The Polikarpov I-16 was a Soviet fighter aircraft of revolutionary design; it was the world's first low-wing cantilever monoplane fighter with retractable landing gear to attain operational status and as such "introduced a new vogue in fighter design." The I-16 was introduced in the mid-1930s and formed the backbone of the Soviet Air Force at the beginning of World War II. The diminutive fighter, nicknamed "Ishak" or "Ishachok" ("Donkey" or "Burro") by Soviet pilots, figured prominently in the Second Sino-Japanese War, the Battle of Khalkhin Gol and the Spanish Civil War – where it was called the Rata ("Rat") by the Nationalists or Mosca ("Fly") by the Republicans. The Finnish nickname was Siipiorava ("Flying Squirrel").
While working on the Polikarpov I-15 biplane, Nikolai Nikolaevich Polikarpov began designing an advanced monoplane fighter. It featured cutting-edge innovations such as retractable landing gear and a fully enclosed cockpit, and was optimized for speed with a short stubby fuselage (similar to the Gee Bee R-1) and a Wright R-1820 radial engine in a NACA cowling. The aircraft was small, light and simple to build.
Full-scale work on the TsKB-12 prototype began in June 1933, and the aircraft was accepted into production on 22 November 1933, a month before it took to the air. The TsKB-12 was of mixed construction, using a wooden monocoque fuselage and wings employing a KhMA chrome-molybdenum steel alloy wing spar, dural ribs and D1 aluminum alloy skinning on the center and leading edges, with the remaining portions of the wings fabric covered. Another modern feature were the ailerons which ran along almost the entire trailing edge of the wing and also operated as flaps (in the manner of more modern flaperons) by drooping 15°. The cockpit was covered by a 40-centimetre-wide (16 in) canopy which featured an Aldis-type tubular gun sight which could slide back and forth on runners fitted with rubber bungee cords. A 225 l (59.4 US gal) fuel tank was fitted directly in front of the cockpit. The main landing gear was fully retractable by a hand crank. The armament consisted of a pair of 7.62×54mmR (0.30 in) ShKAS machine guns in the wings, mounted on the outboard side of the main gear and 900 rounds of ammunition.
These features were proposed at first by Andrei Tupolev; however, the NII VVS was more concerned about the stresses a typical combat aircraft was subjected to in combat, and initially considered the risk too great. However, TsAGI, with the help of the 3rd Design Brigade under the leadership of Pavel Sukhoi and Aleksandr Putylov, eventually convinced NII VVS that what was being proposed was not only feasible, but would enhance the aircraft's performance.
The TsKB-12 was designed for the Wright Cyclone SR-1820-F-3 9-cylinder radial engine (rated at 529 kW/710 hp); a license to build this engine under the supervision of the Shvetsov design bureau in the Soviet Union was being negotiated. As the license was not yet approved, Polikarpov was asked to settle for the less powerful M-22 (Soviet-built version of the Gnome-Rhone Jupiter 9ASB, which itself was a licensed version of the Bristol Jupiter VI) with 358 kW (480 hp). This was deemed acceptable because the projected top speed still exceeded 300 km/h (185 mph).
The M-22-powered TsKB-12 first took to the air on 30 December 1933 with the famous Soviet test pilot Valery Chkalov at the controls. The second TsKB-12, with a Cyclone engine and three-bladed propeller, flew in January of the following year. Initial government trials in February 1934 revealed very good maneuverability, but the aircraft did not tolerate abrupt control inputs. Thus the TsKB-12 was deemed dangerous to fly and all aerobatics were forbidden. The M-22 version was preferred due to the vibration of the Cyclone-powered aircraft. Pilots commented early on about the difficulty of climbing into the cockpit, a trait that persisted through the I-16's service life. Before continuing test flights the designers had to answer the question of spin behavior. Wind tunnel testing suggested that the TsKB-12, with its short tail, would enter an unrecoverable flat spin, but real-life trials were necessary to confirm this. Since Cyclone engines were rare, it was decided to risk the M-22 prototype for this purpose. On March 1 and 2, 1934, Chkalov performed 75 spins and discovered that the aircraft had very benign stall behavior (dipping a wing and recovering without input from the pilot when airspeed increased) and intentional spins could be easily terminated by placing the controls in the neutral position. The stories of vicious spin behavior of the I-16 perpetuated in modern literature is unfounded (perhaps extrapolated from Gee Bee experience). In fact, the I-16's stablemate, the biplane Polikarpov I-153, exhibited much worse spin characteristics.
Service trials of the new fighter, designated I-16, began on 22 March 1934. The M-22 prototype reached 359 km/h (223 mph). The manually retracted landing gear was prone to jamming and required considerable strength from the pilot. Most of the test flights were performed with the gear extended. On 1 May 1934, the M-22 prototype participated in the flyover of Red Square. Approximately thirty I-16 Type 1 aircraft were delivered, but were not assigned to any VVS fighter squadron. Most pilots who flew the I-16 Type 1 for evaluation purposes did not find the aircraft to have many redeeming characteristics. Regardless of pilot opinion, much attention was focused on the Cyclone-powered aircraft and the M-25 (the license-built Cyclone). On 14 April 1934, the Cyclone prototype was damaged when one of the landing gear legs collapsed while it was taxiing.
The third prototype with a Cyclone engine incorporated a series of aerodynamic improvements and was delivered for government trials on 7 September 1934. The top speed of 437 km/h (270 mph) no longer satisfied the Air Force, who now wanted the experimental Nazarov M-58 engine and 470 km/h (290 mph). Subsequently, the M-22-powered version entered production at Factory 21 in Nizhny Novgorod and Factory 39 in Moscow. Because it was the fourth aircraft produced by these factories, it received the designation I-16 Type 4. Aircraft fitted with these new engines required a slightly changed airframe, including armor plating for the pilot and changes to the landing gear doors to allow for complete closure.
The M-25 fitted I-16, the I-16 Type 5, featured a new engine cowling which was slightly smaller in diameter and featured nine forward-facing shuttered openings to control cooling airflow, a redesigned exhaust with eight individual outlet stubs, and other changes. The M-25 was rated at 474 kW (635 hp) at sea level and 522 kW (700 hp) at 2,300 m (7,546 ft). Due to the poor quality of the canopy glazing, the I-16 Type 5 pilots typically left the canopy open or removed the rear portion completely. By the time the Type 5 arrived, it was the world's lightest production fighter (1,460 kg/3,219 lb), as well as the world's fastest, able to reach speeds of 454 km/h (282 mph) at altitude and 395 km/h (245 mph) at sea level. While the Type 5 could not perform the high-G maneuvers of other fighters, it possessed superior speed and climb rates, and had extremely responsive aileron control, which gave it a very good roll rate, which led to precision maneuvers in loops and split-Ss.
A total of 7,005 single-seat and 1,639 two-seat trainer variants were produced.
Initial service experience revealed that the ShKAS machine guns had a tendency to jam. This was the result of the guns being installed in the wings upside-down to facilitate the fit. The problem was addressed in later modifications. Evaluations from pilots confirmed the experience with prototypes. Controls were light and very sensitive, abrupt maneuvers resulted in spins, and spin behavior was excellent. An aileron roll could be performed in under 1.5 seconds (roll rate over 240 degrees/second). The machine guns were fired via a cable and the required effort, coupled with sensitive controls, made precision aiming difficult. The rear weight bias made the I-16 easy to handle on unprepared airfields because the aircraft was rather unlikely to flip over the nose even if the front wheels dug in.
The I-16 was a difficult fighter to fly. The pilots had poor visibility, the canopy tended to become fouled with engine oil, and the moving portion was prone to slamming shut during hard maneuvers, which caused many pilots to fix it in the open position. The front section of the fuselage, with the engine, was too close to the centre of gravity, and the pilot's cockpit too far to the rear. The Polikarpov had insufficient longitudinal stability and it was impossible to fly the aircraft "hands off".