O.Thetford, P.Gray German Aircraft of the First World War (Putnam)
Zeppelin-Lindau (Dornier) Rs III
Built between April and November 1917, this third flying-boat of Dornier's was a pure monoplane which incorporated the lessons learned from its two precursors. Tail booms were now replaced by a square-section fuselage of steel longitudinals and alloy frames with fabric covering, mounted on top of the wing.
The short, wide, shallow-draught hull supported the two power eggs, in which the tandem-mounted engines were installed, upon a system of stout steel struts. Further struts, springing from the power eggs, supported the massive wing, which was now braced with cables only. Flettner-type servos now assisted aileron operation. A cumbersome biplane tail assembly was featured which had undergone various modifications during development.
Maiden flight of Rs III (No. 1431) was on 21st October 1917, and a delivery flight from Friedrichshafen to Norderney was made in approximately 7 hours on 19th February 1918, where it underwent further trials at that station and also at Warnemunde. The aircraft survived the war, but whether it made any operational sorties has not been established; it is recorded as having made one flight of at least 10 hours' duration.
Engines, four 245 h.p. Maybach Mb IVa. Span, 37 m. (121 ft. 4 7/8 in.). Length, 22.75 m. (74 ft. 7 3/4 in.). Height, 8.2 m. (26 ft. 10 7/8 in.). Area, 226 sq.m. (2,441 sq.ft.). Weights: Empty, 7,200 kg. (15,840 lb.). Loaded, 10,670 kg. (23,474 lb.). Speed, 135 km.hr. (84.35 m.p.h.). Climb, 2,000 m. (6,560 ft.). Duration, 10 hr. Armament, three machine-guns.
G.Haddow, P.Grosz The German Giants (Putnam)
The third flying-boat to emerge from the Dornier works at Seemoos was externally a completely new design having little in common with its forerunners. The reason for the great difference in appearance was, as Dornier pointed out, to incorporate all the lessons learned from the extensive Rs.II test programme. The Dornier Rs.III was indeed an unusual aircraft, yet every line in its frame had purpose and reason.
The most noticeable change was the square fuselage mounted on top of the wings. This singular configuration was chosen for three reasons: (1) to keep the tail surfaces as far above the water line as possible; (2) to facilitate the mounting of armament above the wings (a Navy requirement); and (3) to provide a centre-section structure to support the wing-bracing cables. It had been decided to replace the wing struts by cables in order to save weight. The high-set fuselage could function as, and thereby replace, the usual centre-section pylon.
The fuselage was constructed of four steel longerons riveted to duraluminium frames. The forward portion of the fuselage was covered with duraluminium sheet and the remainder with fabric. A small soundproof wireless cabin was located in the fuselage nose, followed by a large gun position fitted with mountings for two machine-guns. A small ladder situated between the engine mounts provided access to the raised fuselage through a trap-door in the wing.
A simple biplane tail unit was mounted at the rear of the fuselage high above the water. Both fixed tailplanes were attached to the fuselage by struts at a mid-gap position and were fitted with unbalanced elevators. Balanced rudders were mounted to a single hinge-post above and below the fuselage. The large fins originally fitted were later reduced to a small fin surface between the tailplanes.
The broad wing, with the characteristic low aspect ratio of the Dornier R-flying boat, was mounted above the engines on short, robust struts. The 6•5 metre chord and the rib section of the Rs.II were retained, but the span was increased about 4 metres. The wing structure as in the Rs.II consisted of three steel spars and duraluminium ribs. The ailerons were hinged to false spars and actuated by two large faired horns, on top of which small balancing aerofoils were mounted.
The four ungeared 245 h.p. Maybach Mb.IVa engines were mounted in tandem between the hull and the wings on a network of heavy struts. The layout of the Rs.III, with its high fuselage and closely grouped engines, is somewhat reminiscent of the Gotha G.I and UWD seaplane designed by Oskar Ursinus in late 1914. Placing the engines close to the flight axis reduced the effect of an asymmetrical engine-off condition on the flight characteristics. It was no doubt planned to cruise the Rs.III on three engines to save fuel on long patrol flights.
The boat-shaped hull, constructed for the first time entirely of duraluminium, was very broad in order to act as a "stable platform" in the water. This feature made it possible to eliminate the lower stub wing, to which auxiliary floats would have been mounted. The hull was fitted with a nose machine-gun position placed directly in front of the large pilots' cockpit. Another cockpit for the flight mechanic was located in the centre of the hull. From here, the mechanics could regulate the fuel flow from the tanks and climb up to service the engines, if necessary. The mechanic sat between eight 380 litre fuel tanks which were securely fastened to the hull and not suspended by straps as in the Staaken and other R-planes. The fuel was pumped to four 25 litre gravity tanks located in each engine nacelle. The Rs.III design configuration was awarded a German patent (No. 373.171) on 18 December 1917.
Detail design work on the RS.III began in February 1917, and construction started in April coinciding with the receipt of a firm German Navy order for one flying-boat (Navy Number 1431) on 25 April 1917. The work proceeded rapidly, and by 21 June Dornier was able to write to the Navy:
... the construction of the Rs.III will be so far advanced by the end of July that the greater number of our workers will be idle. In order to plan ahead, we ask if we can count on building more R-flying-boats for the Navy. We should like to point out that an order for several R-planes based on the Rs.III would give us time to work on advanced designs in less hectic circumstances than has heretofore been the case; in particular, the 2000 h. p. machine of which project drawings have been prepared.
On 31 October 1917 the Rs.III, built in the comparatively short time of seven months, was launched and performed its first taxying tests. Four days later, flown by Navy pilot Oberflugmaat Weiss, it successfully completed its maiden flight. From the beginning only minor problems were encountered. On 27 November the Dornier firm notified the Navy that: "The RS.III has reached a stage of readiness that, with exception of several modifications, will permit the delivery flight to Norderney to take place in the very near future.'"
The Navy was anxious to test the Rs.III in the North and Baltic Seas, where water and weather conditions were of a different nature than on Lake Constance. Norderney, a small island in the North Sea. was chosen after a Navy commission had flown along the coast to look for a suitable base. Norderney already was one of the most active seaplane stations on the North Sea. A large hangar for the Rs.III was constructed on the base rather than over the water as at Warnemunde. The Navy constructed a large beaching wagon and a turntable to handle the launching and retrieval of the Rs.III.
While the facilities at Norderney were being completed, preparations were made to fly the Rs.III along the route; Lindau-Rottweil, across the Black Forest to the Rhine, along the Rhine to Duisburg-Munster-Emden-Norderney. Because the flight was to be made within the operational radius of Allied aircraft, fighter cover was to be provided from Rottweil to Duisburg. Furthermore, the Flak batteries along the route had to be notified; the weather reports from stations along the route had to be co-ordinated; and a guide aircraft had to be provided from Cologne to Emden to steer the Rs.III clear of the Dutch border. The extensive arrangements for the delivery flight to Norderney were carried out in great secrecy.
In the meantime, the Rs.III was put through extensive tests by company engineers, pilot Schulte-Frohlinde and Navy pilots Weiss and Hammer. Their flight reports stated:
The take-off tests (numbers 1-4) showed that very light forward pressure on the control column was sufficient to push the nose down and in about 5-10 seconds the aircraft was on its hull-step. It then alighted without effort by pulling back lightly on the controls (30-51 seconds). The foregoing illustrates that the Rs.III has absolutely reliable take-off characteristics ... when taking-off against the wind in a calm sea only very slight control movements were required. This means that a large steering reserve is available when taking-off in heavy seas. The take-off characteristics are sufficient with the present hull-steps. A further clarification of the improvement of take-off and sea-keeping qualities is only possible after tests on the North Sea.
Regarding the flight characteristics of the Rs.III, another report states:
As soon as one has become slightly accustomed to the natural oscillations which every R-plane possesses, then the Rs.III is relatively easy to fly. Elevator and rudder controls can be moved hard-over without undue exertion. Similarly, the ailerons are easily actuated up to 40 per cent of their movement. When the load is properly distributed it is possible to fly the aircraft for longer periods of time without using the elevator controls whether with the engines running or in a glide. The flying of sharp turns requires practice. Suddenly throttling the engines will cause the aircraft to pitch up as is the case in all flying-boats.
The Rs.III was flight-tested for three months during which time small modifications were made. By the end of January it was standing-by for the delivery flight, but until 18 February 1918 the flight route was covered with fog. On that day, the weather service reported the first signs of a break in the weather: "Weather situation again stable. Strong gusty winds over the Black Forest and along the coast." Finally, on 19 February, the weather reports were favourable all along the projected route.
The Rs.III was prepared for the flight. The delivery crew, consisting of pilots Triller and Weiss, Schulte-Frohlinde and mechanic Heinzelmann, was weighed, and the take-off weight of the Rs.III was put at 9350 kg. Launching was delayed because an oil drainage plug had pulled out as it was being tightened and fell under the floor of the hangar. Meanwhile a floatplane had flown around the immediate area and signalled that there were no wind gust.
All was now ready, and at 09.30 hours the Rs.III was launched. The Rs.III taxied in front of the easterly wind for 10 minutes, then turned and took-off at 09.40 hours in a south-east direction. The Rs.III climbed through a thin cloud layer at 1000 metres and crossed the Black Forest at 2200 metres. It was calm, very clear and cold. Near Rottweil the fighter escort attached itself to the flight and remained until Duisburg. The Rhine valley was covered with a mist of varying thickness, and the Rs.III climbed or descended according to the visibility. In the vicinity of Mannheim and in the Ruhr valley visibility was very poor, forcing the Rs.III to fly a more westerly course and to descend to 400 metres until Duisburg. Even so the ground was lost from view for a period of time which would have made an emergency landing impossible.
Just past Dusseldorf, the mechanic reported low oil pressure in Engine No.2. This potentially dangerous situation during periods of fluctuating visibility required extreme watchfulness on the part of the crew, and the weather report which had been spread out on the Goltzheimer Heath was forgotten. The mechanic determined that the oil pressure in the oil filter was sufficient to allow the engine to run, and it did so for the remainder of the flight. Upon landing, it was discovered that a cotter pin on the oil pump had broken.
Duisburg was the location of the emergency supply depot previously established for the planned Rs.II delivery flight. It was strategically located at the three-quarter distance just prior to the portion of the trip on which water landing facilities were not available. The Rs.III dropped some airmail over Duisburg and continued on, climbing to 1500 metres for the remainder of the flight. It reached Munster and flew to Emden along the Ems River, and finally landed at Norderney at l6.45 hours, completing the impressive delivery flight in exactly 7 hours. During the flight the engines were throttled back and the average speed was 120 km.h. Fuel for 2 hours was still left in the tanks when the Rs.III landed.
The Rs.III was thoroughly tested under varying sea and wind conditions for a period of four months. The chief pilot in charge of these tests was Fritz Hammer of the SVK. The machine was accepted by the Navy on 13 June 1918, and it was turned over to the SVK on 27 August 1918 for further Navy tests. The following statements are excerpts from the Report of the Sea Trials with Flying Boat Marine No. 1431 at Warnemunde.
Bringing the flying-boat out of the hangar was extraordinarily smooth and easy. The floating pier to which the aircraft was fastened was swung out quickly and safely. Two of the engines were started while the aircraft was still secured to the pier, which permitted the aircraft to cast-off with two engines running. The pier made it possible to communicate between land and aircraft up to the last moment.
The Rs.III could be sailed in a wind velocity of 11-12 m/sec. without using the engines. With the rudders and ailerons fully deflected, the Rs.III can be headed four points from the wind and the machine turned to the side on which the aileron was deflected down. The aircraft can taxi forward using the engines while it is simultaneously blown sideways by the wind. The pilot has complete control by using the throttle for forward motion and the flight controls for sideway motion.
The take-off in Seegang 3-4 (Seegang is a German sea-scale system based on both wind velocity and wave height. Because no direct English comparison has been found, the original German system is retained for clarity) (wave height 3/4-4 metres, wind force 4-5) occurred without any difficulties. The nose of the aircraft was raised steeply as soon as the throttles were opened, so that the sea ran cleanly underneath the hull. The propellers were completely free of spray. The landing was very soft and agreeable with no unusual occurrences.
The rudder and elevator did not come in contact with the water while taxying directly into the wind, in spite of a heavy pitching motion caused by the waves. In its lowest position the rudder was about 1 metre above the water.
With the wind off the beam and with the seas running against the sides of the hull, the Rs.III behaved very well and the motion was agreeable and calm due to the aircraft's low metacentric height.
The Rs.III was easy to fly. After getting accustomed to the rather luggish lateral reaction, the aircraft could be flown by any average pilot. On the other hand, take-off and landing required more practice. Engine accessibility was criticized because the strong propeller wash made the mechanic's task of performing small engine repairs extremely difficult if not impossible. Furthermore, it was found that the centre-section wing fabric would part from the ribs due to the propeller wash. Double fabric layers and stitching solved this problem, and in future aircraft the rib pacing was to be reduced.
In conclusion, the report stated that the Rs.III was released for front-line service under test category Class V to the Navy R-plane Command (on 27 October 1918). Pilots Hammer and Niemeyer performed several 10-12 hour evaluation flights from Norderney (The Report of the Aircraft Section of the Allied Naval Armistice Commission states: ''The primary function of giant seaplanes is long-range reconnaissance, and that seen at Norderney (Rs.III) was intended for use against the Grand Fleet. It was hoped that reconnaissance as far off as Scapa Flow would be made." ) before the end of the war.
The Rs.III was not immediately destroyed after the war but enjoyed a relatively long post-war life. As late as January 1920 it was to be found on a roster of 100 mine-spotting Naval aircraft still operated by the Germans for clearing mines in the North Sea and Baltic Sea. The Rs.III was destroyed by order of the Allies on 30/31 July 1921, at which time the German Government valued it at one million marks.
The Dornier Rs.III was the only giant seaplane to achieve active service with the Navy, even if it was only limited operational duty.
The characteristics of the Dornier flying-boat received favourable comment in a contemporary publication:
The R-seaplane tests proved the superiority of the four-engined monoplane flying-boat over the four-engined twin-float biplane. The Dornier flying-boat was faster and easier to fly, possessed better flight characteristics and had greater in-flight reliability than the Staaken machine. The sea-handling characteristics of the Dornier flying-boat were superior to those of the Staaken float-plane, but which aircraft had better seaworthiness could not be determined before the war's end.
The fact that the Rs.III had such good qualities, that it was built of metal and that it passed its sea trial successfully constituted a notable achievement in those days of wooden aircraft. Its success speaks highly for the technical competence and vision of the men who conceived and built the R .III.
Colour Scheme and Markings
Until the late spring of 1918 the Rs.III carried the cross Patee superimposed on a square white background; these crosses were painted on both surfaces of the wing near the tip, on the upper and lower tailplanes at about two-thirds of the span and on the middle of the fuselage. The white square on the wing only utilized half the wing chord. Later a standard narrow Latin cross, that also only spanned half the chord, was carried on the wings. The Latin cross with a large white border was painted on the tailplanes. A U.S. Navy Technical Note stated that the exterior duraluminium surfaces were painted but that the interior was left bright.
Type: Dornier Rs.III
Manufacturer: Zeppelin-Werke Lindau G.m.b.H., Seemoos, Lake Constance
Engines: Four 245 h.p. Maybach, Mb.IVa engines
Span, 37 m. (121 ft. 4 1/2 in.)
Chord, 6•5 m. (21 ft. 4 in.)
Length, 22•75 m. (74 ft. 8 in.)
Height, 8•2 m. (26 ft. 11 in.)
Tailspan, 8-4 m. (27 ft. 6 1/2 in.)
Chord, 2•15 m. (7 ft.)
Propeller diameter, 3 m. (9 ft. 10 in.)
Propeller centres, 3•3 m. (10 ft. 10 in.)
Hull length, 12•57 m. (41 ft. 3 in.)
Hull beam, 4•7 m. (15 ft. 5 in.)
Wings, 226 sq. m. (2432 sq. ft.)
Tailplane, 23•6 sq. m. (254 sq. ft.)
Elevator, 10 sq. m. (108 sq. ft.)
Fin, 1•4 sq. m. (15 sq. ft.)
Rudder, 4-4 sq. m. (47 sq. ft.)
Ailerons, 21 sq. m. (226 sq. ft.)
Empty, 7865 kg. (17,339 lb.)
Fuel, 2260 kg. (4982 lb.)
Disposable load, 545 kg. (1202 lb.)
Loaded, 10,670 kg. (23,523 lb.)
Hull, 1580 kg. (3484 lb.)
Wing loading: 46•9 kg./sq. m. (9,6 lb./sq. ft.)
Maximum speed, 135 km.h. (83'9 m.p.h.)
1000 m. (3281 ft.) in 15•5 mins.
2000 m. (6562 ft.) in 35 mins.
Duration, 10 hrs.
(Note: The duration could be extended to about 12 hours by flying on three engines provided the total weight had been reduced by fuel consumption.)
3140 litres (691 Imp. Gals.)
Oil, 220 litres (48-4 Imp. Gals.)
Armament: Provision for one machine-gun in the nose and two machine-guns in top of the fuselage
Service Use: Norderney, August to November 1918