Suchoj Su-7 B in museums in Poland. 2020r. - Construction
Suchoj Su-7 B in museums in Poland.
Su-7 BKŁ chassis structure.
The Su-7 BKŁ aircraft has a specific landing gear. The designation of the BKŁ is the Russian Koliesno-Łyżny, Колесно-Южный, in Polish Kołowo-Płozowe). In addition to the classic three wheels, the main landing gear legs feature small skids. The Su-7 BM, U planes have a classic landing gear. The chassis is three-support with a front wheel. It is retracted and released hydraulically. The front leg is equipped with a vibration damper, which also acts as a steering (steering) wheel. All wheels are single. The main landing gear tires measure 860x230. Oil-air shock absorbers. The front landing gear retracts towards the front of the fuselage and is covered with two symmetrical flaps. The main landing gear retracts towards the fuselage, towards the wings. The tires have a relatively large diameter (860x230) for the use of ground airports and are high pressure. The spacing of the main chassis is 3.83 m. The base of the chassis is 4.78 m.
During the first months of operation, it turned out that the aircraft had difficulties taking off and landing at ground airports. Especially in early spring and autumn, when the ground is saturated with water.
In 1959, work began on a prototype designated as S-23 (based on Su-7), which replaced the classic wheeled chassis with special skates (skids). They were made of an aluminum alloy by the casting method. Smooth underside, visible reinforcing ribs from the top. The runners were approximately 60 cm x 18 cm.
During the tests, it turned out that on heterogeneous ground, with puddles, mud and frozen places, the skid has different friction and can push the plane aside when moving. The system worked well when the plane landed, as the skates did not get stuck in the relatively soft ground. On the other hand, during take-off, the skids put up much more resistance than the wheels. Other problems also arose. After a frosty night, the skids froze to the ground and the plane could not be moved.
At that time, the designers used an additional installation to lubricate the skid with oil. There are several holes in the bottom of the skid through which the oil leaks, which caused equal dad on both amphibians and cooled the skids. With such a plane, it was even possible to perform a "controlled skid" to throw the plane backwards. The system proved successful, but only at airports with unpaved surface. It turned out, however, that the additional installation complicates the structure of the aircraft, and the aircraft cannot be operated on a concrete surface.
In 1962, another prototype of the S-22-4, built on the basis of the Su-7 B, was tested. The skids were mounted on the outside of the main wheel and were automatically lowered by two hydraulic cylinders when the plane landed on the ground. When taking off or landing from a concrete base, the skids remained in their initial position and the plane used only standard wheels for this. The tests were discontinued for unknown reasons.
However, the CCCP intelligence obtained information that the West had developed special bombs to destroy concrete surfaces at airports. Research on the wheel-and-skate chassis was resumed in 1963 with the development of the S-26 prototype, based on the Su-7 BM aircraft. The plane received a revised version of the skates known from the S-23 (the wheel remained in the front) to reduce their resistance to take-off, especially from a concrete ground, the system received a special skid-cooling installation (with oil). It was planned to implement the aircraft for production under the designation Su-7 BŁ, but eventually, due to the complexity of the installation, this solution was abandoned.
Su-7 BKŁ (rosyjskie Koliesno-Łyżnyj, Колесно-Южный).
In the years 1962-1964, factory tests were carried out, and in 1964-1966, state tests of the S-22-4 aircraft using some elements from other prototypes. The aircraft was recommended for serial production in 1965 as the Su-7 BKŁ. In addition to the aforementioned (in the case of the S-22-4 description) wheel and skid system, the plane received SPRD-110 launch accelerators, enlarged fuel tanks, two large braking parachutes (all innovations were tested on the S-25 aircraft), the ASP-semi-automatic sight PF-7 (replaced with ASP-PFM-7 in the 1970s), KS-4 ejection seat (allowed to leave the plane from 0 m altitude with a minimum speed of 140 km / h), AL-7 F1-250 engine (250 hours, engine weight 2,103 kg, fuel consumption 0.9 kg / kG / h), thinner armored windshield for improved visibility, reinforced aerodynamic combs, air brakes, new additional fuel tanks, 950 liters each. The armament was strengthened, and thanks to the reinforcement of the wings for new spare fuel tanks, the plane received an additional mounting point under each wing (installed from 1969) with a capacity of 2 x 250 kg.
The wheel-skid undercarriage of the main legs has been developed so that there are fewer changes to the undercarriage chamber. The skid has dimensions of 40 cm x 12 cm. Its main element was made by casting from an aluminum alloy. The skid received a metal skid in the form of a form placed on the skid and fixed with bolts. When it was worn out, only the metal slide was replaced, not the entire slide. The skid is lowered by two hydraulic cylinders that are attached to the rocker arm. Sometimes in the descriptions of the Su-7 BKŁ aircraft you can find the term - Terrain skid.
The production of the Su-7 BKŁ aircraft was carried out in the years 1965-1971 and ended with the construction of about 500 copies in series numbers 57-80.
Written by Karol Placha Hetman