O.Thetford, P.Gray German Aircraft of the First World War (Putnam)
Zeppelin-Lindau (Dornier) Rs II
Dornier's Rs II differed considerably from his first machine. Construction began in December 1915, and the first flight was made on 30th June 1918. The lower wings were now no more than rudimentary, and this flying-boat was, in effect, a monoplane. The shortening of these wings obviated the tendency for the tips to "dig in", as had been experienced with the Rs I when anything like a swell was running. It had been found, after initial trials, that the widened hull of the Rs II had ample lateral buoyancy of itself. The hull, which was mainly metal skinned except for some of the decking, which was fabric, was also drastically shortened and the tail surfaces carried on tubular steel outriggers. Initially these were of biplane format, but were later modified to monoplane surfaces with twin fins and rudder, both above and below the tailplane.
Engine arrangements were originally as in the first machine, but later power was increased to four engines, and these were mounted in tandem pairs in two power-eggs between the wing and fuselage. Two massive pairs of steel vee struts on each side braced the fabric-covered wing, with its horn-balance ailerons, to the top longeron.
After its trial programme was completed this flying-boat (No. 1433) was dismantled in order that resources might be devoted to the Rs III.
Engines, three 240 h.p. Maybach Mb IV. Span, 33.2 m. (108 ft. 11 1/4 in.). Length, 23.88 m. (78 ft. 4 1/4 in.). Area, 257 sq.m. (2,776 sq.ft.). Weights: Empty, 7,100 kg. (15,620 lb.). Loaded, 9,300 kg. (20,460 lb.).
After modification: Engines, four 240 h.p. Maybach Mb IV. Span. 33.2 m. (108 ft. 11 1/4 in.). Length, 21.7 m. (71 ft. 2 3/8 in.). Area, 257 sq.m. (2,776 sq.ft.). Weights: Empty, 7,180 kg. (15,796 lb.). Loaded, 10,000 kg. (22,000 lb.). Speed, 130 km.hr. (81.25 m.p.h.).
G.Haddow, P.Grosz The German Giants (Putnam)
Spurred on by the sudden loss of the Rs.I, Dornier and his staff worked vigorously to complete their second flying-boat, the Dornier Rs.II. The design and construction drawings for the aircraft had been prepared during 1915, and the relatively short construction time suggests that portions of the Rs.II had already been completed prior to the destruction of the Rs.I. Although built of the same materials, steel and aluminium, the Rs.II design had little else in common with its predecessor. The most important change was the broad hull, which was the first step towards Dornier's distinctive and practical inherently-stable hull designs. Other changes included a broad, low-aspect-ratio upper wing, an open tail boom structure and an unusual tail configuration.
It is interesting to speculate on the reasons why Dornier continued with the buried-engine system after it had failed in the Rs.I. It is surmised that the transmission had been completed and perhaps even bench-tested and that consideration of expense and expediency warranted giving the buried-engine system another chance. However. Dornier was not satisfied with remotely-driven propeller systems, and he began a series of experiments in early 1916 to thoroughly investigate the feasibility of other propeller and engine arrangements. The first experiment, an examination of the efficiency of the tandem engine lay-out, was conducted by Dornier's long-time associate, Dipl.-Ing. Schulte Frohlinde. In the test the engine were mounted back to back on a large test stand and the thrust of individual tractor and pusher propeller was compared with the thrust of the propellers running in tandem. The tandem arrangement, contrary to expectations, was found to have a negligible thrust loss, and this fact had important consequences for the R .II and many subsequent Dornier types. This system was used on many Dornier aircraft throughout the Thirties until the end of World War II.
Initially, the Rs.II was powered by three 240 h.p. Maybach HS (or Mb.IV) engines mounted within the hull and driving three pusher propellers supported between the upper wing and hull on a lattice-girder framework. The engine radiators were mounted as a wide slab on top of the hull behind the pilot's cockpit.
The upper wing had a very low aspect ratio of 5:1 in order to distribute the great weight of the aircraft over a small beam length. The wing was mounted above the hull by two great N strut at the centre section. The bottom apex of the N was a pivot point from which ran two V struts that supported the outer wing structure. As with the Rs.I, the length of the forward leg of the N could be changed to vary the angle of incidence of the wings. The small lower wings were used, in Dornier's words, primarily to support wingtip floats, but initial flotation tests proved these floats unnecessary and they were removed. This marked the origin of the Dornier inherently stable flying-boat hull, which was destined to be a Dornier trade-mark for many years. The small stub wings were retained for the amount of extra lift they provided, but they were discarded on succeeding flying-boat designs.
Three built-up girder spars of triangular section formed the primary wing structure, which included the Dornier practice of aluminium ribs spaced fairly wide apart. The wing fabric was sewn to special eyelets attached to the framework at evenly spaced intervals. This method of holding down fabric proved very successful and later found application in airships. The ailerons were unbalanced. To improve lateral control at near stalling speeds the tip incidence was "washed-out".
The crew was situated near the bow in a large open cockpit protected by a raised streamlined coaming. The central and rear hull section contained the buried engines and fuel tank. The hull was built of steel bulkheads and stringers. The sides and bottom of the hull were covered with duraluminium, whereas fabric was used for portions of the decking. The tail was supported by an uncovered box framework composed of four tubular booms joined by a series of vertical and horizontal struts reinforced by diagonal cables. The two lower booms ran directly into the end of the hull, while the upper booms were attached to a combination lattice-girder V-strut structure. The lattice girders continued the boom line and were anchored deep in the hull. The tail was left uncovered to avoid buffeting from the water pray, which Rs.I taxying trails had shown to be unpleasantly high and heavy. The tail assembly consisted of a small fixed tailplane mounted to the upper booms. Two rudders, which were rather small for an aircraft this size, were hinged between the booms below the tailplanes. An all-flying biplane cellule was awkwardly hinged behind the rudders and took the place of elevators. The tail assembly does not seem to have been thoroughly engineered, and its appearance suggests that structural simplicity was forfeited for questionable control advantages.
Such was the appearance of the Dornier Rs.II when it left the Seemoos hangar for its taxying trials on 17 May 1916. Test pilot Schroter recalls the event:
Graf Zeppelin was present to observe the launching. With him aboard his motorboat Wurttemberg were Dornier, Oberingenieur Durr, Generaldirektor Colsman. Direktor Winz and Dr. Eckener. At my request Ing. Schulte-Frohlinde occupied the co-pilot's position. With the engines throttled back, I taxied to the Swiss side of the lake, there we turned and, heading into a mild breeze, I gave full throttle. The Rs.II sped forward, but the speed obtained was insufficient to raise the hull on to its step in such placid water conditions. Later the Wurttemberg attempted to break up the water surface for us, but it was useless and the Rs.II was brought back to the hangar.
A total of six water-borne tests were conducted during which it became necessary to increase the rudder area to improve manoeuvrability. This was done by adding a tall third rudder between the tail booms and controlled by cables attached to the outer two rudders. The Rs.II was now ready for its maiden fight.
On the next attempt (30 June 1916) we had a sufficient wind. Aboard, besides myself, were Schulte-Frohlinde, Dr.-Ing. Adolf Rohrbach, Ing. Lindner, and four engine mechanics. After manoeuvering into position, I opened the throttles and we sped over the lake. I gave a slight tug on the controls, the Rs.II raised on its step. The wind had helped. With precise motion, I gently pulled the wheel toward my chest. Now a springy jump from wave to wave. The jolts ceased; we were in the air. How easy it all was, almost as if I had a fully-proven aircraft in my hands.
Carefully I prepared to land. With throttled engines, the Rs.II willingly lowered its nose and glided to a landing. A sudden splash, a lurch forward and we came to a halt.
Triller, a Dornier mechanic who also was aboard the Rs.II on its first fight, described the event somewhat differently:
On 30 June 1916 the Rs.II was ready for its maiden fight. Aboard were Schroter (pilot), Schulte-Frohlinde and myself. The aircraft at the time weighed 7045 kg. The first two takeoff attempts were made with the angle of incidence set at one degree and two degrees respectively. On the third attempt with the angle of incidence increased to three degrees, the Rs.II rose into the air on its maiden flight at 07.30 hours. Two more flights were made that day.
Three further flight proved the need for further modification. The lattice-girders that held the upper booms were replaced by large-diameter metal tubes to give the tail structure greater rigidity and strength. Also, the area of the central rudder was reduced and fixed fins fitted between the boom ends to improve seaworthiness, changes were made to the hull's planing surface.
On 17 July 1916 the modified Rs.II was ready to continue its test programme. These tests were not entirely satisfactory. Various incidence settings were tried to improve take-off qualities. In fight the aircraft reacted well to the elevator and rudder but was sluggish in answering to the ailerons. With the starboard engines throttled back no sinking or stalling was observed, even so, in this condition full rudder was insufficient to maintain straight flight. Trouble was experienced with the port transmission, which vibrated regularly, and it was the failure of this that put an end to the first part of the Rs.II story.
At 10 to 12 metres altitude after 1/2 minute fight duration, the port propeller broke. Mr. Schroter quickly cut the throttle and we found ourselves at once on the water. Since the hull had remained water tight we were in no danger.
About the incident, Schroter wrote later:
We had drifted and took off again when the middle propeller transmission broke loose and severely damaged the tail boom. We were lucky to land safely in a confusion of torn and bent struts and loose wires.
It was decided to completely rebuild the Rs.II, and the work, which was started in July, was finished on 5 November 1916. On the next day Bruno Schroter, by now the most experienced giant sea plane test pilot, flew the new Rs.II on its maiden flight. The rebuilt RS.II was virtually a new aircraft, now powered by four engines mounted above the fuselage in a tandem configuration, which Schulte-Frohlinde had thoroughly tested and proven. The propellers were driven through reduction gears and the engines were initially left uncowled. The changed engine location required altering the upper tail boom attachment structure to avoid the propeller arcs. Therefore, the booms were attached to two corners of a triangular tube frame, the third corner of which was joined to a single tube fixed to the centre decking of the hull. The unusual biplane elevator cellule was retained, but a broad fin and rudder of greatly increased area were fitted to improve manoeuvrability on the water. For the same reason, the hull section was refined, and the step moved farther aft to reduce the take-off run, which in spite of these changes still remained too long.
The wing gap was decreased and the wing struts now met the hull about halfway down the sides instead of on top. The only change in the upper wing was to fit aerodynamic balancing surfaces to the ailerons. The lower stub wings were entirely reconstructed with rounded tips and decreased chord. They were placed well aft on the hull and given pronounced dihedral to prevent them from dipping into the water. Tests proved that the Rs.II had adequate stability in wind force 5, although the bottom wing did come in contact with the water under these extreme conditions.
The old experimental Maybach motors in the Rs.II gave no end of trouble during the test programme, much to the irritation of the naval test pilot Lt. Tille. He complained about the galling of pistons and leaking cylinder-heads. The cause of the latter mishap was attributed to the forced slipstream cooling of the uncowled Maybach engines, rather a paradox, for generally the great problem of the Maybach engines was overheating. However, when the engines were cowled and tested again the complaint voiced was that the radiators were too small. In addition, raw material shortages were making themselves felt; spark plugs burned through and valve seats and parts failed regularly, seriously retarding the flight programme.
Lt. Tille wrote the following report about the Rs.II after he had flown the machine in November 1916:
The flight characteristics, with the exception of a few minor points, are thoroughly satisfactory. The seaworthiness is satisfactory, with exception of the take-off and landing phases. A slight widening of the hull to increase lateral stability is recommended. To insure adequate take-off capability, a complete reconstruction of the hull is absolutely necessary. And last but not least the effect of salt water on duraluminium must be exhaustively investigated.
Lt. Tille was convinced of the future of the R-flying boat when he concluded by saying: "A new flying-boat, built using lessons learned and to be learned, promises the beginning of a development cycle which will extend the flying-boat far beyond the borders of present-day performance characteristics." A prophetic statement indeed, for a flying-boat was, in a few years time, to be the first heavier-than-air machine to cross the Atlantic. On the other hand, with the same optimism shown by Army staff officers at the time, Admiral von Capelle, Commander of the German High Seas Fleet, prematurely expected the arrival of the Rs.II on the coast and he wrote on 28 December 1916: "It is believed an R-flying-boat now under test on Lake Constance will soon come to the coast for further tests. It will then be seen how the R-seaplane can be developed and in what direction." As it turned out, the hopes of the Naval High Command were never fulfilled, for only a single R-seaplane was placed in limited operational service during the war.
Dornier and his staff began to work on their third giant flying-boat, using much of the valuable data gathered from the Rs.II test programme. During May 1917, while practising landings, the Rs.II came down hard on the water, breaking the central boom support. The pilot did not notice that the tail hung by only four cables and attempted to take-off again. The sagging tail forced the Rs.II back on to the water, causing the cables to snap. The lower booms broke and the tail sank to the bottom of Lake Constance. But the hull remained absolutely water-tight during these severe landings and proved to Dornier the soundness of its construction.
By July 1917 the cumbersome tail unit had been replaced by a cleaner design consisting of a single tailplane with finely-formed fins and rudders mounted at the end of each boom. The new boom structure was much stronger and of simpler design. At a later date the wingspan was slightly lengthened to increase lift, the effect being to enclose the ailerons in the wingtip outline.
Up to the summer of 1917 the RS.II carried out a large number of experimental flights. In the period 23-26 June 1917 it was thoroughly evaluated by the SVK (Seeflugzeugs-Versuchs-Kommando - Seaplane Testing Command). During these, the Rs.II was used to investigate the effect of various engine-off configurations on flight performance. For instance, on Lake Constance it was found possible to take-off and slowly climb with one forward engine off. It was also definitely proven that the efficiency of the pusher propeller was higher when the forward engine was shut off than the efficiency of the tractor propeller with the rear engine off.
The Rs.II was easy to manoeuvre on the water, and took-off after a 20-30 second run, but in horizontal flight proved to be very tail heavy. Nevertheless, the flight characteristics were considered quite sufficient for long-range reconnaissance missions. The maximum level speed was remarkable, 128 km.h. had been attained over Lake Constance.
By August 1917 preparations were completed to deliver the Rs.II to the Naval seaplane base at Norderney for seaworthiness tests on the North Sea. An emergency supply depot with tools, spares and fuel had been established at Duisberg on the Rhine in case of a forced landing. Essentially, all that remained was for the Rs.II to complete a 6-hour simulated delivery flight over Lake Constance as required by Naval authorities. Plagued by minor but recurring engine failures (especially the valves) and the increasingly deteriorating quality of aviation fuel, the programme dragged on. The fuel was so poor that engine revolutions were often reduced some 50 to 70 r.p.m., which prevented the Rs.II from achieving optimum delivery conditions. With much difficulty sufficient high-grade fuel was collected, and sometime in August 1917 the Rs.II finally took-off on its practice delivery flight. It had been in the air about 2 hours when, at 1100 metres, number 4 engine backfired violently. As Navy mechanic Blume went to investigate he was thrown into a corner of the hull by flying propeller splinters. Engines number 4 and number 1 (which had completely lost its propeller) were shut off and the Rs.II descended safely in a slow glide.
Because of the extensive damage suffered by engines, hull and wings from the volley of propeller fragments, the Dornier company recommended scrapping the Rs.II. Its parts were to be systematically tested and results applied to the design and calculation of new R-planes.
Although built before the Rs.III, the Rs.II was not placed on Naval status until late April 1917, some months after the Rs.III had been ordered. Consequently the Rs.II received a higher Naval number (1433) than the Rs.III (1431).
The Dornier Rs.II never did achieve the role intended for it: service tests on the North Sea. Even so, it was an extremely useful test bed. Valuable lessons learned from all its modifications were to have a great influence on Dornier's future work. The broad hull, the tandem engines, the sophisticated metal construction and the adaptability of design became Dornier hallmarks.
Colour Scheme and Markings
The national markings of the Dornier Rs.II consisted of the Patee cross on a square white background. The wingtip markings were rather small, occupying less than a third of the available wing chord. Although the earlier four-engined version had crosses on the tail, no tail markings were carried on the final version of R .II.
Type: Dornier Rs.II (first version) Dornier RS.II (second version)
Manufacturer: Zeppelin-Werke Lindau G.m.b.H., Seemoos, Lake Constance
Engines: Three 240 h.p. Maybach HS (or Mb.IV) engines Four 245 h.p. Maybach Mb.IVa engines
Span upper, 33•2 m. (108 ft. 11 in.) 33•2 m. (108 ft. 11 in.)
Chord, 6•5 m. (21 ft. 3 1/2 in.) 6•5 m. (21 ft. 3 1/2 in.)
Span lower, 15•96 m. (52 ft. 4 in.) 16•0 m. (52 ft. 6 in.)
Chord, 3•58 m. (11 ft. 9 in.) 3•58 m. (11 ft. 6 1/2 in.)
Length, 23•88 m. (78 ft. 4 in.) 23• 8 m. (78 ft. 4 1/4 in.)
Height, - 7•6 m. (24 ft. 11 1/2 in.)
Hull length, - 11•8 m. (38 ft. 8 1/2 in.)
Beam, - 4•15 m. (13 ft. 7 1/4 in.)
Hull height, - 2•15 m. (7 ft. 4 in.)
Wing, 257 sq. m. (2765 sq. ft.) -
Wing upper, - 234•25 sq. m. (2521 sq. n.)
Wing lower, - 41•75 sq. m. (443 sq. ft.)
Tailplane, - 18•8 sq. m. (202 sq. ft.)
Elevators. - 8•1 sq. m. (87 sq. n.)
Rudder, - 11•60 sq. m. (125 sq. ft.)
Ailerons, - 14 q. m. (151 sq. ft.)
Empty, 6475 kg. (14.275 lb.) 7278 kg. (16.045 lb.)
Loaded, 7323 kg. (16,144 lb.) 9158 kg. (20,190 lb.)
Wing Loading: - 33 kg./sq. m. (6'76 lb. sq. ft.)
Maximum speed, 100-105 km.h. (62-65 m.p.h.) Maximum speed. 128 km.h. (79'5 m.p.h.)
Climb, - 2500 m. (8202 ft.) in 50 mins.
Fuel: 2000 litres (440 Imp. Gal.) 200 litres (440 Imp. Gals)
Service Use: None