The Junkers J 1, nicknamed the Blechesel ("Sheet Metal Donkey"), was the world's first practical all-metal aircraft. Built at a time, early in World War I, when aircraft structural and materials technology relied almost completely on wooden construction, with woven cloth covering materials to enclose the airframe, the Junkers J 1 (not to be confused with the later, armored all-metal Junkers J 4 sesquiplane, accepted by the Luftstreitkräfte as the J 1, using a Roman numeral) was one of the first true revolutions in aviation, being built and flown only 12 years after the Wright Brothers had flown their pioneering "Flyer I" biplane aircraft in December 1903. This experimental aircraft never received an official "E-series" monoplane designation from the Luftstreitkräfte, most likely from the fact it was primarily intended as an airworthy demonstration of Junkers' metal-based aviation structural concepts, and was officially only known by its Junkers factory model number of J 1.
Design and development
Hugo Junkers, who had already established his engineering credentials in the invention of one type of calorimeter, and in 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 began construction of his all-metal canard design, that he named the Ente ("Duck"), with Junkers' help. Hugo's firm built the flying surfaces of Reissner's design, and also built its radiator. The effort 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 1910 Germany, and when World War I broke out, his mind turned to military matters.
After the asassinaton of Archduke Franz Ferdinand precipitated the outbreak of World War I, as 1914 progressed, Hugo Junkers and his company's research institute, or Forschungsanstalt, began the engineering work to realize Herr Junkers' idea of creating aircraft designs that dispensed with the drag-producing exterior bracing that the wood-and-fabric materials of that era of aviation depended on. Junkers' work on Reissner's Ente design convinced him 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 characteristic flaws 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 her first attempts at 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-like" forms, 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 pioneering cantilever structure for the wing panels allowed their exterior surfaces to be strikingly smooth, being likewise covered in chordwise sheet steel panels, with a wing root that had a depth of roughly 75% of the height of the fuselage at the root's thickest point, and that had at least three airfoil changes-along with tapering of the leading and trailing edge angles - while going from the wing's root out to the wingtip, which (for the airfoil changes) would become a Junkers design hallmark on the later, 1918 vintage Junkers D.I single seat all metal fighter design, made with, and covered with corrugated duralumin. The J 1 also relied on spanwise corrugated steel panels as a structural element, hidden under the smooth outer metal covering, to increase the already advanced thick wing's strength. This particular design element of the J 1 was later used on later all-metal aircraft, in just the same way, as an example, on the wings of the United States' famous 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 "straight-six" engine's crankcase and lower cylinder areas, and a very advanced, well-thought-out engine radiator design layout placed the radiator in a ventral belly position under the forward fuselage, with the front of the radiator housing's opening just behind the front gear strut's attachment points on the fuselage, and with the radiator's housing having a width equal to that of the fuselage above it.
Like the Fokker Eindecker, the vertical tail surfaces were 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.
Before the Junkers J 1 could fly for the first time, however, IdFlieg, the Inspektorat der Fliegertruppen, 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 got underneath the starboard wing panel during the "hop" as the J 1 descended, with the port wingtip scraping the ground and the J 1's portside fuselage was bent inwards as a result, towards the rear of the port 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 January 18, 1916. This flight had the J 1 taken up to only 80 m (260 ft) altitude, following a 200 m (660 ft) takeoff run, and the variable incidence setting on the stabliizer had been mistakenly set to lift the tail excessively, from 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, and up for another flight attempt, this time flying as high as 900 m (2,950 ft) of altitude, 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 and fortunately only bent 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 almost strutless J 1 up to a top speed of 170 km/h (106 mph), which had never been achieved before 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 figure, 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, versus the J 1's experimental steel structure, the Rumpler two-seater had a much better climb rate than the J 1.
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 likely not flown again after January 1916, and it survived to the end of World War I and beyond for roughly a quarter century further in time, as a museum piece 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.
Rumors 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, likely built by some of the same people at the Junkers firm, that could have constructed the original aircraft.
The 1915 design of the J 1 was the first revolution in aviation materials and structural technology, as Hugo Junkers had originally sought to simply eliminate the exterior bracing systems of the wood-and-fabric aircraft of his era. By the time of the Junkers J 4 in early 1917, duraluminum had replaced the much heavier steel, except for the armored steel "bathtub" forward fuselage structure enclosing the engine and crew, and this aircraft was all metal apart from a canvas 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, primarily because of the German government's insistence that Fokker work with Junkers' firm during much of World War I's later years, to jointly innovate more producible aircraft than either could have done alone. Fokker's aircraft had wood material, cantilevered flying surfaces in the manner of Junkers' all-metal designs, which were, even in the era before 1930, still largely an era of biplanes.
Fokker's use of cantilever wing structure concepts essentially started with his rotary engine powered Fokker V 1 and V 2 experimental aircraft of 1916-17, and blossomed with the designs of Reinhold Platz for Fokker's firm, starting with the V 1 and V 2, resulting with Fokker's wooden-structure cantilever wing version of the all-metal structural concepts of Junkers being used in the famous Fokker Dr.I triplane and Fokker D.VII biplane fighters during the war, and many high wing monoplane designs that were to come, shortly after World War I's end.
Junkers' use, and methods of using, metals for aircraft structures inspired American engineer William Stout and Russian aviation designer Andrei Tupolev to adopt Junkers' developments for the creation of all-metal aircraft of their own design in the 1920s and early 1930s.
It was not until the 1970s that it could be said that a person of similar stature and importance to Hugo Junkers, in creating another "revolution" in the field of aerospace materials innovation, emerged when American aerospace engineer Burt Rutan came onto the scene, as Rutan's use of nonmetallic composite materials, to replace the metal structures originally pioneered by Junkers some six decades earlier, has become as prevalent as it has in the 21st century.
Published - July 2009
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