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Dornier Do X flying boat

The Dornier Do X flying boat, which first flew in 1929, was one of the most remarkable aircraft, either landplane or flying boat, ever built. This unique flying boat was the largest aircraft ever constructed up to that time and had a gross weight variously listed as 105 820 or 123 459 pounds. The lower value was probably the design gross weight: the higher, likely an allowable overweight condition for special long-range flights.

By 1927 the Swiss subsidiary of Dornier was about to embark on the design and development of the "DO X" flying boat, a twelve-engine giant larger even than the famous Boeing Clippers of Pan-American, to meet the requirement for a patrol seaplane capable of landing and refueling at sea. Only an unacceptably low service ceiling made it necessary to abandon this design.

A great effort was then being made in a number of countries to develop aircraft suitable for airline use across the oceans, particularly the North Atlantic. The aircraft were of two main types: rigid airships of the Zeppelin type and large flying boats like Dornier’s. The wing of the DO-X projected from the top of an ample hull with a span of 157 feet and a chord of 30 feet. Lateral stability on the water was obtained by the use of sponsons, or short and stubby winglike structures that projected from the bottom of the hull on each side. Constructed in the late 1920s at Altenrein, Switzerland, on Lake Constance and near Friederichshafen, Germany, the DO-X could accommodate sixty-six passengers comfortably over a range of 700 to 900 miles, but could not lift any kind of payload over transatlantic distances, the minimum such distance being roughly 2,000 miles. On one flight from adjoining Lake Constance, though, 169 [or 170, by other accounts] people were crowded into the airplane (nine were stowaways by one account), making quite a record at that time.

In the United States, no aircraft exceeded the gross weight of the Do X until the one-of-a-kind experimental Douglas XB-19, at a gross weight of about 140,000 pounds, flew in 1941, and the Boeing B-29 bomber of on World War II fame was the first production aircraft to have a higher gross weight. Incredibly, the Do X was powered by 12 engines. They were positioned in six nacelles, tractor-pusher style, strut-mounted on top of the wing. Tests were made with several different engines in the three Do X aircraft that were built. British-made Jupiters were rated at 550 horsepower each, providing a total of 6600 horsepower. The 12-cylinder Curtiss V-1570 water-cooled engines of 640 horsepower each provided a total of 7680 horsepower. Perhaps fortunately, no other aircraft has ever been equipped with so many engines. One can only guess at the difficulties encountered in keeping all of them operating simultaneously in an efficient manner.

The monoplane wing was mounted flush with the top of the hull-fuselage, and the six engine nacelles were located on struts above the wing. Instead of lateral stabilizing floats, short stub wings, called sponsons, projecting from the sides of the hull near the waterline provided lateral stability in the water. Each wing was braced by three struts extending upward and outward from near the tip of the sponsons. In turn, the sponsons were braced by three additional struts extending downward and outward from near the top of the hull to about the mid- point of the sponsons. Two sets of horizontal tail surfaces of different size were configured in a sesquiplane arrangement, and directional stability and control were provided by a single fin and rudder. To reduce pilot control forces needed to maneuver so large an aircraft, small surfaces connected to the main control surface, called park-bench balances, were mounted above and ahead of the elevator and aileron hinge lines.

The all-metal hull had a modified vee bottom with a single transverse step and an afterbody that tapered to a sharp vertical stern post. To assist in maneuvering on the water, a small rudder was mounted at the stern post. Accommodations in the hull were divided among three decks. On the top deck were the pilots' compartment, navigation room, captain's cabin, and flight-engineers' compartment. Instruments and controls for operating the battery of 12 engines were located at the engineers' station.

The passengers were carried on the second deck, which had several cabins with seats and sleeping accommodations as well as a bar and smoking and writing rooms. Cabins were spacious, and appointments included wood paneling, rugs, and other features of contemporary luxury liners of that day. Perhaps some of the weight of this equipment was more appropriate to a surface ship than an aircraft where lightness is an essential ingredient of efficient flight. But the 66 passengers for which the aircraft was configured no doubt traveled in a regal style unknown today. (On one occasion, 150 passengers, 10 crew, and 9 stowaways were carried on one short record flight.) Fuel and stores were carried on the lower deck.

The wing and tail surfaces consisted of a metal framework covered mostly with fabric. Having an area of 4844 square feet and a span of 157.5 feet, the large wing was sufficiently thick to incorporate walkways on which a person could pass through the inside of the wing. Access to the engines for maintenance was provided by hatches above the walkways at each engine nacelle position. Whether or not work on the engines was performed in flight is not known.

With a maximum speed of 134 miles per hour and an estimated cruising speed of 122 miles per hour at 75 percent power, the performance of the Do X appears, at first glance, to be very creditable (table IV). The zero-lift drag coefficient of 0.0472 was also as low or lower than most contemporary flying boats. Because of the low wing aspect ratio of 5.12, however, the maximum lift-drag ratio was a low 7.7, a full 22 percent lower than that of the Consolidated Commodore. In addition, the useful load fraction (ratio of payload weight plus fuel weight to gross weight) was only 27 percent for the 105 820-pound gross weight condition. By comparison, the useful load fraction for the Commodore was 35 percent and for the World War I vintage NC-4, 42 percent.

The useful load fraction of modern jet transport varies between 45 and 55 percent. The combination of low maximum lift-drag ratio and small useful load fraction doomed the Do X as a commercial airplane capable of carrying an economically attractive payload on transoceanic routes. For example, estimates suggest that at a gross takeoff weight of 105,820 pounds, including 66 passengers and a crew of 6, the maximum achievable range was between 600 and 650 miles at a cruising speed of 108 miles per hour at sea level.

In 1930, the Dornier Do X went on a world trip which took it to New York via South America. Ground effect is a phenomenon that has been noticed for some time. Early aviators noticed the increased lift on landing when their aircraft approached the ground. A number of early aircraft used the additional lift from ground effect to increase their efficiency. The transatlantic Dornier DO-X flew just above wave height. The increased lift to drag ratio due to ground effect gave the Dornier the required range to complete its mission. The Dornier DO-X could only make it across the Atlantic in ground effect.

A total of three Do X aircraft, including two for the Italian government, were constructed. None of these proved to be commercially viable. The Dornier Do X suffered from a number of mechanical problems and minor disasters during its service. On a trip from Amsterdam via Lisbon, Rio de Janeiro and Miami to New York, the Do X was dogged by troubles and took from 2 November, 1930 until 27 August, 1931 to complete the trip. Yet the aircraft must be considered as an engineering achievement of considerable magnitude for the time; it showed that very large aircraft could be built and indicated some of the problems of such aircraft. Preserved for many years in a Berlin museum, the original Do X was destroyed in an Allied bombing raid during World War II.