J.Herris Fokker Aircraft of WWI. Vol.1: Spinne - M.10 & Watercraft (A Centennial Perspective on Great War Airplanes 51)

Fokker M.9

  The M.9, work number 129, was Fokker's first design after the outbreak of war. The Army wanted an airplane that could defend itself against Allied fighter attack, which was then just hypothetical. Fokker's response was an unusual two-engine, twin-boom biplane. The booms were M.8 fuselages without engines; in their place was a cockpit for a gunner with flexible machine gun. The pilot sat in the central nacelle with two 80 hp Oberursel U.0 engines mounted in push-pull configuration. The booms and nacelle were held together by strengthened center sections and the outboard wings were derived from M.7 wings but were of equal span.
  The construction was standard Fokker; welded steel tube fuselages and tail surfaces and wooden wings, all fabric covered.
  Fokker flew the prototype and quickly discovered it had poor flight characteristics and was tail heavy. In addition, the center section was too flexible. A second flight was made with mechanics in the gunner's cockpits to correct the tail heaviness, but the other problems were not resolved and Fokker dismantled the M.9.

Fokker M.9 Specifications
   Engine: 2 x 80 hp Oberursel U.0

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

Fokker M 9 (K1)
   April 1915 saw the appearance of the M 9, the only twin-engined Fokker aircraft of the First World War. The twin fuselages were modified M 8s, each with its own tail surfaces, there being no rigid connecting surface aft. The central nacelle housed a crew of three, and 80 h.p. Oberursel rotaries were mounted at either end. The Kl designation implied Kampfflugzeug but was soon modified to the G classification. Only one M 9 was built, and relatively few flights were made.

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

FOKKER K I (M 9) Germany

   Original in concept in having a push-pull engine configuration and twin fuselage booms, the M 9 was developed without official encouragement as an offensive fighter, the sole prototype being completed in April I 1915. Also known by the designation K I (the "K” prefix indicating Kampfflugzeug, or ‘‘Battle Aircraft”), the M 9 utilised two fuselages, complete with tail assemblies from conventional M 7 two-seat sesquiplanes. These I were married by means of a biplane structure to a central nacelle which carried single 80 hp seven-cylinder rotary Oberursel U O engines fore and aft, with the pilot seated between. The nose of each M 7 fuselage accommodated a cockpit for a gunner. No rigid structure connected the two fuselages aft of the nacelle and, in consequence, the booms tended to twist when the wings warped. The M 9 was perfunctorily flight tested by Anthony Fokker. He complained of the flexing of the tailbooms and the marked tail heaviness which rendered control difficult. As Fokker was by then preoccupied with testing the M 5K/MG (E I) monoplane, further development of the M 9 was abandoned. No data relating to this short-lived experimental fighter have apparently survived.