German aircraft designations, and what constitutes an "R-Plane".
All aircraft serving with the German Army in WWI were classified by function with a letter designation:
A.-unarmed scout monoplane
B.-unarmed two-seat observation biplane
C.-armed two-seat biplane
Cl.-armed, light, two-seater biplane (includes late-war monoplanes)
D.-armed single-seat biplane (includes late-war monoplanes)
Dr.-armed single-seat triplane
E.-armed single-seat monoplane
G.-multi-engine bomber
J.-armored ground-attack two-seat biplane
K.-heavily armed, lightly armored, multi-seat, multi engine biplane (obsolete designation after 1915)
R.-long range, multi-engine bomber with in-flight engine serviceability
Official specifications (not finalised until 1917) for the "R" type ("R" for Riesenflugzueg-giant aircraft), differentiate from the "G" type (Grosskampffluzeug-large battle aircraft) specifications by the requirement for in-flight engine serviceability, aside from load carrying capability and flight duration. Just because an aircraft was very large, it wasn't automatically an R-Plane; several experimental G type aircraft were relatively "giant", and possessed 4 engines. The opposite could also be true; the 3-engined Siemens-Schuckert Werke (SSW) R.I was originally categorised as a G type.
In contrast with all other German aircraft of WWI, the R-Planes had serial numbers that ran sequentially over the span of the war.
Why this is significant for the study of WWI German aircraft is that a hypothetical Albatros C.III built in 1916 could carry the serial number C 1501/16, indicating that it's the 1,501st aircraft ordered out of a batch of 1916's procurement. An Albatros C.VII ordered in 1917 could also carry the serial 1501, but expressed as C 1501/17. Theoreticaly, two different aircraft of the same type could carry the exact same serial number (these are not factory serial numbers, but official army designations). Granting the condition of two small orders for Pfalz D.IIIs in the same year, you could conceiveably have two different aircraft listed as Pfalz D.III 200/17.
R-Plane serial numbers ran from the SSW R.I 1/15 to the Zeppelin-Staaken R.XIVa 86/18; owing to these small numbers, referring to individual R-Planes by their serial number is a convenient identification method.
The official requirement for engine serviceability was percieved as critically important for long-range flying; aircraft powerplant development was at an early stage, and was recognised as such. In the attempt to increase reliability by carrying in-flight mechanics, however, engineers were led to introducing an undesireable amount of mechanical complexity into what was already pushing the limits for the state of the art.
That complexity went quite some way to negating the reliability which was the entire rationale of the official specifications in the first place.
Most major aircraft manufacturers in Germany were involved in the R-Plane program, though only a fraction of the designs made it to prototype stage.
While engine installations in these aircraft were widely varied, there were basically two general themes in providing powerplant accessability; centralized (engines mounted in the fuselage), and de-centralized (engines in external nacelles).
Of the two types, generally, the more common centralized-type installation required self-defeatingly complex arrangements of drive shafts, clutches, and gearing, although some firms produced better results than others.
Crew training was also a large factor to take into consideration in regards to reliability.
SSW Pioneering Efforts
Engineer Brothers Franz and Bruno Steffen were active before the war cooperating on a small airship, and other aircraft designs. They co-owned a small aircraft factory and flying school in Neumünster, founded in 1908.
Bruno, flying the Steffen design Falke II, broke the German endurance record in 1913.
At the outbreak of war, both brothers enlisted and served as pilots; Franz on the Western Front, Bruno on the eastern. While serving there, apparently Bruno had the chance to observe the Siskorsky Ilya Mouromets in action; analyzing what was then the world's only large combat aircraft he concluded that ideas he and Franz had for large aircraft design before the war were correct:
That the engines should concentrated near the center of gravity to improve manouverability; be fully enclosed to reduce drag and allow mechanics to attend the engines during prolonged flights.
Bruno Steffen was on convalescent leave in Berlin and coincidentally met Franz who was on leave after completing a tour of duty. Seeing this as an opportunity to return to their profession, they visited Idflieg Headquarters in the hopes to obtain a production contract for their bomber design. While meeting with success in pitching their ideas to the authorities, they were forced to abandon any idea of using their own facilities in Neumünster for construction owing to the lack of capitol, facilities, and equipment.
With this situation as impetus, they managed to secure an agreement with SSW in December 1914 to construct the bomber.
Ordered as SSW G.I 31/15, construction on what would become R 1 1/15 began almost immediately, and in early 1915 components were shipped to Neumünster for final assembly. The choice of Neumünster for assembly was an attempt to keep the plane's existence a secret until after initial tests.
R 1 was the prototype of the SSW R series, of which seven were built. Later aircraft were built with the same configuration, differing only in details and size.
SSW R.I
Engines - 3 Benz Bz.III 150hp
Propellor rpm - 900
Wingspan - 28 m. (91 ft. 10 in.)
Length - 17.5 m. (57 ft. 5 in.)
Height - 5.2 m. (17 ft. 1 in.)
Wing area - 138 sq. m. (1485 sq. ft.)
Dry weight - 4000 kg. (8820 lbs.)
Loaded weight - 5200 kg. (11,466 lbs.)
Wing loading - 37.6 kg./sq. m. (7.7lbs /sq. ft.)
Maximum speed - 110 kph. (68.4 mph.)
Climb - 2000 m. (6562 ft.) in 35 min.
Ceiling - 3700 m. (12,139 ft.) in 45min.
Duration - 4 hours.
Range - 520 km. (323 m.)
The R 1, represented in model form (model built by WWI enthusiast Karen Rychlewski) here, here, here, and here, presented a unique and really bizarre appearance.
The three Benz engines were mounted with two side-by-side in the extreme nose with the crankshafts facing aft, and the other underneath and slightly behind the pilot's cockpit. Their engine bearers formed the basis of the forward structure of the machine.
Each engine was connected to a common gearbox with a combination leather-cone and centrifugal-key clutch. The purpose of the leather cone clutch was to friction-start the propellors from a standing position. As soon as a certain RPM was achieved, the centrifugal-key clutch engaged automatically, and the leather-clone clutch was disengaged by the mechanic.
If an engine were to slow down or stop, the centrifugal clutch would automatically decouple below certain RPM.
The massive gearbox was designed to be as light as possible, using lightweight gears with fairly high gear-tooth pressures.
Although difficulties plagued the clutch system on Steffen-designed SSW R-Planes, the gearbox itself was reliable and relatively trouble-free when carefully serviced, and was used in the same basic form on the other six Steffen giants.
Two transmission shafts transferred power from the gearbox to propellor gear housings mounted between the wings. The original laminated-spring couplings and universal joints that absorbed vibrations and mis-alignments had to be later reinforced with stiffener tubes running parallel to the transmission shafts.
Maiden flight of the R 1 was on May 24th, 1915, with Franz and Bruno Steffen at the controls. While neither of them knew what really to expect, the flight revealed the aircraft to have pleasant flying characteristics, with no heaviness in the controls; Steffen R-Planes were said to fly like a standard two-seater.
Both were apparently concerned about landing a 5 ton aircraft, but were relieved by the ease with which they brought it down safely.
This flight was marred by gearbox overheating, which was resolved by rigging it with a forced lubrication system and thinner grade oil.
I'll quote directly from G. W. Haddow and Peter M. Grosz on the final acceptance flight...
Apparently, after the champagne cork was popped, a party-like atmosphere developed onboard, and on arrival at Döberitz the officers who greeted the R 1 showed surprise at the number of people disembarking.G. W. Haddow and Peter M. Grosz in The German Giants wrote:These flights took place some time in June, 1915, and the three-man Idflieg acceptance commission arrived at Neumünster to participate in a combined acceptance delivery flight.
Bruno Steffen, as pilot, had purposely kept 600 kg. free in hopes of being allowed to carry seven passengers (some sources say nine). His wish was granted, and after the useful load and passengers were weighed and verified, the R.I took off for for Döberitz. Steffen had placed two light armchairs in the cabin and hidden a bottle of champagne in a corner.
After take-off his sole attention was fixed on reaching the acceptance altitude in the specified time, not an easy task in the sultry and bumpy summer weather. As he fought to achieve altitude, he noticed clouds and lightning ahead over Lübeck. This did not escape the acceptance commission, who asked Steffen to please avoid the storm. Steffen answered that he was very sorry, but he had to achieve the required altitude. In a short while the answer came from the commission, who stated that they could see from the barograph that the R.I would reach the altitude in the specified and therefore it was not necessary to do so...but to please avoid the storm.
In August of 1915, soon after delivery, R 1 crashed on a training flight shortly after lift-off from a height of 8 meters with 5 people on board, including Bruno Steffen, who was instructor on the flight. The cause was traced to foreign material in the fuel tanks, causing immediate suspicions of sabotage. No one was seriously injured in the crash, but R 1 suffered extensive damage.
By October 13, 1915, the repaired R 1 was at Slonim, attached to Feldfliegerabteilung 31. Perhaps predictably for such an experimental aircraft, R 1 never flew any combat missions while with FFA 31 due to numerous mishaps and technical difficulties.
In March of 1916, R 1 was dismantled and loaded on railcars for transport back to Berlin; the fuselage was wrecked while passing thru the Spandau fortress due to lack of clearance. Repaired yet again, R 1 served as a trainer with the Training Section of Riesenflugzeugersatzabteilung (Rea) Döberitz until 1918, possibly up to the armistice.
Parts of R 1 were preserved in a Berlin museum after the war, but like so very many German aviation relics of WWI, they were completely destroyed by Allied bombing in WWII.
With a career stretching from 1915 to 1918, SSW R.I R 1/15 was the longest-lived of all the R-Planes.
...meanwhile, elsewhere at Siemens-Shcukert Werke...
SSW's other early venture into large aircraft was a design by Villehad Forssman, a Swedish engineer who spent some time in Russia before the war working on airships. His design for SSW greatly resembled a smaller, and somwhat naive copy of a Sikorsky Ilya Mouromets.
Where the Steffens analyzed the Sikorsky aircraft with an eye to it's faults, Forssman attempted a loose copy.
While pre-dating the R designation, lacking in-flight engine access, and with a wingspan of only 24m (78.75'), it doesn't truly merit inclusion as an R type, but it was an early attempt at solving long-distance flight and load carrying.
The SSW-Forssman "Giant" was...a total failure; during initial tests it crashed once and was rebuilt twice. Before it's final acceptance flight it was actually extensively modified no less than five times.
It possessed near-impossible flight characteristics, early flights being no more than short hops, and the final iteration being so tail-heavy and awkward that correcting for a violent updraft during the acceptance flight nearly led to uncontrollable oscillations.
The pilot on that flight, Bruno Steffen, invited five people to join him as on the earlier R 1 acceptance/delivery flight, including members of the official Idflieg Acceptance Comission; all declined the offer. It broke it's back in a crash after acceptance (to reduced specifications and at a reduced price) and was never rebuilt.
SSW-Forssman "Giant"
(final version)
Engines - 2 Mercedes 220hp. D.IV, and 2 Mercedes 110 hp. D.II
Wingspan - 24 m. (78 ft. 9 in.)
Length - 16.5 m. (54 ft. 2 in.)
Wing area - 140 sq. m. (1506 sq. ft.)
Dry weight - 4000 kg. (8820 lbs.)
Loaded weight - 5200 kg. (11,466 lbs.)
Maximum speed - 120 kph. (75 mph.)
Climb - 2000 m. (6562 ft.) in 28 min.
Barring any protest, I'll continue with adding to this. Sources of info are the already mentioned The German Giants by Haddow and Grosz, The Sky On Fire by Raymond H. Fredette, and other works by Peter M. Grosz, as applicable.
