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The Dornier Do X

An extraordinary aircraft

by Julian Edgar

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The Dornier Do X of 1929 was an astonishing aircraft. At the time, it was the largest flying boat in the world – and with its 12 engines, it looked the part, too. In 1930, Flight magazine said of the Do X: “The size is beyond anything previously accomplished; the lines of the machine are unusual; and the power plant arrangement is novel.”

So what’s the story of this enormous craft – and what became of it?

The man

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Claude Dornier was born in the southern German town of Kempten on May 14, 1884. After completing his studies in mechanical engineering, he began working for the Zeppelin airship manufacturing company in 1910. At the time, Zeppelin airships were among the world’s most advanced structures in terms of their size, lightness and strength.

Dornier’s talent was soon recognised by Count von Zeppelin, who placed Dornier in charge of his own experimental and design department. The department became an independent company in 1917, with Claude Dornier becoming the sole shareholder in 1932.

The company concentrated on the development of flying boats, many of which were produced outside of Germany to avoid restrictions placed on the country following WWI.

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A succession of successful flying boats was produced by the Dornier company, including the Wal, Superwal and Delphin models. More than 300 Wal aircraft were built; one took explorer Roald Amundsen to the North Pole and back in 1925.

And it was in 1925 that Dornier received funds from the German government to develop a giant flying boat.

The Do X

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The Do X was built in Altenrhein, on the Swiss side of Lake Constance, with construction starting on December 19, 1927. It took 570 days to build the huge craft, with the launching and first trial flights taking place on July 12, 1929. By the fourth day of testing, the aircraft was making half-hour flights around Lake Constance. On the sixth day, it was taking off with a loaded mass of 48.6 tonnes!

The central hull of the flying boat used a V-shaped design. Two wing stubs projected from near the base of the hull - these provided stability in the water and also located three struts per side that braced the large, semi-cantilever wing. On top of the wing were located no less than 12 engines, mounted two per nacelle with a single front and rear propeller on each nacelle.

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The wingspan was 48 metres and the hull had a length of 40 metres. The maximum height from the base of the hull to the tips of the propeller blades was 10 metres. Unloaded weight was just over 27 tonnes.

The Do X was the first aircraft to have three separate decks. The upper deck contained the pilot’s room (note: ‘room’, not cockpit!), the commander’s room, the engine control room and rooms for the radio and auxiliary motors. (These motors provided power and compressed air for main engines starting.)

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The middle deck was designed exclusively for passengers. It had a length of 23.5 metres, a height of about 2 metres and a width at its maximum point of 3.5 metres.

The large bottom deck contained fuel, freight and passenger luggage.

The wing, rectangular in plan view with rounded ends, used three internal spars. The central spar was positioned at approximately the thickest part of the wing, with the two additional spars 2.8 metres distant from the central spar.

Some references suggest that steel was used to form one of the spars, but an account by Dornier himself says that “with the exception of a few steel fittings, the whole wing structure is made from duralumin… this material was used because it was impossible to procure, in time, steel sections of the requisite dimensions.”

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The spar flanges were formed from pressed duralumin angles and plates, similar - Dornier says – to those used in bridge building.

Perhaps a better analogy might be that the structure shows in some respects great similarity to airship construction. Certainly, it was an approach then unknown in aircraft.

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Testing of the wing spar was problematic, with the building of substantial foundations needed on the test stand.

With an ultimate load of 42,500kg needing to be supported by the spar in testing, the usual method of loading the test specimen with sandbags or iron ballast was not feasible. Instead, tanks were attached to the spar and then filled with water.

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As can be seen in this diagram that shows measured and (hand!) computed deflections, at 100 per cent load, the tip of the spar deflected more than 1.1 metres.

The wings were large enough to crawl within and in fact were designed with internal frames shaped to allow this to occur.

The leading edge of the wing used metal panels and the rest of the wing used a mixture of fabric and sheet metal. The elevator, rudder and ailerons were covered in fabric.

Incidentally, the window frames, handles and cleats were made from cast aluminium.

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The hull used frames placed 0.7 metres apart, with 58 transverse frames used in total.

The frames, which varied in design according to their location, used a riveted press-metal construction. (Again, this was identical to contemporary airship construction.)

This pic shows a modern recreation.

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Three of the frames used a mixed construction of steel and duralumin. These frames had plates at the top for attaching the wing spars and at the bottom, and a double band of steel angles that extended into the lower wing stubs and formed supports for the outer struts bracing the stubs to the wings.

Throughout the whole craft, duralumin rivets were used for joining metal parts. Rivet diameters were as great as 8mm, with the riveting generally done with compressed air tools.

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Five longitudinal girders (keelsons) were used to provide hull with bending stiffness. The main keelson, which extended from the bow to the stern step, was 23.3 metres long and 2.12 metres in maximum height.

Two additional smaller keelsons, with a mean height of about 0.4 metres, were located on each side of the main keelson, being positioned 0.9 and 1.58 metres from it. The floor of the passenger compartment rested on the main keelson; either side of this in the lower hold were located the fuel and oil tanks, freight and luggage.

Strong duralumin plates, 3mm thick and each 0.63 metres square, covered the area of hull bottom subjected to the action of waves.

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The hull was divided into nine watertight compartments. Each of the lower stub wings contained four water-tight compartments. Dornier commented: “The reserve displacement [ie, extra buoyancy] is exceptionally large… the displacement of the stubs alone is 43.5 cubic metres.” Contemporary observers often commented on how high the flying boat sat in the water. There was sufficient buoyancy to keep the aircraft afloat even if two compartments were flooded.

The initial engine choice comprised twelve 500hp, radial Siemens-Jupiter engines. The engine nacelles, while appearing to be in a line, were actually alternately staggered by 20cm, so preventing the tip airflow of one propeller affecting its neighbour. The engines used 2:1 reduction gearing, with an engine speed of 2100 rpm corresponding to a propeller speed of 1050 rpm.

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Each wooden propeller had a diameter of 3.55 metres and a (fixed) pitch of 3.65 metres. The leading edges of each propeller were covered in copper sheathing. The engines weighed 5.1 tonnes, and the propellers and hubs, 1.1 tonnes.

After about a year of testing, it was found that the air-cooled Siemens-Jupiter engines had a tendency to overheat – especially the rear engines. The engines were replaced with more powerful, 610hp Curtiss V-1570 12 cylinder inline engines. With the engine change, the streamlined pillars on which the engine nacelles originally sat, joined by a wing-like appendage, were replaced with simple struts.

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The engines were largely under the control of the engineer working in a separate control room; however, the pilots were able to control the engines by means of two adjacent throttles – one for the starboard engines and the other for the port engines.

All engines could be stopped simultaneously from the pilot’s seat.

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The fuel supply comprised four cylindrical tanks of 3000 litres each, and four other tanks of 1000 litres each, giving a total capacity of 16,000 litres.

These tanks were mounted on the floor of the hull.

The tanks fed a collecting pot from which fuel was pumped to two 300 litre tanks located in the leading edge of the wing.

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Ensuring redundancy, no less than three different pumps were available to move fuel – a wind-driven pump, an electric pump and a hand pump.

If extra range was required at the expense of pay load, six further tanks of 1300 litres were able to be carried, giving a total potential fuel load of nearly 24,000 litres.

An oil capacity of 1,600 litres was provided.

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The aircraft was luxuriously appointed for passengers.

The Art Deco interior was arranged in a series of airy and spacious rooms. The walls were covered in rich floral patterns and carpeted floors featured oriental rugs and stylish club chairs. This is a faithful, modern day recreation.

There was also a smoking room with a bar. A dining saloon was provided along with seating for 66 passengers – these seats could be converted to sleeping berths.

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Passengers dined on Dornier crockery.

On November 3, 1930, the Do X left Friedrichshafen on Lake Constance in Germany for a flight that would take it to the United States. The journey to New York took 9 months.

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At Lisbon on November 29 a fire occurred that destroyed most of the port wing; after repairs, the aircraft continued along the west coast of Africa, crossing then to Brazil and making its way northwards to the US.

Another 9 months were spent there as the engines were overhauled. The aircraft departed New York on May 21, 1932, flying back to Germany via Newfoundland.

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The Do X undertook other promotional flights and was damaged by a heavy landing during one of these flights. It was repaired and in 1936 placed on display in a German aviation museum. During WWII, an Allied bombing raid on Berlin destroyed the museum – and also the Do X.

Three Do X aircraft were constructed – the additional aircraft built for Italian interests. These aircraft were powered by Fiat engines. They were broken up for scrap in 1937.

Now all that remains of the Do X is a section of fuselage in the Dornier museum in Friedrichshafen, Germany.

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