OKB-155 started work on modifying the I-230 with the new AM-39A engine, rated at 1700/1800 hp, in September 1943. The new aircraft was designated I-231, or 2D. At the same time, a new high altitude fighter with the same engine was built. This was the I-220, or A; it was a far larger aircraft than the MiG-3, despite a superficial resemblance. The I-231 was considered as a backup program for this more advanced fighter.

In comparison with I-230, the I-231 had some differences:

• the new AM-39A engine;

• the cowling and nose panels were slightly modified;

• a new enlarged radiator with 30 m² effective cooling area was installed; in this was integrated the additional cooling loop (7 dm:) of the intercooler taking air from the supercharger to the cylinders;

• the fuselage tank was enlarged to 500 l.

• the rear fuselage was all metal, with a lower spine behind the cockpit;

• the rear of the canopy had no central frame, to improve visibility;

• horizontal tail surfaces were lowered by 200 mm (compared to the MiG-3) to avoid vibrations noted on the I-230;

The first flight was made on October 19, 1943. The team was headed by test pilot Captain V.M.Savkin, chief engineer V.Fufurin and motor engineer I.V.Kotov. On 5 November, a malfunctioning of the supercharger obliged the pilot to make an emergency landing at Noginsk airport. Despite the emergency, the landing was executed with the undercarriage lowered, and didn't damage the aircraft; Savkin was awarded the Order of the Red Star for this.

The tests were resumed on November 23, after repairs and an engine change. The top speed achieved was 707 km/h at an altitude of 7100 m, and 5,000 in was reached in 4.5 min. This was the highest speed reached by a Soviet fighter to that date. The prototype was transferred to NII-VVS for state tests, after some modifications involving the replacement of the propeller with an AV-5L-126E, which weighed 25.3 kg less. The tests were soon interrupted by an accident, when during a landing the flaps didn't function properly and the brakes were damaged, so the aircraft went off the end of landing strip and turned over. The damaged aircraft was returned to OKB 155 for repairs, and was back again at NII-VVS on 12 May. But on May 19, the engine failed while testing the boosted regime. The delivery of a new AM-39 was delayed, and then the whole AM-39 program was abandoned due to its unreliability, and thus work on the I-231 was abandoned too. The I-231 was the last of the MiG-3 family.

Photo	Description
	Drawing I-231 (2D)
	The I-231 (2D) fighter powered with AM-39 engine.

Bibliography

• "The history of designs of planes in USSR 1938-1950" /Vadim Shavrov/
• "MiG-3 - first high-altitude fighter" /Alexander Medved`, Dmitri Khazanov/
• "The Soviet planes" /Alexander Yakovlev/