The Hien (swallow) was unique among Japanese first-line warplanes of the Pacific War in World War II in being powered by a liquid-cooled inverted V-12 engine, although this Kawasaki Ha-40 unit was in fact a German engine, the Daimler-Benz DB 601A, built under licence in Japan. So untypical was the Hien of contemporary Japanese design concepts, indeed, that Allied intelligence officers at first though that the type was a licence-built version of either the Messerschmitt Bf 109 or an unspecified Italian fighter, this latter possibility resulting in the type’s Allied reporting name of ‘Tony’.
Other notable features of the Hien, designed by Takeo Doi and his deputy Shin Owada, were its creation from the outset with self-sealing fuel tankage and a measure of armour protection for the pilot, both of these being features that were at the time unknown in Japanese aircraft but thought necessary by the design team as a result of information it had received about the nature of air combat over Europe in 1939 and 1940. Although the Hien’s V-engined configuration came as a shock to the Allies, it was in fact a logical development of the Kawasaki philosophy that had evolved during the 1920s, when the company had greatly benefited from the design experience of Dr Richard Vogt (later chief designer of Blohm und Voss) and had also been the leading Japanese exponent of the water-cooled engine. The company’s first success in this field had been with licence-built versions and then developments of the BMW VI liquid-cooled V-12 engine, but when the Imperial Japanese army air force selected the radial-engined Nakajima Ki-27 monoplane fighter over Kawasaki’s Ki-28 with the Ha-9 liquid-cooled V-12 engine, the company appreciated that the BMW VI and its derivatives had reached the end of their useful lives and accordingly looked for a modern engine of the same basic type and accordingly secured an April 1940 licence both to make and undertake local development of the DB 601A in Japan, where the first Ha-40 ran in July 1941 and received type approval in November of the same year.
As it was preparing the engine for Japanese production, Kawasaki was starting the process of designing a fighter to use the new engine, and this was schemed under the supervision of Doi in parallel with the company’s Ki-60 response to the Imperial Japanese army air force’s requirement for a cannon-armed and heavily loaded interceptor in which speed and climb rate were more important than agility. The resulting Ki-61 was clearly related in conceptual terms to the Ki-60, but was planned as a fighter to which a multi-role capability could be added, and was therefore considerably more lightly loaded with a wing of greater span and area selected for a combination of agility and long range.
The structure was comparatively lighter than that of the Ki-60 although, as noted above, it was still fairly heavy by Japanese standards as a result of the design team’s decision to build into the design a fair measure of resistance to combat damage. The Ki-61 therefore matured as a very clean monoplane of basically all-metal construction with fabric-covered control surfaces, an oval-section fuselage of the semi-monocoque type, a cantilever tail unit, a low-set cantilever wing that was dihedralled, tapered in thickness and chord, and carried across its trailing edges the standard combination of outboard ailerons and inboard split flaps, fully retractable tailwheel landing gear including wide-track main units that hinged inward into door-covered bays in the wing roots, and the powerplant of one Ha-40 engine installed in the nose and cooled by a radiator located in a ventral bath under the fuselage in line with the trailing edges of the wing.
The first of an eventual 12 Ki-61 prototype and pre-production aircraft recorded its maiden flight in December 1941 with the fixed forward-firing armament of two 0.5 in (12.7 mm) Ho-103 (Type 1) machine guns in the upper part of the forward fuselage and two 0.303 in (7.7 mm) Type 89 machine guns in the leading edges of the wing. The powerplant was one Ha-40 engine rated at 1,175 hp (876 kW) for take-off and 1,100 hp (820 kW) at 13,780 ft (4200 m) and driving a three-blade metal propeller of the constant-speed type. The prototypes revealed excellent performance and good handling, and although some service pilots were critical of a wing loading that was high by Japanese standards, most pilots approved of the Ki-61’s protection, comparatively heavy armament, high diving speed and good acceleration.
The Imperial Japanese army air force had already ordered the type into production as the Ki-61-I, and this initial model entered service during February 1943 with the powerplant of one Ha-40 (Army Type 2) engine rated at 1,175 hp (876 kW) for take-off and 1,080 hp (805 kW) at 11,480 ft (3500 m). The two initial models were the Ki-61-Ia (full designation Army Type 3 Fighter Model 1A) with the fixed forward-firing armament of two 0.5 in (12.7 mm) Ho-103 (Type 1) machine guns in the upper part of the forward fuselage and two 0.303 in (7.7 mm) Type 89 machine guns in the leading edges of the wing, and the Ki-61-Ib (Army Type 3 Fighter Model 1B) with a fixed forward-firing armament of four 0.5 in (12.7 mm) Ho-103 (Type 1) machine guns. The Ki-61-Ia had the fully retractable tailwheel and under the wing, outboard of the wheel bays, two hardpoints each able to carry one 33 Imp gal (39.6 US gal; 150 litre) drop tank or a light bomb; the subvariant’s maximum take-off weight was 6,504 lb (2950 kg). The Ki-61-Ib reflected the fact that the Ki-61-Ia’s armament had soon been discovered to be relatively ineffective against the always more sturdily built Allied aircraft and the tailwheel retraction mechanism to be unreliable. The subvariant was therefore modified with two 0.5 in (12.7 mm) Ho-103 machine guns in place of the wing-mounted 7.7 mm (0.303 in) machine guns, requiring modification to the upper-wing bulges that accommodated the breeches, and the tailwheel doors were removed and the tailwheel locked in the extended position although the retraction mechanism was left intact; the subvariant’s maximum take-off weight was 6,900 lb (3130 kg).
The Ki-61 entered combat over New Guinea in April 1943, and immediately transformed the capabilities of Japanese air power in a theatre that had previously relied on the lightly armed and essentially unprotected Nakajima Ki-43 Hayabusa as its primary fighter. The Ki-61-I was difficult to handle on the ground and the Ha-40 engine was both unreliable and difficult to maintain, but in the air was basically superior to the US and Australian fighters by which it was opposed. Production of the Ki-61-I series lasted to July 1944 and totalled 1,380 aircraft.
From the beginning of the Ki-61 programme it had been planned that the two fuselage-mounted machine guns should eventually be replaced by a pair of Japanese-designed 20 mm cannon, but delays in the development of these weapons meant that 388 Ki-61-Ia and Ki-61-Ib fighters were modified during production to carry two 20 mm MG 151/20 cannon, which were German weapons of which 800 had been imported, together with ammunition, by submarine, in place of the wing-mounted machine guns. The advent of the 20 mm Ho-5 cannon coincided with Kawasaki’s determination to improve the maintainability of the Hien and at the same time simplify construction and strengthen the airframe.
This resulted in the Ki-61-I Kai-c (Army Type 3 Fighter Model 1C), the Kai indicating ‘modified’ or ‘improved’, which entered production in January 1944 with a powerplant of one Ha-40 engine rated at 1,180 hp (880 kW) for take-off and 1,100 hp (820 kW) at 12,795 ft (3900 m ), a lengthened fuselage with a detachable tail section, a fixed tailwheel, a strengthened wing with two hardpoints for the carriage of 551 lb (250 kg) bombs, and a fixed forward-firing armament of two 20 mm Ho-5 cannon with 120 rounds per gun in the fuselage and two 0.5 in (12.7 mm) Ho-103 (Type 1) machine guns with 250 rounds per gun in the leading edges of the wing.
The fuselage was lengthened, engine bulkhead being moved forward 7.5 in (19 cm) and a small fuel tank added. Trial fittings found that the cannon could be installed in the existing wing if the weapon was laid on its side and a fairing was provided on the underside of the wing for clearance of the breech mechanism. Some 388 examples of the Ki-61 were so modified, the first being completed in September 1943 or January 1944 depending on source, and the last in July 1944. In other respects the Ki-61-I Kai-c differed from the Ki-61-Ib in details such as its internal fuel capacity of 121 Imp gal (145.3 US gal; 550 litres), external fuel capacity of 88 Imp gal (105.7 US gal; 400 litres) in two 44 Imp gal (52.8 US gal; 200 litre) drop tanks, length of 29 ft 4 in (8.94 m), empty weight of 5,798 lb (2630 kg), maximum take-off weight of 7,650 lb (3470 kg), maximum speed of 318 kt (360 mph; 590 km/h) at 13,980 ft (4260 m) declining to 313 kt (360.5 mph; 580 km/h) at 16,405 ft (5000 m), climb to 16,405 ft (5000 m) in 7 minutes 0 seconds, service ceiling of 32,810 ft (10000 m), maximum range of 971 nm (1,118.5 miles; 1800 km) with drop tanks, and typical range of 313 nm (360.5 miles; 580 km) with standard fuel.
The designation Ki-61-I Kai-d has been suggested but not confirmed for a subvariant with the fixed forward-firing armament of two 0.5 in (12.7 mm) Ho-103 machine guns in a strengthened wing with provision for external drop tank or bomb carriage using fixed underwing pylons, and a fixed tailwheel. As in the Ki-61-I Kai-c, the forward fuselage was lengthened by 7.5 in (19 cm) between the exhaust line and forward edge of the windscreen to make room for the installation of Japanese 20 mm Ho-5 cannon in the fuselage decking as the continued supply of MG 151/20 cannon via submarine could not be guaranteed and the Ho-5 was ready for service though inferior to the Mg 151/20 in several respects. A number of internal changes were also made, these including the simplification of several systems for increased reliability and greater ease of maintenance.
The rear section of the fuselage was also made more easily removable to further facilitate repair work.
What is not doubted, however, is a Ki-61-I-KAI-d interceptor subvariant with two 0.5 in ( 12.7 mm) Ho-103 fuselage-mounted machine guns and two 30 mm Ho-105 wing-mounted cannon.
It is worth noting that one Ki-61-I was adapted for tests of an evaporative engine cooling system using wing-mounted radiators in a manner modelled on that of the Heinkel He 100. This was the fastest Ki-61, achieving 340 kt (391.5 mph; 630 km/h), and was the last Ki-61 with a retractable tailwheel. No serious consideration was given to production as the evaporative cooling system’s large wing radiators would have been extremely vulnerable to gun fire damage.
Production of the Ki-61-I Kai series extended between January 1944 and January 1945, and totalled 1,274 aircraft.
Better altitude performance
Although the Ki-61 was a very good fighter at low and medium altitudes, it performance declined rapidly with altitude. In an effort to rectify this failing, Doi evolved the Ki-61-II with a wing of slightly different section and also enlarged by some 10% to an area of 236.81 sq ft (22.00 m²), and the powerplant revised to the Ha-140 engine, which was a Japanese development of the DB 601A with ratings of 1,500 hp (1118 kW) for take-off and 1,250 hp (932 kW) at 18,700 ft (5700 m). The cowling panels were redesigned and the supercharger air inlet was lengthened. A redesigned windscreen, incorporating an extra panel, was mounted farther forward, and the transparency to the rear of the sliding canopy was redesigned to increase rearward fields of vision.
The first of 11 Ki-61-II prototypes recorded its maiden flight in December 1943. The flight trials were disappointing, for the engine suffered from crankshaft failures, the enlarged wing was structurally suspect, and the handling characteristics were poor. As a result only eight of the prototypes were flown, and the ninth was converted to the Ki-61-II Kai standard with the Ki-61-I’s wing panels, a larger rudder, and the fuselage lengthened to 30 ft 0.625 in (9.16 m). These changes removed the Ki-61-II’s structural and handling problems; 30 pre-production aircraft were built to this standard, and the Imperial Japanese army air force ordered the Ki-61-II Kai into production for service as the Army Type 3 Fighter Model 2. In other respects, the Ki-61-II Kai differed from the Ki-61-Ib in details such as its empty weight of 6,261 lb (2840 kg), normal take-off weight of 8,333 lb (3780 kg), maximum take-off weight of 8,433 lb (3825 kg), maximum speed of 329 kt (379 mph; 610 km/h) at 19,685 ft (6000 m), climb to 16,405 ft (5000 m) in 6 minutes 0 seconds, service ceiling of 36,090 ft (11000 m), maximum range of 863 nm (994 miles; 1600 km) with drop tanks, and typical range of 594 nm (684 miles; 1100 km) with internal fuel.
The Ki-61-II Kai entered production in September 1944, and was built in two forms as the Ki-61-II Kai-a (Army Type 3 Fighter Model 2A) with the fixed forward-firing armament of two 20 mm cannon and two 0.5 in (12.7 mm) machine guns, and the Ki-61-II Kai-b (Army Type 3 Fighter Model 2B) with the fixed forward-firing armament of four 20 mm cannon. Production totalled 274 airframes up to August 1945, but only 99 of these were completed to Ki-61-II Kai standards because of engine unreliability and then, in January 1945, the total destruction of Kawasaki’s Akashi aero engine factory by a bombing raid. The Ki-61-II was one of only very few well-armed Japanese aircraft able to reach the operational altitude of the Boeing B-29 Superfortress strategic heavy bombers raiding Japan, and most of B-29 bombers lost to Japanese fighters were shot down by Ki-61-II aircraft.
The Ki-61-III was a single prototype with a cut-down rear fuselage and a canopy design which was later used on the Ki-100-II. Total production of the Ki-61 series was 3,159 aircraft.
After World War II numbers of captured Ki-61 fighters were used by each side in the Chinese civil war, and in 1945 the Indonesian People’s Security Force seized a small number of aircraft at numerous Japanese air bases. Most of these aircraft were destroyed in the fighting between the Netherlands, the returning colonial power, and the newly-proclaimed Republic of Indonesia during the Indonesian national revolution of 1945/49.
Kawasaki Ki-61-Ib Hien
Manufacturer: Kawasaki Kokuki K.K.
Accommodation: pilot in the enclosed cockpit
Fixed armament: two 0.5 in (12.7 m) Type 1 fixed forward-firing machine guns with 250 rounds per gun in the upper part of the forward fuselage with synchronisation equipment to fire through the propeller disc, and two 0.5 in (12.7 mm) Type 1 fixed forward-firing machine guns with 200 rounds per gun in the leading edges of the wing
Disposable armament: none
Equipment: standard communication and navigation equipment, plus a reflector gun sight
Powerplant: one Kawasaki Ha-40 (Army Type 2) liquid-cooled inverted V-12 piston engine rated at 1,175 hp (876 kW) for take-off and 1,080 hp (805 kW) at 11,485 ft (3500 m)
Internal fuel: 145.3 US gal (121 Imp gal; 550 litres)
External fuel: up to 105.7 US gal (88 Imp gal; 400 litres) in two 52.8 US gal (44 Imp gal; 200 litre) drop tanks
Dimensions: span 39 ft 4.25 in (12.00 m); area 215.28 sq ft (20.00 m²);length 28 ft 8.5 in (8.75 m); height 12 ft 1.75 in (3.70 m); wheel track 13 ft 1.5 in (4.00 m)
Weights: empty 4,872 lb (2210 kg); normal take-off 6,504 lb (2950 kg); maximum take-off 7,165 lb (3250 kg)
Performance: maximum level speed ‘clean’ 319.5 kt (368 mph; 592 km/h) at 15,945 ft (4860 m); cruising speed 216 kt (249 mph; 400 km/h) at 13,125 ft (4000 m); climb to 16,405 ft (5000 m) in 5 minutes 31 seconds; service ceiling 37,730 ft (11600 m); maximum range 594 nm (684 miles; 1100 km) with drop tanks; typical range 324 nm (373 miles; 600 km) with standard fuel