The Polikarpov I-15, nicknamed Chato (Spanish: Flat-Nose) by Spanish Republican forces, and misidentified as a Curtiss by the Nationalists, was a Soviet biplane fighter aircraft used during the Spanish Civil War, the Soviet-Japanese border conflicts, and the opening defensive actions of Operation: Barbarossa.
The I-15 was the direct successor to the I-5, another Polikarpov fighter design from the early 1930s. The I-15 was unique in that its upper wings had no middle section, and connected into the fuselage in a diagonal gull-wing shape. This offered increased maneuverability and forwards visibility, but was a very controversial design.
Many blamed the gull-wing for restricting a pilot's view during take off and landing. The gull-wing was also noted to make the aircraft highly unstable and very sensitive to controls. The I-15's stability, however, improved upon higher speeds.
The I-15 was a sesqiplane, meaning that its lower wing was much smaller than its upper wing. The fighter was designed to be powered by an imported American radial engine, the Wright "Cyclone" SGR-1820/F-3, later produced under license in the Soviet Union as the Shvestov "Cyclone" M-25. The M-25 engine was capable for achieving 635 horsepower at sea level and 715 horsepower at high altitude.
However, the M-25's production had not yet finished by the time the fighter entered service. The earlier I-15s were powered by the M-22 radial engine, a license-built version of the British Bristol Jupiter. This older engine was already in stock, and was the engine used on the I-5 series of fighters. The M-22 was much less powerful, generating a meager 480 horsepower.
Armament on the fighter consisted of four PV-1 light machine guns mounted between the radial engine's cylinders. Two of the machine guns were near the middle of the engine, with the other two towards the top. Many other fighters of the time were equipped with only two machine guns, giving the I-15 an edge over the competition. The biplane could also carry bombs or rockets under its lower wings.
The I-15 lacked many of its planned innovations, such as motorized landing gear and a pressurized cockpit. These additions were present in the first prototypes, but never made it to production.
In the 1930s, the VVS's ideal fighter strategy required two types of craft; fast monoplanes and agile biplanes. The monoplanes would attack and break apart larger formations, allowing the biplanes to target the disorganized chaos that followed.
In 1932, Nikolai Polikarpov collaborated with fellow designer Pavel Sukhai to create a suitable pair of aircraft which could replace the aging fighters that were in use at the time. Sukhai designed the I-14, a high speed, cannon-armed monoplane that was often considered to be one of the most advanced fighters of its time. Its biplane pair would be the Polikarpov I-13, a sesqiplane with an inline V-16 engine.
Ilyushin, the head of Polikarpov and Sukhai's brigade, was still worried that Sukhai's design would not be accepted into the VVS. As such, two other sesquiplanes were designed, the I-14A and I-14B, in case the I-14 monoplane failed its trials.
The I-14A was undertaken by Polikarpov. It featured a gull-style upper wing and a radial engine, as well as retractable landing gear and an enclosed cockpit. Both the I-14 and I-14A were accepted, but the I-14A was much more successful, and eventually evolved into the I-15.
The first I-15s arrived in Spain at Cartegena in 1936, and were adopted by Spanish Republican forces. Around 155 aircraft were delivered, 231 more produced under license in Spanish factories in 1937.
In November of 1936, the I-15 saw its baptism by fire in the skies of Madrid. Here it dueled with German Heinkel 51s and Italian Fiat CR. 32s. It proved to be superior to the German fighters, but were not quite
nimble enough to face the Italian fighters. None the less, the I-15 was a successful implement in combat.
At first, the I-15s were flown only by Soviet-supplied pilots under Soviet commanders, all with fake Spanish names. But by November of 1937, the first true Spanish pilots arrived to fly the fighter. It was by these pilots that it was given its famous name, Chato. Among Nationalist pilots, the I-15 was commonly believed to be an American Curtiss Sparrowhawk, and was called a Curtiss. Throughout the war, foreign volunteers from America, Canada, Ireland, Great Britain, the Balkans, Cuba, Belgium, and other countries would operate the Chato in combat.
By 1938, the I-15 Chato was passed its prime. The newer German Bertha and Clara, and eventually the Emil, completely outclassed the older Soviet biplane. Other monoplanes, such as the Italian G.50 Freccia, also helped to give the Nationalists the advantage. Even the Nationalist bombers were superior; the Heinkel He 111 could outrun and evade a Chato.
By 1940, the I-15 was still in the Soviet arsenal, but only for ground attack missions. A number of Soviet I-15s were deployed in the Winter War, some of these captured by Finland and used against their original operators.
In total, 6,578 I-15s were manufactured.
- TsKB-3: Development name for the first prototype This version was much like the standard I-15, but also featured an enclosed cockpit, and retracting landing gear.
- I-15: The standard model. 384 completed
- I-15 M-25: The standard I-15 with the 635hp M-25 engine.
- I-15 M-22: A number of I-15s with the older low-performance 430hp M-22 engines. This version suffered in combat.
- I-15bis or I-152: A later variant with a straight upper wing, instead of a gull-wing. 2,408 completed.
- I-15ter or I-153: A highly advanced version with retractable undercarriages and heavier armor and armament. Used until 1943. 3,437 completed.
- I-153BS: A variant armed with two 12.7mm UBS machine guns in place of the four 7.62mm PV-1s. Only few UBS machine guns were built, and so many received only one UBS, and two PV-1s.
- I-15GK: For "Germeticheskij Kabina" (Russian: Pressure Cabin) Based on the first I-15 variant with a pressurized cockpit. A few were produced, none saw service
- I-152TK: An I-152 with TK-3 Superchargers. The supercharger's weight rendered its extra speed useless. Never progressed past test flights.
- I-152DM: Another I-152 with two DM-2 Ramjets. The jets only had minimal effect while active, and actually slowed the fighter in standard flight. 54 successful test flights were made, but the vehicle never saw service.
- I-152 DIT-2: Two-seat trainer version of the I-152. The craft proved to be too difficult for a standard trainer, and was instead used as a conversion trainer for more advanced pilots.
- I-153 DM-2: An I-153 with the DM-2 Ramjets that were used on the I-152DM. Once again, the jets proved ineffective.
- I-153UD: A variant with a wooden fuselage, made in an attempt to reduce the usage of valuable metals. Never entered service.
- I-153P: An I-153 with two 20mm ShVAK cannons. Eight were produced in total.
- Gunston, Bill, "The Osprey Encyclopedia of Russian Aircraft 1875–1995", London: Osprey, 1995.
- Green, William and Swanborough, Gordon, "Of Chaika and Chato... Polikarpov's Fighting Biplanes". Air Enthusiast Issue 11, November 1979