O.Thetford, P.Gray German Aircraft of the First World War (Putnam)

Brandenburg W 20
   This little single-seat flying-boat was designed during 1917 for carrying aboard U-boats. It could be dismantled in 1 3/4 min. and stowed into space measuring only 20 ft. X 6 ft.; re-assembly took 2 3/4 min. As the type of submarine for which it was intended never went into service, only three examples of the W 20 were built. The first, No. 1551, had no interplane struts; these members were, however, added to the structure of Nos. 1552 - 1553, which also had the lower wing increased in span. Engine, 80 h.p. Oberursel UO. Span, 5.8 m. (19 ft. 0 3/8 in.), "1551"; 6.8 m. (22 ft. 3 3/4 in.), "1552/3". Length, 5.91 m. (19 ft. 4 5/8 in.), "1551"; 5.925 m. (19 ft. 5 1/8 in.), "1552/3". Area, 14.95 sq.m. (161 sq.ft.), "1551"; 15.82 sq.m. (171 sq.ft.), "1552/3". Weights: Empty, 396 kg. (871 lb.). Loaded, 568 kg. (1,250 lb.). Climb, 1,000 m. (3,280 ft.) in 14.9 min. Duration, 1 1/4 hr. Armament, none.

C.Owers Hansa-Brandenburg Aircraft of WWI Vol.2: Biplane Seaplanes (A Centennial Perspective on Great War Airplanes 18)

Type W.20 and Submarine-Borne Aeroplanes

   In the last half of 1917 and the first half of 1918, three Brandenburg W.20 flying boats were delivered to the German Navy. The W.20 was a small reconnaissance flying boat designed as Class Bu, to be carried by a submarine. The history of the decision to build such an aircraft has not been uncovered to date, but aircraft had liaisoned with submarines and the submarine-borne aircraft was a logical way to develop a means to widen the range of vision of a U-boat on patrol. The success of the raider SMS Wolf was due in no small part to the use of the Friedrichshafen 33E floatplane that had been carried by that ship. While the Wolf returned to Germany on 24 February 1918, and the development of a submarine-borne aircraft was already proceeding, it seems logical to infer that the use of an aircraft by a ship or submarine was not lost to the German Navy, indeed it would have reinforced the desire to produce a successful machine to be capable of being used with the large U-Boat cruisers being developed.
   Brandenburg received an order on 30 April 1917, for three floatplanes (MNs 1478 - 1483) and three flying boats (MNs 1551 - 1553) to meet the requirement for a submarine-borne aircraft. The floatplanes were cancelled on 9 November. The flying boat was given the designation W.20 and was a single-seat reconnaissance flying boat designed to be dismantled to fit into a 20 foot x 6 foot space in order to enable it to be carried by a submarine. It took 2 3/4 minutes to reassemble the aircraft and 1 3/4 minutes to dismantle. Part of the specification was that no tool other than a hammer should be used to assemble/disassemble the aircraft. This was to prevent delays that could prove disastrous to a submarine if tools were mislaid. A full-sized mockup was built to check that the proposed machine would fit into the cylinder that would be carried outside the submarine's pressure hull.
   It appears from the testimony of Ernst Rothenburg, who had come to Brandenburg from Dornier as chief engineer in charge of metal construction, that the W.20 series were used to develop the concept and once the ideal machine was derived, an all-metal aircraft would be built for active service.
   Apart from its small size the W.20 was a conventional Brandenburg construction aircraft with wooden hull and fabric-covered wooden wings. A Brandenburg three-view drawing of 22 June 1917, shows the W.20 with the upper wing of lesser span than the lower wing and interplane struts. On the first boat, MN 1551, the upper wing was supported by the centre-section struts with two long struts from the hull to about one third span. The upper wing was of greater span than the lower wing. The lower wing would have been of cantilever construction similar to that of the W.17. This arrangement proved insufficient in practice. MN 1551 suffered severe damage in October 1917, possibly during trials. The second version of the W.20 had the upper and lower wing increased in span and interplane struts for a greater margin of safety. Two of these flying boats were built, MNs 1552 - 1553, and 1551 was modified with interplane struts to a similar configuration. MN 1552 was delivered to the SVK in March 1918. Tested during the period 1 to 15 April, 1552 was flown several times but could not perform the trials required as modifications requested had not been carried out by the factory. Nevertheless, the W.20 was accepted by the Navy and the aircraft was modified by Brandenburg and the repaired Marine Numbers 1551 and 1553 were shipped to Warnemunde.(9)
   It appears that nothing further was done with these aircraft. The three W.20 boats were stored at Warnemunde when inspected by the Allied Naval Armistice Commission on 7 December 1918. The machines were dismissed as "an abandoned experimental design". Only the US appears to have acknowledged the advanced concept that the Germans were working on with regards to submarine borne aircraft.(10)
   Heinkel states that the W.20 was very important to himself as it enabled him to re-enter the aviation industry and eventually to set up his own company.
   Brandenburg received a contract for three all-metal monoplanes in June 1918, (MNs 2590 - 2592). Ernst Rothenburg was actively working on these aircraft that were apparently built and tested; however he does not describe whether they were flying boats or floatplanes, nor is their designation known. Work stopped in November 1918. There were two orders for monoplanes for submarine service that fit Rothenburg's recollections:
Orders for Monoplanes for Submarine Service
Marine No. Class Engine Notes
2590 - 2592 Bu Ur.II Work stopped November 1918
2790 - 2791 Bu Ur.II Work stopped November 1918

(9) The only known photograph showing 1551 after modification appeared in the German magazine Flugzeug, No.6,1991, P.67.This shows the wing cellule was modified to match that of 1552, confirming the Atlas drawings relating to the three machines, MNs 1551 - 1553.
(10) The British had sent out Sopwith Schneider floatplanes on the deck of the submarine E.22 in April and May 1916. They were launched and flew back to Felixstowe. The idea was not pursued owing to the difficulty of hoisting the aircraft when the submarine needed to submerge.


Brandenburg W.20 Specifications
Source Typenschau Gray & Thetford MN 1551 Gray & Thetford MN 1552 - 1553 SVK MN 1552 (1551 -1553)
Span, m 6.80/6A0 5.8 6.8 6.800/6.400
Chord, Upper, m 1.19 - - 1.190
Chord, Lower, m 1.51 - - 1.510
Length, m 5.93 5.91 5.925 5.925
Height, m - - - 2.900
Hull Length, m 5.45 - - 5.450
Hull Breadth, m 0.80 - - 0.800
Areas in m2
Wings 15.82 14.95 15.82 15.82
Ailerons 1.25 - - 1.25
Elevators 0.64 - - 0.64
Rudder 0.375 - - 0.375
Weights in kg
Empty 395 - 396 395
Loaded 567 - 568 567
Performance
Speed in km/hr 117 - - -
Time to 800 m 11.4 minutes - - 11.4 minutes
Time to 1000 m 14.9 minutes - 14.9 minutes 14.8 minutes
Endurance - - 1 1/4 hour -
Motor 80-hp Oberursel 80-hp Oberursel U.O 80-hp Oberursel U.O 80-hp Oberursel