Junkers J 1

The Junkers J 1, nicknamed the Blechesel ("Tin Donkey" or "Sheet Metal Donkey"), was the world's first practical all-metal aircraft. Built early in World War I, when aircraft designers relied largely on fabric-covered wooden structures, braced with struts and exposed rigging lines, the Junkers J 1 was a revolutionary development in aircraft design, being built and flown only 12 years after the Wright Brothers had first flown the "Flyer I" biplane in December 1903 which used the period's characteristic wood and fabric construction, still widely used for aircraft design at the outbreak of World War I. Herr Junkers' experimental all-metal aircraft never received an official "A" nor an "E-series" monoplane designation from IdFlieg and the then-designated Fliegertruppe, probably because it was primarily intended as a practical demonstration of Junkers' metal-based structural ideas, and was officially only known by its Junkers factory model number of J 1. It should not be confused with the later, armoured all-metal Junkers J 4 sesquiplane, accepted by the later Luftstreitkräfte as the Junkers J.I (using a Roman numeral).

Design and development
Hugo Junkers, who had already established his engineering credentials by the invention of a type of calorimeter and in the construction of internal combustion engines, first got interested in aviation in 1907 when a colleague named Hans Reissner, a professor at the Technische Hochschule in Aachen, approached Junkers for assistance in aircraft construction. Five years later, Reissner, with Junkers' help, began construction of his all-metal canard design, that he named the Ente ("Duck"). Junkers' firm built the flying surfaces of Reissner's design, and also built its radiator. The problems encountered in creating the Ente got Hugo's active mind working on the problems of airframe design, and solving the problem of eliminating the then-prevalent exterior bracing from airframes, placing all such structural elements within the covering of the airframe. He patented the concept of the flying wing aircraft in Germany in 1910. When World War I broke out his mind turned to military matters.

After the assassinaton of Archduke Franz Ferdinand precipitated the outbreak of World War I, Hugo Junkers and his company's research institute, or Forschungsanstalt, began the engineering work to realize  Junkers' idea of creating aircraft designs that dispensed with  drag-producing exterior bracing. His work on Reissner's Ente design had convinced him of the necessity to use metal as the main structural material, but since the apparently "ideal" metal alloy for aircraft construction, duralumin, had only been invented some six years earlier in Germany, and was initially prone to flaking and other undesirable characteristics  when worked in sheet metal form, Junkers had to use sheets of heavier electrical steel instead for his first all-metal aircraft designs, similar to the types of ferrous sheet metals used in laminated-core AC electrical transformers.

The Junkers firm got its first aircraft construction contract in July 1915 from the German government, No. 96/7.17 A7/L, to produce an example of a two-seat all-metal aircraft that would be capable of a 130 km/h (81 mph) top speed, wing loading of 50 kg/m² (10.2 lb/ft²) and use a 75 kW (100 hp) engine for power. Junkers engineers Otto Mader, head of Junkers' Forschungsanstalt, and Hans Steudel, director of Junkers' structural materials and testing department, started the work on the design of what would become the Junkers J 1 in September of that year, and by November 1915 the completed J 1 was ready for initial flight testing.

Pioneering design features
When the Junkers J 1 was first rolled out for government examination by the Kaiser's military aviation experts in November 1915, they were looking at what was the pioneering example of future aviation design — a sleek, well-thought-out low drag design that had completely eliminated the need for major exterior bracing struts, except on the empennage, for support of the horizontal stabilizer (which had a variable incidence feature built into it) and tailskid structures, and only two crossed bracing cables along the front vertical plane of the main landing gear struts, exposed outside the structure.

The fuselage used welded strip-steel angle stock and I-beam sections along with some steel tubing to form its main internal structure, with 42 cm (17 in) wide sheet steel panels wrapped around the fuselage to form its covering. The innovative cantilever structure for the wings allowed their exterior surfaces to be strikingly smooth, being likewise covered in chordwise sheet steel panels. The wing root had a depth of about 75% of the height of the fuselage at the root's thickest point, and the wing had at least three airfoil changes, along with tapering of the leading and trailing edge angles between the wing's root and the wingtip. These changes in wing section would become a Junkers design hallmark on the later 1918 Junkers D.I single seat all metal fighter design, which was covered with corrugated duralumin. The J 1 also relied on steel panels with "spanwise" corrugations, running from root to tip as a structural element, hidden under the smooth outer metal covering, to increase the wing's strength. This particular design element of the J 1 was used on later all-metal aircraft, such as the wings of the American Boeing B-17 Flying Fortress heavy bomber design of 1935.

The 90 kW (120 hp) Mercedes D.II six-cylinder liquid-cooled inline engine selected for the design had a simple, clamshell-like horizontally split cowling enclosing the engine's crankcase and lower cylinder block, and an advanced engine radiator layout, placing the radiator in a ventral position under the forward fuselage, with the front of the radiator housing's opening just behind the front gear strut's attachment points to the fuselage, and with the radiator's housing having a width equal to that of the fuselage above it.

Like the Fokker Eindecker, the single vertical tail surface was of an "all-flying" design (with no fixed fin) and the entire tail surface structure and covering also consisted of formed and sheet steel, much like the wings, with the stabilizer capable of having its angle of incidence adjusted on the ground.

Operational history
Before the Junkers J 1 could fly for the first time, IdFlieg, the Inspektorat der Fliegertruppen, the aviation administration arm of the German army, required that static load tests be done on the J 1, with the usual early era static loading trials carried out on the J 1's structure with sand bags for weight, loading and strength tests, as well as a test of the static thrust that would be obtained with the chosen engine and propeller combination. The static tests were completed on 3 December 1915, preceding the engine thrust tests. The Junkers factory did not yet have its own test field in Dessau, so the completed J 1 was taken to the Fliegerersatzabteilung 1 (FEA 1) airfield in Döberitz just west of Berlin for its flight testing program.

On 12 December, Leutnant Theodor Mallinckrodt of FEA 1 was assigned to taxi and briefly "hop" the J 1, which he managed to do successfully up to almost a 3 m (10 ft) altitude, but a gust of wind caught the starboard wing during the "hop" as the J 1 descended, with the port wingtip scraping the ground and the J 1's left side of the fuselage was bent inwards towards the rear of the  wing mount. Repairs were made through the holiday period at the end of 1915, and more static load tests were carried out to check on the integrity of the repairs.

The second attempt at flight for the J 1 was carried out at Döberitz by Gefreiter (Private) Paul Arnold of the FEA 1 unit, on 18 January 1916. This flight had the J 1 taken up to an altitude of only 80 m (260 ft), following a 200 m (660 ft) takeoff run, and the variable incidence setting on the stabilizer had been mistakenly set to lift the tail excessively, in the belief that the J 1 was too tail-heavy. Later that day, after the stabilizer's incidence adjustment was corrected to give level flight trim, Leutnant Mallinckrodt took another turn at the J 1's controls for another attempt at flight, this time flying as high as 900 m (2,950 ft), with a shorter takeoff run than before. The in-flight handling of the J 1 was acceptable, and it was stable in flight, but upon landing the J 1's main landing gear wheels ran over a small ditch, fortunately only bending one of the landing gear struts, with no damage to the airframe itself.

On the following day, 19 January, Mallinckrodt once again took the J 1 up for its only known "high performance" flight test, which consisted of a 7 km (4 mi) course, at varying altitudes from 200–300 m (650–985 ft), and managed to get the J 1 up to a top speed of 170 km/h (106 mph), a speed  which had never been achieved with a liquid-cooled 90 kW (120 hp) engine. The J 1 was compared to the popular Rumpler C.I two-seat armed observation biplane, which was some 30 km/h (19 mph) slower in its top speed, even though the Rumpler biplane had the more powerful Mercedes D.III engine, but due to the lighter weight of the Rumpler's wood-and-fabric structure it had a much better climb rate than the J 1.with its experimental steel structure.

By the end of January 1916, Junkers had been given a contract to further develop his all-metal concept, and the later Junkers J 2 single seat fighter, which would never see front line service, was the follow-on to the J 1.

Ultimate fate of the J 1
The Junkers J 1 was probably not flown again after January 1916. However, it survived World War I and was placed on display in a Berlin aviation museum. Sadly, it met its end during one of the earliest Royal Air Force bombing raids on Berlin, during World War II.

Rumours have existed that an accurate scale display model, made of metal, of the J 1 had been built by the Junkers factory workers during the years following its initial flights, and exhibited at the Franklin Institute in Philadelphia, Pennsylvania after the close of World War I, but no word exists on the eventual fate of the miniature replica of the J 1, probably built by some of the same people at the Junkers factory that constructed the original aircraft.

Legacy
The 1915 design of the J 1 was a revolution in the technology of airframe structure. By the time of the Junkers J 4 in early 1917, the metallurgy of duraluminum had improved to the point where it was able to replace the much heavier steel as a primary  airframe structural material, except for the armored steel "bathtub" forward fuselage structure enclosing the engine and crew, and this aircraft was all metal apart from a fabric covered rear fuselage. The late-1917 J 7 experimental fighter monoplane prototype was entirely duraluminium and both these aircraft had the corrugated stressed skinning that characterised all subsequent Junkers designs up to the 1932 Ju 60. After that Junkers aircraft became smooth skinned though still as in the J 1, they were all metal. Almost all Junkers designs were aerodynamically clean cantilever monoplanes. Only Anthony Fokker's aircraft were as advanced aerodynamically. Although Fokker briefly collaborated with Hugo Junkers, the later Fokker designs, starting with the experimental Fokker V.1 were independent developments by Rheinhold Platz from 1916 onwards. The Fokker company's use of wooden cantilever wing structures started with the Fokker V.1 and V 2 experimental aircraft of 1916-17. The two Fokker factory experimental projects led to later fighter aircraft such as the Fokker Dr.I triplane and the Fokker D.VII biplane which both dispensed with steel-cable rigging to brace their wings, solely depending on their cantilever structures and interplane wing struts for strength. Junkers' methods of using metal for aircraft structures inspired both American engineer William Stout and Russian aviation designer Andrei Tupolev each to independently adopt Junkers' developments for the creation of all-metal aircraft in the 1920s and early 1930s, leading to Stout's popular Ford Trimotor all-metal airliner in 1926, and to Tupolev's enormous, eight-engined Maksim Gorki, the largest aircraft anywhere when it was first built in 1934.