O.Thetford, P.Gray German Aircraft of the First World War (Putnam)
Zeppelin-Lindau (Dornier) Rs I
This gigantic flying-boat was conceived by Claude Dornier before the First World War. Count von Zeppelin was so impressed with this young engineer's latent brilliance that he authorised special facilities for him to develop his ideas at Friedrichshafen. Dornier's research eventually produced encouraging results, which were looked upon more as essays in techniques from which valuable lessons could be learned rather than designs suitable for production aircraft.
Construction on Rs I was started in January 1915, and the machine was launched in October of that year. The three engines were buried in the hull and drove three pusher airscrews through shaft and bevel-gear transmission. Hull longitudinals and wing spars were of high-tensile steel, the lower part of the hull covering being alloy sheet and the wing covering of fabric. Ribs were of metal, as were the Warren truss arranged interplane struts. These struts converged upon the centre spar of the lower wing, around which the whole cellule could be rotated to adjust the incidence. The Rs I was wrecked before flight trials began, but results thus far obtained were sufficiently encouraging to proceed on the Rs II development.
Engines, three 240 h.p. Maybach Mb IV. Span, 43.5 m. (142 ft. 8 7/8 in.). Length, 29 m. (95 ft. 1 7/8 in.). Height, 7.2 m. (23 ft. 7 1/2 in.). Area, 328.8 sq.m. (3,551 sq.ft.). Weights: Empty, 7,500 kg. (16,500 lb.). Loaded, 10,500 kg. (23,100 lb.).
G.Haddow, P.Grosz The German Giants (Putnam)
The giant Dornier flying-boats signified the beginning of an aircraft enterprise that still exists today as the oldest German aircraft concern. Its longevity is due in great measure to its brilliant founder, Claudius Dornier, who, after having built and lost an aircraft empire, has had the satisfaction of experiencing its rebirth.
Dornier, born in 1884, had joined the experimental design offices of the Luftschiffbau Zeppelin in 1910 after three years work with several steel construction firms. At first he was engaged in a variety of airship problems, among them: stress calculations, propeller theory, preliminary investigation of a metal-clad airship and the design of a rotating airship hangar which earned him a prize and a patent, one of the many assigned to him in his lifetime. From the beginning, Dornier proved himself a thorough and creative engineer, endowed with the ability to reduce ideas to practical use. These talents soon caught the eye of Graf Zeppelin, with the result that in 1913 Dornier was transferred to Graf Zeppelin's private design bureau in Friedrichshafen to work on an 80,000 cubic metre steel airship capable of flying across the Atlantic. This behemoth, three times the size of existing airship, was to have been completed for the Dusseldorf World's Fair of 1916, from which it was to fly to America and then on to the San Francisco Exposition. However, war intervened, and the ambitious project was abandoned.
As described elsewhere, Graf Zeppelin organized the VGO-Staaken venture in 1914 to construct a giant wooden bombing aircraft as quickly as possible. At the same time, however, Graf Zeppelin, believing in the ultimate superiority of all-metal construction, commissioned Dornier to undertake the more formidable task of building all-metal seaplanes. Dornier collected his small design staff and moved into all old airship hangar at Seemoos, on the shores of Lake Constance, to begin his revolutionary work. The new organization was known as Zeppelin-Werke Lindau. G.m.b.H ., although the aircraft built by this firm were all designated "Dornier".
In 1914 metal was an uncommon construction material for aircraft, but Dornier and his engineering staff approached the problem of developing lightweight structures with vigour and ingenuity. Aluminium was available, but in its unalloyed state it was a relatively low-strength metal. Progress had been made with aluminium alloys such as the newly-developed duraluminium, an alloy which doubled the strength of aluminium, but, being new, still suffered from a number of faults. A Dornier engineer had this to say about the material:
Dural was a brand new material in 1914, available only for experimental use. It had many drawbacks. For instance, it was not produced in consistent quality; more often than not a rolled Dural sheet would exfoliate like the leaves of a book. Impurity inclusions caused frequent embrittlement cracking and after brief periods of storage, the Dural sheet had the unpleasant tendency to disintegrate in spots to a white powder.
Although an airship using duraluminium parts was placed into service in early 1915, the very promising metal was not yet suitable for all-round aircraft use. Dornier therefore decided in favour of a mixed construction technique using principally high-strength steel for his first all-metal aircraft, with less-stressed sections formed from duraluminium.
At first glance it would appear that steel tubing would make an ideal structural medium, for it was light, easy to produce and possessed a high strength-to-weight ratio. Yet from Dornier's point of view it was far from perfect. Tubes are extremely difficult to join with bolts and rivets, and invariably a weakening of the structure takes place. Welding, at the time, was considered still unreliable; inspection of the welded joint was not foolproof, and the high welding temperatures involved had a tendency to weaken the metal, although this situation was to change later in the war. Finally, the many sizes of tubing required would entail extensive storage facilities and place Dornier at the disadvantage of being dependent on the producer for special tube sizes.
Dornier chose to exploit the ingenious construction method which he had developed and was perfecting for the transatlantic airship project. The starting material was alloy steel strip of varying widths and thicknesses, which had the advantage of being easy to fabricate, ship and store. The strip was then readily drawn or rolled into hollow-flanged profiles (for example, open flanged, "clover leaf" and V-shapes) of different dimensions as the occasion required. An inverted U profile was then placed into the open end of the "clover leaf" and joined with rivets. The extended flanges provided a surface to which other profiles and structural parts could easily be fastened. This method of forming structural members and variations thereof not only remained Dornier's basic structural philosophy for years, but virtually was the foundation for modern all-metal aircraft construction techniques the world over. These shapes possessed higher buckling strengths and had better resistance to breaking than tubing of similar weight and cross-section. Furthermore, an infinite variety of shapes could be formed to suit every structural requirement of the aeronautical engineer.
The design of the all-metal Dornier Rs.I was begun in August 1914, and by January of the following year the actual assembly work had begun. Former Dornier test pilot, Erich Schroter, recalls that Graf Zeppelin followed with deep interest every detail of the Rs.I assembly. In spite of his advanced years, he would not hesitate to climb an extended fire ladder to watch the huge wings being lowered on to the hull. The Rs.I was a private venture, and as such it was not assigned an official Navy identification number. (The first two Dornier R-seaplanes were also at one time designated by the company as FS.I and FS.II (for Flugzeug Seemoos or Flugschiff), but the Rs designation later became standard)
The framework of the Rs.I was constructed primarily of alloy steel using Dornier's ingenious double-flanged shapes, perforated girders, built-up triangular sections and stamped sheet. The uncovered frame of the Rs.I had all the earmarks of contemporary airship structure. In spite of its shortcomings, duraluminium was used for light unstressed structural parts as well as for the lower fuselage covering.
The wings were of relatively narrow chord considering their great span. The upper wing was constructed around four built-up truss spars, two of which formed the supporting structure, while the remaining two were designed to take pure bending loads which were transferred to the main structure at the junction points. Duraluminium rib profiles were riveted to the spars to form the airfoil section, and they were internally braced by diagonal cross-members. The ribs were wrapped with linen strips to which the wing covering was sewn. The lower wing consisted of a similar three-spar structure that was attached to a fuselage pivot point, in a manner that permitted the angle of incidence of both wings to be varied by changing the length of the forward fuselage strut. The lower wing had slight dihedral, and two small duraluminium stabilizing floats were mounted at the wing tips. (Early project drawings show the stabilizing floats mounted just outboard of the engines.) Unconventional triangular wing struts, reminiscent of the Warren truss when viewed head-on, eliminated the need for interplane cables. The huge unbalanced ailerons were unusual in that they spanned half the length of the upper wing.
The Rs.I hull was of orthodox design, single-stepped, with the tail section rising gently to keep the tail well above water. The planing surface and the sides of the hull were covered with sheet duraluminium, while the top and rear were covered with fabric. The pilots sat in a large square cockpit in the nose behind a rather awkward Cellon canopy which must have seriously restricted their forward view. A machine-gun ring was mounted in the cabin roof just behind the pilots' seats. Consistent with Graf Zeppelin's "bomb in harbour" theory, provisions were made to carry a 1000 kg. bomb in the hull. In what was perhaps the first test of its kind, model of the RS.I hull were extensively tested in the indoor tow tank of the Kgl. Versuchsanstalt fur Wasserbau und Schiffbau (Berlin) beginning in September 1914. Today, of course, such tests are taken for granted.
The simple square monoplane tail assembly was strut-braced and a small triangular fin supported a perfectly square rudder. The tail surfaces were balanced by hingeing them some distance behind their leading edges. All surfaces were covered with linen.
In October 1915 the Rs. I emerged from its hangar at Seemoos, the largest aircraft in the world, with a wingspan of over 142 feet. This in itself was a remarkable achievement made even more incredible by the fact that the Rs.I was constructed wholly of metal. The machine at this point was powered by three 240 h.p. Maybach HS (or Mb.IV) engines, two of which were buried in the hull and drove two pusher propellers mounted on the lower wings. The drive system, similar to that used on airships, consisted of the usual outrigger shafts and bevel gear-boxes to transmit power. The third engine, directly driving a pusher propeller, was mounted in the wing gap above the hull. The buried engines were cooled by two radiators fixed to the fuselage decking, while the third radiator was mounted in front of the central engine. The Rs.I taxying trials began on 12 October 1915 with test pilots Hellmuth Hirth and Erich Schroter at the controls. Further tests, conducted on 15 and 16 October, resulted in a taxying speed of 40-50 km.h. This was still some 30-40 km.h. below the calculated lift-off velocity. A sudden end to tests came on 23 October when the port propeller or drive fractured and fragments tore the trailing edge of the upper wing. Dornier himself wrote that the unreliability and complexity of the outrigger drive system, its inability to transmit high forces and the problem of mounting it rigidly, finally forced him to dispense with the buried engine. Another reason, perhaps, for modifying the Rs.I may have been to raise the propellers. Fixed to the lower wings as they were, they may have been badly battered by the heavy spray which the Rs.I was capable of kicking up. In any case, the three Maybach engines were placed between the wings on an integral engine mount consisting of broad streamlined girder struts fixed to the fuselage, completely independent of the wing structure. The engines were connected to each other by a streamlined cat-walk fitted with hand-rails, so that mechanics could move among and service the engines in flight.
Presumably the Rs.I carried out scores of taxying trials, for the inhabitants on the shores of Lake Constance made fun of it with this doggerel:
That is the flying-boat from Seemoos.
From the lake it can't come loose.
They were right. The Rs.I never did fly. On its fifth and final trial it came to grief.
Here is what the Dornier test pilot, Erich Schroter, wrote of that fateful day, 21 December 1915, when the RS.I was destroyed:
Preliminary engine and taxying trials were being carried out when the treacherous Fohn wind, common to the Lake Constance area, made its sudden and unexpected appearance. We immediately made preparations to bring the Rs.I back into the hangar, when the slipway on which the aircraft rested sideways to the wind, jammed. The Rs.I was in great danger of immediate destruction, but we managed to relaunch it. Using all engines to keep the aircraft heading into the violent gusts, we succeeded in mooring the Rs.I to a buoy. It soon became evident that the mooring would part, threatening the Rs.I with destruction on the rocks. Night fell quickly. About 11 p.m. a packet boat ventured near in an attempt to take off the storm-tossed crew, but was unable to do so as it was in danger of foundering. The only chance of saving the Rs.I appeared to be for one of the large lake steamers from Friedrichshafen to tow the aircraft out to the middle of the lake, there to ride out the storm away from the danger of the nearby rocks. However, owing to the extreme conditions, all steamship traffic on Lake Constance had been halted. Those on land saw the giant aircraft buffeted by the waves until the early morning when the moorings broke, and the RS.I was driven on to the waiting rock and there pounded to pieces.
This incident was to cause great consternation among naval engineers and constructors concerning the best means to shelter these fragile aircraft from the elements.
The Rs.I, although it never took to the air, provided a valuable proving ground for new structural techniques, many of which were incorporated in later Dornier designs with much success. As an aircraft, the all-metal Rs.I will always stand high among the great aircraft of the world for its daring concepts, great size and technological pioneering.
Color Scheme and Markings
The black cross Patee superimposed on a square white background was painted on both upper and lower wing surfaces of both wings, and on the rudder surface.
Type: Dornier RS.I
Manufacturer: Zeppelin-Werke Lindau G.M.b.H., Seemoos, Lake Constance
Engines: Three 240 h.p. Maybach HS (Mb.IV) engines
Span upper, 43•5 m. (142 ft. 9 in.)
Chord, 4•6 m. (15 ft. 1 in.)
Span lower, 37•75 m. (123 ft. 10 in.)
Chord, 3•6 m. (11 ft. 10 in.)
Length, 29•0 m. (95 ft. 1 1/2 in.)
Height, 7•2 m. (23 ft. 7 1/2 in.)
Hull length, 27-4 m. (89 ft. 10 1/2 in.)
Hull beam, 3•5 m. (11 ft. 6 in.)
Wings, 328•8 sq. m. (3538 sq. ft.)
Tailplane, 27 sq. m. (291 sq. ft.)
Elevator, 16 sq. 111. (172 sq. ft.)
Rudder, 10 sq. 111. (108 sq. ft.)
Fin, 5•3 sq. m. (57 sq. ft.)
Ailerons, 24•8 sq. 111. (267 sq. ft.)
Empty, 7500 kg. (16,537 lb.)
Loaded, 10,500 kg. (23,153 lb.)
Service Use: None