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What cross was repainted over the Maltese cross on the Dr.I?
Short answer
The Balkenkreuz, the black straight-armed cross outlined in white. It was applied in the field over the earlier cross pattée, whose points are still visible in the surviving fabric. The Luftstreitkräfte officially adopted it in mid-April 1918, about a week before Richthofen's death.

The Balkenkreuz over the cross pattée
The insignia repainted over the earlier one was the Balkenkreuz, applied "in the field" over an existing cross pattée, whose points are clearly visible in the surviving fabric. The emblem consists of a black cross outlined in white, all over a background painted red. One of these large black crosses, about 4 by 4 feet, was painted over the red paint covering the aircraft's fabric. The piece is a good example of a palimpsest, an object reworked or overwritten to serve another purpose.
The official change of insignia in 1918
The Luftstreitkräfte, the air service of the German imperial army, officially adopted the Balkenkreuz in mid-April 1918. The stated reason was to improve recognition of friendly aircraft. The decision came about a week before the death of Manfred von Richthofen.
The triplane that carried these markings
The Fokker Dr.I became famous as the aircraft in which Manfred von Richthofen scored his last 17 victories. The triplane originated in April 1917, when Anthony Fokker examined a captured Sopwith Triplane during a visit to Jasta 11. Fokker then commissioned Reinhold Platz to build a triplane.
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