on 22 June 1941, 425 Yak-1s had been built, although many were en route or still incomplete. 92 machines were fully operational in the Western Military Districts but most were lost in the first days. The Yak-1 was built as an escort fighter for Il-2 tactical bombers and combats took place below , where the Yak-1 performed the best. The Yak-1 proved to have a significant advantage over its Soviet competitors. A full circle turn took just 17 seconds in the Yak-1M. The MiG-3, which had the best high-altitude performance, did poorly at low and medium altitudes and its light armament made it unsuitable for ground attack. The LaGG-3 experienced a significant degradation in performance (as much as on some aircraft) compared to its prototypes due to the manufacturer's inexperience with its special wooden construction, which suffered from warping and rotting when exposed to the elements. The Yak-1's plywood covering also suffered from the weather, but the steel frame kept the aircraft largely intact. Early aircraft suffered from fuel leaks, spot-welded fuel tanks failing from vibration. The canopy could not be opened under certain conditions in earlier models and some pilots had the sliding portion of the canopy removed. The first 1,000 Yak-1s had no radios; wireless equipment became common by spring 1942 and obligatory by August 1942 but Soviet radios were notoriously unreliable and short-ranged, so they were frequently removed to save weight. The M-105 could not tolerate negative G forces which starved it of fuel and suffered from breakdowns of magnetos, speed governors and emitted oil from the reduction shaft. The Yak-1 was better than the Bf 109E but inferior to the Bf 109F – its main opponent – in rate of climb at all altitudes, although it could complete a circle at the same speed (20–21 seconds at ). The Bf 109, with its automatic wing slats, had a lower stall speed and was more stable in sharp turns and vertical aerobatic figures. A simulated combat between a Yak (with M-105PF engine) and a Bf 109F revealed that the Messerschmitt had only marginally superior maneuverability at , though the German fighter could gain substantial advantage over the Yak-1 within four or five nose-to-tail turns. At , the capabilities of the two fighters were nearly equal, as combat was essentially reduced to head-on attacks. At altitudes over , the Yak was more manoeuvrable. The engine's nominal speed at