I always wondered why the word Wunderwaffe is applied only to the late-WWII German projects.
In the history of WWI there is a whole lot of weird and scary "wonderweapons". And if you think that the DFW giant bomber was the only one with Mercedes inside, you're totally wrong. I've got some more.
The AEG R.I (see the picture above) was unusual for a multi-engined aircraft in that rather than mounting propellers directly to the engines and mounting these in nacelles, the R.I carried all its engines within the fuselage and turned its propellers via a system of drive shafts.
A single prototype was completed and flew in 1916. Initial flights were quite successful, the aircraft being considered very maneuverable, but on 3 September 1918, a newly assembled propeller, which had not been given sufficient time for glue to cure, disintegrated. The vibrations resulting from that failure caused the complex transmissions and shafting connecting all four engines to both propellers to tear loose, which then cut a center section strut, resulting in the break up of the aircraft, killing all seven crew on board. Of the seven further AEG R-1's planned or under production when the war ended (R.21, R.22, R.59, R.60, R.61, R.62, R.63 and R.64) only the R.21 was finished and R.22 partially complete.
Same concept, same designation, another make. Using encouraging data from test at the Göttingen laboratory Linke-Hofmann designed the Fuselage of the R.I (above) to completely fill the interplane gap of the widely separated biplane wings, unfortunately the anticipated performance advantage was not realised. The capacious fuselage housed the crew compartments as well as the four engines and their clutches and combining gearboxes. Construction of the R.I was largely of wood covered by transparent Cellon in the first prototype, R I (8/15), and lozenge camouflage fabric in the second aircraft, RI (40/16).
Cellon was used with the intention of making the aircraft partially transparent and so less visible. Unfortunately, the Cellon reflected sunlight, making the aircraft more visible, before quickly yellowing due to the effect of ultra violet radiation. It also shrank and stretched due to in-flight temperature changes, with detrimental effect on trim.
The forward section of the fuselage was divided into three levels. The top deck housed the pilots and wireless station, the middle the engine compartment and the lower the bombardiers, fuel tanks and payload. It has to be noted that this configuration would have made the aircraft top heavy when, after expending its fuel and weapon payload, it eventually landed.
The Linke-Hofmann R.I was powered by four in-board Mercedes D.IVa engines, rated at 260hp (194kW), coupled to a gear-box assembly which transferred power through shafts to two tractor propellers mounted between the wings, giving the plane a maximum speed of 140 km/h.
Now take a look at this monster:
The Linke-Hofmann R.I had disappointing performance and handling, as well as structural weakness with both prototypes crashing. Linke-Hoffman took a radically different approach for their second Riesenflugzeug, the Linke-Hofmann R.II. The R.II was an approximately three-fold scale-up of a conventional single-engined biplane, powered by a single 6.90 meter (22 ft 7.5 in) diameter propeller, the largest single propeller ever used to propel any aircraft in aviation history. Power was supplied by four 252 hp (188 kW) Mercedes D.IVa engines. These were arranged in pairs in the central fuselage and drove the propeller through clutches, shafts and gearboxes. The Linke-Hoffman R.II, probably the largest single propeller driven aircraft that will ever be built, had a wing span of 41.16 m (135 ft 0 in), length of 23.3 m (76 ft 5 in) and height of 7.1 m (23 ft 4 in).
The airframe was constructed largely of wood, with plywood covering the forward fuselage and a steel-tube v-chassis main undercarriage with two wheels and a tail-skid at the aft end of the fuselage. Two examples of the R.II had been completed by the time the Armistice was signed: R 55/17 and R 58/17.
Flight testing of R 55/17 was carried out after the Armistice in 1919, demonstrating acceptable performance and handling, being able to fly happily with only two engines driving the propeller. Normal endurance was estimated to be 7 hours, but with adjustment of load and a cruising speed of 74 mph (119 km/h) it was estimated that the R.II could stay aloft for 30 hours.
Source: Haddow, G.W.; Peter M. Grosz (1988). The German Giants - The German R-Planes 1914-1918 (3rd ed.). London: Putnam