O.Thetford, P.Gray German Aircraft of the First World War (Putnam)
Rumpler C IX (7C 1) (first version)
This experimental two-seater appeared late in 1917. C IV wing panels were used with single 'T'-type interplane struts. The oval, multi-stringered fuselage had pleasing lines, but the pivoted, "all-moving" rudder was a particularly vulnerable component, and was subsequently modified. It was intended the type should be a two-seat fighter, but the machine could not have been successful, as it did not go into production. Engine, 160 h.p Mercedes D III.
Rumpler C IX (second version)
Although the trailing edge of the rudder is unfortunately clipped in this illustration, the general revision of the tail surfaces to include a fin may be seen.
J.Herris Rumpler Aircraft of WWI (A Centennial Perspective on Great War Airplanes 11)
"Everything is set on achieving the greatest possible performance, particularly with regard to climbing ability and speed." (Edmund Rumpler)
Extreme designs and their performance indicate the leading edge of technology for their time. However, racing automobiles and record-breaking aircraft are rarely practical on a daily basis; the main production emphasis is generally on what is practical and useful. In the case of long-range reconnaissance aircraft that must have the speed, climb, and ceiling to avoid interception, the leading practical WWI German aircraft was the Rumpler C.IV and its derivatives that shared its airframe. Evolution of the C.IV airframe into something lighter, more compact, and streamlined culminated in the excellent Rumpler C.X that appeared too late to see operational service. This section profiles another Rumpler design for the same role that pushed the state of the art too far. Like its single-seat counterpart the 7D1, the Rumpler 7C1 could not be developed into a practical production aircraft and is an example of what is sometimes ruefully known as the 'bleeding edge' of technology.
Postwar Edmund Rumpler stated, "Before the end of the war the latest new developments needed to totally diverge from those of the previous types." This applied particularly to completely new aircraft types, whose flight performance was to be substantially increased. Preliminary planning had begun as early as the beginning of 1916; however, actual flight testing took place in 1917 and 1918.
Intended for the long-range reconnaissance role was an optimized, extremely light and aerodynamic biplane layout, with Idflieg (Inspektion der Fliegertruppen = Inspectorate of Flying Troops) requirements of a minimum ceiling of at least 5,000 meters and a maximum speed of at least 165 km/h at that altitude. Preliminary plans for the 7C1 reconnaissance plane and its 7D1 single-seat fighter counterpart envisioned a short fuselage with an oval cross section, single-bay wing surfaces that organically flowed from the fuselage, small rudder without fixed fin, and smooth, streamlined surfaces. Rumpler had stipulated circular fuselage cross-sections since 1911, but no Rumpler series production aircraft had used it yet. Experience with the strength and rigidity of single-bay wings was first provided to the Rumpler design team by their 5DDD biplane followed by the Rumpler 6A2 prototype aircraft.
At the end of November 1916 a mockup of the front fuselage section for the Rumpler 7C1 and 7D1 was completed for demonstration purposes. The factory mockup exhibited extremely lightweight construction and uncompromising effort to achieve the lowest aerodynamic drag - characteristics later displayed in the postwar Rumpler Tropfen-Auto streamlined automobile. The fuselage had an oval cross section with extremely thin stringers. Separate upper wing panels bent downward at the wing root in gull-wing configuration to rest directly over the completely enclosed 160 hp Mercedes D.III engine. The engine was mounted within the fuselage under the integrated cabane structure, which also included the radiator. For this design method Rumpler obtained a patent in 1919, which noted "with mid-wing monoplane aircraft the fuselage enclosing the engine is used for attaching the upper wing."
In December 1916 Idflieg contracted Rumpler to build three prototypes. In January 1917 these three prototypes, the two-seat 7C1 and the 7D1 and 7D2 single-seat biplanes, were under construction. The wings of all three aircraft were braced with a single I-strut. The fuselage nose started with a large propeller spinner blended into the fuselage contours. The plywood-covered empennage was entirely cantilevered and without fixed fin to save weight. According to a patent issued in early 1915 the fuselage received a double-layer covering of Cellon-soaked fabric, diagonally wrapped, that was extremely light and resistant to warping.
During the flight tests, which took place from March through August 1917, the prototypes demonstrated a top-speed of 170 km/h, noteworthy considering the 160 hp Mercedes D.III engine used. The flight tests also showed, however, that the sophisticated aerodynamics detracted from the handling of the aircraft. The broad I-Struts and the cabane structure integrated into the fuselage obstructed the pilot's view. The cabane structure also negatively affected the flow of air from the propeller, so that the engine did not achieve its full revolutions, and thus its full power.
There were other practical difficulties. During installation the engine had to be slid in from the front, and during routine maintenance it was nearly inaccessible. In addition, test pilot Friedrich Budig determined during the first flight with the 7C1 that the rudder and ailerons worked irregularly. Aileron ineffectiveness was largely due to use of a worm-gear actuating mechanism, rare among aircraft designs.
Moreover, directional stability was inadequate due to the short fuselage and lack of a fixed fin. After attachment of a fixed fin brought insufficient improvement, the fuselage was lengthened. Pitch stability was improved because the tailplane had an inverted wing profile but was still insufficient, particularly when the airplane lost speed, which caused the nose to pitch up. Lateral stability was sufficient, but the limited aileron effectiveness gave marginal control and a slow roll rate. The effect of the ailerons improved somewhat after the outer portion of the aileron washout was bent downward. Finally, the aircraft were plagued with in-flight vibration and twisting of the fuselage, clear indications that the light-weight structure was not robust enough for combat operations.
Even before flight testing was completed, development of a two-seat fighter based on the 7C1 was considered. Contrary to initial planning, Idflieg communicated on 15 May 1917 that it required a "non-bomber reconnaissance aircraft as a replacement for the 7C1." Rumpler proposed that altitude tests should be undertaken as soon as possible, which were to be conducted by test pilot Gustav Basser. Among the structural alterations considered it was determined that the aircraft should be "an X-Strutter like the 6A2." However, despite this suggestion, the I-struts remained in place.
Despite a reference stating the Rumpler 7C1 saw limited production as a batch of 20 aircraft designated Rumpler C.IX, this is highly unlikely. First, no photos have been found of such an aircraft in service; such photos surely would have been taken given the striking appearance of the 7C1. Second, the late Peter M. Grosz, dean of WWI German aircraft historians, stated that the Rumpler C.IX was in fact based on the Rumpler C.IV and C.VII airframe, with power from a 245 hp Maybach Mb.IVa engine and equipped with a long-range fuel tank. Third, the long, troubled, and ultimately unsuccessful development of the companion Rumpler 7D1 fighter design into the Rumpler D.I has also been well-documented by Peter M. Grosz. Compared to the extensive information and photographs of that protracted development, the trail of the 7C1 stops with a photograph of the attractive last version of the design with large fixed fin and test pilot Gustav Basser in the cockpit.
The extensive development difficulties that prevented the Rumpler D.I from reaching operations despite intense engineering work apparently derailed the Rumpler 7C1 even earlier in its development in favor of the more practical Rumpler C.X. Rather than totally diverging from existing types as Edmund Rumpler favored, the successful C.X was an evolutionary design. In contrast, the unsuccessful 7C1 diverged too far from existing types.
The 7C1 also suffered from a common malady of German and Austro-Hungarian aircraft designs in both world wars; trying to save weight with a tail that was too small and a fuselage that was too short. The result was an unstable design that had to be modified with larger tail surfaces and longer fuselage for safe flight characteristics.
Moreover, by mid 1917 it was certainly realized that, for performance competitive with powerful late-war fighters, a photographic two-seater would require a much more powerful, and thus larger and heavier, engine than the 160 hp Mercedes D.III in the 7C1. In fact, this should have been obvious in 1916 when the 7C1 was being designed because Rumpler itself was building the more powerful C.III and C.IV. This lack of foresight made a complete redesign necessary and, with the intractable D.I undergoing development along with other types, Rumpler did not have enough engineering resources for that.
The delicate, highly aerodynamic design of the Rumpler 7C1 was more appropriate to a racing or record-breaking aircraft than a robust, reliable frontline combat aircraft.
Rumpler 7C1 Specifications
Engine: 160 hp Mercedes D.III
W.Green, G.Swanborough The Complete Book of Fighters
RUMPLER 7C 1 Germany
Late in 1916, the Rumpler team headed by Edmund Rumpler initiated design of both a two-seat and a single-seat fighter embodying a novel, if complex, method of fuselage construction. This, protected by a patent filed early in 1915, sought to combine minimum weight with maximum strength. The fuselage was built up from plywood frames with numerous thin stringers, the whole being covered by two layers of doped fabric strips applied diagonally in opposite directions and intended to provide the necessary torsional stiffness. A close-cowled 160 hp Mercedes D III engine was attached to a fuselage extrusion supporting the upper wing, which embodied an offset flush radiator. The upper wing was of parallel chord, the lower wing being of so-called Libelle (Dragonfly) form featuring a curved trailing edge, the wing cellule being braced by single broad-chord I-section struts with cables running to two fuselage points. This formula, which resulted in what was, aerodynamically, an outstandingly clean aeroplane, was applied to the 7C1 two-seat fighter and the 7D 1 single-seat fighter. The 7C 1 entered flight test in the spring of 1917, initially with vertical tail surfaces confined to a pivoting rudder. The tail was subsequently redesigned to embody a fixed fin, but development was discontinued at a comparatively early stage, presumably as a result of difficulties similar to those experienced with the parallel single-seat 7D 1. No specification for the 7C 1 appears to have survived.