That modified version was called MiG-3U, with the U standing for "uluchsennyi", which means "improved" in Russian. To achieve those performance results, the fuselage was lengthened without central welded truss, the cockpit was moved back, raised and widened to improve visibility, the main spar of the wing wasn't longer interrupted by the radiator tunnel, horizontal tail surfaces were moved 200mm to avoid interference with the new tail wheel mechanism, it was armed with two 20mm ShVAK guns placed above the engine with 150 rounds each and they could fire synchronised or separately and the chosen engine was the AM-35A with 0.732 reduction gear built from spared AM-35A and AM-38 engines, however, due to engine shortages it was 40kg (88.18lb) heavier than expected.
First prototype was piloted by test pilot V.N.Savkin on 31st May 1943 with 5 more built between June and July of that same year with a total of 28 test flights done, they showed engine overheating problems.
The first prototype was transferred to NII-VVS (Soviet Air Force's test unit) on 23rd July to be tested by the Air Force. It was tested by Cpt. A.S. Rozanov and B.I. Khomyakov. It showed a speed of 656km/h (407.62mph) at 7000m (22965.88ft) with a ceiling of 11900m (39041.99ft) and a good aerodynamical performance with some landing problems though. Cockpit was confortable and up-to-date with the newest Soviet fighters. However it also showed some excessive vibrations and oil overheating which caused leakage specially at high altittudes.
After being repaired and modified all the prototypes were sent to 12th GvIAP for operational testing and were flight tested by Lt. P.A. Zhuravliov and checked by the technicians of the 12th GvIAP. Only three of them passed the acceptance test of the technicians while the other two of them were sent back to OKB-155 until 10th October 1943 when new AM-35A engines arrived.
The second prototype (which didn't pass the acceptance trials of the 12 th GvIAP) was furtherly modified at OKB-155 and was fitted an AM-39A engine rated at 1700/1800hp (1242.26/1342.26Kw), it had modified cowling panels, it received a new radiator, an extended fuselage tank, rear fuselage was reinforced with metal, central frame of the rear canopy was removed in order to improve visibility and horizontal tail surfaces were lowered by 200mm in order to reduce vibrations.
It flew for the first time on 19th Ocotber 1943 manned by test pilot Cpt. V.M. Savkin, chief engineer V.Fufurin and motor engineer I.V. Kotov. On 5th November a malfunction in the supercharger forced the machine to land at Noginsk airport without damaging it thanks to the deployment of the landing gear in time. Due to that landing Savkin was awarded with the Order of the Red Star.
Tests were resumed on 23rd November after the aircraft was repaired and the engine was replaced. It achieved a top speed of 707km/h (439.31mph) at 7100m (23293.96ft) and reached 5000m (16404.2ft) in four and a half minutes. It was the fastest Soviet fighter to the date. Prototype was transferred to the NII-VVS to be tested with new lighter propellers. Tests were however soon interrupted due to a flap malfunction that damaged brakes and provoked a turn-over of the aircraft at the end of the testing strip. It was repaired by the OKB-155 and was active again on 12th May 1944 but, unfortunately, on 19th May the engine failed while testing.
Given the delays and the shortage of AM-39A engines and its unreliability, the whole programme was abandoned and the MiG-3U was the last of the MiG-3.
Sources:
1. http://www.airpages.ru/eng/ru/i230.shtml
2. http://www.airpages.ru/eng/ru/i231.shtml
3. Salamander Books - The Complete Book of Fighters
4. Signal Squadron - Aircraft In action 204 - Early Mig Fighters in Action
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