W.Green, G.Swanborough The Complete Book of Fighters

SPAD S.XX France
  
   Late in 1917, Herbemont designed a new fighter with a monocoque fuselage and a 37-mm Puteaux cannon mounted in the Vee of a 300 hp Hispano-Suiza 8G engine and firing through the propeller shaft, as on the S.XII. This, the S.XVIII (the SFA designation being Spa.XVIII Cal-2), was under construction in April 1918, and was described as a monoplace protege, being intended to be flown as a single-seater in combat, but having provision for a second crew member. Both wings were two-spar structures, the upper having pronounced sweepback, and the single interplane struts were of broad-chord I-type. Development problems with the 300 hp engine-cannon combination led to revision of the design to take a standard 300 hp direct-drive HS 8Fb engine and an armament of twin 7,7-mm Vickers guns, the designation being changed to S.XX and the prototype appearing in August 1918. Official trials were conducted in the following month and the S.XX was ordered into production with a planned output of 200 aircraft monthly. Initially a Lewis gun was provided for the second occupant, whose function was now to fend off stern attacks by enemy fighters, and later twin Lewis guns on a T.O.3 mounting were provided. Production was scaled down with the Armistice, and only 95 S.XXs were built, these remaining in French service until 1923. One example was purchased by Japan in October 1921, being designated Hei 2.

Max speed, 142 mph (229 km/h) at 6,560 ft (2 000 m).
Time to 6,560 ft (2 000 m), 4.6 min.
Loaded weight, 2,438 lb (1106 kg).
Span, 32 ft 2 1/2 in (9,80 m).
Length, 23 ft 11 1/2 in (7,34 m).
Height, 9 ft 5 in (2,87 m).
Wing area, 312 sq ft (29,00 m2).