It was a remarkable aircraft - huge, all-metal, with an enclosed pilots' cabin, seats for eighteen passengers, complete with a luggage compartment, kitchen and toilet. And it was built as early as in 1920!
The all-metal Zeppelin-Staaken E.4/20 four-engined monoplane, the last creation of the dying R-plane industry, was visible proof of the progress which the Germans had made in the design and construction of large all-metal aircraft during the war years. Standing on its own merit, the Staaken E.4/20 was truly the world's first modern transport aircraft, and its functional lines belie the fact that it was built as early as 1919-20.
The construction of the E.4/20 was begun in May 1919 at the direction of Alfred Colsman, general manager of the Zeppelin combine; one of the stipulations was that the fully-loaded aircraft should be able to remain airborne on the two engines. It was planned to place the E.4/20 in service as a commercial passenger transport between Friedrichshafen and Berlin. There can be no question that the design of the E.4/20 owed much to the all-metal monoplane bombers which Staaken was developing under Idflieg contract in 1918. Although strictly a transport aircraft, the E.4/20 could not have differed appreciably in layout from these projects. With an additional engine mounted in the nose, the E.4/20 could well have been the Staaken five-engined bomber design described by von Bentivegni in 1919.
As related previously, Dr. Adolf Rohrbach, an engineer who had been very active in the development of metal construction techniques at Dornier, was transferred to Staaken in 1916-1917 to apply his talents to all-metal R-landplane design. Dr. Rohrbach had been hand-picked by the head of the Staaken R-plane Construction Department, Prof. Baumann, to be his successor, and this he became in 1919, when he assumed technical leadership of the Staaken organization.
Viewed in relationship to the wood and fabric civil aircraft of the immediate post-war era, the E.4/20 was indeed a pioneering achievement of the first magnitude. With the exception of refinements in detail, the construction techniques were very similar to those common today. The airframe was built wholly from specially formed duraluminium profiles riveted together and covered with a thin-riveted duraluminium skin. This construction method was simply an extension of the process devised by Dornier which found expression in a very modern airframe.
The most striking feature was the large cantilever wing, which had a wingspan of 31.0 metres (101'7 feet) and featured a thick wing section based on the aerodynamic investigations of Professor Junkers. The wing was built around a single riveted box-spar of very large dimensions; its width constituted one-third of the wing chord, and at the wing roots its depth was 1.5 metres (4'9 feet). The leading and trailing edge wing sections and engine bearers were attached to this massive central spar. The leading-edge skin was formed of 4 mm. thick duraluminium sheet, the box-spar covering 3 mm. thick and the trailing edge 2 mm. thick, while the remainder of the wing was covered with fabric. The box-spar was pierced by an oval opening which permitted a mechanic to crawl behind the engines, although it would seem difficult to perform extensive in-flights repairs in the small space provided.
The wing loading-extremely high for its day-was 80 kg./sq.m., a value almost double that of a comparable monoplane, the Dornier Rs.IV (46.5 kg./sq.m.). Because practical experience with such high wing loads was not available, it was deemed prudent to provide supplementary wing-bracing cables primarily as a safety feature. However, the wing structure had been calculated to withstand the dynamic flight loads without any additional supporting cables or struts.
The four 245 h.p. Maybach Mb.IVa engines were mounted on a robust framework extending from the central box-spar, and their outline was neatly cowled into the wing. The engines were placed ahead of the leading edge in a manner shown feasible by tests conducted at the Dornier works in 1917. Fuel was contained in two large tanks placed in the leading edge between the engines and four smaller tanks situated in the trailing-edge wing section.
The fuselage consisted of fourteen rectangular frames attached at each corner to four angle-shaped longerons running the whole length of the fuselage. The duraluminium skin riveted to the fuselage framework provided the required stiffness in the absence of diagonal bracing between adjacent frames. In the rear of the fuselage light metal traps riveted across the corners of each frame gave torsional stiffness. The two frames which supported the wing were solidly constructed of riveted box-sections. To prevent buckling and resonance vibrations of the skin, light reinforcing stringers were riveted to the inside of the skin, giving an appearance of extra longerons.
The open pilots' cockpit was located atop the fuselage slightly forward of the wing leading edge and commanded an excellent view. At a later date the nose decking was lowered and a fully-enclosed pilots' cabin with Cellon windows was installed. A door in the nose allowed passengers to enter the fuselage, which could accommodate twelve to eighteen persons seated two abreast. A large mail compartment, toilet, separate wash-room and luggage space were located in the rear of the fuselage.
The tail assembly consisted of a fixed tailplane and fin of cantilever construction covered with duraluminium sheet. The elevator and rudder were aerodynamically-balanced and covered with fabric. The ailerons, of very small chord, were fully balanced and hinged to the ends of the wing spar. The twin wheels were supported by a very simple undercarriage made up of one heavy strut, which contained the shock absorber and was attached to the wing spar between the engines. Two horizontal struts were affixed to the lower corner of the fuselage and converged at the axle.
While the aircraft was under construction the Inter-Allied Control Commission inspected the framework and permitted Staaken to complete the E.4/20 in spite of the prohibition clauses in the peace treaty. The Commission felt that they could not pass judgement on the aircraft’s military potential until it was completed and had performed flight tests. Naturally, Allied engineers attached to the Commission followed the assembly progress with a high degree of interest.
The E.4/20, completed 30 September 1920, performed a number of exceedingly promising flights with test pilot Carl Kuring at the controls. During one flight made under optimum conditions the aircraft was clocked at 225 km/h, an unheard-of speed for an aircraft of that size. It was found, however, that it was not possible to maintain flight on two engine as had been hoped. Further test flights were prohibited by the Inter-Allied Control Commission, which was not convinced that the E.4/20 did indeed have military potential and as such violated the treaty stipulation. Director Colsman wrote that the Commission also refused the E.4/20 to be sold or given away, leaving Staaken no recourse but to scrap the machine. It was Colsman's opinion that the Commission acted as it did in order to financially cripple the Zeppelin concern and destroy its ability to perform further work in this field. This action can be understood in the light of the Commission's charter to prevent the resurgence of a military aircraft industry in Germany. With its superb performance, the E.4/20 could have been a bombing weapon to fear.
The aircraft was scrapped on 21 November 1922, destroying in the process the forerunner of the modern transport aircraft. The significance of the E.4/20 in future aircraft development has been largely forgotten, and it is a pity that Rohrbach and Staaken never had a chance to develop its full peacetime potential. To Dr.-Ing. Adolf Rohrbach must go the credit for having conceived an aircraft formula which virtually all large transport and military aircraft were to follow for a great many years.
* Source: The German Giants, The Story of the R-planes 1914-1919, G.W. Haddow & Peter M. Grosz, Putnam & Company Limited, 42 Great Russell Street, London, First Published July 1962