|First flight||March 1936|
|Primary user||Polish Air Force|
The LWS-6 Żubr (PZL.30, wisent (En. Bison) ) was a Polish twin-engined medium bomber, produced by the LWS factory before World War II. It was produced in limited numbers and was used for training only, as it was inferior to the contemporary PZL.37 Łoś design.
The PZL.30 was designed by Zbysław Ciołkosz for PZL in the early 1930s, initially as a 12-passenger airliner. When the Polish Airlines LOT bought Douglas DC-2 aircraft instead, the design was converted to a bomber, with a projected bomb load of 1,200 kg. The PZL.30 was developed as a less-advanced, alternative medium bomber design, in case the more modern bomber design, the PZL.37 Łoś would fail. The first prototype, designated PZL.30 (or PZL.30BI) was flown in March 1936 by Bolesław Orliński.
The maiden flight of the PZL.30 took place only three months before the PZL.37 Łoś prototype flew. Although developed at the same time as the PZL.37, the PZL.30 was inferior to its counterpart, being slower and having a much smaller bomb load than the Łoś, yet produced at a similar price.
The PZL.30 prototype, initially powered with two 420 hp Pratt & Whitney Wasp Junior engines, was further refitted as the PZL.30BII, with more powerful Bristol Pegasus VIII engines. The aircraft was accepted for a limited production run by the LWS state factory in Lublin, of which Ciołkosz became the technical director and project lead. The second prototype was designated LWS-4.
It was planned to produce 16 aircraft for the Polish Air Force, with the designation: LWS-6 Żubr. The Żubr was given a military code number 71 and the series was to have factory numbers 71.1 to 71.17, but the aircraft were given different code numbers eventually due to secrecy.[Note 1]
The production aircraft incorporated further changes, among others the landing gear retracting into the fuselage sides was changed to more conventional one, retracting into the engine nacelles.  After a prototype crashed on 7 November 1936, caused by de-laminating plywood wing skins, the wing had to be reinforced. Due to this increased weight, the LWS-6 had a smaller bomb load than expected. A modified prototype was made with a double tail fin, and flown at the end of 1937. The definitive variant, however, in the course of production, returned to a single enlarged tail fin configuration. A series of 15 aircraft were built in 1938.
In 1937 the factory proposed to build a floatplane torpedo bomber variant LWS-5, instead of developing LWS-1 (R-XXA) design by Jerzy Rudlicki. It was supposed to be fitted with two Short floats of the Lublin R-XX prototype. At first the Polish Navy exhibited interest and work on the LWS-1 was cancelled despite its progress, but then due to problems with LWS-5 development, work on the prototype was canceled. It was evident that the underpowered LWS-5 could not carry an effective offensive load. [Note 2]
Apart from the Polish Air Force, the Romanian Air Force showed an interest in the Żubr prototype in 1936, and considered a purchase of 24 aircraft. On 7 November 1936, after the Pegasus-powered prototype encountered a mid-air structural failure and crashed at Michałowice, all on board dying, including two Romanian officers on board, Romania ordered the PZL.37 Łoś instead. [Note 3]
The LWS-6 was a conventional high-wing cantilever monoplane. It featured mixed construction (metal and wood). The fuselage was rectangular in cross-section, made of a metal frame, covered with metal (upper fuselage) and canvas (sides and bottom) with the front section made of duralumin. Wings were of wooden construction and plywood covered. There was a crew of four: pilot, commander-bombardier, radio operator and a rear gunner. The bombardier was accommodated in the glazed nose, with a forward twin machine gun turret with a notable pointed "beard" below the turret. The pilot's canopy was located above the fuselage, offset to the left. The rear gunner operated twin machine guns in a semi-retractable upper dorsal turret, elevating to a working position, along with a "trap" ventral position, operating the same way. The main undercarriage retracted into engine nacelles. The aircraft was powered by two Bristol Pegasus VIII radial engines, normal power: 670 hp (500 kW), take-off power: 680 hp (520 kW). Bombs were carried in a bomb bay in the fuselage, the maximum load was 660 kg.
A total of 15 LWS-6 bombers were delivered to the Polish Air Force in 1938–1939. From the outset, they were considered obsolete, and were assigned to training units, mainly to the 3rd Training Dyon. In operational service, the LWS-6 revealed several faults, for example, the undercarriage retracted on some aircraft during landing. Reportedly, the crews later flew with the undercarriage permanently locked down. As training aircraft they had their armament removed. There was only one crash, however, without fatal injuries.
During the invasion of Poland in 1939, Żubrs were not used in combat. Several were destroyed on the ground, along with many other training aircraft. The Germans captured several LWS-6s, including the twin-tailfin prototype, and used them for blind flying training until at least 1942 (among others, in Blindflugschule Schleissheim). The Luftwaffe service of this bomber was longer than the Polish one.
The Soviets captured four aircraft after their invasion of Poland and subsequently used them as communication aircraft.
- Polish Air Force for training
- Soviet Air Force used four captured aircraft for communication.
Data from Polish Aircraft 1893–1939
- Crew: 4 (pilot, commander-bombardier, radio operator, rear gunner)
- Length: 15.4 m (50 ft 6 in)
- Wingspan: 18.5 m (60 ft 8 in)
- Height: 3.5 m (11 ft 6 in)
- Wing area: 49.5 m2 (533 sq ft)
- Empty weight: 4,788 kg (10,556 lb)
- Gross weight: 6,747 kg (14,875 lb)
- Max takeoff weight: 6,876 kg (15,159 lb)
- Powerplant: 2 × PZL Pegasus VIII 9-cylinder air-cooled radial piston engines, 490–510 kW (660–680 hp) each
- Propellers: 3-bladed Hamilton/PZL variable-pitch propellers
- Maximum speed: 380 km/h (240 mph, 210 kn) at 4,500 m (14,764 ft)
- Cruise speed: 300 km/h (190 mph, 160 kn) at 4,500 m (14,764 ft)
- Range: 1,250 km (780 mi, 670 nmi)
- Service ceiling: 8,000 m (26,000 ft)
- Rate of climb: 6.8 m/s (1,340 ft/min)
- Wing loading: 138.9 kg/m2 (28.4 lb/sq ft)
- Power/mass: 0.0893 hp/lb (0.1468 kW/kg)
- 2 × 7.7 mm Vickers F machine guns in nose turret
- 2 × 7.7 mm Vickers F machine guns in upper dorsal rear turret
- 1 × 7.7 mm Vickers F machine gun in ventral underbelly position
- 660 kg (1,450 lb) of bombs
Aircraft of comparable role, configuration, and era
- Bloch MB.131
- Bristol Blenheim
- Dornier Do 17
- Fokker T.V
- Junkers Ju 86
- Lockheed Hudson
- Martin B-10
- Mitsubishi G3M
- Potez 540
- PZL.37 Łoś
- Tupolev SB
- In many publications, the Żubr was erroneously designated the LWS-4, a designation also used for a light fighter project (PZL. 39).
- The factory continued design work on the Żubr, and, in 1939, developed a lighter wing of steel construction and a refined fuselage, but these changes were not incorporated due to the outbreak of World War II.
- The factory published a cover-up story, that the crash was caused by one of Romanians opening the door during flight.
- Fitzsimmons, Bernard, ed. "Żubr, L.W.S. 4." The Illustrated Encyclopedia of 20th Century Weapons and Warfare. London: Latimer House Ltd., 1979. ISBN 978-0-9067-0400-4.
- Glass, Andrzej. Polskie konstrukcje lotnicze 1893-1939 (in Polish) ('Polish aviation constructions 1893-1939 . Warsaw: WKiŁ, 1977, no ISBN.
- Morgała, Andrzej . Samoloty wojskowe w Polsce 1924-1939 (in Polish) (Military aircraft in Poland 1924-1939). Warsaw: Bellona, 2003. ISBN 83-11-09319-9.
- Taylor, John W. R. "PZL L.W.S. 4 and L.W.S. 6 Żubr." Combat Aircraft of the World from 1909 to the Present. New York: G.P. Putnam's Sons, 1969. ISBN 0-425-03633-2.
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