LWS-6 Żubr


The LWS-6 Żubr was a Polish twin-engined medium bomber, produced by the LWS factory before World War II. A short series was used for training only, because it was inferior to the contemporary PZL.37 Łoś design.

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 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.

Development

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.
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 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.
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.

Technical design

The LWS-6 was a conventional high-wing cantilever monoplane. It featured mixed construction. The fuselage was rectangular in cross-section, made of a metal frame, covered with metal and canvas 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, take-off power: 680 hp. Bombs were carried in a bomb bay in the fuselage, the maximum load was 660 kg.

Operational history

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. 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.

Operators

Citations