The LWS-6 Żubr (PZL.30 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 PZL.37 Łoś design.
In 1933, Państwowe Zakłady Lotnicze in Warsaw received an order for a passenger plane from PLL LOT from the Civil Aviation Department of the Ministry of Transport. The new aircraft was to replace Fokker F-VII B / 3m aircraft used in PLL LOT. The plane was to be powered by two engines and take 12-14 passengers on board.
In 1931, the PZL-27 aircraft was designed in PZL in Warsaw, and was to be a communication aircraft. Its designer was the engineer Zbysław Ciołkosz. PZL-27 was a three-engine, 7-seat aircraft. Mixed metal-wooden construction. At that time, it was estimated that an all-metal plane would be heavier than a mixed-material aircraft. Not all metal working processes had been mastered.
As work on the PZL-27 was nearing completion, work on a new passenger plane was commissioned to engineer Zbysław Ciołkosz. The passenger plane was designated PZL-30. The aircraft was designed in a high-wing system with two engines. The fuselage was designed as a welded structure made of steel. Wings with wooden structure covered with plywood. The plane was to take 12 passengers or 1,000 kg of cargo on board. Range of a minimum of 1,000 km. Two Pratt Whitney Wasp Junior star engines with 295 kW (400 hp) were selected for the drive. These engines were known for their good quality and reliability.
At that time, the Department of Aviation of the Ministry of Military Affairs planned to order a new bomber plane. Therefore, engineer Jerzy Dąbrowski started working on the PZL-37 Łoś bomber. Because the PZL-37 Łoś project was a high-risk project, it was decided to convert the PZL-30 aircraft into a bomber. The idea was propagated by Colonel Tytus Karpinski, who convinced Colonel Ludomił Rayski, head of the department on the idea. The arguments were: ready design, the possibility of building an aircraft from domestic materials and mastered technologies.
Engineer Zbysław Ciołkosz redesigned the PZL-30 aircraft. The new aircraft was designated PZL-30 BI, and the army gave the name Żubr. The plane was to have a bomb load of 1,200 kg and four crew members. According to the doctrine of Douhet and on French recommendations, the bomber plane was not to have a high flight speed, but should have strong defensive armament. Therefore, the bomber was equipped with 5 machine guns.
The PZL-30 BI aircraft were equipped with movable gun turrets for the first time in Poland. The rear turret was retractable. It was developed by Zbysław Ciołkosz and Ludwik Białkowski. The turret was patented – Polish Patent No. 22638 filed on 1935-01-26 and granted on 1936-01-14.
The PZL-30 BI aircraft were equipped with a three-point landing gear, with a tail wheel. The main undercarriage is retractable. The way of hiding is unusual. The wheels hid into the sides of the fuselage – similar to flying boats. The mechanism is screw, crank operated; about 200 turns. The bolt is placed vertically in the fuselage and pulls the two chassis half shafts. It is 1.50 m long, 60 mm in diameter and therefore very heavy. This way of retracting the chassis was patented – Polish Paten No. 21888, filed 1934-04-19, and granted 1935-08-19.
The problem is that the benefit of retracting the chassis was a speed increase of only 14 km / h. This system was abandoned and the classic way of retracting the chassis in the engine nacelles was used. French electric motors were used to retract the chassis
The construction of the prototype of the Żubr aircraft began at the factory in Mokotów and was completed at the factory in Okęcie-Paluch. It was the first aircraft built in the new factory PZL Wytwórnia Płatowców Nr 1. Żubr aircraft received military type No. 71, and the first prototype No. 71.1. Aircraft PZL-30 BI No. 71.1, the first flight was made in March 1936. The pilot was Bolesław Orliński. On 1936-04-24, the aircraft was transferred to the Institute of Aviation Technical Research. By 1936-07-03, the aircraft had flown in 45 hours. The assessment found that the plane was flying correctly, but due to the weak engines, the performance was insufficient.
In order to improve performance, the engines were replaced with PZL-Bristol Pegasus VIII engines, 500 kW (680 HP), which was produced in Poland for PZL-23 Karaś aircraft. Engine replacement has been extensively analyzed. PZL-30 BI aircraft No. 71.1 was rebuilt into PZL-30 BII. Received: new engines, PZL-Bristol Pegasus VIII, new chassis retraction system. The weight of the aircraft increased from 2,891 kg to 4,004 kg. The aircraft was tested from 1936-09-23 to 1936-10-28. 35 hours of testing
As a result, the Polish Army ordered 15 PZL-30 BII aircraft, with some changes. Several times the tail was changed on the plane: it was single, double and single, but widened. Aircraft production was placed in Lublin at LWS, which at that time had spare capacity. Initially, the aircraft was to bear the designation LWS-4, and eventually LWS-6.
Even before the production of Type 71 aircraft began in Lublin, work was started on a bomber-torpedo plane for Naval Aviation. The plane was to receive floats. The aircraft was to replace the R-XX (LWS-1) aircraft. The new aircraft received the designation LWS-5, but remained in the design phase.
Fifteen LWS-6 aeroplanes were delivered to the Polish Air Force in 1938-1939. From the beginning they were considered obsolete, and were assigned to training units. The Polish Army received the first LWS-6 aircraft in August 1937. The price of one aircraft was set at PLN 211,000. Over time, the price increased to PLN 280,000, and with weapons – PLN 300,000. In use they revealed several faults – for example, the undercarriage retracted on some planes during landing. However, there was only one crash in a military aviation, without fatal injuries. Reportedly, they flew with the undercarriage fixed in the open position later. As training aircraft they had their armament removed. The Żubr was inferior to its counterpart the PZL.37 Łoś, developed at the same time. For a similar price, it was of now obsolete design, slower, with inferior performance, and a much smaller bomb load.
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.
After the outbreak of war, two planes found their way to Grójec. As the front approached, one plane was evacuated. The other was ordered to burn because of a lack of crew. At that time, two mechanics: Platoon Sowa and Platoon Nowak, without flight training, took off in this plane and flew to Warsaw. This proves that the Żubr aircraft is very easy to fly. This aircraft was transferred to Lviv to Skniłów airport. Here the plane was bombed by the Germans. Other planes were destroyed in Małaszewicze and Świdnik. The planes being in Dęblin were taken over by the Germans.
The Germans captured several LWS-6, including the twin-tailfin prototype, and used them for blind flying training until at least 1942 (among others, in Blindflugschule Schleissheim). Ironically, the Luftwaffe service of this bomber was longer than the Polish one.
The Soviets captured four aircraft after their invasion on Poland and next used them in communication aviation.
Apart from the Polish Air Force, Romania showed an interest in the Żubr prototype in 1936, and wanted to buy 24 planes. However, after the prototype crash on November 7, 1936 over Michałowice with two Romanian officers on board, they ordered the PZL.37 Łoś instead (the factory published a cover-up story, that the crash was caused by one of Romanians opening the door during flight).
Detailed tests have revealed that the disaster occurred due to the lack of a bottom strut between the girders. This led to the wing twisting and pulling the engine out of the wing. The directional arrangement of the covering plywood was also incorrect. The cover and girders worked poorly together. There were also technological disadvantages of the gluing process.
The disaster had further consequences. In 1937, engineer Zbysław Ciołkosz left Lublin and was employed at the Podlasie Aircraft Factory. Work on improving the LWS-6 Żubr aircraft was undertaken by engineer Jerzy Teisseyer.
To cover the wings of the aircraft, a thicker plywood of 6 mm was used. The new aircraft received type number 71.2 and officially the name LWS-6. A double vertical tail was also used. The plane had a mass 4,480 kg. The first flight was made in December 1937. On 1938-01-07, the aircraft was sent to Warsaw for testing (Aviation Technical Institute).
In the summer of 1937, series production of the aircraft resumed under the designation LWS-6 A. The aircraft received a single vertical tail. 15 aircraft were built, which received nos. 71.3 – 71.17. The army picked them up in the summer of 1938. In 1938-11-03, one of these aircraft was exhibited at the Salon in Warsaw.
The aircraft was conventional in layout, high-wing cantilever monoplane, of 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), front part was made of duralumin. Wings were of wooden construction, plywood covered. There was a crew of four: pilot, commander-bombardier, radio operator and a rear gunner. The bombardier was accommodated in a glazed nose, with a forward machine gun turret and a significant pointed “beard” below. The pilot’s canopy was above a fuselage, offset to the left. The rear gunner operated an upper turret, elevating to a working position. The main undercarriage retracted into engine nacelles. The plane was powered by two Bristol Pegasus VIII radial engines, normal power: 670 hp (500 kW), take-off power: 700 hp (522 kW). Bombs were carried in a bomb bay in the fuselage, the maximum load was 660 kg.
In Lublin, they did not stop at modernizing the aircraft. Construction of a metal wing, lighter than wooden, began. The hull was redesigned and the front part of the hull was significantly changed, which was similar to the Liberator bomber. On the day the war broke out (1939-09-01), the new aircraft was almost complete.
Crew: four (pilot, commander-bombardier, radio operator, rear gunner)
Length: 15.40 m (50 ft 6 in)
Wingspan: 18.50 m (60 ft 8 in)
Height: 4 m (13 ft 2 in)
Wing area: 49.5 m² (532.6 ft²)
Empty weight: 4,788 kg (10,533 lb)
Loaded weight: 6,747 kg (14,843 lb)
Useful load: 1,959 kg (4,319 lb)
Max. takeoff weight: 6,876 kg (15,127 lb)
Powerplant: 2 × Bristol Pegasus VIII 9-cylinder air-cooled radial engine, 700 hp (522 kW) each
Maximum speed: 341 km/h (212 mph)
Cruise speed: 280 km/h
Range: 750-1250 km (466-776 mi)
Service ceiling: 6,700 m (21,975 ft)
Rate of climb: 408 m/min (6.8 m/s) (1,338 ft/min)
Wing loading: 129 kg/m² (26.4 lb/ft²)
2 × 7.7 mm Vickers F machine guns in nose turret
2 × 7.7 mm Vickers F machine guns in upper rear turret
1 × 7.7 mm Vickers F machine gun in underbelly
660 kg (1,450 lb) of bombs