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The L.W.S.5 Zubr was a floatplane version of the disastrous L.W.S.4, and had to be cancelled while the prototype was under construction after an investigation into the crash of the original prototype of the L.W.S.4 meant that the structure had to be strengthened, making the floatplane version too heavy to carry a useful payload.
When the new L.W.S. company was formed in December 1935 it inherited a number of designs from the old Lublin firm (Plage and Laskiewicz) including the Lublin R-XXA twin engined float plane torpedo bomber. This was the largest float plane to be developed in Poland, and one of the largest aircraft to be developed in the country in the inter-war period, and the prototype made its maiden flight in the summer of 1935. However Major Sipowicz, the managing director of L.W.S., believed that it would be inefficient to set up a production line for the relatively small number of R-XXs that the Navy was expecting to order (six in the first batch and probably six more each year after that), and was able to convince the Navy that it would be more economical to develop a floatplane version of the L.W.S.4, which at the time was expected to enter production in quite large numbers.
The Navy was convinced, and replaced their order for six R-XXAs with a letter of intent to order six L.W.S.5s once the type had been proven. The Navy’s version would be powered by their choice of engine, two 750hp Bristol Pegasus III radial engines. Work on the prototype of the L.W.S.5 then got underway. The L.W.S.5 would have been a rather ungainly looking aircraft. It would have had shoulder mounted wings, and an unusually deep fuselage. The undercarriage caused problems on the L.W.S.4, and the vertical struts for the floats would have been unusually lengthy.
The entire Zubr programme began to crumble on 6 November 1936 when the prototype L.W.S.4 crashed while carrying two Romanian air force delegates, killing everyone on board. The Romanians immediately ended their interest in buying the aircraft, and a comprehensive investigation into the cause of the crash was carried out. This found that the wing wasn’t strong enough to cope with the more powerful Pegasus engines, problems with the type of glue being used, and that damage caused by a landing with the undercarriage only partly down had been missed.
As a result the structure of the aircraft had to be strengthened, increasing its weight. This reduced the usefulness of the land plane version (and would mean that the few that had been built were unable to operate from the temporary airfields used in 1939), but it was a disaster for the floatplane L.W.S.5, which already had to cope with the extra weight of the floats. As a result the military payload was reduced so much that the type became effectively useless. The Navy cancelled their letter of intent and ordered L.W.S. to stop work on the prototype, which was at an early stage of construction. The Navy then went on to order the Italian Cant Z.506B floatplane, a rather superior design (although only one was actually delivered).
If it had worked as hoped the L.W.S.5 would have been an improvement on the R-XX, with longer range, a higher ceiling, and probably higher speed – the land plane version was expected to have a cruising speed of 186.4mph, faster than the top speed of the R-XX. However the floats would have increased weight and drag, so the L.W.S.5 wouldn’t have performed as well as the L.W.S.4/ L.W.S.6. In contrast the Cant 506B’s proven speed as a float plane was the same as that of the L.W.S.4, and it had much longer range and endurance.