Despite maneuverability inferior to that of the Ki-43, the new aircraft won the approval of pilots. They especially liked the Demon Ki-44’s spin performance and lateral stability under gunfire. Ki-44 became the fleetest high-speed fighter in the Japanese army. But not eager to see it go into mass production, the Air Force command ordered Nakajima to manufacture another trial group of 40 aircraft, this time armed with four large-caliber machineguns. The first production models received the designation Ki-44-I and were sent to the same 47th squadron, which was reorganized into a sentai. The new division was stationed at the Narimatsu airfield near Tokyo and served as the Air Force command reserve division.
Only in September 1942 did Ki-44 assume a full-fledged place in the arsenal. It was effectively the only interceptor able to perform its particular duties in the Japanese army at that time, and it held this position for a year and a half until the appearance of the Ki-84.
The plane appeared to be more modification-friendly, than the Ki-43. As early as August 1942 Koyama undertook improvements to the Ki-44-II model, in part by equipping it with the more powerful Nakajima Ha.109 1520-hp engine with a two-stage supercharger. As a result of this improvement, the top speed rose from 580 to 605 km/h.
The new Ki-44-IIc model with two external fuel tanks.
Ki-44-IIb with Ho.301 40mm guns in the wings. The telescopic sight still remained.
From the late fall of 1942, the new aircraft began to play an increasingly active role in the Japanese air defense, especially with respect to the defense of the large industrial cities. By the spring of 1943, the Ki-44 appeared on practically every front, defending the air bases in Taiwan and Philippines and the oil producing centers in Sumatra from enemy bombers. By late 1943, five fighter sentai were re-equipped with Ki-44s.
In June 1943, the new series prototype Ki-44-III, equipped with an even more powerful Nakajima Ha.145 2000-hp engine, took off. But the aircraft of this type never went into mass production—only 4 planes were manufactured before it was decided to abandon work on the fighter’s modernization. Nevertheless, Ki-44 had the best rate of climb of all the Japanese fighters until the end of the war!
In January 1945, after a total of 1225 aircraft had been issued, production of Ki-44 was discontinued. But the aircraft remained in service till the end of the war.
In late 1941, specialists at Nakajima under the leadership of Koyama had already begun to contemplate the replacement of the Ki-43 and Ki-44 with something better. Both models eventually gave way to the best Japanese army fighter, the Ki-84.
Enjoying the confidence and trust of the army leaders, Nakajima’s specialists enabled the firm to receive the order without any competition. Issued in January 1942, the order included the following specifications: powerful weaponry to include two 20mm guns and two large-caliber machineguns, level flight top speed range of 640-680 km/h, pilot and fuel tank protection.
Work on the future prototype was also assigned to the team of Tamura Koyama, who had accumulated vast experience in creating the Ki-27, Ki-43 and Ki-44. The engineers first studied the record of single-engine fighters in European combat, consulting with German military specialists who also provided the development team with the latest German fighter, the Fokke-Wulf FW.190. Koyama derived considerable profit from the German experience and built his aircraft around the Nakajima-Homare Ha.45 1800-hp engine, which was similar to the German BMW.801 engine.
The development of the first prototype was finished in March 1943, and that of the second by June of the same year. Both planes made a brilliant showing in their tests, which turned up no significant flaws. But battle conditions subsequently revealed a number of defects, which were difficult to correct “on the fly.” Koyama began by introducing the new large-diameter four-bladed Sumimoto Pe.32 propeller with automatic pitch adjustment. The landing gear had to be enlarged, which led to an increase of the landing angle and, consequently, extremely poor forward view on take-off and landing. But the flaws connected with the engine and the landing gear would become apparent later, and for now, following the tests, Nakajima received an order to build the first 83 aircraft. The prototypes’ top horizontal speed of 624 km/h was declared sufficient. The new fighter was officially named “Heavy army fighter/interceptor type 4 model 1A” and also earned the colorful designation “Hayate” (“Devastating Tornado” or “Storm”, according to some translators). It was added to the arsenal in August 1943. In October the first experimental squadron was formed and ordered to challenge the aircraft in battle conditions and to report possible engineering flaws.
One of the first Ki-84 type 4 model 1a near the “Nakajima” hangars in Ohta.
Ki-84-1b on an airstrip, 1944.
From the very start it was evident that the Japanese now had a fighter of unprecedented sophistication. Despite handling somewhat more clumsily than that of its predecessors, the plane earned the respect of the pilots, who had already become accustomed to high-speed aircraft. The “Fowler” combat flaps provided the Hayate with significant advantages in dogfight, and the aircraft’s respectable rate of climb not only enabled it to compete with the latest US fighters, but even ensured its superiority in some cases. The first aircraft could climb to 5000 m in 6.25 minutes, a rate inferior to that of the Ki-44 but still satisfactory. When diving, Hayate could reach up to 800 km/h and it didn’t suffer from the vibrations so typical of all other Japanese aircraft.
The principal advantage of Ki-84 was its high manufacturability, a benefit acquired by relying on the experience of the Fokke-Wulf. The aircraft parts required much less equipment and fewer production tools than those of other Japanese aircraft. The plane was divided into manufacturing units, which could be produced independently and then quickly attached to each other. This feature enabled production of the aircraft to proceed at several subcontractor factories at once while conserving time and resources. However, the engine of the Ki-84 was a cause of some trouble, as the Nakajima factory in Musashi, the sole supplier, could not reach the projected production amount. Only in April 1944 did the factory succeed in producing 100 engines per month.
In March, the production of the first trial group of 83 aircraft was finished, and in April, a second trial group of 42 aircraft was produced, together with the serial Ki-84-Ia. The planes of the second trial group were used for testing such innovations as the separate engine exhaust manifolds, which increased the engine’s jet propulsion. The central external fuel tank pylon was replaced by an underwing pylon so that the plane could now carry two 200-l external fuel tanks or two 250-kg bombs. The engine became more sophisticated with the replacement of the Nakajima Ha.45 model 11 with the model 12, which offered 25 hp more power and enhanced reliability in comparison with its predecessor. This improvement increased the level flight speed by almost 15 km/h.
The 22nd Fighter Sentai were the first to employ the Hayate in combat, in China in March 1944. The new aircraft proved itself to be a formidable opponent and resulted in considerable losses for the Allied Air Forces. The US Air Force had pursued “hit-and-run” tactics in Europe in accordance with a directive stipulating that Allied fighters avoid direct engagements with German fighters unless they outnumbered the latter by a factor of 6. Encounters with the Ki-84 led to implementation of a similar directive, though with no ratio specified but a general recommendation to refrain from openly engaging the Japanese aircraft all together!
After the defeat at Midway, the main part of the Japanese air forces was transferred to Philippines, which became the scene of the most massive battles involving the Ki-84, as the Japanese tried to regain air superiority in the Pacific at all costs. It was also here that the Allies began to call the fighter “Frank”. At the Philippines, 10 fighter sentai were equipped with the Ki-84. The planes were primarily intended for interception of B-29 and B-24 bomber groups taking off Tinian Island. But another flaw of the Ki-84 came to light—its non-hermetical cockpit and lack of speed at high altitudes. Moreover, its weapons were also too weak to fight strategic bombers and the army pilots had no experience of open sea flights.
With the aim of making the weapons stronger, Ho.5 20mm guns replaced Ho.103 12.7mm machineguns under the hood of the Hayate. The new series, named “type 4 model 1B”, was added to the arsenal in late 1944. But the engineers were still concerned about the engine problem. Due to low quality of assembly and materials, the Ha.45 engine required very high quality service. It was necessary to cleanse and rebuild it after each flight, and after a few flights the engine had to undergo an overhaul with the replacement of the cylinder block. Given the lack of spare parts and qualified personnel, the need for this kind of engine care became a serious problem. Moreover, the overall quality of the aircraft assembly left much to be desired, which led to an unacceptable growth of non-combat losses. For example, breakage of the landing became a scourge for the inexperienced pilots. Another “peculiarity”, which plagued the Ki-84 throughout its career was the flawed fuel system, where as a consequence of low assembly precision, the pressure could fall rapidly. The “childhood diseases” of the Hayate progressed along with the decreasing quality of the plane’s assembly and service. Urgent measures had to be taken to increase the engine’s manufacturability.
