by a British 450-hp Napier Lion with internal supercharger. It was labeled the CK CO-2 and had no provisions for military equipment during Army tests.
The Engineering Division next offered the CO-5, which used the wings of the second TP-l fighter, but reversed them, putting the larger one above the fuselage to improve visibility downward. The first version was tested in October 1923 with 375-square-foot, single-bay wings and frontal radiator, but in November 1924 reappeared with wider 438-square-foot wings and a “chin” radiator. The data given here is for this version, but the picture shows the aircraft after a turbosupercharger and wider, two-bay wings were installed to enable it, as the XCO-5A, to set an American altitude record of 38,704 feet on January 29, 1926.
The last Engineering Division prototype was the XCO-6 (by now experimental aircraft had the X prefix). Designed by Jean Roche, it was first fitted with an inverted Liberty, but was tested on November 30, 1924, with a standard engine and chin radiator (later called the XCO-6B), and finally, became the XCO-6C test bed for an inverted air-cooled Liberty. Performance and flying qualities were said to be excellent, but the double-bay, wire-braced wing was bad for maintenance work. Three guns were provided.
The officer’s board which met in December 1924 to evaluate observation planes considered the XCO-6 the best of the observation types discussed here so far, followed in order of merit by the AO-l, CO-5, CO-8 and the Cox-Klemin, but the Board stated that the new Curtiss and Douglas types were “so far superior to all of the others that no consideration should be given to the rest.” After this, the Army left all aircraft construction to private firms, and the McCook Field facility limited itself to testing. Since 1918, the Engineering Division had designed and built 27 prototypes, some of which had made important advances in the “state of the art.”
The observation board also agreed that the worst of the observation types tested was the “very poor” CO-7 that Boeing made by combining a standard DH-4M-l fuselage with a tapered wing and new landing gear. Three airframes were made: one without engine for static tests, the XCO-7A with a standard Liberty on November 29, 1924, and the XCO-7B flown with an inverted Liberty.
The last corps observation series aircraft was the Atlantic XCO-8. After that, all observation planes were numbered in a series from O-l to 0-63. The XCO-8 was simply a DH-4M-2 with the Loening wings used in the COA-l. Performance was better than on the standard DH-4, but inferior to the other types being considered. Twenty sets of Loening wings were ordered for use on DH-4s in airmail service.