With its liquid-cooled engine, long tapered nose and wings of high aspect ratio, the Ki-61 was unique among Nipponese fighter aircraft of World War II and it marked the first attempt by the JAAF to incorporate in a fighter design the armour protection and self-sealing fuel tank which had been shown to be indispensable by early war reports received from Europe. The Hien (Swallow) was so un-Japanese in its appearance that it was initially reported as being a licence-built version of either the Messerschmitt Bf 109 or of an unspecified Italian aircraft, the latter report earning for it the Allied code-name ‘Tony’.
During the twenties and thirties, initially under the guidance of Dr Richard Vogt, the German engineer who later became the chief designer of Blohm und Voss, Kawasaki Kokuki KK were the leading exponents in Japan of liquid-cooled engines and held manufacturing rights for the German BMW VI, a V-12 engine, which powered most of their aircraft during the period. Following the Army’s selection of the Nakajima Ki-27 over their own Ki-28, Kawasaki decided to negotiate with Daimler-Benz for a licence for the new series of twelve-cylinder inverted-vee engines which the German company had developed. Negotiations were successfully concluded in April 1940 when a Japanese technical team brought back from Stuttgart the blueprints for the DB 601A as well as a number assembled engines to serve as production patterns. Adaptation to Japanese production techniques began immediately at Kawasaki’s Akashi plant and the first Japanese-built DB 601A, designated Ha-40, was completed in July 1941. Four months later the Ha-40 had successfully passed all ground tests and production started under the designation Army Type 2 (Kawasaki Ha-40) liquid-cooled engine, rated at 1,175 hp for take-off and 1,100 hp at 4,200 m (13,780 ft).
While negotiating with Daimler-Benz, Kawasaki had approached the Army with initial design studies for various fighter aircraft making use of this engine. As reports from the air war in Europe were showing the apparent superiority of aircraft powered by liquid-cooled engines, the Koku Hombu instructed Kawasaki to proceed with two aircraft of this type: the Ki-60, a heavy interceptor fighter, and the Ki-61, a lighter all-purpose fighter, priority being given to the heavier aircraft. In December 1940, however, the emphasis shifted to the Ki-61 for which Takeo Doi and Shin Owada were responsible. The Ki-61 was a low-wing monoplane of all-metal construction with a wide-track inward-retracting main undercarriage and a retractable tailwheel. It was characterised by a large ventral radiator bath under the fuselage just beneath the wing leading edge. The pilot sat in an enclosed cockpit with a backward-sliding canopy. The aircraft, powered by a Kawasaki Ha-40, showed in its design the strong influence left by Dr Vogt on his Japanese pupils. To provide good manoeuvrability and to obtain long endurance a wing of high aspect-ratio and large area was selected by Takeo Doi, considerable attention being given to weight and drag reduction. An armament of two 12.7 mm (0.5 in) Type 1 (Ho-103) machine-guns mounted in the upper fuselage decking and either two 7.7 mm (0.303 in) Type 89 or two 12.7 mm (0.5 in) Type 1 (Ho-103) wing-mounted machine-guns was selected, this armament representing a one hundred per cent increase over that carried by the Ki-43-I then just entering service.
One year after receiving authorisation from the Koku Hombu to proceed with the design, the first aircraft was completed at the Kagamigahara plant where flight tests began in December 1941. Prior to this event, Kawasaki had been authorised to prepare for production and to purchase the necessary tooling and material. Fortunately the wisdom of this decision was vindicated when the prototype met the most sanguine hopes of its designers and the Army staff. Eleven additional prototypes and pre-production machines were built in the early part of 1942, and following handling and performance tests during which a maximum speed of 591 km/h (367 mph) at 6,000 m (19,685 ft) was reached, Service trials began. The wing loading of 146 kg/sq m (29.9 lb/sq ft), high by Japanese standards of the time, was criticized by military pilots, but the majority of those who flew the aircraft were impressed by its high diving speed, and its armour protection, self-sealing fuel tanks and armament were also commented upon favourably.
The thirteenth Ki-61, the first machine to be built with production tooling, was completed in August 1942 and differed from the prototypes only in minor equipment details, the deletion of a small window on each side of the fuselage ahead of the windshield providing the only recognition feature. During competitive trials against prototypes of the Nakajima Ki-43-II and the Ki-44-I, an imported Messerschmitt Bf 109E and a captured Curtiss P-40E, the Ki-61 was judged to have the best overall performance and to be an effective weapon against enemy aircraft.
Consequently, late in 1942, the fighter was accepted for Service use under the designation Army Type 3 Fighter Model 1A when armed with two fuselage-mounted 12.7 mm (0.5 in) Type 1 machine-guns and two wing-mounted 7.7 mm (0.303 in) Type 89 machine-guns and Model 1B when the wing guns were the 12.7 mm (0.5 in) Type 1. Initial deliveries of the aircraft were made in February 1943 to the 23rd Dokuritsu Dai Shijugo Chutai at Ota, which acted as a pilot conversion and training unit. Combat operations began two months later when the 68th and 78th Sentais were deployed to Wewak on the north coast of New Guinea. Immediately these units proved that the Ki-61s, then named Hien, were better suited to combat the US and Australian aircraft than the Ki-43s, which they supplemented in this theatre, due to their heavier armament, good protection and high diving speed – a performance required to overcome the enemy fighters which favoured hit and run attacks from higher altitude against the nimbler Nipponese fighter aircraft. The idiosyncrasies of the liquid-cooled Ha-40 which powered the Hien caused the aircraft to be difficult to handle on the ground because of the prevailing hot and damp weather but in the air the Ki-61-I was an outstanding aircraft liked by its pilots and respected by its foes.
At an early stage in the design of the Ki-61 replacement of the fuselage-mounted machine-guns by a pair of 20 mm (0.79 in) cannon had been contemplated. However, as cannon of domestic design were not yet available, 388 Ki-61-Ias and -Ibs were modified on the assembly line to carry one 20 mm (0.79 in) Mauser MG151 in each wing. As space in the wing was limited, the cannon had to be mounted on its side, a small underwing fairing covering the breech, while some local strengthening was required because of the increased recoil force. One other aircraft was modified to test the surface evaporation cooling system which Takeo Doi proposed to use on the Ki-64. This experimental Hien had its large ventral radiator replaced by a smaller retractable unit, for use on the ground, mounted further forward, while in flight cooling was provided by steam evaporation through wing condensers with a total area of 14 sq m (150.694 sq ft). Tests began in October 1942 and thirty-five flights – during which a maximum speed of 630 km/h (391 mph) was attained – were made until the end of 1943 when the purpose of the tests was sufficiently achieved.
Operations in New Guinea, New Ireland and New Britain had shown that ease of maintenance had to be improved and Takeo Doi decided to simplify the Hien’s structure in the next version of the aircraft. With the availability of the indigenous 20 mm (0.79 in) Ho-5 cannon the Ki-61-I-KAIc was produced with a pair of these replacing the two fuselage-mounted 12.7 mm (0.5 in) Type 1 machine-guns. Stronger wings, allowing an increase in diving speed and featuring provision for fixed pylons for external stores outboard of the wheel wells, were mated to a slightly longer fuselage with detachable rear section. On this version the retractable tailwheel was replaced by a fixed unit while minor control modifications were incorporated. production of the Ki-61-I-KAIc began in January 1944, and the type had completely supplanted the earlier versions on the Kagamigahara assembly line in August of the same year. Following the introduction of this version the Hien’s production, which so far had been somewhat slow, quickly gained tempo and the monthly rate reached a peak of 254 aircraft in July 1944. Including a few Ki-61-I-KAIds, which were armed with a pair of 30 mm (1.18 in) Ho-105 cannon in the wings and two fuselage-mounted 12.7 mm (0.5 in) Type 1 machine-guns, a total of 2,654 Ki-61-Is and Ki-61-I-KAIs – of which the former type accounted for over half of the total – were built until January 1945 when production was terminated. Many of the Ki-61-Is and Ki-61-I-KAIs saw operation in the New Guinea/Rabaul area with the two previously mentioned Sentais, but they were mostly active in the Philippines campaign of 1944-45 (17th, 18th and 19th Sentais) and over Formosa (Taiwan) and Okinawa (19th, 37th, 59th and 105th Sentais, and 23rd Dokuritsu Dai Shijugo Chutai). Finally the type played an important role in the defence of the Japanese homeland where the Hien-equipped 18th, 23rd, 28th and 244th Sentais were assigned to the Tokyo Defence Area, the 59th Sentai to the Western Defence Area and the 55th and 56th Sentais to the Central Defence Area. Over Japan the Hiens were engaged against the B-29s, US Navy carrier aircraft and, later, against Iwo Jima based P-51 Mustangs. Against the high-flying B-29s the Ki-61-I lacked the necessary altitude performance, but the type was not really outclassed until the arrival of the superb Mustang.
Soon after commencing production of the Ha-40 at the Akashi plant, the Kawasaki engineering team began developing a more powerful version of this engine, the Ha-140. Primary emphasis was placed on altitude rating, and Takeo Doi, urged by the Army Staff to develop an advanced version of the Hien, decided to mount the Ha-140 rated at 1,500 hp for take-off and 1,250 hp at 5,700 m (18,700 ft) in a specially redesigned version of the Ki-61. Completed in December 1943, the first prototype Ki-61-II had a wing area increased by 10 per cent to 22 sq m (236.806 sq ft) and a redesigned aft canopy providing improved pilot visibility. However, flight trials were disappointing as the Ha-140 had more than its fair share of teething troubles, the crankshaft proving particularly weak. Even the airframe was not without its problems, and the enlarged wings, which had been designed to enhance the aircraft’s manoeuvrability and performance at high altitude, suffered from several failures. The handling characteristics, too left much to be desired. Consequently, only eight of the eleven Ki-61-IIs built were tested and the ninth airframe was modified as the Ki-61-II-KAI before completion in April 1944. The fuselage length was increased from 8.94 m (29 ft 4 in) to 9.16 m (30 ft 0 5/8 in), the rudder area was enlarged to offset the increased wetted area and the larger wings were replaced by standard Ki-61-I-KAI wings. The airframe problems were thus eradicated and, when the engine performed smoothly, Ki-61-II KAI was an outstanding interceptor with a maximum speed of 610 km/h (379 mph) at 6,000 m (19,685 ft) and a climb rate of 5,000 m (16,405 ft) in six minutes. Still confident that the persistent engine teething troubles would be eradicated, the Ministry of Munitions, acting on behalf of the Army, instructed Kawasaki to proceed with the mass production of the aircraft under the designation Army Type 3 Fighter Model 2.
Starting in September 1944 the Ki-61-II-KAI was built in two versions, the Model 2A with an armament of two fuselage-mounted 20 mm (0.79 in) Ho-5 cannon and two wing-mounted 12.7 mm (0.5 in) Type 1 machine-guns and the Model 2B with an armament of four 20 mm (0.79 in) Ho-5 cannon, two in the fuselage and two in the wings. The Ki-61-II-KAI never supplanted the Ki-61-I-KAI in operational units as its engine was still suffering from chronic weaknesses, and the comparatively few Hiens of this model saw only limited operational service in Japan. When the engine was operating smoothly the Ki-61-II-KAI was an effective interceptor and was the only Japanese fighter able to maintain combat formation at the operating altitude of the B-29s. However, the lack of skilled workers was by then being badly felt and seldom did the Ha-140 give its full rated power. Finally, production of the Army Type 3 Fighter Model 2 was dealt a crippling blow when, on 19 January, 1945, the US Army Air Force destroyed the Akashi engine plant. Only 374 Ki-61-II-KAI airframes were built in slightly less than a year but some thirty were destroyed on the ground prior to delivery to Service units and 275 were left without engines until the successful adoption of the Mitsubishi Ha-112-II fourteen-cylinder radial engine which gave birth to the Ki-100. Prior to this conversion it had been proposed to incorporate various modifications in a new version, the Ki-61-III, but only one aerodynamic prototype was built, this aircraft being characterised by having a cut-down rear fuselage and the fitting of an all-round vision canopy to a modified Ki-61-II-KAI.
Among the Japanese aces who flew the Ki-61 was Major Shogo Takeutchi. He flew with the 68th Sentai over New Guinea and claimed 16 enemy aircraft destroyed before being killed in a crash-landing on 21 December, 1943.
Plagued by engine troubles and production difficulties, the Hien never saw as extensive a Service use as the more numerous Nakajima fighters, but during the mid-war years it was the only Japanese aircraft which could successfully engage the fast Allied fighters by combining some of the Nipponese machines’ traditional manoeuvrability with a strong and well protected structure.
17th, 18th, 19th, 23rd, 26th, 28th, 37th, 55th, 56th, 59th, 65th, 68th, 78th, 105th and 244th Sentais. 23rd and 28th Dokuritsu Dai Shijugo Chutais. 8th Kyo-iku Hikotai. 5th,11th, 16th and 18th Lensei Hikotais. Akeno Fighter Training School.
Manufacturer: Kawasaki Kokuki Kogyo KK (Kawasaki Aircraft Engineering Co Ltd).
Type: Single-engined interceptor fighter and fighter bomber.
Crew (1): Pilot in enclosed cockpit.
Powerplant: (Ki-61- prototypes) One 1,100 hp Kawasaki Ha-40 twelve-cylinder inverted-vee liquid-cooled engine, driving a three-blade constant-speed metal propeller; (Ki-61-Ia, -Ib and KAIc and KAId)) One 1,100 hp Army Type 2 (Kawasaki Ha-40) twelve-cylinder liquid-cooled engine, driving a three-blade constant-speed metal propeller; (Ki-61-II and II-KAI) One Kawasaki Ha-140 twelve-cylinder liquid-cooled engine, driving a three-blade constant-speed metal propeller.
Armament: Two (Ki-61-Ia) fuselage-mounted 12.7 mm (0.5 in) Type 1 machine-guns and two wing-mounted 7.7 mm (0.303 in) Type 89 machine-guns; two (Ki-61-Ib) fuselage-mounted 12.7 mm (0.5 in) Type 1 machine-guns and two wing-mounted 12.7 mm (0.5 in) Type 1 machine-guns; two (modified Ki-61-Ia and -Ib) fuselage -mounted 12.7 mm (0.5 in) Type 1 machine-guns and two wing-mounted 20 mm (0.79 in) Mauser MG 151/20 cannon; two (Ki-61-I-KAIc, Ki-61-II and -II-KAIa) fuselage-mounted 20 mm (0.79 in) Ho-5 cannon and two wing-mounted 12.7 mm (0.5 in) Type 1 machine-guns; two (Ki-61-I-KAId) fuselage-mounted 12.7 mm (0.5 in) Type 1 machine-guns and two wing-mounted 30 mm (1.18 in) Ho-105 cannon; two (Ki-61-II-KAIb) fuselage-mounted 20 mm (0.79 in) Ho-5 cannon and two wing-mounted 20 mm (0.79 in) Ho-5 cannon.
External stores: Two 200 litre (44 Imp gal) drop tanks, or (Ki-61-I KAI and -II KAI) two 250 kg (551 lb) bombs.
Dimensions: Span (Ki-61-Ib, -I-KAIc and -II-KAIa) 12 m (39 ft 4 7/16 in); length (Ki-61-Ib) 8.75 m (28 ft 8 1/2 in), (Ki-61-I-KAIc) 8.94 m (29 ft 4 in), (Ki-61-II-KAIa) 9.16 m (30 ft 0 5/8 in); height (Ki-61-Ib, -I-KAIc and -II-KAIa) 3.7 m (12 ft 1 11/16 in); wing area (Ki-61-Ib, -I-KAIc and -II-KAIa) 20 sq m (215.278 sq ft).
Weights: Empty (Ki-61-Ib) 2,210 kg (4,872 lb), (Ki-61-I-KAIc) 2,630 kg (5,798 lb), (Ki-61-II-KAIa) 2,840 kg (6,261 lb); loaded (Ki-61-Ib) 2,950 kg (6,504 lb), (Ki-61-I-KAIc) 3,470 kg (7,650 lb), (Ki-61-II-KAIa) 3,780 kg (8,333 lb); maximum (Ki-61-Ib) 3,250 kg (7,165 lb), (Ki-61-II-KAIa)3,825 kg (8,433 lb); wing loading (Ki-61-Ib) 147.5 kg/sq m (30.2 lb/sq ft), (Ki-61-I-KAIc) 173.5 kg/sq m (35.1 lb/sq ft), (Ki-61-II-KAIa) 189 kg/sq m (38.8 lb/sq ft); power loading (Ki-61-Ib) 2.51 kg/hp (5.53 lb/hp), (Ki-61-I-KAIc) 2.94 kg/hp (6.48 lb/hp), (Ki-61-II-KAIa) 2.52 kg/hp (5.56 lb/hp).
Performance: Maximum speed (Ki-61-Ib) 592 km/h (368 mph) at 4,860 m (15,945 ft), (Ki-61-I-KAIc) 590 km/h (366 mph) at 4,260 m (13,980 ft), (Ki-61-II-KAIa) 610 km/h (379 mph) at 6,000 m (19,685 ft); cruising speed (Ki-61-Ib) 400 km/h (249 mph) at 4,000 m (13,125 ft); climb to 5,000 m (16,405 ft) in (Ki-61-Ib) 5 min 31 sec, (Ki-61-I-KAIc) 7 min, (Ki-61-II-KAIa) 6 min; service ceiling (Ki-61-Ib) 11,600 m (37,730 ft), (Ki-61-I-KAIc) 10,000 m (32,810 ft, (Ki-61-II-KAIa) 11,000 m ( 36,090 ft); range – normal (Ki-61-Ib) 600 km (373 miles), (Ki-61-II-KAIa) 1,100 km (684 miles), – maximum (Ki-61-1b) 1,100 km (684 miles), (Ki-61-I-KAIc) 1,800 km (1,120 miles), (Ki-61-II-KAIa) 1,600 km (995 miles).
Production: A total of 3,078 Ki-61s were built by Kawasaki Kokukai Kogyo KK in their Kagamigahara plant as follows:
12 Ki-61 prototypes – 1941-42
1,380 Ki-61-I production aircraft – August 1942-July 1944
1,274 Ki-61-I-KAI production aircraft – January 1944-January 1945
8 Ki-61-II prototypes – August 1943-January 1944
30 Ki-61-II-KAI prototypes and pre-production aircraft – April-September 1944
374* Ki-61-II-KAI production aircraft – September 1944-August 1945.
Out of 374 Ki-61-II airframes built 275 were completed as Ki-100-Ia.