O.Thetford, P.Gray German Aircraft of the First World War (Putnam)
Two "Giant" machines were ordered from A.E.G. during 1916, R I 21/16 and R. I 22/16. The four engines were placed together inside the fuselage, the airscrews being driven through gear boxes and transmission shafts. Initially, four-bladed airscrews were fitted, but later a two-blade type was substituted. First flights of the R I 21/16 took place during 1916 and were considered to be satisfactory. Later, during flight trials on 3rd September, a propeller disintegrated due to the laminating glue not having been allowed sufficient time to harden thoroughly. As a consequence the transmission shaft tore loose and smashed a centre-section strut, which caused the machine to crash with a loss of seven lives. R I 22/16 was still incomplete at the end of the war and was eventually scrapped. Engines, four 260 h.p. Mercedes D IVa driving two airscrews through transmission geared down to 750 r.p.m. Span, 36.0 m. (118 ft. 1 1/2 in.). Length, 19.5 m. (63 ft. 11 1/2 in.). Weights: Empty, 9,000 kg. (19,845 lb.). Loaded, 12,500 kg. (28,003 lb.).
G.Haddow, P.Grosz The German Giants (Putnam)
By virtue of the experience gained building twin-engined bombers, the Allgemeine Elektrizitats Gesellschaft (AEG) was included in the ambitious R-plane programme of 1916 along with SSW, Schutte-Lanz, Albatros, Aviatik and others. At the time, two giant bombers were ordered, designated R.I 21/16 and 22/16 and followed by an order for an additional six R.I machines numbered R.59 to R.64. Only one aircraft, the R.21, was flown before the end of hostilities. The R.I was born of much engineering skill and technical know-how as represented by AEG's staff of experienced engineers. Oberleutnant Bruckmann, former test pilot of the DFW R.I, was in charge of the venture established by AEG on 1 January 1917. Ing. Sander, formerly with DFW and SSW, was chief engineer, and his assistants were Dipl.-Ing. Werner Zorn, who also had been with DFW, and consultant Prof. Oesterlein of the Technische Hochschule, Brunswick. Design of the R.21 began in a rented eight-room flat in Berlin. An assembly shed was erected next to the Rumpler factory in Johannisthal. In the autumn of 1917 the design bureau moved there to supervise the construction of the R.21.
The R.I's long embryonic period from early 1917 until its first flight on 14 June 1918 reflected careful design, attention to detail and many hours of extensive bench tests. No effort was spared to incorporate lessons learned from the earlier giants, and many improvements and innovations were tried. Particularly noteworthy were the electrically-operated tailplane trim controls, the all-steel fuselage and the mixed steel and duraluminium wings. One of the most obstinate problems, which, perhaps, was never really solved, was engine misalignment and transmission vibration in centrally-powered aircraft. To mitigate this effect the R.I was literally built around a massive reinforced engine mount which supported fuselage, wings, landing gear and engines.
The four 260 h.p. Mercedes D.IVa engines were each linked to the drive system through a combination leather cone and dog clutch, while a fifth clutch connected each bank of engines. The airscrews were driven by two heavy transmission shafts running from a central gear-box to the propeller gear-box mounted between the wings. Articulated, sliding and universal joints were fitted to all connections to compensate for play and misalignment.
All four engines were started with a single Bosch inertia starter by first starting one engine and then "clutch-starting" the remainder. The exhaust gases were led into two large manifolds that were mounted in an indentation running along each side of the engine-room, permitting the exhaust system to operate in the airstream. Originally each engine had its own radiator bolted to the fuselage side, but later these were replaced by two large radiators composed of four separate units mounted on the centre-section struts. For a time, the AEG R.I was driven by four-bladed propellers, but these were soon replaced by two-bladed ones.
Cable-braced steel tubing was used exclusively on the frame-work of the fuselage. It was covered from the nose to the rear of the engine-room with plywood, the remainder was fabric-covered. The layout was typical of central-powered R-planes, with the exception that the pilots' cabin was situated aft of engines and wings. From this position it was considered easier for the pilot to judge his height during the critical touch-down phase, but this configuration, although tried in other R-planes, did not find favour with the crews.
The observer's cabin in the nose could be reached from the ground by a retractable ladder or through the engine-room directly aft. The cabin was fitted with glass windows to allow unobstructed vision to all sides. A spacious machine-gun post capable of holding two men was directly above the observer's cabin. The pilots' position was reached by stepping aft from the observer's cabin through a sliding door, passing through the engine-room (which was situated directly above the landing gear) between the engines on a small catwalk, and then climbing a small ladder to the cockpit. The cockpit seated two pilots and was fully-equipped with standard instruments, dual controls and the like. An engine telegraph similar in operation to those aboard ship, consisting of repeating pointers, provided communication between pilots and engine crew. The wireless cabin was located a few feet behind the cockpit, beneath the dorsal machine-gun post. The large windows on each side of the cabin indicated that beam machine-guns may have been considered. A ventral gun position was located in the floor.
The first R.I version had a curious semi-enclosed cabin which was later modified to a simple open cockpit for better visibility, and the enclosed dorsal gun position was replaced by a standard open gun ring. After modifying the cockpit, the fuselage had its greatest dimensions at this point, a depth of approximately 10 feet and a width of 6 feet.
The wing structure consisted of two chrome-nickel steel tube spars supporting duraluminium wing ribs, cable-braced throughout. Later AEG projects were to use rectangular aluminium girder spars designed and fabricated by Zeppelin-Werke, Lindau (Dornier), pioneers in this method of construction. The three-bay biplane wing structure was of equal span and chord. A curious feature was that the angle of incidence was washed-out towards the tips, giving the wings an unusual twisted appearance. This practice was fairly common on German aircraft of the period and was supposed to improve lateral control.
The controls of the R.I were unconventional, and it is a pity that only scant information is available. The elevator, for instance, was a forerunner of the all-flying tail of supersonic jets today. It was mounted some 5 feet over the tailplane, which itself could be electrically trimmed plus or minus several degrees. Two rudders were mounted between the elevator and tail plane out in the slipstream. Originally the gap between the two horizontal surfaces had been smaller, but after initial flight tests it was modified to improve control by adding more area to fin and rudders. The ailerons, rather than forming an integral part of the wing, were hinged to the outer two rear wing struts at a position midway between the wings. Like the high-set slab elevator, the aileron position was an innovation to avoid loss of control at critical angles of attack, particularly while landing.
The main landing wheels consisted of two wide-spoked hubs, each fitted with three tyres. Attached to each end of the main axle by a ball joint was an auxiliary landing gear, a kind of safety device that came into action only during a hard landing or sudden lurch that might throw the aircraft to one side.
The engine were ground tested on 23 and 30 May 1918. One engine with a broken connecting rod had to be replaced. On it first flight on 14 June 1918 the R.21, carrying a useful load of 1190 kg., flew for 27 minutes and climbed to 100 metres in a little under 10 minutes. The maiden flight was not without difficulties. With the engines throttled back the aircraft proved to be extremely tail heavy, even with full-down elevator the nose continued to rise. Only by shifting all available weight to the front was a safe landing made possible. After this initial flight the R.21 was modified as already described. It was hoped to have the work completed by 15 July 1918.
After a number of test flights it was determined that the propeller were not suitable. Additional sections were glued on at the propeller factory. Dipl.-Ing. Zorn warned Bruckmann that at least ten days would be required for the glue to set, but after only four days on 3 September 1918 Bruckmann ordered the propellers fitted to complete the urgent test programme. About an hour after the start a propeller flew apart, causing the cardan shaft to tear loose and shatter the centre-section structure. The R.21 broke up in the air over Rudow, killing seven men, including Bruckmann, Lt. Otto Reichardt (formerly commander of the R.13) and Lt. Dr. Oskar Wittenstein, both pre-war pilots attached to the Kommando-Riesenflugzeug-Abteilungen. Max Fiedler, who was flying escort in a Rumpler C.I, remembers the accident as follows:
I recall being slightly over the R.21 at about 6000 feet, when Bruckmann celebrating the event waved a cognac bottle as the R.21 swung into a left and then a right bank. There was a flutter and suddenly the wings folded back. It was a frightful sight.
Work on the partially completed R.22 was immediately halted until the cause of the catastrophe could be determined and fully evaluated by Idflieg.
An Idflieg document of 15 March 1918 listed these projected delivery dates for the AEG R.I aircraft: R.21 in April, R.22 and R.59 in June, R.60 in July, R.61 in September, R.62 in October 1918. The delivery dates of the R.63 and R.64 were not listed. In spite of this optimistic schedule, only the R.21 as completed. The R.22 (which was subject of discussions between Idflieg and AEG concerning an increase in wing area as late as 5 November 1918) was partially completed at the end of the war, but not enough to prevent it from being scrapped. Only portions of the R.59 to 64 series were begun, and after the war three R.I fuselage nose sections were photographed in the AEG scrap yard.
Like SSW, AEG possessed enough resources to weather the post-war period, and it is today among the largest of German electrical concerns.
Colour Scheme and Markings
The overall finish was the standard printed camouflage pattern. The design consisted of a repeated pattern of irregular polygons of five or six dull colours, such as grey, mauve, sage green, purple, blue and black. The actual colours and patterns varied with the sources of supply of the different aircraft manufacturers, but were all required to conform to a general standard set by Idflieg. After covering, the fabric was clear doped so that the printed pattern showed through. Latin crosses were painted on the wingtips, fuselage sides and rudders. The first version of the R.I had the early 1918 form of the Latin cross with short, broad arms, but the usual wide white outline was reduced to a thin white band. The final version of the R.I carried the standard Latin cross in use at the end of the war.
Type: AEG R.I
Manufacturer: Aligellleine Elektrizitats Gesellschaft, Henningsdorf near Berlin
Engines: Four 260 h.p. Mercedes D.IVa engines
Propeller Revolutions: 750 r.p.m.
Dimensions: Span, 36 m. (118 ft. 1 1/2 in)
Chord, 3•8 m. (12 ft. 6 in.)
Gap, 4•5 to 4 m. (14 ft. 9 in. to 13 ft. 1 1/2 in.)
Incidence, 5 degrees
Length, 19•5 m. (63 ft. 11 1/2 in.)
Height, 6•35 m. (20 ft. 10 in.)
Tail gap, 1•5 m. (4 ft. 11 in.)
Wheel diameter, 1•3 m. (4 ft. 3 in.)
Propeller diallleter, 5•2 m. (17 ft.)
Propeller centres, 7•82 m. (25 ft. 8 in.)
Propeller pitch, 3 m. (9 ft. 10 in.)
Wings, 260 sq. m. (2798 sq. ft.)
Ailerons, 4-4 sq. m. (47 sq. ft.)
Tailplane, 14•0 sq. m. (151 sq. ft.)
Elevator, 8•5 sq. m. (92 sq. ft.)
Rudders, 2•2 sq. m. (24 sq. ft.)
Empty, 9000 kg. (19,845 lb.)
Useful load, 3700 kg. (8158 lb.)
Loaded, 12,700 kg. (28,003 lb.)
Wing Loading: 49 kg./sq. m. (10-1 lb./sq. ft.)
Performance: Not known
Fuel: 2750 litres (605 Imp. Gals.)
Armament: Provision for five machine-guns
Service Use: None