Figure 11 does not tell quite the complete story of De Haviland production, since in August and September, 204 De Haviland planes which had been built were shipped to France without engines and were there knocked down to provide spare parts for other De Havilands in service. These 204 machines, therefore, do not appear in the production total. Adding them to the figures above, we find that the total output of De Haviland airplanes up to the end of December, 1918, was in number 4,587. [Illustration: DE HAVILAND-4. USED FOR OBSERVATION, RECONNAISSANCE, COMBAT, DAY BOMBING, AND DEFENSIVE FIGHTING. Engine, Liberty 12-cylinder, 400-horsepower. Weight, empty, 2,391 pounds. Weight, full load, 3,582 pounds. Ground speed is 124.7 miles per hour. Speed at 10,000 feet, 117 miles per hour. Speed at 15,000 feet, 113 miles per hour. 10,000 feet is reached in 14 minutes with full load. Ceiling, 19,500 feet.] [Illustration: UNITED STATES DE HAVILAND 9-A. This is the American development of the British DH-4.] [Illustration: FIGURE 11. DE HAVILAND-4 AIRPLANES PRODUCED EACH MONTH DURING 1918. Jan. 0 Feb. 9 Mar. 4 Apr. 15 May. ===== 153 June. =========== 336 July. ================ 480 Aug. ==== 128 Sept. ===================== 653 Oct. ==================================== 1097 Nov. ================================== 1036 Dec. =============== 472] The production of the model machine only served to show us some of the problems which must be overcome before we could secure a standard design that could go into quantity production. Experimental work on the De Haviland continued during December, 1917, and January and February, 1918. The struggle, for it was a struggle, to secure harmony between this English design and the American equipment which it must contain ended triumphantly on the 8th day of April, 1918, when the machine known as No. 31 was completely finished and established as the model for the future De Havilands. The characteristics of the standard American De Haviland-4 were as follows: Endurance at 6,500 feet full throttle, 2 hours 13 minutes. Endurance at 6,500 feet half throttle, 3 hours 3 minutes. Ceiling, 19,500 feet. Climb to 10,000 feet (loaded) 14 minutes. Speed at ground level, 124.7 miles per hour. Speed at 6,500 feet, 120 miles per hour. Speed at 10,000 feet, 117 miles per hour. Speed at 15,000 feet, 113 miles per hour. Weight, bare plane, 2,391 pounds. Weight, loaded, 3,582 pounds. Endurance here means the length of time the fuel supply will last. The ceiling is the maximum altitude at which the plane can be maneuvered in actual service. Ground level means only far enough above the ground to be clear of obstructions.