The engine problems were solved with the new 1990-hp Nakajima Ha.45 model 21 engine, and some hydraulics problems were removed with the even newer Ha.45 model 23 series, but at the cost of power loss – it now produced only 1900 hp. With the new engine, the Hayate became one of the most superior fighters not only in the Pacific, but in the whole world. At the second altitude threshold of 6000 m, the Ki-84 was faster in level flight than any of the advanced Allied models. In particular, it surpassed the North American P-51D Mustang by 5 km/h, and the Republic P-47D Thunderbolt by 35! To raise their altitude, some Hayate were equipped with the 2000-hp Ha.45Pu engine featuring a high-altitude turbocharger. This variant was meant to go into mass production under the name Ki-84-III, but work on the prototype wasn’t finished until the end of the war.
The Japanese army demanded more and more new aircraft. Nakajima production facilities could not satisfy the demand because of irregularities in the raw materials supplies. The plane was designed in such a way, that it required nothing less than high-quality materials which Japanese industry was unable to ensure. In May 1944, Nakajima opened the new Ki-84 assembly line in Utunomiya. The firm subsequently produced a total of 1670 aircraft instead of 3500 as planned. In December 1944 the production rate of Ki-84 was at its peak level – 373 fighters per month. But after the American bombing of the Musashi motor factory, production of the Hayate slowed to a crawl. In spring 1945 Nakajima introduced another assembly line at an underground Mantsui factory in Harbin, Manchuria. However, the factory, nicknamed “the tunnel,” had a very low production rate as a result of interruptions in engine shipments.
Ki-106 destroyer prototype, a variant of Ki-84-II, with a totally wooden hull and a modified tail-group
Meanwhile the shortage of raw materials presented a steadily increasing threat. Designers confronted the task of replanning the frame to accommodate inexpensive raw materials. At first Nakajima elaborated the Ki-84-II version with a wooden tail unit. Subsequently, the Tachikawa designers were directed to extend such work by creating an entirely wooden frame. The construction of Koyama was not intended for such modifications—at great loads the tail empennage was simply ruined. As a result, Tachikawa submitted its prototype Ki-84-KAI, with an entirely wooden frame and two modifications to decrease the load—an enlarged tail area and reduction of the velocity threshold. The aircraft passed the test quite successfully, yet it remained less rapid and maneuverable than Ki-84. The new fighter was equipped with a standard engine Ha.45 model 21 and was set for imminent inclusion in the series under the index Ki-106. However, the Tachikawa specialists were preoccupied with weight decrease and flight characteristics improvement. Thus, they did not manage to start up mass production of the aircraft. Until the end of the war only three Ki-106 specimens were produced as prototypes. Their checkout tests were conducted during July 1945.
Efforts to improve the flight characteristics of the Ki-84 and to lower the cost of its production continued until the end of the war. The version Ki-113, which had all possible aluminum units replaced with steel counterparts, underwent testing as early as the end of 1944; yet, it was not included in the series as a consequence of deterioration in flight characteristics. The version Ki-84N, with an enlarged wing area, became the prototype of a high-altitude interceptor Ki-117, equipped with 2500-hp engine Ha.44 model 13, yet this also remained a mere prototype. The plane vibrated so severely as to make it nearly uncontrollable, and it was also unable to produce aimed fire. In the Ki-84II version, the defects of the preceding prototype were eliminated by increasing the wing area. However, the Ki-84P model was considered more promising. In that plane, the Ha.44 engine was replaced with the less powerful Ha.45 model 44, containing a 2000 -hp three-stage driving supercharger. However, by the end of operations this prototype was only 80% ready. Ultimately, the Ki-116, equipped with a fundamentally different Mitsubishi Ha.33 1500-hp takeoff power engine, became the most promising fighter to be based on the Ki-84. This plane’s prototype, elaborated by the Mantsui firm, recorded perfect results thanks to a decrease in weight of nearly 450 kg. Still, in this case as well, the end of the war prevented its going into serial production.
Altogether, before Japan’s capitulation 3514 samples of the Ki-84 in different versions were produced by 2689 Nakajima factories in Ohta, 727 factories in Utunomia, and 95 Mantsui factories in Harbin.
The Hayate became the most superior Japanese army fighter and participated in nearly all the last-stage Pacific engagements. Moreover, the fighter became one of the world’s best planes in its class. Notwithstanding many technical shortcomings, it could compete with the most up-to-date of its Western counterparts on equal terms. Especially impressive was the success of the Ki-84 in the battle for Okinawa Island. On April 15, 1945, the 100th Sentai, equipped with Hayate aircraft, delivered a severely crippling attack on the American airport in Okinawa, damaging almost all the aircraft stationed there while losing only 8 of its own fighters as a consequence of construction demolition incurred while swooping. However, despite the excellent combat qualities of this fighter, it still required improvements. Besides, its production numbers were far too low to significantly effect the outcome of hostilities.
Throughout the war, the Japanese aircraft industry made several other attempts to create a plane capable of competing with advances in the rest of the world. One such attempt was highly successful and thus merits special attention. The majority of researchers regard the aircraft in question as unique, since it featured an in-line liquid-cooled engine, so untypical, so their reasoning goes, of Japanese planes. Yet, this is a mistake, to say the least. It is enough to mention a variety of Japanese aircraft, such as the Kawasaki Ki-10 and the Yokosuka D4Y.
From 1923 through 1933 the chief designer at Kawasaki was none other than the German engineer Richard Gogt, who later headed up the design team at the German aircraft concern Blom und Foss. Gogt laid the foundations of engineering thought at Kawasaki for years to come. He also provided Kawasaki with priceless experience in the construction of aircraft equipped with liquid engines.
On the eve of the war Japanese engineers kept very close tabs on the newest developments in aircraft construction. Obviously, they could not but notice the success of the Bf.109, the most illustrious creation of the German designer Willie Messerschmitt. In April 1940, the Kawasaki technical team visited the factories of Daimler-Benz in Stuttgart and, to their great pleasure, received a license for manufacture of the newest engine DB.601A. Apart from the technical documentation and the drafts, the Japanese brought along several working samples for possible checkouts. Immediately after the team’s return, Kawasaki began to organize serial production of the engine at its Akashi factory. By July 1941 the Japanese had completed design of a modified Daimler adapted to the army requirements, and in November of the same year the new Kawasaki engine Ha.40 type 2, with a take-off power of 1100 hp, was included in the series.
In February 1940 the Army Command announced a contest for the creation of a multifunction fighter, equipped with in-line engine Ha.40. Kawasaki was charged with the task of designing the heavy fighter-interceptor Ki-60 and multifunction fighter Ki-61, whereas Nakajima was to design the Ki-62 fighter. Later, it was decided to settle on the Kawasaki projects.
The company concentrated primarily upon the elaboration of the Ki-60. During 1941 three prototypes of this aircraft, equipped with imported engines DB.601A, had been constructed. However, after the test flights the pilots were obviously dissatisfied with a too heavy design, high unit wing load and low maneuver characteristics. It was decided to discontinue work on the creation of an interceptor and to concentrate instead on elaboration of a multifunctional Ki-61 version.
The team responsible for construction of the aircraft was headed by the chief engineer at Kawasaki, Takeo Doi. Relying upon experience accumulated in work on the Ki-60, Doi concentrated on improving aerodynamic forms of the predecessor and decreasing weight and unit wing load. Work on the project proceeded rapidly, and already by December 1941 the first prototype of the new fighter Ki-61 rolled out of the Kawasaki hangars. Not waiting for the test-flight results, Aircraft headquarters ordered Kawasaki to create another 11 prototypes. In spring 1942 the program of test flights got underway. It ended with a total success for Ki-61. Participating in instructional combats with the Ki-43-II, Ki-44-I, the Messerschmidt Bf.109E-3 purchased from Germany, and a trophy Curtiss P-40E and LaGG-3 captured in Manchuria, the new fighter demonstrated its full superiority to all the opponents in speed characteristics and was inferior in maneuverability only to the Ki-43-II.
August 1942 saw the startup of mass production for the plane, which received the official designation “army multifunction fighter type 3 model 1” and a gentler nickname “Hien” (“Swallow”). The Ki-61-I was manufactured in two versions: the Ki-61-Ia, equipped with two synchronous 12.7-mm Ho.103 type 1 machine-guns and two wing-based 7.7-mm type 89 machine-guns, and the Ki-61-Ib, equipped with four 12.7-mm Ho-103 machine-guns. Altogether Until the end of 1942 Kawasaki produced 34 serial aircraft altogether.
In March 1943 the first spring Swallows from the 68th and 78th Fighting Sentai received their baptism of fire in the skies above New Guinea. Soon thereafter, the aircraft appeared across a rather extensive front sector stretching from the Philippines to the Solomon Islands. Once again the appearance of a new Japanese aircraft constituted a highly unpleasant surprise. It was no longer so easy to escape from attack by means of the swooping technique, which had been a standard maneuver for the American pilots, as the Ki-61 outdid all the enemy aircraft in swooping speed. Now not only maneuver but also vertical combats put the Americans in danger.
Two out of four existing prototypes of the heavy fighter-interceptor “Kawasaki” Ki-60
One of the first series Ki-61-Ia with retractable tail wheel.
“Daimler-Benz” engine D.601A, serial index “Kawasaki” Ha.40 on engine mount of Ki-61, which was authentically produced by the Japanese.
One of the 368 samples of Ki-61-Ib, equipped with German 20-mm cannon “Mauser” MG.151
