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In July of 1936, a major air war began in Spain prompting requests from General Francisco Franco to Hitler, to provide air support to the Nationalists. At the beginning of the war, only thirty-six domestically built Nieuport Ni D.52s were available of which twenty-nine were in possession of Republican forces.6 Six He-51s were sent to Spain with six German pilots to instruct Spanish pilots. Unfortunately, the airplane was a "handful" for the Spanish, who immediately wrote off two of them. The Germans Condor Legion entered combat and met with such success, that it allowed Luftwaffe pilots to gain invaluable air combat experience. The He 51 was a product of the German Heinkel firm and a design of the Gunter twins, Walter and Siegfried. The Gunters became part of the Heinkel firm in 1931 and made the He 49 their first product with the company. The He 49 was a biplane aircraft developed as a fighter, sporting a pair of machine guns, and developed into three major prototype forms. It was these prototype designs that eventually developed into the He 51a prototype and, ultimately, the He 51 production models.

The origins of the He 51 can be traced back to a series of streamlined fighters beginning with the He 37 in the 1920s, and continued with the He 38, He 43 and He 49.2 The Technischen Amt of the Luftfahrtkommisariat became interested when it was revealed it had a higher performance than the Arado Ar-65E which was planned as the first fighter equipment of the still-secret Luftwaffe, with the same BMW VI engine, but the He 51 now showed promise as a fighter that would meet international requirements.

The initial preproduction He-51A-0 fighter, first flown in 1933, equipped the Mitteldeutsche Reklamestaffel, (Central Germany Publicity Squadron), which would later become the core of Jagdgeschwader, JG132 "Richtofen," when the Luftwaffe was revealed to the rest of Europe in 1935.3 Early accidents were laid to deficiencies in training rather than anything intrinsic to the design.

The He-51B structure was strengthened, including twin-wire bracing of the landing gear, and a provision for a 50-liter drop tank beneath the fuselage was added. As production proliferated in 1936-37, so did the Jagdgeschwader mounted on the elegant-looking fighter, with their colorful unit markings.

The beginning of the end for the He-51 came in January 1936. The Luftwaffefuhrungsstab considered that the Arado Ar-68 offered little over the Heinkel in terms of performance and questioned putting it into production. Ernst Udet, Inspector of Fighter and Dive Bomber Pilots, decided to resolve the question. At comparative trials in Brandenburg, mounted in the Ar-68E, with a very experienced pilot in an He-51, Udet out-climbed, out-dove and out-maneuvered the Heinkel fighter with ease.4 Like the He 51, the Ar-68a was also originally equipped with a BMW VI engine, but citing poor performance, the engine was replaced with Junkers Jumo 210, inverted-Vee engine.5

The He 51a differed from the previous He 49 forms in that it was given a new tail fin, wings, undercarriage and radiator. The He 51 itself would become a successful German fighter in World War 2, becoming a serviceable floatplane derivative and light ground attack platform as well. Despite the limitations imposed on German war machine development following the close of World War 1, the He 51 was none-the-less pursued under the guise of it being an advanced trainer. In many ways, the He 51 was part of the "new-look" German military soon to be unleashed on an unsuspecting world.

The He 51 was given uneven-span biplane wings (also known as "sesquiplane") with the upper span longer (and situated forward) than the lower span. Each wing had a single bay with parallel struts angled forward (in profile) and outward with applicable bracing cables. The engine was positioned at the extreme forward of the contoured fuselage and consisted of a liquid-cooled inline engine powering a two-bladed propeller. The pilot sat directly behind and under the upper wing element in an open-air cockpit protected by a forward windscreen. The empennage was conventional with a rounded vertical tail fin and mid-mounted elevators. The undercarriage was fixed and featured two main landing gears faired over and a tail wheel at rear, also faired over. The seaplane version of the He 51 was very similar in design with the most notable exception being the large pontoon floats affixed in place of the landing gears. Armament consisted of 2 x MG 17 machine guns mounted over the engine, synchronized to fire through the revolving propeller blades, with 500 rounds reserved to a gun. For the light strike roles inherent in the C-models, up to six 22lb bombs could be carried.

The Heinkel He 51 was produced in a handful of variants but represented in only three major production models. The four prototypes represented the aircraft initially beginning with the He 49a featuring a short fuselage, the He 49b with a lengthened fuselage, the He 49c with a revised glycol-cooled engine and the He 51a with its revised vertical tail fin and wings, a new undercarriage and new radiator. The He 51a prototype led to the first nine pre-preproduction aircraft under the He 51A-0 designation. This was followed by the first production-rate examples in the 150 He 51A-1's.

The He 51B-0 appeared as another dozen pre-production examples that saw reinforcement of their structure. The production version became the He 51B-1 of which only another 12 were produced. The He 51B-2 was a floatplane variant to which 46 were produced in total. The He 51B-3 was a dedicated high-altitude version.

The He 51B-1 was powered by a single BMW VI series 7,3Z liquid-cooled V12 engine with an output of 750 horsepower. This allowed for a maximum speed of up to 205 miles per hour with a cruise speed of 174 miles per hour. Range was limited to 354 miles with a respectable service ceiling of 25,256 feet.

The He 51C-1 was the first dedicated light ground attack model. At least 79 of these were delivered to Spain to fight with the Legion Kondor (Germany's "Condor Legion") in Spain's civil war in favor of the Nationalist forces. The He 51C-2 was nothing more than an improved version of the He 51C-1. These fitted improved radio equipment. Twenty-one examples of this type were produced.

In all, some 700 examples represented the He 51 series in slightly varied forms. Operators included the German Luftwaffe, the Spanish State and Bulgaria. Bulgaria received twelve He 51's from Germany.

He 51's were first used in anger in the Spanish Civil War. In many ways, Germany used this battleground to field test a variety of items including aircraft, vehicles and tactics. Six He 51 systems were delivered to the battlefront on August 6, 1936, and quickly proved better systems then their biplane contemporaries. Spanish and German airmen both flew the aircraft in the conflict, the Germans under the Condor Legion banner.

As the war progressed and aircraft technology advanced, the He 51 was just as quickly taken out of the dogfighting fold. Instead, the system was utilized as a ground attack platform with good success, despite losses to more modern monoplane types. Many Spainish He 51's survived the conflict, and the ensuing World War, to be of use through the end of the 1940s. Its limited successes in the Spanish Civil War showcased the limitations of fighters relying on biplane wing arrangements over those with monoplane arrangements. The He 51 was nimble enough thanks to its slower speeds and available wing area but speed was once again at the forefront of dogfighting. Newer systems could simply outrun aircraft like the He 51. This naturally forced the He 51 our of favour against more modern implements though the aircraft did soldier on for the German Luftwaffe as a trainer in the early years of the war. As a front-line fighter, however, the type was dropped from service in 1938. While the sleek Heinkel He 51 looked like it had the "Right Stuff," unfortunately this was not the case as the Heinkel He-51 turned out to be a completely orthodox biplane of undistinguished performance. Part of its shortcoming could have been attributed to the failure of the German aircraft industry to develop a suitable high-performance engine, as was happening in Great Britain at Rolls-Royce. It had been hoped that the He-51 would exceed or match opponents such as its contemporary, the Hawker Fury powered by a Kestrel engine, but it was not to be, since it was nowhere near as maneuverable as that famous classic.

In retrospect, the failure of the He-51 when it entered combat was ultimately a good thing for the Luftwaffe, since it forced the service to bring the Messerschmitt Bf-109 into operational use far earlier than would otherwise have been the case, subjecting that great design to the pressure of wartime development from the beginning of its career, and assuring it of the ascendancy it would hold, when war finally broke out in Europe. The introduction of the Bf-109 into combat, enabled pilots of the Luftwaffe to abandon the outdated three-plane fighter Vic formations, and develop the double-pair or Schwarm formations.10 Height was becoming a significant factor used in dive and zoom attacks to catch the fast and modern bombers coming into use at the time, such as the SM.79, Heinkel He III and Dornier Do.17.11