home   japanese aviation

home « japanese aviation « Ki-61 Hien survey « Ki-61 Hien story

For illustrations and drawings see the corresponding Hien page on this site

Asahl Journal Vol.2.1 (JII, 1994) Kawasaki Ki-61 & Ki-100 Japan Investes and then divestes itself of a liquid cooled fighter by Scott Hoffman   Prologue One might say that the Japanese use of a liquid cooled engine in the development of the Ki-61 was an attempt to mimic the success seen in the other militaries of the world. At that time the British were using the Spitfire and Hurricane, the Germans the Me109 and the United States the P-40, all of which possessed liquid cooled engines. When this, coupled with the advantage in the power to weight ratio, was considered it would seem that the attitude of "if it works for them then it should work for us" might have had something to do with the project.         Due to the contacts developed between Kawasaki and the Germans a deal was struck and the Japanese obtained licensed rights to the DB601 engine. This was the same powerplant residing in the Me109 so, added with the experience Kawasaki had gained in their failed attempt to produce the inline Ki-28, they pushed ahead. The path they began to carve forthemselves led to the development of an aircraftthat, for the Japanese, was very unorthodox when compared to other machines they were currently flying.         Originally two inline engined projects were started at about the same time. Priority was initially given to the Ki-60 heavy fighter which spawned three prototypes in it's development program. Less than ideal pilot reports led to an increase in the second machines wingspan and a redesigned cowl area. Again the pilots were not satisfied and the third machine saw continued work in the redesign of the cowling area and attempts to reduce the weight. Never able to obtain the desired speed and maneuverability the program was abandoned in favor of the Ki-61.

The Ki-61 With their sights now set on designing something a little more like the already existing Japanese aircraft, work on the Hien (Swallow) began to move forward. The DB601 had been slightly redesigned to match Japanese production capabilities and was redesignated the Ha-40. The new power plant was mated to the first airframe in December 1941. With the war underway the military pushed for production to commence as soon as possible. The later prototypes were built using production tooling in an effort to eliminate as many problems from the system as possible before actual combat ready examples were built.         Flight testing soon began and the aircraft's first opportunity to engage in combat was to be on April 18, 1942. While flying one of the early prototypes Lt. Umekawa was told to pursue the fleeing B-25's of Doolittle's raiders. He closed on one of the bandits but had to break pursuit due to lack of fuel and problems with his guns. Since the machine contained many systems that could be considered "state of the art" for it's day the emergence of problems should have been expected. The Army may have had second doubts about the new fighter when several prototypes and their pilots were lost during early testing.         However; when things begin to go wrong solutions are rapidly sought for and, sometimes, implemented with little consideration to the problems this haste might cause. Such seems to have been the case with this project. Withtheirbacks soon to be pressed against the wall in Pacific the Army began almost frantic preparations to throw the fledgling design into the fight.         In early 1943 Ki-61 Ia's were transfered to the Tokorozawa Army Maintenance School (fewer than 150 of the machines had been delivered at this time). Soon after the 68th Fighter Regiment began conversion training and was quickly followed by the 78th. These units were pushed into combat as a Japanese Army build-up was begun in the Rabaul area and became part of the 4th Air Army.         The delivery of the two unit's machines included flying the aircraft considerable distances as they islandhopped their way south. With the continuing design problems that remained large numbers of the aircraft were lost in transit due to mechanical malfunctions. This relegated the 68th and 78th to a minimal amount of equipment during their early deployment. Soon the 33rd Fighter Regiment would also join them in what could only be called combat and advanced prototype analysis.         It was at this time that Allied reconnaissance confirmed the new machines existence and it was given the code name "Tony". The supposition that the Italian based name was assigned thinking the machine was a Japanese built copy of an Italian aircraft seems to be lacking in credibility. There also seems to be a distant possibility that the code name "Mike" was used for a short time while the Allies assumed it was a Japanese built Me109.         By late September 1943 the 4th Air Army had completed it's moved to New Guinea. The combat between the Hien and the pilots of the U.S. Army's 5th Air Force were fierce and, while producing several Ki-61 aces, also created a much higher than desired loss rate for the new machines. Many times the Americans would catch large numbers of the Japanese aircraft on the ground and slaughter them without significant loss to themselves. Also continuing to trouble the Japanese were the temperamental engines, prone to over heating and excessive wear due to the ingestion of dust.         While the mechanical problems badgered the machines the severe lack of replacement part must have been even more frustrating. Vessels carrying spares were continually being destroyed before they had a chance to deliver their much needed cargo. Well trained Hien mechanics were also in short supply. So desperate was the replacement parts problem that one of the production lines in Japan was converted to produce nothing but spare parts. Output of finished fighters never exceeded 150 examples per month.

Improvements A common shortcoming for all Japanese aircraft was their lack of fire power when compared to the machines they continually met in combat. The Ki-61 1 a (also called the K-61 1 a(ko)) was armed with two fuselage mounted 12.7mm guns and two 7.7mm wing mounted units. Deliveries of the Ki-61 lb (called Ki-61 lb(ko)) were initiated during the fighting in New Guinea. It's armament was altered to replace the two wing guns with the same 12.7min units found in the fuselage.         Still lacking in potency a deal was struck with the Germans to provide 800 MG151 cannons to be delivered by submarine. It was found that these could be placed into the existing wing of the machine if the gun was laid on it's side and a fairing was provided on the underside of the wing for clearance of it's mechanism. This modification was undertaken on 388 units and produced the machines designated Ki-61 la(modified) and Ki-61 lb(modified). These are also refereed to as the Ki-61 I a(otsu) and the Ki-61 I b(otsu). It should be noted that the 33rd arrived with otsu models and that the 68th and 78th had some of their existing machines modified in the field.         With the favorable reports on the modification using the German guns the Japanese were quick to develope a version containing a Japanese produced cannon. The continued supply of MG 151's via submarine was not a viable option and soon the HO-5 20mm cannon was ready. These were installed in the fuselage of the Ki-61 1 KAIc. This machine also featured two 12.7mm guns in the modified (stronger) wings, provisions of external storage using fixed underwing pylons, and a non-retractable tail wheel. Several internal changes were also made. These included the simplification of several systems for increased dependability and ease of maintenance. The rear section of the fuselage was also made to be easily removable to further facilitate the ease of repair work.         Further work on the Ki-61 1 KAIc produced the Ki-61 1 KAId. This version had the two wing guns replaced with 30mm Ho-105 cannons. With a now simplifed assembly procedure the output of finished aircraft was able to exceed 250 per month in July of 1944. Apparently both KAI versions saw some action in the New Guinea but were not available in any number until the fighting moved to the Philippines.         By early 1944 the 4th Air Army had been devastated in New Guinea and was evacuated to New Hollandia. The "Tony" pilots could not avoid the Allied onslaught. The 4th was again hammered and forced to move it's remaining strength to Menado in the Celebes. Many valuable pilots and technical personnel could not be evacuated due to the lack of transport facilities and were forced to remain, this was yet another loss the Army could ill afford in it's effort to maintain the Hien as a potent fighting machine.         Pilots of renown from the fighting in the New Guinea conflict and who survived the war included WO Tokuyasu Ishizuka (23 kills), commander of the 33rd Lt Kuroki (33 kills), and Capt. Kanshi Kishkawa (19 kills). Of unknown status at the end of the conflict is Sgt. Susumu Kaffinami of the 68th (18 kills before his 21 birthday) and killed during the fighting was Maj. Takaffi Kimura (26 kills) at the hands of a P-38.         With the battle for the Philippines looming in the near future the Japanese quickly began to rebuild the 4th Air Army for the coming fight. New Hien units were formed for the battle. These included the 17th, 18th, 19th, 26th, 53rd, 55th, 59th, 105th, and the 244th. Their training was principally conducted using the KAIc and d models at the Akeno and Hitachi Air Training Divisions. The 17th quickly found itself based in Manila and was soon joined by the 19th. The 33rd was pulled out of Malaya and stationed at Clark field. The 18th and 55th were moved to Luzon in Novemberof 1944 only to have the 55th's complement of pilots completely wiped out by the end of the month.

Ki-61-II Work on improving the machine's design continued with the hope of providing a more powerful engine in the Ha-140. The new engine brought with it several other changes including a redesigned cockpit section for improved visibility and an enlarged wing. This new design was given the designation Ki-61 II. Testing showed that the larger wing was not desirable and the 9th prototype was altered to use the smaller Ki-61 1 wing. The tail surface was enlarged and the fuselage was lengthened. These changes were designated Ki-61 II KAI. The design was further subdivided into two categories, KAIa with two fuselage mounted Ho-5 20mm cannon and two wing mounted 12.7mm guns, and the KAIb with all four guns being the Ho-5 cannon. The new design was indeed an improvement over the Ki-61 I but with the continued degradation of the engine's assembly line quality it was never able perform as designed. This all became irrelevant when a B-29 raid destroyed the engine plant leaving some 300+Ki-61 II's without powerplants.         The Japanese mainland was soon to become the center of attention for the American B-29's and Hien units were quickly positioned to attempt to slow this new onslaught. The Eastern defense sector, which included the Yokohama-Tokyo-Nagoya area, was assigned to the 18th, 23rd, 28th, 53rd, and 244th Regiments. The middle sector was protected by the 17th, a restaffed 55th, and the 56th. The Western Sector was the responsibility of the 56th and latter a repositioned 55th. The 56th was one of the few units to receive the Ki-61 II design and was commanded by Lt. Col. Furukawa. Other "Tony" aces involved in the defense of the homeland included WO Tokuyasu Ishizuka with four B-29 kills (23 total), Maj. Teruhiko Kobayashi 10+ B-29's (20 total), and Kanshi Ishikawa 19 kills total.         With the extreme operating altitude of the B-29's the Ki-61 was hard pressed to reach the required height for an attack. Many units resorted to removing several of the fighter's guns in an attempt to lighten the machines and thus allow them improved performance at altitude. However, his left the pilots with almost no way of attacking the huge bombers. Given the Japanese dedication to the defense of their homeland with Kamikaze attacks it was no real surprise when special groups were formed for suicide attacks against the B-29's.         A unit was formed and called Shinten-Seiku-Tai "Shuddering Sky" Air Superiority Unit. Their participation was voluntary and their training included information regarding the most vulnerable areas of the bombers in which to perform their Taiatari "body crashing" attacks. Flying aircraft with theirguns removed to enhance altitude permormance these attacks were, for the most part, suicide. However, several examples exist of pilots making successful B-29 kills and surviving the ordeal. Maj. Kobayashi possessed two crash attack kills of B-29's as did a pilot named Nakamura. One of the more interesting crash attacks was conducted by a Lt. Shinomiya who killed a B-29 by crashing into it's tail section and then managed to land his badly damaged machine. His aircraft was proudly displayed in the local Matsuya Department Store during the Christmas season of 1944.         Finally as the Japanese position became more desperate the use of the Ki-61 in Kamikaze attacks against shipping became necessary. Pilots who volunteered were transfered to special units which would fly machines carrying two 550 lb. bombs in an attempt to stop the anticipated invasion forces. One whole unit, the 105th Regiment on Formosa, was converted to special attack early in 1945. Their first sortie was on April 1, 1945 but unable to find targets they returned to base.

The Ki-100 is born The Ki-61 ll's which were made unusable by a severe reduction in the supply of powerplants and then total unavailability with the destruction of the engine manufacturing facility presented a difficult problem to the military. With the desperate need for viable combat aircraft the slow output of useable engines and final cessation of inline productin, represented badly needed resorces going to waste in the 300+ orphaned machines. The possibility of installing available radial engines into the airframes would allow them to contribute to the war effort. However the Ki-61 possessed a very narrow crosssection and to install a large diameter engine on the machine would require extensive redesign. A FW190 was at their disposal having been provided in a technical exchange program with the Germans and this illustrated the mating of a large diameter engine with a narrow fuselage. This combination of the Ki-61 II KAI's with the Mitsubishi Ha-112-II radial produced the Ki-100 Ia.         A prototype of the new machine was flying by February 1, 1945 and was quickly followed by several more. The overall flight characteristics of the aircraft were an improvement over it's forefather in all areas except the maximum speed, which was only degraded a small degree. Training was initiated rapidly at the Akeno Air Training Division. Units rotated through for training included the 5th, 59th, 200th, and the 244th Regiments and the 81st Indipendent Fighter Company.         When the supply of engineless Ki-61 II KAl's had been exhausted Ki-100 Ib (also called the type 5) production moved ahead. This machine featured a bubble type canopy for improved visibility and was purposely built as a radial engined machine. Training in these machines was provided to the 17th, 1 8th, 25th, 111th and 125th. The machine proved to be an efficient weapon in the hands of an experienced pilot and provided the novice with a machine that would at least provide him a chance of learning to fly the aircraft before it killed him in a landing accident.         The inexperienced pilots, even when flying the agreable Ki-100, suffered dire consequences due to their lack of skill or when they strayed from tight formations and were hunted down as stragglers. Head on attacks of the B-29 proved to be most effective. The pilots would approach singly and alter their approach path as they neared the bombers. To fail in changing ones incoming flight path was to bring certain death as the coordinated firepower of the Superfortress quickly chewed an aircraft to pieces. Pilots of renown who flew the Ki-100 included Capt. Hidea Inayama (22 kills) who became the leader of the 111th, S/Maj Goro Miyamoto (26 kills) and Lt. Morikichi Kanae (32 kills) both of the 25th, and Capt. Akira Onozaki (28 kills) of the 59th.         The death knell of the type 5 came with the bombing of the Kagamigahra plant and the slow delivery of components from the satellite plants. Production fell to 12 units between May and July and finally ended due to the bombing. Other plans in develpment for the Ki-100 were directed at the production of a high altitude version for interception of B-29's. This was to be accomplished through the installation of a supercharger and was designated the Ki-100 II. Four examples of the machine were built. One was transferred to the 3rd Operational Training Unit but was never used in combat against the Allies.

Epilogue In the end it can be said that the Ki-61 was an example of a new idea that was pushed into service too quickly. Had Kawasaki been allowed to develope the machine and work out the "bugs" it would have been much more viable. However, given all of the hardships placed on the pilots and mechanics, which lived and died with the Hien, it represented itself well.         The Ki-100 was an example everything falling into place at the right time. The Allies were lucky in that the bombers and lack of fuel forced the production of this example into extinction or inactivity. It must be ranked with the Ki-84 and considered as one of the Army's best combat participants.   Scott Hoffman, 1994   Editors note Since this article first appeared in the Asahi Journal Vol.2.1 in 1994, I've found that it was Matsumi Nakano's a/c, not Lt. Shinomiya's, that was displayed in front of the Matsuya Department Store in December 1944 (see my corresponding Hien page). Also, Scott Hoffman's choice of the designations for the various model changes may differ somewhat from what my own research has made me believe, as illustrated in the text and profile drawings on the corresponding Hien page.   Mark Kaiser, 1999