The L.W.S.4/ L.W.S.6 Zubr (Bison) was an unsuccessful design for a medium bomber that was produced in limited numbers as an operational trainer and as backup in case the more advanced P.Z.L P.37 Los failed, but that was unable to operate from temporary airfields and thus made no contribution to the fighting in 1939.
Work on the aircraft began at P.Z.L. (as the P.Z.L. 30), but late in 1935 it’s designer, Zbyslaw Ciolkosz, was appointed as technical director of the newly formed L.W.S. company, which had taken over the bankrupt Plage and Laskiewicz company (generally known as Lublin), and the project came with him.
The aircraft had originally been designed as a ten or twelve passenger civil airliner, to be powered by two 400-450hp Pratt & Whitney Wasp Junior radial engines. It was hoped that the Polish airline Lot would purchase the new aircraft to replace its Fokker F.VIIIb-3m aircraft. Lot weren’t interested in the design, but P.Z.L. continued work on it, in the hope that the airline would change its mind once a prototype had been built. However in 1935 Lot decided to purchase Douglas and Lockheed aircraft, ending any chance of success for the civil airline version.
At the same time Ciolkosz had been working in a medium bomber development of the design, and late in 1933 the Polish Air Force decided to fund a further study of the design. By 1935 the Air Force was more interested in the P.Z.L. P.37 Los, which would be the most modern aircraft in Polish service in 1939, but the Department of Aeronautics decided to order a single prototype of the P.Z.L.30, with the aim of using it as an operational trainer and as a cheaper second line bomber.
The P.Z.L.30 was a rather ungainly looking aircraft. It was a shoulder-wing cantilever monoplane, with twin engines. It was of (very) mixed construction, with a two-spar wooden wing covered in plywood. The front of the fuselage used a duralumin structure with a smooth duralumin skin and the rear of the fuselage had a frame of chrome-molybdenum tubes covered with fabric. It had a rather odd retractable undercarriage where the main wheels folded inwards towards the fuselage. The wheels were sheltered within the fuselage, but the long legs remained exposed while in flight. The fuselage itself was narrow looked, but only because it was very deep, with a rather ugly sharp ‘chin’ that contained the bomb aimer’s position (this perhaps demonstrates the close contact between the Polish and French aircraft industries, as several French aircraft of the period were equally ugly looking!). The combination of the deep fuselage and high wing position meant that the fuselage was almost touching the ground when landed, which did at least make it easier for the crew to enter the aircraft. It was at least well armed with twin guns in the nose turret and dorsal turret and space for one or two guns in the ventral position. The pilot and radio operator sat in tandem cockpits above the leading edge of the wing, rather oddly positioned on the left hand side of the fuselage. It could carry ten 220lb bombs in racks inside the fuselage.
The P.Z.L.30/I prototype was powered by two 406hp Pratt & Whitney SB nine-cylinder radial engines. It made its maiden flight at Warsaw in March 1936, and was the first Polish twin engined bomber to fly. It then went for more comprehensive tests, certification and utilization trials, where it proved to be reliable and comfortable for its crews, but rather underwhelming in performance, with a top speed of 172mph and a ceiling of only 15,090ft. As a result the Department of Aeronautics ordered Ciolkosz to give the prototype more powerful 680hp P.Z.L. Pegasus VIII radial engines and modify to operational standards, as an alternative in case the P.37 was a failure. The original aircraft became the P.Z.L.30A, while the Pegasus VIII powered version became the P.Z.L.30B/I. It was later given a retractable ventral turret and became the P.Z.L.30B/II. The aircraft was given the name Zubr (Bison).
Early in 1936 Ciolkosz moved from P.Z.L. to Lublin, and the P.Z.L.30 followed him. The P.Z.L.30B was redesignated as the L.S.W. 4 Zubr, and the prototype was moved to Lublin to be converted to the new standard. L.S.W. also received a contract to build fifteen L.W.S.4s, which were to equip two bomber squadrons. At this point the design looked to have a strong future – the Polish Navy was convinced to replace an order for six Lublin R-XXA floatplanes with one for six L.S.W.5s, which would have been a floatplane version of the L.S.W.4. The Romanians were also interested in the type and indicated that they would probably order 24 examples powered by Gnome-Rhone Mistral engines.
Things began to go wrong with the conversion of the prototype into the first L.W.S.4. The fuselage was barely modified to cope with the more powerful Pegasus engines. A new type of undercarriage, with the wheels retracted into the engine nacelles, was ordered from France, but proved to be troublesome, and on at least one occasion the aircraft had to land with the wheels only partly down. Damage to the wing spars was missed, and on 7 November 1936 the prototype crashed while carrying two Poles and two Romanians from an airforce delegation. All four men were killed. Unsurprisingly this ended Romanian interest and they ordered the Italian Savoia Marchettil S.M.79B instead.
In the aftermath of the crash Zbyslaw Ciolkosz was replaced as technical director by Ryszard Bartel, and a comprehensive investigate was carried out. Unsurprisingly these revealed that the wings were too weak to cope with the more powerful engines. Problems also emerged with the casein glue that had been used in its construction, which proved to be weaker than believed. L.W.S. then produced their own aviation glue, sold as Formalit, which was one of the company’s more successful products. A new team leader, Jerzy Teissyre, took over the development of the Zubr.
The new team was forced to strengthen the fuselage, which made it heavier and reduced its useful payload. This ended the Navy’s interest, and the L.W.S.5 was cancelled (instead they ordered the rather superior Cant Z.506B from Italy, although only one had arrived by the outbreak of war in 1939). The changes were implemented on the first of the L.W.S. built aircraft, which was also given a new twin fin and rudder tail assembly, becoming the L.W.S.6.
Despite these setbacks production of the fourteen L.W.S.4s remaining from the original order resumed in 1938. The production version was given a new single fin and rudder tail. The French undercarriage continued to cause problems, with the French blaming L.W.S. for using it on a heavier aircraft than it was built for and the Poles blaming faulty locking mechanisms and weak electrical motors. Eventually most aircraft had their undercarriage locked in the down position. Work was also underway on a new chrome and molybdenum wing, with one prototype almost ready when the war began.
After all of the effort that went into building it, the L.W.S.4 (or possibly L.W.S.6 – sources disagree on the designation for the production aircraft) had a very limited service career. Some were used as bomber trainers by the IIIrd (Training) Dyon. Not all of them had been delivered by the time the Germans invaded, and the ones that were with the Air Force couldn’t take off with a useful payload from the temporary airfields used by the dispersed Polish Air Force. As a result most of them spent the entire campaign unused at Malaszewicze, where some were destroyed by German bombing. Some were captured intact by the Germans, and may have been used as operational trainers.
Engine: Two P.Z.L. Pegasus VIII nine-cylinder air-cooled radial engine
Span: 60ft 8.75in
Length: 50ft 6.25in
Height: 11ft 6in
Maximum Weight: 15,158lb
Maximum Speed: 236.1mph at 14,763ft
Cruising Speed: 186.4mph at 14,763ft
Range: 776 miles
Bomb load: Ten 200lb bombs inside fuselage, two 220lb or four 110lb bombs under the wings