09/30/2013. Remarks by Johan Visschedijk: "With the advent of more powerful rotary engines in the spring of 1918, Idflieg supported the development of compatible fighters, one such being the D.IX powered by the new 160 hp Siemens-Halske Sh III rotary.

The first D.IX prototype, fitted with a four-bladed propeller, demonstrated excellent performance at the first D-type contest in January 1918, before being destroyed as a result of a freak accident. While the D.IX was in flight, the pilot's seat collapsed, its occupant inadvertently pulling the aircraft into a tight loop in which the resultant g forces propelled him through the bottom of the fuselage before the aircraft broke up.

At the time, two more prototypes were in assembly, the first of these being successfully load-tested in April prior to resumption of flight testing. The second D.IX (pictured here) differed in having enlarged tail surfaces and unbalanced ailerons, and a 210 Sh IIIa engine. Armament comprised twin synchronized 0.311 in (7.9 mm) LMG 08/15 machine guns. It appeared in the second D-type contest in May, but, according to the debriefing minutes, was considered unsuited for series production.

The third D.IX had been completed meanwhile with overhung, unbalanced ailerons and a larger horn-balanced rudder. The D.IX was not taken into production."