To increase directional stability and improve aiming conditions, military experts recommended N.N. Polikarpov to return to the usual scheme of the upper wing. Although the designer did not agree with this proposal, he was forced to give in, and from 1938 the I-15bis fighter went into production. Compared to the I-15, the upper wing of this fighter was straight (without a "gull"). The designers have improved the cowling of the engine and strengthened the design of the aircraft. The I-15bis was equipped with a more powerful and high-altitude M-25V engine and had stronger armament.

The forced M-25V engine developed a power of 750-775 hp. at an altitude of 2900 m (for comparison, the power of the M-25 was 720-730 hp at 2500 m). Engine - with a metal propeller of constant pitch with a diameter of 2.8 m. The width of the blades is 217 mm, in later series - 250 mm. The propeller sleeve is covered with a spinner with a diameter of 640 mm. The engine was equipped with an oil cooler. The Townend ring covering the engine cylinder heads was replaced with a NACA type hood. The propeller hub was covered with a spinner.

The aircraft had a normal biplane-type upper center section without a seagull. The span of the upper wing was increased to 10200 mm, the bearing area was increased to 22.3 m² due to the increased flight weight and the desire to provide the same wing load.

Performance. The aircraft was described in the instruction manual as follows:

“The I-15bis aircraft has slightly higher horizontal speeds near the ground and at altitudes compared to the I-15 Chaika and has a good speed range (from 100 to 370 km / h), which is of great importance in the combat use of the machine.

The I-15bis aircraft dives well and steadily at all angles of inclination of the longitudinal axis up to a vertical dive.

The I-15bis quickly changes speed, responding well to gas supply. It has good directional, longitudinal and lateral stability and normally performs all aerobatics. Mastering the piloting technique on the I-15bis is not difficult, because. the plane reacts sluggishly to errors in piloting.

Good maneuverability and stability of the I-15bis provide easy targeting and aimed fire in any position in the air, which gives advantages in combat over aircraft with high speeds, but worse maneuverability.

Due to the overweight, the I-15bis is somewhat inferior in terms of maneuverability and rate of climb to the I-15 Chaika aircraft (the duration of the turn is 2-3 seconds longer). In addition, the I-15bis has a tighter control of the ailerons, there is no energetic transition from figure to figure, as on the "Seagull" aircraft.

The flight of an aircraft with full machine-gun armament and 4 bombs of 10 kg each has no special features, the aircraft freely performs maneuvers and dives without any noticeable deterioration in piloting qualities.

The aircraft can fly with a full bomb load of 150 kg and a full ammunition load of machine guns (3050 pieces) with a fuel reserve of 225 kg (full tank). The flight weight in this case is 1870 kg. At the same time, stability does not deteriorate, the efforts on the handle increase, the control of the ailerons becomes tighter.

Soviet volunteer pilots fought on the I-15bis fighter in the skies of Spain, China and Mongolia. The combat activity of the I-15bis basically ended in 1943.

The Soviet aviation industry produced 2408 I-15bis fighters (1938-1939)

Photo	Description
	Drawing I-15bis
	The I - 15bis #3615 with 50-liter fuel tanks. The Soviet AF Scientific Research Institute. April 1939.
	The I-15bis with 68-th onboard number. The airplane was applied after 1943 - a red star in a white edging.

First flight: 1937 Training fighter Polikarpov

The need for educational institutions and combat units of the Air Force to have a two-seat version of the I-15, intended for training and training, has been recognized for a number of years. The solution of the problem began already during the launch of the I-15bis into mass production. The project for modifying the aircraft at the Polikarpov Design Bureau was prepared during the summer of 1937. A few months later, all the changes made to the design of the I-15bis were approved and the two-seater was sent into production. The aircraft received the designation DIT - two-seat training fighter. Appointment - a transitional machine during the development of I-15bis and I-153.

An additional front cabin DIT was equipped in place of the main fuel tank. Gasoline was placed in three containers: one gas tank for 70 liters on the left side of the first cabin, the second - for 170 liters - under the floor of the first cabin, the third - for 80 liters - under the floor of the second cabin. The total amount of fuel remained the same - 320 liters. Aircraft control and instrumentation were available in both cockpits, while foot and hand controls in the rear cockpit changed compared to the I-15bis. Cable wiring was a little higher, corresponded to that on the I-153, so the rudder was placed from this aircraft. For the convenience of landing in the front cockpit, the center section cutout, compared to the I-15bis, has noticeably increased, holes were made in its trailing edge for gripping with hands. The pilot's visor in the front cockpit with flat edges, according to the I-153 type, is equipped with a PAK-1 sight. In the rear cockpit, the visor was cut off; in its front part there was a headrest for the pilot sitting in front. Ring sight, mounted on the side of the visor. The small arms of the DIT consisted of two synchronous ShKASs with a supply of 250 rounds of ammunition per machine gun. Given the higher rate of fire of the ShKAS machine guns compared to the PV-1, the density of fire remained at the level of the I-15bis. DIT bomber armament was made according to the I-15bis type, load from 40 to 100 kg.

It is known about three built copies of DIT: they had the designation DIT-1, DIT-2, DIT-3. It was supposed to be a series at the aircraft factory No. 207 (Dolgoprudny), which was not implemented. DIT-1 No. 5128 and DIT-2 No. 5337 aircraft factory building no. 1 flew in the spring and summer of 1939, DIT-3 appeared at the end of the summer. Pilots Pumpur, Laryushkin, Davydov, Zhukov, Turzhansky took part in the overflights. The overall impression of the car was quite favorable, the piloting technique did not differ from that of the I-15bis.

"The DIT aircraft provides full control from both the rear cockpit and the front cockpit in all flight modes. Stability in all three axes is good, it makes mistakes in piloting ... Having a fuel reserve of 320 liters, it ensures flight duration in medium operating modes motor about 2 hours 20 minutes Normally enters and exits the spin. "

The proximity of the instrument panel to the pilot in the second cockpit was considered a disadvantage - the small size of the machine affected, forcing a dense layout of the internal filling.

Although DIT aircraft were not widely used, the three samples built were used as efficiently as possible. One copy was used for official flights at the Air Force Research Institute. So, on May 18, 1940, pilot Davydov and observer Eryshev flew on a DIT to observe the spin of the I-180 aircraft. The other two copies were sent to the Sevastopol region in 1940, and later they participated in hostilities. It is known that two DITs were used as scouts and spotters of artillery fire on the Southern Front in the Novorossiysk region in 1942-1943. The planes were painted, according to the recollections of eyewitnesses, with small spots of sand color.

Bibliography

• "History of aircraft in the USSR before 1938" /Shavrov V.B./
• "Planes of Stalin falcons" /Konstantin Kosminkov and Dmitriy Grinyuk/
• "Fighter I-15" /Michael Maslov/
• "The Soviet planes" /Alexander Yakovlev/