O.Thetford, P.Gray German Aircraft of the First World War (Putnam)

Friedrichshafen D Type
  Built during 1917, this quadruplane (vierdecker) was a purely experimental machine and by no means successful, for it crashed on its first flight. The engine was a 160 h.p. Mercedes D III, and the three-ply skinned fuselage bore unmistakable Albatros resemblance.

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

FRIEDRICHSHAFEN FF 54 Germany

  The Flugzeugbau Friedrichshafen specialised in the design of robust seaplanes and sturdy bombers, proving unsuccessful in attempting to produce lightweight structures needed for small single-seat fighters. The FF 54, designed by Dipl-Ing van Gries, proved no more successful than preceding warplanes of this type from the Manzell factory, albeit less conventional in design approach. The FF 54 was conceived as a single-seat quadruplane powered by a 160 hp Mercedes D IIIa water-cooled engine and armed with two synchronised 7,92-mm MG 08/15 machine guns. The two narrow-chord middle wings were mounted on the centreline and baseline of the fuselage with little gap, interconnected between themselves and the bottom wing by broad, aerofoil-section interplane struts. On 31 October 1917, the FF 54 was in final assembly with flight testing anticipated within 3-4 weeks. When this began, however, the quadruplane arrangement proved unsuccessful and, in April 1918, the prototype was rebuilt as a triplane, changes being restricted to removal of the lower middle wing and redesign of the vertical tail surfaces. Flight testing of the FF 54 in its revised form began in May 1918, and continued until the following September, when a crash resulted in abandonment of further work on the aircraft. No data are available for the FF 54 in either quadruplane or triplane form.