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Bartini Stal 6

 

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Stal-6 fighter prototype.
 
The work started by AI Putilov in the OOS of the NII GVF on the use of steel in aeronautical construction was followed by other groups of engineers.On April 30, 1930, Bartini proposed to the management of the Central Construction Bureau (TsKB) the creation of a fast airplane with a totally clean aerodynamically airframe. The project was analysed and approved, but its realization was very far away.
 
First of all, Roberto Bartini , despite being a communist, was Italian and therefore could not be allowed to have access to the secrets of the Soviet aeronautical industry. Second, most airplane builders at that time did not believe in the "wonderful foreign engineer" and therefore did not wish to share their experience with him.
 
Fortunately, help came from the VVS chief Pyotr Ivanovich Baranov and the RKKA chief of armaments Mikhail Nicolaevich Tukhachevsky. They succeeded in starting in 1932 the fast plane project works at the CNII (Factory No. 240).
 
The Stal-6 was originally conceived by Bartini as a fighter under the name EI ( Э кспериментальный И стребитель - Experimental Fighter). As it was developed within the structure of a civilian research institute, no weapons were installed.
 
Structurally, it was conceived as a low-wing single-seater monoplane, with a wing area of ​​14.3 sq.m. The tail unit was of the monoplane type. One of the most unusual features of the new aircraft was the presence of a single 800 x 200 mm wheel on the front centre portion of the fuselage with two stabilization skis on the underside of the wings. Both the wheel and the skis were fully retractable. The retraction system was manual using a rope system.
 
The structure was made of chromium - moly steel tubes (hence the name Stal - Steel) and the skin was an aluminum-magnesium alloy (AltMag, Electron) on the entire front and Bakelite on the tail. The wings featured a steel tube structure with a stainless-steel coating. The wing ribs were made of U-type steel profiles. The fuselage was noted for its aerodynamic cleanliness with the cockpit deck flush with the top of the fuselage. The weight of the aircraft with the 660 hp engine was only 850 kg.
 
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Stal-6 structural drawing
 
The aircraft used an evaporative water-cooling system inside the wings. The engine's cooling water was heated to steam. The steam was circulated through the double wing cladding made of 'enerzh-6' (stainless) steel. There it condensed to become a liquid state again and cooled to return to the engine. This configuration avoided the use of external radiators, improving the aerodynamics.
 
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The control of the elevators could be adjusted so that the pilot could vary the pressure to exert on the control stick depending on the flight regime and speed.
 
To minimize air resistance, the cockpit was inserted into the fuselage, without protruding. Visibility of the runway during takeoff and landing operations was ensured by moving the pilot's seat upward, with the pilot's head protruding above the level of the fuselage. During the flight the seat was returned to its normal position.
 
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In 1933, under the direction of Bartini, the construction of the single-seat Stal-6 (Russian Сталь-6) began.
 
The first flights began on October 7, 1933. In one of the first flights 400 km / h were reached. Pilots A.B.Yumashiev and P.M.Stefanovsky demonstrated that take-off and landing operations with a single centre wheel were as safe and easy as with conventional gear.
 
The speed reached at sea level was 420 km / h, almost 100 km / h higher than that of contemporary fighters and was a new world speed mark for aircraft in its class. It is noteworthy that the main RKKA fighter at the time was the Polikarpov I-5 that reached only 280 km / h and the new fighter also devised by Nikolai Polikarpov that was preparing to be tested at that time, the I-16 with an M-22 engine, only reached 359 km / h.
 
The reaction that the Stal-6 provoked was not what was expected. VVS authorities immediately ordered its builders to work on increasing the speed of their fighters. The builders began to protest, claiming that increases in speed were impossible to achieve in such a short time.The GVF (Civil Aviation Fleet) was ordered to build a fighter based on the Stal-6. This fighter was named Stal-8.
 
Important discussions centred on the vulnerability of the use of the vaporization cooling system. All water-cooled systems are generally more vulnerable than air-cooled systems, unless they are specially armored as in the Ilyushin Il-2. However, the increased area of ​​the cooling surface in the vaporization system, which increases the risk of impact, is offset by the less impact of a hole in the entire system.
 
During the Stal-6 flight tests, the pilot ABYumashiev carried out several flights with the cooling system switched off to mimic damage to the cooling system. Each flight lasted more than 30 minutes. Taking into account that the autonomy of the majority of Soviet fighters of the time did not exceed one hour, it follows that the damaged fighter could not only return to its base satisfactorily, but could also finish its combat without problems.The Stal-6 used a single cooling unit and this could have been a development point as well, as segmented systems tend to be less vulnerable.
 
Engine: 1 × V-1570 Curtiss Conqueror, 469.8 kW (630 hp)
Wingspan: 9.0 m (29 ft in)
Wing area: 14.3 sq.m (154 sq.ft)
Length: 6.88 m (22 ft 6-3/4 in)
Height: 7.316 ft / 2.23 m
Empty weight: 850 kg (1,874 lb)
Gross weight: 1,080 kg (2,381 lb)
Fuel capacity: 135 kg
Maximum speed at height: 420 km/h (261 mph)
Maximum speed at sea level: 350 km / h
Landing speed: 110 km / h
Ceiling: 8000 m
Rate of climb: 21 m/s (4,135 ft/min)
Climb to 1000m: 0.8min
Climb to 2000m: 2.0min
Climb to 3000m: 3.0min
Endurance: 1.5 hours
Practical range: 180 km
Take-off run: 150 m in 10 seconds
Landing roll: 280 m in 20 seconds
Crew: 1
 
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