Fokker Dr.I Fokker Dr.I (serie)

The production Dr.I derived directly from the V.5 prototype and entered production in the summer of 1917 following acceptance by the German army (Idflieg); its entry into front-line service is dated to 1 September 1917. Around 320 units were built up to 1918 (the German Wikipedia cites up to 420 ordered/registered, a figure that probably includes assigned serial numbers not all of which were delivered; the discrepancy between sources is noted here). It carried a nine-cylinder Oberursel Ur.II rotary engine of 110 hp (81 kW), a German copy built under licence from the French Le Rhône 9J; the shortage of castor oil following the Allied naval blockade forced the use of lower-quality substitute lubricants (such as «Voltol»), which shortened engine life and penalised performance at altitude. Part of the fleet, especially in units with supply difficulties, flew with original captured enemy Le Rhône engines, which were more reliable than the Ur.II. Between mid-September and early November 1917 a series of fatal accidents caused by structural failure of the upper wing forced the fleet's temporary withdrawal: on 29 October 1917 the ace Heinrich Gontermann crashed while performing aerobatics when his triplane broke up in flight, and two days later, on 31 October, Günther Pastor suffered wing failure in level flight. The investigation found that poor construction and the lack of waterproofing varnish had allowed moisture to damage the upper wing ribs, which disintegrated and caused the ailerons to come away. Production was halted while Fokker reinforced the ribs and improved the varnishing of the wing surfaces; sources differ on the exact date of return to service (28 November 1917 according to the English Wikipedia, February 1918 according to the German), although both agree on the technical cause. Armament consisted of two 7.92 mm Spandau LMG 08/15 machine guns, fixed and synchronised to fire through the propeller arc; as a pure single-seat fighter, it carried no external ordnance load or passenger capacity.