The Kawasaki Ki-61 Hien & Ki-100

v1.2.7 / 01 aug 16 / greg goebel

* The Kawasaki Ki-61 Hien ("Swallow") fighter represented a major departure for Japanese aircraft design in World War II. While other Japanese fighters were designed with air-cooled radials and were optimized for maneuverability, the Ki-61 used a liquid-cooled in-line engine and was designed for speed and power. In fact, the Ki-61 was so different from other Japanese fighters that when the type was first encountered in combat over New Guinea in June 1943, the Allies thought it wasn't a Japanese design at all. At first they believed it was a copy of the German Messerschmitt Bf 109, then suspected it was a copy of the Italian Macchi C.202 Folgore or similar Italian fighter. By the summer of 1943, the Allies were finally convinced the Ki-61 was in fact a Japanese design.

The Hien proved initially successful in combat against American fighters. As the war in the Pacific ground on, however, the Ki-61 found itself increasingly outclassed, but it soldiered on until the end of hostilities. This document provides a history and description of the Ki-61 and its radial-engine derivative, the Ki-100, and the experimental twin-engine Ki-64.

Kawasaki Ki-61 Hien

[3] KI-61-II / KI-100 / KI-64


* The confusion of the Ki-61 with German and Italian fighters had some basis in the aircraft's origins. Between 1923 and 1933, Kawasaki Aircraft Engineering Company's head designer was a German named Dr. Richard Vogt, who returned to Germany in 1933 to take a similar position at the firm of Blohm und Voss. Not surprisingly, Kawasaki continued to be strongly influenced by Dr. Vogt's beliefs after he left, particularly his faith in the merits of liquid-cooled inline engines. This made Kawasaki something of a heretic among Japanese aircraft manufacturers, with their preference for air-cooled radials.

In March 1938, Kawasaki signed an agreement with Daimler-Benz of Germany for manufacturing rights to the liquid-cooled inline engines then under development by the German firm. In April 1940, a Kawasaki engineering team visited Daimler-Benz in Stuttgart to obtain plans and samples of the DB 601A engine, then being used in the Bf-109.

The Kawasaki engine team managed to increase the take-off power of their version of the engine to 875 kW (1,175 HP) and reduce its weight slightly. The engine was put into production in November 1941. It was designated the "Ha-40", or "Army Type 2", though it would be later redesignated the "Ha-60" in a combined Army/Navy nomenclature.

In the meantime, certain officers at the Air Headquarters of the Imperial Japanese Army (IJA) were very interested in the new fighters powered by liquid-cooled engines being developed in Britain, the USA, the USSR, Germany, and France. The Japanese Army also had unpleasant experiences in air combat against Soviet Polikarpov I-16 fighters during the severe beating the Japanese took from the USSR in a border clash over Manchuria in the summer of 1939. This experience suggested that the single-minded focus on agility above all that characterized Japanese fighter design doctrine might need to yield to a focus on speed plus improved armor protection and firepower.

* In February 1940, the IJA initiated work with Kawasaki on two single-seat fighters based on the DB-601A derivative engine: a heavy interceptor, designated the "Ki-60", and a general-purpose fighter, designated the "Ki-61". Kawasaki decided to build the Ki-60 first, and the design team, under Kawasaki chief designer Takeo Doi and his deputy Shin Owada, constructed three prototypes of the interceptor in 1941.

Kawasaki Ki-60

The Ha-40 engine was not available at that time, so the three aircraft were powered by sample DB 601A engines obtained from Germany. The Ki-60 was a low-wing monoplane, with plenty of power and heavy armament by Japanese standards. The first and second prototypes had two 12.7-millimeter (0.50-caliber) Ho-103 machine guns mounted on the nose in front of the pilot and two 20-millimeter Mauser MG 151/20 cannons, one mounted in each wing, for a total of four guns. The third prototype had four 12.7-millimeter guns.

   _____________________   _________________   _______________________
   spec                    metric              english
   _____________________   _________________   _______________________

   wingspan                10.5 meters         34 feet 5 inches
   wing area               16.2 sq_meters      174.4 sq_feet   
   length                  8.47 meters         27 feet 9 inches
   height                  3.7 meters          12 feet 2 inches

   empty weight            2,150 kilograms     4,740 pounds
   loaded weight           2,750 kilograms     6,063 pounds

   max speed at altitude   560 KPH             348 MPH / 303 KT
   service ceiling         10,000 meters       32,800 feet
   _____________________   _________________   _______________________

Flight tests began in March 1941. The trials showed that the Ki-60 lacked maneuverability; it also didn't meet its speed and climb requirements. Various tweaks to improve the aircraft failed, and so the Ki-60 was abandoned.

* The experience was valuable, however. Design work on the Ki-61, whose development had been proceeding in parallel with the Ki-60 since December 1940, incorporated new features derived from the lessons learned by the Ki-60 program:

The first prototype was rolled out in early December 1941, and its performance proved excellent. Eleven more prototypes were delivered to the IJA for intensive trials. The Ki-61 was pitted against other Japanese fighters, as well as against the Messerschmitt Bf 109E-3, of which two had been bought by the IJA from the Germans, and the Curtiss P-40E, several of which had fallen into Japanese hands after the capture of the Dutch East Indies.

While test pilots were a little skeptical of the new aircraft at first, pilots with combat experience appreciated the Ki-61's self-sealing fuel tanks, heavier armor and armament, and fast diving speed. The air combat tests showed the Ki-61 to be faster than all its adversaries, and it easily out-maneuvered everything it went up against except the Japanese Nakajima Ki-43.

The 13th Ki-61, a production prototype, was delivered in August 1942. The IJA gave the green light for production, and the fighter began to roll off the assembly line, with 34 delivered by the end of 1942. The type was formally known as the "Army Type 3 Fighter Model 1 Hien", or "Ki-61-I".

The production machines differed from the prototypes only in tweaky details. Initial production consisted of two variants: the "Ki-61-Ia", with two 12.7-millimeter guns in the nose and a 7.7-millimeter gun in each wing, for a total of four guns; and the "Ki-61-Ib", with 12.7-millimeter guns in both fuselage and wings. These aircraft could be fitted with two 200-liter (53 US gallon) drop tanks. A total of 1,380 Ki-61-Ia/b machines was built in all.



* The Hien entered combat in the spring of 1943 in the New Guinea war zone, covering New Guinea, the Admiralty Islands, New Britain, and New Ireland. The new Japanese fighter caused some pain and consternation among Allied pilots, particularly when they found out the hard way that they could no longer go into a dive and escape as they had from lighter Japanese fighters. 5th Air Force Commander General George Kenney found his P-40 Warhawks completely outclassed, and begged for more P-38 Lightnings to counter the threat of the new enemy fighter.

The Ki-61 demonstrated only a few teething problems in field use, such as a tendency towards engine overheating during ground running under tropical conditions. However, despite the heavier armament, it still didn't have the punch to easily knock rugged and well-armed Allied bombers out of the sky.

The Kawasaki designers had foreseen this problem. The Japanese Ho-5 20 millimeter cannon wasn't available at the time, but the Japanese obtained 800 Mauser MG 151/20 20-millimeter cannon from Germany in August 1943, and modified 388 Ki-61-I airframes on the production line to carry the German weapons in place of the two 12.7-millimeter wing guns. The cannon had to be mounted on their sides to fit into a wing, with an underwing blister for the breech, and some reinforcements were added to the wing to absorb the heavier recoil.

Once the Ho-5 cannon finally became available, Kawasaki designers then reversed the arrangement of the guns, putting the 20-millimeter cannon in the nose and the 12.7-millimeter guns in the wings. While they were making these modifications, they also made a few changes to streamline manufacturing and simplify field maintenance.

Kawasaki Ki-61-I, Ki-61-II

This new variant was designated the "Ki-61-I KAIc" (where "KAI" was for "kaizo", or "modified"). It was 19 centimeters (7.5 inches) longer than its predecessors, and also featured a detachable rear section; a fixed tailwheel instead of the earlier retractable tailwheel; stronger wings; and stores pylons outboard of the main landing gear, allowing it to carry two 250 kilogram (550 pound) bombs.

   _____________________   _________________   _______________________
   spec                    metric              english
   _____________________   _________________   _______________________

   wingspan                12 meters           39 feet 4 inches
   wing area               20 sq_meters        215.3 sq_feet   
   length                  8.94 meters         29 feet 3 inches
   height                  3.70 meters         12 feet 2 inches

   empty weight            2,630 kilograms     5,800 pounds
   loaded weight           3,470 kilograms     7,650 pounds

   maximum speed           590 KPH             366 MPH / 318 KT
   service ceiling         10,000 meters       32,810 feet
   range                   1,800 kilometers    1,120 MI / 975 NMI
   _____________________   _________________   _______________________

The Ki-61-I KAIc went into production in January 1944 and ultimately replaced production of all earlier models in August 1944. A few "Ki-61-I KAId" bomber interceptors were also built in late 1944. These aircraft featured two 12.7 millimeter guns in the fuselage and a 30 millimeter Ho-105 cannon in each wing. Total production of the Ki-61-I KAIc/d was 1,274 aircraft.


[3] KI-61-II / KI-100 / KI-64

* Even before the Hien saw combat, the IJA had been pressing Kawasaki for an improved version of the same aircraft. To this end, Kawasaki engineers focused on a derivative of the Ha-40 engine known as the "Ha-140", which was expected to have a take-off power of 1,120 kW (1,500 HP).

The first prototype of the new fighter variant, the "Ki-61-II", flew in August 1943. Ten more prototypes were ordered. They featured a wing with 10% more area, and an improved canopy to provide better rearward field of view, but the Ha-140 development program ran into troubles and only eight of the prototypes were fitted with engines. Even then, they suffered from engine troubles, structural failures due to weaknesses in the new wing, and handling problems.

In an attempt to fix the problems, after delivery of the eighth Ki-61-II prototype, the ninth prototype was extensively modified during manufacture. The extended wing was replaced with the original Hien wing, the fuselage was lengthened, and the rudder area increased. The result was the "Ki-61-II KAI", with initial flight of the prototype in April 1944. It was followed by 30 more prototypes. As long as the temperamental Ha-140 engine worked properly, the Ki-61-II KAI proved to be a promising interceptor, with a fast climb rate and good high-altitude flight characteristics.

The military situation was becoming increasingly desperate, and so the Ki-61-II KAI was put into production in September 1944 in spite of the problems with the engine. Two versions were produced: one designated "Ki-61-II KAIa", with 12.7-millimeter guns in the wings and 20-millimeter cannon in the nose; and the other designated "Ki-61-II KAIb", with four 20-millimeter cannon.

374 Ki-61-II KAI airframes were built and 99 of them fitted with engines, giving a total production of 404 Ki-61-II KAI machines built, if not completed. On 19 January 1945, US Army Air Forces B-29 Superfortresses turned the plant at Akashi that was producing the Ha-140 engine into cinders and rubble. That abruptly ended concerns over the reliability of the Ha-140 engine, but left 275 airframes sitting around without powerplants.

* However, in November 1944 concerns over the availability (or lack thereof) of the Ha-140 engine had led the Munitions Ministry to request that Kawasaki redesign the Ki-61-II KAI to use another engine. Company engineers performed a lightning design effort to mate the fighter to the 1,120 kW (1,500 HP) Mitsubishi Ha-112-II 14-cylinder double-row air-cooled radial engine. The engineers inspected the radial engine installation of a sample Focke-Wulf FW-190 fighter obtained from Germany and -- in an example of interservice cooperation that was far more the exception than the rule between the Imperial Japanese Army and Navy -- took advantage of Navy efforts to use the Ha-112-II.

The first prototype conversion of three took to the air on 1 February 1945. Sometimes improvisations work poorly, sometimes they work surprisingly well; the new variant demonstrated excellent performance. The rest of the engineless Ki-61-II KAI airframes were then converted to the new fighter type, which was designated the "Ki-100-Ia". They retained the armament of the Ki-61-II KAIb, consisting of 12.7-millimeter guns in the wings and 20-millimeter cannons in the fuselage. Performance was roughly the same but engine reliability was vastly improved. The Ki-100 was in fact an excellent fighter, a nasty customer for Allied aircraft to deal with while being surprisingly comfortable and easy to fly -- an important consideration when experienced Japanese pilots were in increasingly short supply.

Kawasaki Ki-100-II, Ki-61-I

A batch of 118 new-production Ki-100s was built as well, incorporating an all-round vision canopy, tested on a modified Ki-61-II KAI, with this variant given the designation "Ki-100-Ib". Work was then done to add a turbosupercharger and water-methanol engine boost to the Ki-100 to provide improved high-altitude performance. The first prototype of this variant, designated the "Ki-100-II", flew in May 1945, with two more prototypes completed before Japan's surrender ended plans for production.

   _____________________   _________________   _______________________
   spec                    metric              english
   _____________________   _________________   _______________________

   wingspan                12 meters           39 feet 4 inches
   wing area               20 sq_meters        215.3 sq_feet   
   length                  8.82 meters         28 feet 11 inches
   height                  3.75 meters         12 feet 4 inches

   empty weight            2,525 kilograms     5,567 pounds
   loaded weight           3,495 kilograms     7,705 pounds

   max speed at altitude   580 KPH             360 MPH / 315 KT
   service ceiling         11,000 meters       36,000 feet
   normal range            1,400 kilometers    870 MI / 755 NMI
   _____________________   _________________   _______________________

* As Allied forces pushed in the bounds of Japan's overextended ocean empire, the Hien fought in the South Pacific, in the Philippines, on Okinawa, and finally in defense of the Japanese home islands themselves. Some Hien units also served on in China and on Formosa.

The home defense units operating on Japan used a mixed bag of Ki-61 variants, essentially anything they could get their hands on. The most famous of these units was the 244th Sentai (Fighter Group) under Japanese ace Major Tembiko Kobayashi. Major Kobayashi encouraged his men to perform frontal attacks on B-29s and press their assaults to close range, even ramming if that was what was needed. Major Kobayashi set an example by fighting with almost suicidal determination.

By this time, the Hien was outclassed by American Mustangs and other late-model US fighters. In fact, when attacking B-29s Hiens often had to be protected by Ki-100s. Operations faded away as planes and pilots continued to fall in combat and fuel and spare parts dried up. By the end, the home defense units were no longer capable of posing a real threat to the Americans.

Of the thousands of Hiens built, not many survived the war. Several were evaluated by the Americans and one was presented back to Japan by the US Air Force later. This was the only surviving Hien as of the mid-1960s. American aircraft restorer Kermit Weeks has a Ki-61 airframe in slow process of restoration.

The following table summarizes Ki-60/61/100 variants:

   variant       built   notes
   Ki-60             3   Prototype interceptor, not produced.

   Ki-61            12   Hien prototypes.
   Ki-61-I       1,380   Initial production (3 armament configurations).
   Ki-61-I KAI   1,274   Rearranged armament, longer fuselage.
   Ki-61-II          8   Ha-140 engine.
   Ki-61-II KAI    129   Simplified production with Ha-140 engine.

   subtotal:     2,803

   Ki-100            3   Ki-61-II KAI with Ha-112-II radial.
   Ki-100-Ia       272   Ki-61-II KAI conversions.
   Ki-100-Ib       118   Ki-100 with all-round vision canopy.
   Ki-100-II         3   Turbocharged prototypes.

   subtotal:       396

   TOTAL:        3,202

* The Ki-61 wasn't the only fighter design to be powered by the Ha-40 inline engine. Kawasaki also experimented with an unorthodox twin-engine design conceived by Takeo Doi in 1939 and approved for prototype development in October 1940 as the Kawasaki "Ki-64".

Kawasaki Ki-64

The Ki-64 had both engines in tandem, straddling the cockpit and driving a single contra-rotating prop system with twin three-bladed props. The combined engine was known as the "Ha-201", and generated 1,755 kW (2,350 HP) take-off power. It featured an unusual steam cooling system with radiators in the wings; the front engine used the right wing for cooling, while the rear engine used the left wing. The Ki-64 had a vague configurational resemblance to the Hien, though it was larger.

   _____________________   _________________   _______________________
   spec                    metric              english
   _____________________   _________________   _______________________

   wingspan                13.5 meters         44 feet 4 inches
   wing area               28 sq_meters        301.4 sq_feet   
   length                  11.03 meters        36 feet 2 inches
   height                  4.25 meters         13 feet 11 inches

   empty weight            4,050 kilograms     8,929 pounds
   loaded weight           5,100 kilograms     11,245 pounds

   max speed at altitude   690 KPH             429 MPH / 375 KT
   service ceiling         12,000 meters       39,400 feet
   range                   1,000 kilometers    620 MI / 540 NMI
   _____________________   _________________   _______________________

One prototype was finished in December 1943, but the rear engine caught fire on the fifth test flight. The aircraft made an emergency landing and survived. However, it was never repaired and the project was abandoned in the middle of 1944.



* The Ki-61 is fairly well known in the current day by the Allied codename "Tony" -- derived from "Antonio" because of the suspicion that it was a copy of an Italian Macchi Folgore. However, one of the problems with referring to Japanese aircraft by the Allied codenames is that the codename system wasn't in widespread use until well into 1943, and so any usage of it in a context before that time is uncertain. In addition, German types were given similar codenames, for example a Junkers Ju 87 Stuka was codenamed "Irene", but for the German types they never really stuck. In sum, the codenames end up being historically more bother than they're worth, and the most a document can sensibly do with them is mention that they existed.

Kawasaki Ki-100-IB at RAF Museum

* As concerns copyrights and permissions for this document, all illustrations and images credited to me are public domain. I reserve all rights to my writings. However, if anyone does want to make use of my writings, just contact me, and we can chat about it. I'm lenient in giving permissions, usually on the basis of being properly credited.

* Sources include:

* Revision history:

   v1.0   / 28 feb 97 
   v1.1   / 01 aug 99 / Review & polish.
   v1.2.0 / 01 jul 03 / General cleanup and update.
   v1.2.1 / 01 jun 05 / Review & polish.
   v1.2.2 / 01 may 07 / Review & polish.
   v1.2.3 / 01 apr 09 / Review & polish.
   v1.2.4 / 01 nov 10 / Review & polish.
   v1.2.5 / 01 oct 12 / Review & polish.
   v1.2.6 / 01 sep 14 / Review & polish.
   v1.2.7 / 01 aug 16 / Review & polish.