Early in the Spanish Civil War, a small group of six Heinkel 51 biplanes were sent to the Spanish Nationalist forces by Nazi Germany. The Heinkels were assigned codes 2-1 through 2-6. Spanish pilots, being unfamiliar with the German aircraft, soon reduced the original complement of six aircraft. A flying accident on August 18, 1936 resulted in one He-51 being written off (page 16), while another two He-51s were stricken from the roster as a result of accidents on August 24, 1936 (page 18). Unfortunately the numbers assigned to these aircraft were not identified.
The German pilots in Spain, anxious to join the fray, convinced the authorities to allow them to fly the Heinkels to prevent further attrition due to accidents. These remaining Heinkels were flown exclusively by Hannes Trautloft, Kraft Eberhardt and Herwig Knuppel (page 19).
Hannes Trautloft was shot down in a He-51 on August 30, 1936 and this excerpt from his diary, found on page 22 states, "... my good old He 51 with the number 4 met it's end".
The drawing of Hannes Trautloft's He-51 that he flew during the summer of 1936, is based on a photograph of one of the early He-51s sent to Spain displaying the coding 2-4 (page 16). Only the port side is shown. It is assumed that the same markings are applied to the starboard side of the aircraft.
The aircraft is overall grey, either RLM02 or RLM63. All the markings are black. Note that the rudder is white, but the white follows the line of the rudder hinge only. Also note the unusual shape of the 4 and the offset of the numerals from the black disc on the fuselage.
As this was early in the war, there are only solitary black discs on the port and starboard sides of the wings. On the upper wing they are on the upper surfaces, and on the lower wing they are on the lower surfaces. The back faces of the prop are black and the portion of the spinner between the propeller blades along with a small portion of the blades are left in their natural wood finish. There appears to be a remnant of a serial number on the rudder. All that remains is a portion that resembles an "E" and the number 2.
All page references are from the excellent book, Legion Condor Karl Ries & Hans Ring Schiffer Military History, West Chester, PA 1992 ISBN: 0-88740-339-5 LC: 91-62741