Service and Combat Record
The Polikarpov I-15 was a gull-winged biplane that made its name fighting for the Republican cause in Spain, where it earned its nickname of 'Chato', or 'Snub Nose'.
Soviet doctrine during the 1930s called for two types of fighters that would operate together. Fast monoplanes would be used to break up enemy formations, and manoeuvrable biplanes would take advantage of the resulting confusion to dogfight with the scattered enemy aircraft. This plan appeared to work well in exercises, where an artificially high level of coordination was maintained between the two types of aircraft, but in actual combat conditions soon fell apart, leaving the slower biplanes unable to actually catch their opponents.
Polikarpov's first design for a biplane to replace his own I-5 was produced during 1932 as the I-14a (alongside Sukhoi's I-14 monoplane). This aircraft shared many features with the I-15, and was a gull-winged sesquiplane powered by a radial engine. In other ways it was more advanced, with a retractable undercarriage and fully enclosed cockpit.
Both projects were approved for development. Polikarpov's design was given the military designation I-15 (standing for the fifteenth fighter design), and the design bureau designation TsKB-3. This stood for Tsentrainoe Konstruktorskoe Byuro (Central Design Bureau), an organistion that had originally been created in September 1926 as part of the wider Aviatresta organisation. This was replaced by the VAO, a national association responsible for aircraft design in 1930. At this point Polikarpov was actually working with the Soviet prison system, at the Internal Prison Design Bureau, known at KB VT. This was absorbed into the new TsKB VAO later in 1930, but remained under the overall control of the OGPU (Main Political Directorate, the precursor of the KGB). This arrangement lasted until 1931 when an increase in the number of design teams resulted in their take-over by the Central Aero- and Hydrodynamics Institute. Polikarpov's team were now part of the TsKB TsAGI. Finally, at the start of 1933 the bureau came under the control of the newly formed Main Directorate of the Aviation Industry (GUAP), becoming TsKB GUAP. The TsKB-3 was the third design to be allocated a number under this new system.
Work on the I-15 began in February 1933. The new aircraft was a single-bay sesquiplane (with a much smaller lower wing). The retractable undercarriage and enclosed cockpit of the earlier I-14a were gone, replaced by a more conventional fixed undercarriage and open cockpit. The upper gull wing was retained, meaning that the upper wing had no centre section and was instead connected directly to the upper fuselage by diagonal sections. This eliminated the drag caused by the central wing section and its associated struts, reduced the wing loading of the aircraft and thus increased its manoeuvrability. It also gave the pilot's a better view directly above the aircraft, but restricted the view to left and right.
The fuselage was made with a framework of welded steel tubs. The front of the fuselage was covered in duralumin and the rest in fabric. The wings had two wooden spars but steel structural members. The central 'gull' section had a metal tube frame.
The prototype was powered by an imported Wright Cyclone SGR-1820/F-3 engine, producing 630hp at sea level and 715hp at 6,500ft. The engine was surrounded by a Townend ring, a narrow band that just covered the engine cylinders. The Cyclone engine was a significant improvement on the 480hp M-22 engine used in the earlier I-5, itself a clone of the British Bristol Jupiter radial engine.
The I-15 was armed with four PV-1 machine guns carefully mounted to fire between the engine cylinders. Two were level with the propeller and two towards the top of the fuselage. Many aircraft only carry two of these guns. The aircraft could also carry bomb racks under the wings.
The first prototype was completed in October 1933, and began factory evaluation tests on 18 November. These were interrupted by an accident of 23 November, but were still completed in only 26 days. During these tests the aircraft was equipped with ski landing gear, and achieved a top speed of 201mph at sea level and 218mph at 6,500ft. This increased to 228mph at 10,000ft when wheels were installed.
The second prototype was completed in December 1933, and had the wheeled undercarriage, with the wheels contained in streamlined spats. This aircraft was used for the State Acceptance Trials. The first stage of these trials took place in the same month, the second in April 1934.
Work on putting the I-15 into production began at the start of 1934, before the type had completed it acceptance trials. The aircraft was to be produced at two state aircraft factories, Zavod 1 and Zavod 39, both in Moscow. The first aircraft from No.39 underwent trials in August-September 1934, the first from No.1 in December. The two aircraft were of rather different quality, with the aircraft from zavod 1 markedly inferior. It was over-weight and of poorer build quality.
The prototypes and pre-production (or very early production aircraft) used imported Wright Cyclone engines. Production of the licence built version of this engine, the M-25, was slow to begin, and so during 1935 several hundred I-15s were built with the earlier 480mhp M-22.
The I-15 was not a universally popular aircraft. The gull wing configuration was generally to blame, restricting the pilot's view during take-off and landing. It was also blamed for the aircraft's poor directional stability, although this improved at higher speeds. Towards the end of 1935 the Air Force asked for production to be stopped and the aircraft phased out of service. Polikarpov met privately with Stalin, who now held him in high regard, and convinced the Soviet dictator to keep the aircraft in production and in operations.
Despite Stalin's support for the type production of the I-15 was suspended during 1936. According to some sources work resumed during 1937. Zavod No.1 was given responsibility for producing the I-15, which was finally powered by the licence-built M-25 Cyclone engine. Perhaps more likely is that this reflects a program to re-engine older aircraft with the M-25.
Different sources give slightly different production figures for the I-15. Some suggest that fifty nine were produced with imported Cyclone engines, 400-404 with the M-22 and 270 with the M-25. Others give a lower overall figure of 384 aircraft with all engines. Either figure make it numerically the least important of the I-15 family, and it was followed by 2,408 I-152s and 3,437 I-153s, for a total production run of 6,578 aircraft.
Service and Combat Record
The I-15 had a fairly short front-line career in the Soviet Union. The gull wing configuration was not popular at high level, or with some pilots, and the weight savings that helped improve its performance also meant that it wasn't quite as sturdy as earlier aircraft. Most I-15s had been replaced in front line service by 22 June 1941, but the type was still being used by the 3rd Fighter Regiment of the Black Sea Fleet at Sevastopol.
The I-15 made its biggest impact in Spain, where it was provided in large numbers to the Republican government. The Spanish Civil War began on 18 July 1936 when the fascists rebelled against the elected government. The Spanish Government asked for assistance from the Soviet Union, and in August Stalin agreed to provide equipment and men.
The I-15 was supplied to Spain in batches of 31 aircraft, the equivalent of a full Soviet squadron. The total number of aircraft delivered is unclear, with either five or six batches delivered, for a total of either 155 or 186 aircraft.
Even the details of the arrival of the first batch are uncertain. All agree that the first aircraft arrived at Cartagena by sea on 13 October 1936, but disagree on numbers, with either 13 or 18 arriving in this first batch. Either 12 or 7 arrived three days later, for a total of 25, and the last 7 arrived on 23 October.
The I-15 was also built under license in Spain. 300 aircraft were ordered and around 231 were completed, operating alongside the Soviet built aircraft from mid 1937.
These aircraft were used to form two fighter squadrons of 12 aircraft, each manned by Soviet pilots and commanded by Soviet officers, although with false Spanish names. The first trained Spanish pilots arrived in November, and gave the aircraft its familiar nickname of Chato (snub-nose). They were followed by a wide variety of international pilots, including a number of Americans.
The I-15 made its combat during the Battle of Madrid of November 1936. This brought it up against German Heinkel He 51s and Junkers Ju 52/3ms and Italian Fiat CR.32s. The I-15 proved itself to be at least the equal of the He 51, but was less successful against the CR.32. Nine I-15s were lost in combat around Madrid during November, and played a major part in the failure of the fascist attack on the city. The I-15 also played a major part in the defeat of an Italian attempt to encircle Madrid (battle of Guadalajara) in March 1937.
By the spring of 1938 the number of serviceable I-15s had dropped to under twenty. Although numbers did increase, the improvements were only temporary, and the type was becoming increasingly outclassed as the Nationalists received the first Messerschmitt Bf 109s from Germany and Fiat G.50s from Italy. The Heinkel He 111 bomber was another nasty surprise, proving to be faster than the biplane fighter.
A number of experimental versions of the I-15 were produced, including the I-15GK (Germeticheskij Kabina or Pressure Cabin). The SK-IV pressure cabin was designed by Alexey Shcherbakov and was made of riveted steel with rubber seals. Three successful test flights were carried out in October-November 1937.
I-15 with M-25
Engine: Shvetsov M-25 air-cooled radial (Cyclone)
Power: 635hp at sea level
Wing span: 32ft (upper wing), 24.6ft (lower wing)
Length: 20ft 8in
Height: 7ft 4in
Empty Weight: 2,601lb
Maximum Weight: 3,135lb
Max Speed: 223mph
Service Ceiling: 32,808ft
Range: 450 miles
Armament: Four 7.62mm PV-1 machine guns
Bomb-load: Two 25kg or two 50kg bombs or eight RS-75 missiles under lower wing.