The Corrugated Catalyst

Believe it or not, the F-13 probably represents the high point of technical and commercial innovation in Hugo Junkers’ privately-owned Junkers Flugzeugwerke. Forget all the Ju-somethings of W.W.II, they came after both Hugo’s death in 1935 and the Nazis’ sequestration of the company. If you’re looking for a testimony to the vision of the Rhinelander who was born when George Cayley was still articulating the relationship between weight, lift, thrust and drag, but whose 1910 patent for a flying wing anticipated Jack Northrop by nearly 40 years, then the F.13 is it.


From the modern perspective, the folded-cardboard lines of the F.13’s ribbed skin may look rather quaint, but in 1919 they were the cutting edge of aviation. Only eight years before, don’t forget, in a world where flying machines were made of wood, fabric and wire, all-metal aircraft were no better than penguins. Frenchman Leon Levasseur, for example, had designed the radical Monobloc, whose cantilevered wings not only did away with external rigging wires but housed the fuel tanks and engines. The problem was that it required more power to lift than the engines of the time could provide.

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Although Hugo Junker’s practical background lay in gas engines and steam boilers, as an engineer he had a strong interest in technological developments in general, and aeronautics in particular. His approach to the problem of power, then, was to increase the wing’s lift by using deeply cambered profiles – so cambered, in fact, they could house all of an aircraft’s structural elements, which is how his flying wing design came about.

In 1911, Junkers applied his thinking to the Ente, an aeroplane being built by a colleague, Hans Reissner, which became the world’s first all-metal flying machine, partly due to Junkers’ steel-skinned wing. It’s success lay not only in its camber, but in the corrugated construction that would characterise Junkers aircraft for the next 20 years. By increasing the rigidity of a metal skin, these corrugations reduced the need for stringers and ribs in the aircraft’s structure, thus saving weight and complexity.


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Now, it may have been that Junkers was naturally inclined towards heavy engineering, after all, the Junkers Motorenbau – which gave the Jumo prefix to its aero engines – originally built diesel engines for ships. The Ente experiment, however, seems to have confirmed his idea that the future of aviation lay in the strength and efficiency of metal monoplanes. Such was his conviction that he invested in the construction of another aviation milestone, the J1, which in 1915 gave the world its first flying all-metal cantilever-wing aircraft.

The J1’s nickname, the Tin Donkey, reflected the wartime restrictions on aluminium which obliged Junkers to again use steel in its construction. The result was that the 120hp single-seater weighed 2029 lbs compared to, say, the 929 lbs of a 130hp F1/3 Sopwith Camel. Having said that, the J1’s 106mph top speed wasn’t far short of the Camel’s 113mph, which tells you something about the aerodynamics of in-line engine metal monoplanes and rotary engine fabric-covered biplanes. And the J1, remember, was flying nearly two years before the Camel.

As ever, private enterprise lit a fire under ministerial trouser seats and the German government came forward with money, materials, and the suggestion that Junkers join forces with Fokker, the Dutch designer who’d been press-ganged into helping the German war effort. Unsurprisingly, perhaps, the collaboration between the two men wasn’t fruitful, and certainly nothing like as successful as the combination of Junkers’ aircraft and duralumin. This aluminium alloy, which is both light and workable yet comparable to soft steel when hardened, was used for the first time on the Junkers J7 and in the subsequent J9 single-seat fighter, J10 two-seat ground attack aircraft and J11 naval patrol floatplane.

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At the end of the war, Fokker and Junkers went their separate ways, their factory in Dessau was renamed Junkers Flugzeugwerke and Junkers turned his attention to civil projects. In part, this was a matter of necessity – the terms of the Versailles Treaty prohibited Germany from manufacturing military aircraft – but the pursuit of these civil opportunities was also Junkers’ primary interest. As early as 1914, he’d designed an aircraft to carry four passengers and it was to this formula he now applied the experience gained in building military machines, to produce the F.13.

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Beneath the stressed skin of the deeply cambered wing, the Junkers used nine tubular spars braced in Warren girder-fashion with diagonal members. The use of duralumin meant that the early BMW-engine F.13 weighed in at just over one ton empty, but was strong enough to carry another half ton before reaching its all-up mass. Indeed, with a few notable exceptions (see Not entirely Unbreakable), this strength contributed to its excellent record for survivability when things went wrong.

The F.13’s light weight must also have extended to its handling: not only were the control surfaces horn-balanced, but the aircraft could also be trimmed by pumping fuel to and from a trim-tank mounted in the tail. This sophisticated set-up (which is still used on modern jets) must have further improved the efficiencies of the design because it would have obviated the need for coarse trim tab settings which add considerably to an airframe’s drag.

One of the factors that made this sort of efficiency so important, no doubt, were the further limitations imposed by the Inter-allied Aeronautical Commission of Control upon the size and power of the aircraft sold to Germany’s domestic market, wherein the F.13 was only approved for use with a 185bhp motor. These restrictions only added to the challenge Junkers’ faced in selling the F.13, which also had to compete with pensioned-off military machines that were being converted to civil use.

Junkers’ solution was to pursue sales in foreign markets, which he helped to grow by leasing or lending F.13s, selling them on favourable terms, and sometimes gifting them. Sixteen European operators were supplied in this way, though the F.13’s role as a catalyst extended even further afield to South America, Africa and Australia, where it played a part in establishing early air services; it also dipped a toe in the Japanese market, and saw military service in China.

Junkers Flugzeugwerke itself entered the airline fray by taking a share in Persia’s new state-owned airline, and collaborating with Albatros and Norddeutscher Lloyd (NDL) to establish Lloyd Ostflug in 1920. When NDL and Albatros withdrew a year later, the enterprise continued to develop routes, first as Junkers’ Department of Air Traffic, and from 1924 as Junkers Luftverkehr.

F.13s were key to this operation: around 60 machines formed the backbone of the airline’s fleet, which between 1921 and 1926 is claimed to have flown 9,500,000 miles and carried 281,748 passengers on its Europe-wide network, whose infrastructure included maintenance facilities and even a pilot training school. In 1925 alone, Junkers Luftverkehr and its allied crop-spraying and cargo operations were, apparently, responsible for 40% of air traffic worldwide.


In July 1930, Flight magazine reported (in its 7 November edition of the same year), that G-AAZK – which was owned by the Croydon-based Walcot Airlines – broke up during a flight from Berck to London in which it encountered a strong and gusty westerly wind and rainstorms. “It [was] seen to fly into a cloud,” Flight magazine said, “and very shortly thereafter the personnel, pieces of tailplane, a wing, the engine, the fuselage and various items fell from the cloud.” The unfortunate F.13 then became the first all-metal aircraft to undergo a comprehensive crash investigation, in which the wreckage was reconstructed at Farnborough by the Royal Aircraft Establishment.

The British findings pointed to the tailplane, suggesting (Flight recorded in the 26 February 1932 edition) that, “buffeting caused the tail to break, the breakage of the wings and the tearing away of the engine from the machine being subsequent events.” The Germans, however, conducted their own investigation and concluded that, “the machine accidentally got into a steep dive in the clouds and that in pulling out too suddenly and at high speed, stresses were set up in the wings which proved too great, and the wings collapsed.” What lent strength to the German view, Flight noted, was the fact that a similar accident had taken place previously, “in which the observer was saved by parachute and was able to corroborate the evidence of eye witnesses on the ground.”


In pursuit of further overseas sales, Junkers entered into a series of foreign ventures, the most successful of which was A.B. Flygindustri in Sweden. Effectively, this was a Junkers Flugzeugwerke subsidiary that provided a cover for the export of the F.13’s twin-engine derivative, the G23: the aircraft were shipped out of Germany in low-powered, civilian guise, then re-engine and equipped for military service in Sweden.

In the USA, Junkers signed in 1919 to supply the US Navy, the US Army Air Corps, and the US Post Office with F.13s via a partnership with a John M Larsen. Unfortunately, despite a promising start which saw the aircraft being used on long distance and trans-continental postal routes, a series of accidents brought an end to the arrangement just two years later. Apparently, the high-octane fuel used in the US corroded the F.13’s fuel lines, leading to fires and explosions.

The financial loses arising from the North America partnership were quickly followed by those in the USSR where, encouraged by subsidies from the German government, Junkers built a factory near Moscow to help the Soviets to establish an aviation industry. Again, this scheme came to nothing: the Russians weren’t persuaded by the Junkers aircraft, while the German government’s attention quickly turned to other markets, so that in 1925 it not only withdrew funding, but demanded Junkers repay all previous subsidies. At around the same time, a similar co-operative venture with the Turkish government also failed, and the cumulative loses so undermined Junkers Luftverkehr that, in 1926 in a flurry of corporate transfers and amalgamations, the airline was transferred from private to government ownership, and amalgamated with Deutscher Aero Lloyd, to create Deutsche Lufthansa.

Though Hugo Junkers retained control of Junkers Flugzeugwerke and Junkers Motorenbau – at least until the Nazis came knocking – he now fades from the F.13’s story, surviving just long enough to witness the Depression begin the diminution of the Junkers empire. During the war, an air raid on Dessau destroyed the Junkers museum which contained the legacy of his early work in aviation. In the post-war division of Germany, meanwhile, all hopes for Junkers’ future were ended as all but one of its factories fell into the hands of the Soviets, who dissolved them; the sole surviving plant in the West was absorbed into Messerschmitt Bolkow Blohm (MBB) in 1969.

For its part, the F.13 remained in production until 1932, during which time 322 examples were built in a series of variants that embodied numerous modifications which operated on skis and floats as well as wheels, and used several powerplants, including Junkers’ own 310bhp L2, and – records suggest – Armstrong Siddeley’s Puma and Jaguar engines, and Pratt & Whitney’s Wasp.

Even when manufacturing ceased, the F.13 continued to serve with Lufthansa until 1938, by which time, of course, the cutting edge of the all-metal monoplane had evolved from the Junkers’ angles to the curves of aircraft like the DC-3. If, however, it was the streamlined aluminium aeroplanes of the ‘30s that came to epitomise the new era of airline travel, it was an age that Hugo Junkers and his boxy metal monoplane helped to usher in.

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