World War One saw a rapid evolution within the field of aviation between 1914 to 1918. By 1918, military aircraft on all sides, especially the British and Germans, had evolved from slow and relatively cumbersome planes to fast and agile ones. Another aspect of aircraft by 1918 was a clear demarcation between types of aircraft. By the end of World War One, most of the warring powers had an array of aircraft classes such as fighters, interceptors, bombers, reconnaissance planes, fighter-bombers, and even ground support aircraft. If one examines the breadth of German aircraft design during the World War One years, there were a number of rather odd aircraft which appeared. It should be remembered that the start of World War One in 1914 was over ten years after the Wright Brothers made the first controlled and sustained flight in December 1903. As such, if a theory for some aeronautical advantage was determined to be feasible, it was investigated by aircraft designers and in some cases, turned into an actual airplane to fully put the idea to the test. One such aircraft was the German bomber, the Linke-Hofmann R I.
The Modellversuchsanstalt der Motorluftschiff-Studiengesellschaft (MLStG; Institute for Testing of Aerodynamic Models of the Powered Airship Society), established in 1907 in Göttingen, Germany, had designed and tested a model aircraft concept in which it was seen that if the space between a biplane's wings (the interplane gap) was completely enclosed, it provided for a more aerodynamically clean surface by eliminating some of the drag induced by exposed struts, pilot windscreens, engine radiators, and oil coolers. The study by the MLStG was taken up by the Linke-Hofmann company (founded in 1834) who sought to compete in the Riesenflugzeuge (giant aircraft) arena in which several German aviation companies built large three to four engine bombers for the Deutsche Luftstreitkräfte (German Air Force). Despite Linke-Hofmann being well known for designing and constructing locomotives and not aircraft, the company brought in former A.E.G. engineer Paul Stumpf to design and oversee the new bomber which was designated the R I. What resulted was evidence that what works in the laboratory does not always translate to success when put into production.
To facilitate the MLStG's study results, the fuselage of the R I was immense in size. Within the cavernous interior were two pairs of Mercedes D IVa engines, each of the four engines rated at 260 horsepower. Also inside the fuselage were the gearboxes and clutch systems for each pair which were connected to a driveshaft that drove a large, two-bladed wooden propeller on each side of the aircraft. The propeller mountings, along with dual radiators, were situated between the wings. The propeller mountings were not connected to the wings and thus did not pass on vibration forces to them. If required, the engines, gearboxes, and clutches could all be serviced by flight mechanics without exposure to the elements. So large was the interior of the R I that it had three decks. The top deck contained the cockpit and the wireless radio operator's station. The middle deck was the engine compartment and the lower deck contained the bombardier's position, the fuel tanks, and the bomb bay. The R I utilized all-steel tires rather than rubber and used a multi-spring suspension system. The first prototype, R I 8/15, was completed and used Cellon for skinning over the wooden airframe. Cellon was cellulose acetate and since it was clear, the intent was to make the R I less visible in the sky. The maiden flight of R I 8/15 took place in the spring of 1917 with test pilot Hauptmann Krupp at the controls.
Suffice it to say, a host of problems quickly manifested themselves. The wings were found to be less than robust due to their light weight and tended to warp in flight, leading to sluggish aileron flight controls. Another problem was the engines did not get adequate cooling and significant vibration from the engines while running was felt throughout the aircraft. The Cellon skinning was also a problem. While clear, it was highly reflective which had the opposite effect on reducing the visibility of the R I. In addition, Cellon very quickly yellowed in sunlight and any dirt and grime easily stained the material. Finally, Cellon flexed when temperatures changed, causing shrinking and expansion on control surfaces which impacted performance. Another concern was the poor pilot who, sitting some 20 feet up in the cockpit, had a very difficult time in landing the R I. The R I was found to have a handful of qualities in that the rudder and elevator controls were adequate and the unusual fuselage design did not overly result in reducing the handling in the air. However, it was not anywhere close to what the MLStG study determined would be the resulting benefit. In May 10, 1917, the R I 8/15 was flying at low altitude when the wings collapsed and the aircraft plummeted to the ground. Amazingly, of the six man crew (which included the pilots Dr. Wittenstein and Leutnant Hebart), only two (both mechanics) perished in the crash. The damage was too extensive and so the bomber was written off but work on a second aircraft, R I 40/16, had already been underway (shown in the photograph). Some of the lessons learned in the first prototype were worked into the second with the most notable being the replacement of the Cellon with regular fabric which was painted in a multi-color lozenge pattern (Buntfarbenaufdruck or Lozenge-Tarnung). The handling of the second prototype was marginally better but the overall performance was not to expectations. The end of the R I program came when the second prototype landed hard, broke an axle, and nosed over, crushing the forward portion of the bomber. It was never repaired and although two additional aircraft had been finished by January 1918, R I 41/16 and R I 42/16, it is believed that neither the third nor fourth bomber prototypes were ever flown.
The dimensions of the R I were huge. The R I had a length of 51.2 feet, a height of 21.1 feet, a wing span of 108.1 feet, and a wing area of 2,851 square feet. Empty, the R I weighed 17,640 pounds and when loaded, 24,696 pounds. The four engines gave the R I a maximum speed of 81 miles per hour with enough fuel to give the bomber a five hour endurance. It took the R I two hours to attain an altitude of 9,840 feet. Defensive armament was not fitted to any of the R I aircraft though dorsal, ventral, and beam machine-gun positions were provisioned.
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