The Kawasaki Ki-61 Hien
("Swallow") fighter represented a major departure for Japanese aircraft
designers of World War II. While other Japanese fighters were designed
with air-cooled radials and were optimized for manoeuvrability, 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
Me-109, then suspected it was a copy of the Italian Macchi C.202
Foglore. For this reason they gave it the code-name "Antonio", or
"Tony", though by the summer of 1943 the Allies realized 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.
Kawasaki Ki-61-KAIc of the 1st Chutai, 244th Sentai, Chofu, Tokyo,
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 during the war. Not surprisingly, Kawasaki continued to be
strongly influenced by Dr. Vogt's beliefs after he left, particularly a
faith in the usefulness of liquid-cooled inline engines. This made
Kawasaki something of a heretic among Japanese aircraft manufacturers,
who preferred air-cooled radials.
In March 1938, Kawasaki
signed an agreement with Daimler-Benz of Germany for obtain
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 Me-109. The Kawasaki engine
team managed to increase the take-off power of their version of the
engine to 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
In the meantime,
certain officers at the Air Headquarters of the Imperial Japanese Army
were very interested in the liquid-cooled inline powered fighters being
developed in Britain, the USA, the USSR, Germany, and France. The
Japanese Army also had unpleasant experience in air combat against the
Soviet Polikarpov I-16 during the beating the Imperial Army took in
their Manchurian border clash with the USSR in 1939. This experience
suggested the the single-minded focus on agility above all that
characterized Japanese fighter design doctrine might need to yield to a
focus on speed and improved armour protection and firepower.
In February 1940, the
Army initiated work with Kawasaki on two single-seat fighters based on
the DB 601 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.
The Ha-40 engine was
not available at that time, so the three aircraft were powered by some
of the sample DB 601 engines obtained from Germany. The Ki-60 was a
low-wing monoplane, with plenty of power and heavy armament by Japanese
standards. The new fighter had two 12.7 millimeter guns mounted on the
nose in front of the pilot and two 20 millimeter Mauser MG-151 cannons,
one mounted in each wing.
Flight tests began in
March 1941 and showed that the Ki-60 had unpleasant handling
characteristics. The aircraft didn't meet its performance specs,
various tweaks to improve the aircraft failed, and the Ki-60 was
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 using the lessons learned by the Ki-60 program.
Aerodynamic refinements were added, the wing was increased in size and
length to improve manoeuvrability and the fuselage was slimmed down to
Armament was reduced by
replacing the two 20 mm cannon in the wings with either two 12.7 mm
(0.50 in) or two 7.7 mm (0.303 in) machine guns. Fuel capacity was also
increased, as required by offensive fighter operations, which dictated
a longer range than that required by an interceptor.
The landing gear track
was widened to allow use from primitive forward airfields. The first
prototype was rolled out in early December 1941, and its performance
delighted its designers. 11 more prototypes were delivered to the Army,
which performed intensive trials with them. 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 Army from the Germans,
and the Curtiss P-40E, several of which had been captured during the
seizure of the Dutch East Indies.
While the test pilots
were a little sceptical of the new aircraft at first, pilots with
combat experience appreciated the Ki-61's self-sealing fuel tanks,
heavier armour and armament, and fast diving speed. The air combat
tests showed the Ki-61 to be faster than all its adversaries in the
tests, and easily out-manoeuvred all of them except the Japanese Ki-43
The 13th Ki-61, a
production prototype, was delivered in August 1942. The Army gave the
production go-ahead, 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.
consisted of two variants: the Ki-61-Ia, with 12.7 millimetre guns in
the fuselage and 7.7 millimetre guns in the wings, and the Ki-61-Ib,
with 12.7 millimetre guns in both fuselage and wings. These aircraft
could be fitted with two 200 litre (53 US gallon) drop tanks.
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 operations 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 millimetre cannon
wasn't available at the time, but the Japanese obtained 800 Mauser
MG-151 20 millimetre cannon from Germany in August 1943, and modified
388 Ki-61-I airframes to carry the German weapons in place of the two
12.7 millimetre wing guns.
Once the Ho-5 cannon
became available, Kawasaki designers then reversed the arrangement of
the guns, putting the 20 millimetre guns in the forward fuselage and
the 12.7 millimetre guns in the wings. While they were making these
modifications, they also made a few changes to streamline manufacturing
and simplify field maintenance. This new variant was designated the
Ki-61-I KAIc (where "KAI" was for "kaizo", or "modified) was 19
centimetres (7.5 inches) longer than its predecessors, with a
detachable rear section, a fixed tailwheel instead of the retractable
tailwheel previously used, stronger wings, and stores pylons outboard
of the main landing gear, allowing it to carry two 250 kilogram (550
Kawasaki Ki-61-KAIc of the HQ Chutai, 244th Sentai, Chofu, Tokyo,
The Ki-61-I KAIc went
into production in January 1944, and ultimately replaced production of
all earlier models in August 1944. The Ki-61-I KAIc would become the
heavily-produced version of the Hien, accounting for over half the
total number built. A few Ki-61-I KAId bomber interceptors were also
built in late 1944. These variants incorporated two 12.7 millimetre
guns in the fuselage and a 30 millimetre gun in each wing.
Even before the Hien
saw combat, the Army had been pressing Kawasaki for an improved version
of the same aircraft. To this end, Kawasaki engineers focused on an
improved version of the Ha-40 engine known as the Ha-140, which was
expected to have a take-off power of 1,500 HP.
The first prototype of
the new variant, the Ki-61-II, flew in August 1943. Ten more prototypes
were built by the end of the year, featuring a wing with 10% more area
and an improved cockpit. However, the Ha-140 development program ran
into troubles, and only 8 of the prototypes received engines. Even
then, they suffered from engine troubles, structural failures, and
handling problems. In an attempt to deal with the problems, 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. The first prototype was modified from the 9th Ki-61-II
prototype and flew in April 1944, 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 operating characteristics.
Despite the problems
with the engine, the military situation was increasingly desperate, and
so the Ki-61-II KAI was put into production anyway in September 1944.
Two versions were produced, one designated Ki-61-II KAIa, with 12.7
millimetre guns in the wings and 20 millimetre guns in the fuselage,
and the other designated Ki-61-II KAIb, with four 20 millimetre guns.
374 Ki-61-II KAI
airframes were built and 99 of them fitted with engines. Then, on 19
January 1945, US Army Air Force B-29 Superfortresses turned the plant
at Akashi that was building 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. The
airframes could not be left to go to waste, so Kawasaki engineers
performed a lightning design effort to mate them to the 1,500 HP
Mitsubishi Ha-112-II 14-cylinder double-row air-cooled radial engine.
Type 3 Fighter Model 1 Hien "Swallow" - Kawasaki Ki-61-Ic)
Seat Fighter Interceptor
1,175 hp (876 kw) Kawasaki Ha-40 12-cylinder inverted Vee engine.
Maximum speed 348 mph (560 km/h) at 16,405 ft (5000 m); service ceiling
32,810 ft (10000 m).
Range: 1,181 miles (1900 km) with internal fuel stores.
5,798 lbs (2630 kg) with a maximum take-off weight of 7,650 lbs (3470
39 ft 4 1/4 in (12.00 m); length 29 ft 4 1/4 in (8.95 m); height 12 ft
1 3/4 in (3.70 m); wing area 215.59 sq ft
(20.00 sq m).
fuselage mounted 12.7 mm (0.50 in) machine guns and two wing mounted 20
mm Ho-5 cannon of Japanese design and manufacture. All versions had
provisions for underwing drop tanks, but Ki-61-Ic and all Ki-64-II's
could carry two 551 lbs (250 kg) bombs in place of the drop tanks.
(12 initial prototypes), Ki-61-I (Army Type 3 Fighter Model 1),
Ki-61-Ia (wing mounted cannon instead of machine guns), Ki-61-Ib (four
12.7 mm (0.50 in) machine guns), Ki-61-Ic, Ki-61-Id (two 30 mm cannon
instead of the two 20 mm cannon), Ki-61-II (eight prototypes), Ki-61-II
KAI, Ki-61-IIa, Ki-61-IIb (four 20 mm Ho-5 cannon), Ki-61-III (proposed
prototype but the destruction of the Akashi engine factory ended any