W.Green, G.Swanborough The Complete Book of Fighters

RUMPLER 7D 1 Germany

   Evolved in parallel with the two-seat 7C 1 and embodying similar aerodynamic and structural features, the 7D 1 single-seat fighter was the recipient of an Idflieg (Inspectorate of Flying Troops) contract for three prototypes, flight testing commencing in the spring of 1917. The Idflieg requirement called for a speed of 103 mph (165 km/h) at 16,405 ft (5 000 m), that altitude being attained in 31.5 minutes, and an endurance of 1.5 hrs. An armament of two synchronised 7,9-mm LMG 08/15 machine guns was specified and provision for oxygen breathing apparatus requested. A second identical prototype was designated 7D 2. Flight testing revealed that the pilot’s field of vision was seriously impaired by the broad-chord interplane struts and there were aerodynamic problems associated with the upper wing/fuselage junction. Furthermore, there were servicing difficulties related to the engine installation. In consequence, the 7D1 and 2 were abandoned in favour of a more conventional fighter, the 7D 3.

Max speed, 109 mph (175 km/h).
Service ceiling, 22,965 ft (7 000 m).
Span, 26 ft 10 3/4 in (8,20 m).
Length, 19 ft 4 1/4 in (5,90 m).
Height, 8 ft 6 1/3 in (2,60 m).


RUMPLER 7D 3 Germany

   The shortcomings displayed by the 7D 1 and 2 led the Rumpler team to evolve a more conventional derivative fighter retaining the same structural precepts. This, the 7D 3, retained the Mercedes D III engine, but a more orthodox installation was adopted, the flush radiator being centrally mounted in the wing centre section which was raised above the forward fuselage decking by means of a cabane structure. The broad-chord I-type interplane struts were discarded in favour of more conventional twin struts. The 7D 3 was tested during the summer of 1917, but it may be presumed that results were not entirely satisfactory, as, by the late autumn, work had begun on an entirely new aircraft, the 7D 4, intended to participate in the first D-type contest that was to take place at Adlershof early in the following year. No further details of the 7D 3 are available.


RUMPLER 7D 4 Germany

   To compete in the first D-type contest (20 January - 12 February 1918), intended to select single-seat fighters for service introduction in mid-1918, Rumpler built two prototypes of the 7D 4. One prototype was completed with a conventional twin-strut cellule and the other with a cellule employing "reverse-C” interplane struts braced by fabric-wrapped triple cables. The fuselage structure remained unchanged, but in an attempt to eradicate some torsional problems experienced earlier with this type of construction, a thin plywood veneer skinning was applied to the nose and tail sections to increase rigidity. Again, the Mercedes D III engine was retained and specified armament was two 7,9-mm LMG 08/15 synchronised guns. Flight testing of the 7D 4 had commenced by October 1917, and during the D-type contest the example fitted with "reverse-C” interplane struts attained an altitude of 16,405 ft (5 000 m) within 23.8 min. This prototype was considered to afford excellent visibility from the cockpit, but was, by consensus, somewhat temperamental in handling and difficult to land. Nevertheless, it appeared to possess sufficient promise to warrant an order for a pre-series of 50 examples of a developed version (7D 7) for further investigation and possible operational evaluation. Another prototype was completed as the 7D 5, this differing essentially in having an automobile-style frontal radiator. No specification for the 7D 4 is available.


RUMPLER 7D 7 Germany

   Too late to participate officially in the first D-type contest, the 7D 7 was an improved derivative of the 7D 4 with the "reverse-C” type interplane struts. A new Gottingen aerofoil was employed for the wing, which had control surfaces of marginally reduced area, the cockpit was smaller and was moved forward 13 4/5 in (35 cm), and the buried wing radiator gave place to ear-type radiators mounted immediately above the lower wing roots. Flight testing proved the 7D 7 faster than the 7D 4, and an unofficial climb to 16,405 ft (5 000 m) within 18 min was reported. The 7D 7 was powered by the 160 hp Mercedes D III engine and possessed an armament of two synchronised 7,9-mm LMG 08/15 machine guns, static load testing and flight evaluation occupying the period 22 February to 1 May 1918. Some disconcerting twisting of the tail was encountered during certain manoeuvres, calling for structural reinforcement, the Idflieg reporting in May 1918 that the 7D 7 was "unacceptable for the Front and would be rebuilt.”


RUMPLER 8D 1 (D I) Germany

   To overcome the lack of rear fuselage rigidity experienced during testing of the 7D 7, the fabric-wrapped, multi-stringered fuselage structure first featured by the 7C 1 and 7D 1 was finally and reluctantly abandoned in favour of a stronger, more conventional semi-monocoque of diagonally-wrapped strips of glued plywood. The wing cellule was reinforced, balanced ailerons were fitted, and the fin and rudder were redesigned and enlarged. Designated 8D 1, this revised fighter provided the standard for the 50 pre-series aircraft previously ordered from Rumpler as D Is. Three pre-series D Is powered by the 160 hp Mercedes D IIIa engine and one powered by the 180 hp D IIIau high-compression engine participated in the second D-type contest (27 May - 28 June 1918), the last-mentioned attaining an altitude of 16,405 ft (5 000 m) in 18.7 min compared with 27 min required by the D IIIa-engined D I. Excellent climb and altitude capabilities notwithstanding, the evaluation pilots’ consensus of the D I was unfavourable, particularly criticised being aileron response - which was considered slow and erratic - the gliding and landing characteristics, and the level of vibration. One D I powered by a 185 hp BMW ma engine participated in the third D-type contest (10-22 October 1918), being the only contender to attain an altitude of 26,900 ft (8 200 m), but the fighter was deemed of ‘‘limited usefulness” in close-in combat. Military acceptance testing of the Rumpler fighter had still to be completed at the time the conflict terminated when 22 had been built. Another 27 were completed after the Armistice. The following data relate to the standard D IIIa-powered D I.

Max speed, 112 mph (180 km/h) at 16,405 ft (5000 m).
Time to 16,405 ft (5000 m), 23.75 min.
Endurance, 1.5 hrs.
Empty weight, 1,356 lb (615 kg).
Loaded weight, 1,775 lb (805 kg).
Span, 27 ft 7 1/2 in (8,42 m).
Length, 18 ft 10 2/5 in (5,75 m).
Height, 8 ft 4 3/4 in (2,56 m).
Wing area, 179.33 sq ft (16,66 m2).

J.Herris Rumpler Aircraft of WWI (A Centennial Perspective on Great War Airplanes 11)

Rumpler D.I

   The story of the prolonged development of Rumpler's only production land-based fighter, the D.I, and all the prototypes that led up to it is a story of great vision and innovative engineering that ultimately failed to produce a practical fighter despite intensive effort.
   Design studies for the Rumpler single-seat fighter began in early 1916 in parallel with its two-seat, high-altitude reconnaissance counterpart, the 7C1. Like the 7C1 the fighter prototype, known internally as the 7D1, was based on extensive investigation into new aerodynamic features supported by new structural techniques. To achieve the desired climb, ceiling, and speed at altitude the aircraft had to be very light and streamlined. As a result the 7C1 and 7D1 prototypes resembled aircraft designed for breaking speed and altitude records and did not have the flying qualities and robust structures needed for practical combat aircraft.
   By November 1916 Rumpler was able to build a mock-up of the 7D1 and the next month Idflieg ordered three prototypes of the fighter. Like the 7C1, these were powered by the 160 hp Mercedes D.III, then the best available fighter engine. Idflieg required a climb to 5000 m in 33 minutes and a maximum speed of 165 km/h at that altitude, and also required installation of supplementary oxygen for the pilot, proof that exceptional climb and ceiling were planned.
   Rumpler completed the 7D1 prototype (and an 'identical' 7D2 which is otherwise unknown) in spring 1917. The 7D1 was a highly-streamlined biplane with top wing supported by the engine cowling. It was built with a series of light-weight fuselage stringers covered with doped fabric for streamlining and light weight. Single I-struts on each side braced the wings and only two bracing wires were used for each bay.
   Flight test were disappointing. Not only did the engine cowling obscure the pilot's forward vision, a serious problem for any aircraft and especially for a fighter, but the airflow interference between the propeller and the cowling and upper wing structure prevented the engine from developing its full RPM and thus its full power. Moreover, the buried engine was virtually inaccessible for maintenance.
   As a result, Rumpler designed a revised prototype with conventional center-section, the 7D3 (according to chief test pilot Friedrich Budig). The 7D3 was probably not successful because little is known about it, and the following 7D4 prototypes were much more widely tested. The first 7D4 prototype, powered by a 160 hp Mercedes D.III, was flight-tested in October 1917 and demonstrated a climb to 5,200 m in 20 minutes, an exceptional rate of climb for a fighter powered by a water-cooled, in-line engine.
   Two different 7D4 prototypes competed at the First Fighter Competition in January 1918, two years after start of development. The U-strut 7D4 prototype climbed to 5,000 m in 23.8 minutes, and the parallel-strut 7D4 climbed to the same altitude in 28.0 minutes. The significantly slower climb of the parallel-strut 7D4 motivated the development team to focus on the U-strut configuration for the production fighter. By this time Idflieg had designated the Rumpler fighter as the Rumpler D.I regardless of the details of internal company designations. Pilots at the competition were impressed by the Rumpler fighter's climb but reported it was excessively touchy and difficult to fly.
   According to engineer Theo Rockenfeller the Rumpler 7D5 was a fighter fitted with an automobile-type radiator in the nose that was built and tested; unfortunately, nothing else is known about it. The 7D6 was a cantilever quadraplane fighter design of late 1917 that fortunately was not built. The next Rumpler prototype fighter to fly was thus the 7D7, which appeared too late to compete at the First Fighter Competition.
   The 7D7 design was modified from the U-strut 7D4 by using a new airfoil and smaller control surfaces. The smaller cockpit was moved forward 35 cm and the inefficient airfoil radiator used in the 7D4 prototypes was replaced by a pair of ear radiators attached to the fuselage above the lower wing. The new radiators initially were not effective and significant engineering work had to be done to fix the problem. The improved 170 hp Mercedes D.IIIa engine was used, and flight tests showed the 7D7 was faster than the 7D4 and demonstrated a climb to 5,000 m in 18 minutes, a significant improvement over an already impressive climb rate.
   The Fokker D.VII won the First Fighter Competition and went on to win fame over the Western Front. The Rumpler D.I, now based on the 7D7 prototype, was viewed as promising enough that Idflieg ordered an evaluation batch of 50 aircraft, something that was also done for the Roland D.VI and SSW D.III. The Rumpler D.I production batch was given serials D.1550/18 to D.1599/18.
   The D.I, based on the 7D7, underwent Idflieg’s standard test and evaluations procedures starting in February 1918. Load tests lasted from February 22 to May 1 with a number of failures and fixes during that time. The summary report of May 18 called for additional reinforcement of various items, indicating that the Rumpler team had left little margin for error when designing the aircraft for minimum weight. The light-weight fuselage of the 7D7 twisted during flight maneuvers, and flight tests in April 1918 revealed problematic flight characteristics. Idflieg stated that the type was "unacceptable for the front and would be rebuilt."
   To solve the fuselage twisting problem and improve flight characteristics, a completely new, stronger fuselage design had to be used for the production fighters. The revised design, based on a fuselage wrapped diagonally with strips of plywood similar to the Wimpelrumpf technique developed by LFG Roland, was known internally as the 8D1, and it became the basis for the production Rumpler D.I.
   Three Rumpler D.I production fighters based on the 8D1 design competed at the Second Fighter Competition in May-June 1918. Two of these, D.1552/18 and D.1553/18, were powered by the overcompressed 180 hp Mercedes D.IIIau high-altitude engine. Flown by Gustav Basser, these climbed to 5,000 m in 18.7 minutes, an excellent performance. Despite the excellent climb rate, the front-line pilots who tested the Rumpler D.I agreed that it was wholly unsuitable for combat use; it did not glide well, oscillated in flight, and the flying surfaces vibrated and made the pilots feel insecure about the aircraft's structural strength.
   The aileron control came in for particular criticism. In the D.I the linkage between the pilot's stick and the ailerons was a worm-gear transmission that turned control rods in the wing. It was light and created no drag, but the resulting aileron control was slow and erratic. Such a control system might be acceptable in a racing or record-breaking aircraft but not in a fighter; fast, consistent roll control is essential to a fighter in combat. Disappointed, Idflieg decided that testing and evaluation should continue but mass production was out of the question until the aircraft was improved.
   A parallel Idflieg flight evaluation on production airframe D.1550/18 was summarized on September 9, 1918 and stated that the D.I fitted with aerodynamically-balanced ailerons (the 8D1) flew much better than its predecessor (7D7) with unbalanced ailerons. Maneuverability was very good and most in-flight vibrations were eradicated, but the aircraft was not easy to land and roll-rate was still slow.
   Moreover, Idflieg identified 42 installation faults with the prototype! While most were not serious and fairly easy to remedy, the great number of defects calls the competency of the Rumpler design team, and especially its management, into question. What had the design team been doing since the start of type-testing in February?
   During load testing between September 16 and October 2 on production airframe D.1550/18 the airframe failed the vertical nose dive requirement and a 4 mm drag cable had to be installed between the U-strut and the nose to pass the test. But the aileron control mechanism failed again, a critical deficiency in a fighter.
   Rumpler D.I 1581/18, powered by a 185 hp BMW.IIIa over-compressed engine, was entered and flown at the Third Fighter Competition. This was the only D.I powered by a BMW engine, which was fitted especially for the Third Fighter Competition that specified use of the BMW. The aircraft reached 7,000 m in 26.9 minutes, 8,000 m in 53.3 minutes, and was the only fighter in the competition to reach 8,200 m. The climb and ceiling were clearly superior and flying qualities were reported as good, and the Rumpler D.I was declared one of the two winners of the competition (the other winner was the Fokker V29 parasol monoplane derivative of the Fokker D.VII). However, the D.I was appraised as of limited use in a dogfight against other fighters because it lost height in turns and spun easily, and the short fuselage limited rudder effectiveness.

In Retrospect

   Although the D.I had potential as an interceptor, by the Armistice only 22 production aircraft had been built and the type had still not passed military acceptance requirements! Extolled as a 'super plane' by the postwar German aeronautical press, the D.I was never a practical front-line fighter. The basic design was too extreme and despite great effort and the investment of tremendous engineering resources, Rumpler was simply unable to develop the aircraft into an effective, combat-worthy fighter.


Rumpler 7D7 Specifications
Engine: 170 hp Mercedes D.IIIa
Wing: Span Upper 8.42 m
Span Lower 7.38 m
Chord Upper 1.32 m
Chord Lower 1.09 m
Gap 1.365 m
Area 16.66 m2
General: Length 5.75 m
Height 2.56 m
Empty Weight 615 kg
Loaded Weight 805 kg
Maximum Speed (at 5000 m): 180 km/h
Climb: 5000m 23.7 min


Rumpler D.I (8D1) Specifications
Engine: 185 hp BMW.IIIa
Wing: Span Upper 8.42 m
Span Lower 7.38 m
Chord Upper 1.32 m
Chord Lower 1.09 m
Gap 1.365 m
Area 16.66 m2
General: Length 5.75 m
Height 2.56 m
Empty Weight 615 kg
Loaded Weight 805 kg
Maximum Speed (at 5000 m): 180 km/h
Climb: 5000m 13.2 min
6000m 17.8 min
7000m 26.9 min
8000m 53.3 min
Ceiling: 8200m

M.Dusing German Aviation Industry in WWI. Volume 2 (A Centennial Perspective on Great War Airplanes 85)

Rumpler-Werke A.G. in Berlin-Johannisthal (Ru)

Aircraft Development:

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  In 1918, the company released a promising fighter aircraft, but too late. The development history was as follows:
   7D1 - single-seat fighter prototype (based on 7C1) with 160 hp Mercedes D.III, 3 samples built in spring 1917.
   7D2 - same as 7D1, not successful tested.
   7D3 - revised 7D2 with conventional center section, no progress.
   7D4 - flight tested in October 1917, two different variants (J- and parallel struts).
   7D5 - Tests with new surface radiator between the upper wings.
   7D7 - final D.I configuration with 170 hp Mercedes D.IIIa engine. Not accepted by Idflieg due to twisting behavior.
   8D1 - final production configuration of Rumpler D.I.
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