 The first prototype was completed on 26 February 1945 , although the VRDK was not yet ready . It was fitted with a new 3 @.@ 1 @-@ metre ( 10 ft ) AV @-@ 10P @-@ 60 propeller before making its first flight on 4 April according to Gordon and Komissarov , although Belyakov and <unk> say 3 March . It was armed with three 20 mm Berezin B @-@ 20 cannon with 160 rounds each . One gun was fitted on each side of the nose and the third fired through the hollow propeller shaft . The VRDK was tested for the first time on 8 April in a dive , attaining a speed of 710 km / h ( 440 mph ) , but a leak was found in the oil cooler after landing and the VRDK had to be returned to the factory for repairs . The VRDK was reinstalled by 14 April , but was still troublesome . It twice reached 809 km / h ( 503 mph ) at about 7 @,@ 000 m ( 22 @,@ 966 ft ) during these early tests , but its service life had been exceeded by 30 May and it had to be returned to TsIAM , which postponed any further tests until the second aircraft was finished . 
