Kawasaki Ki-61 Hien (Swallow) 'Tony'

Introduction
Development
Combat Record
New Guinea
The Philippines
Formosa and Okinawa
Defence of Japan
Variants
Stats (Ki-61-I KAIc)

Introduction

The Kawasaki Ki-61 Hien (Swallow) was the only inline-engined fighter aircraft to see service in Japan during the Second World War, and was designed around a licence-built version of the German Daimler-Benz DB 601A engine. 

Development

The Kawasaki design team was lead by the German engineer Dr Richard Vogt for ten years, before he returned home in 1933 (where he became chief designer for Blohm und Voss). During this period Kawasaki negotiated the right to build a number of German liquid-cooled engines, and used them to power some of their most successful aircraft of the period. This tradition continued after Vogt's departure, and in the late 1930s Kawasaki purchased a licence to build the Daimler Benz DB 600 inline engine. This was soon followed by a licence for the DB 601, the engine that powered the Messerschmitt Bf 109E. In April 1940 a team from Kawasaki returned from Germany with a set of design drawings and several engines. Over the next year Kawasaki worked on producing a Japanese version of the engine, and the first Kawasaki Ha-40 was completed in July 1941. The new engine was slightly lighter than the DB 601A, and provided more power at take-off, but would also suffer from overheating problems, making it less reliable than the German original. The engine was quickly accepted by the Japanese army and entered production as the Army Type 2 engine.

During the 1930s all modern Japanese fighter aircraft were powered by radial engines, and were built with manoeuvrability in mind. By 1939 the Japanese Army was using the Nakajima Ki-27, a radial-engine powered monoplane, and the more advanced Nakajima Ki-43 Hayabusa was under development. However, in the west the most advanced fighters were powered by liquid cooled in-line V-12 engines, and in particular the DB 601 of the Bf 109 and the Rolls Royce Merlin of the Hurricane and Spitfire. Kawasaki believed that they could produce a superior fighter using their own version of the DB 601, and approached the Japanese Army with a number of different designs. In February 1940 (before the DB 601 plans and engines had reached Japan) Kawasaki were ordered to begin work on two of their designs, the Ki-60 heavy fighter and the Ki-61 all-purpose fighter, with the Ki-60 getting the higher priority.

The first prototype Ki-60 made its maiden flight in March 1941. The aircraft was designed to have a high top speed, a good rate of climb and to be more heavily armed than contemporary Japanese fighters. It had a comparatively small wing, and a narrow but quite tall fuselage. The new aircraft was unpopular with Japanese pilots - its small wings meant it had a high wing loading (by Japanese standards), which lowered its manoeuvrability and increased its landing speed. Most disappointing was its top speed, which even on the third, and significantly modified prototype, only reached 354mph, 20mph slower than expected. Work on the Ki-60 was abandoned and attention switched to the lighter Ki-61.

The Kawasaki design team, lead by Takeo Doi and Shin Owada, had already done some preliminary work on the Ki-61. It was similar to the Ki-61 in overall shape and length, but the fuselage was reduced in cross-section (and in height). A new longer wing was designed, with a higher aspect ratio (longer and thinner). The wingspan of the Ki-61 was five feet wider than that of the Ki-60. The new wing reduced the wing-loading, making the Ki-61 more manoeuvrable than the Ki-60 (and than the Allied fighters it would encounter when it first entered combat), but less manoeuvrable than other Japanese army and navy fighters.

The first prototype of the Ki-61 made its maiden flight early in December 1941. As expected, the Ki-61 was faster than the Ki-60, but the relative lack of manoeuvrability worried the Army's test pilots. Veteran pilots returning from the front were much more enthusiastic. The lighter, more manoeuvrable, fighters had won Japan control of the skies in late 1941 and early 1942, defeating both the out-dated Allied fighters in the Pacific in 1941 and the first of the more modern fighters that began to arrive in the area during 1942. This began to change as the Allies worked out how to fighter the nimble Japanese aircraft.

The newer Allied aircraft tended to be faster, more heavily armed, better armoured, have a better rate of climb and be faster in the dive than their more nimble Japanese opponents. The favoured Allied tactic was now to make 'hit and run' attacks on Japanese aircraft, diving on them from above, then climbing away without getting involved in a dog-fight. The Ki-61 promised to negate this tactic, allowing the Japanese pilots to dive away from the Allied fighters (or to catch Allied pilots attempted to dive away from trouble). The self-sealing fuel tanks and pilot armour were also popular, as was the increase in firepower.

The fate of the Ki-61 was decided by a series of competitive trials, flown against an imported Bf 109E-3, a captured Curtiss P-40E, and the Nakajima Ki-43-II and Ki-44-I. The Ki-61 was the fastest of these aircraft, and was only out-manoeuvred by the Ki-43-II. The Ki-61 was ordered into production, and given the designation Army Type 3 Fighter Model 1 Hien (Swallow).

When the Ki-61 was first encountered by Allied pilots it caused a certain amount of confusion. Every other Japanese-designed and built fighter used a radial engine. The new aircraft shared the same general shape as many DB-601 powered fighters, which gave it a slightly 'droopy' nose, with the propeller mounted in a distinctive low position, and a sloped upper fuselage running back to the cockpit. The Bf 109 had a similar, but more angular nose, and at first some believed that the new Japanese fighter was a licence-built version of the German fighter (this had been expected for some time, and a code-name, 'Mike', allocated to the Bf 109). After a few more sightings of the new fighter it became clear that it wasn't a version of the Bf 109, but it did resemble some of the Japanese fighters powered by the same engine, and in particular the Macchi C.202, which had a similar (although longer) smooth nose. In order to reflect this visual resemblance, the new fighter was allocated the code name 'Tony', short for Antonio. 

Production of the Ki-61-I ended in January 1945, after somewhere between 2,654 and 2,734 aircraft had been built. The KAIc accounted for the vast majority of the 1,274 of the KAI models to be built. Although 374 Ki-61-II airframes were built, 275 of these didn't receive their inline engines and were instead completed as Ki-100s, while of the remaining 99 about two thirds reached front line units while the rest were destroyed by American bombing.

Combat Record

The Ki-61 entered service with the 23rd Dokuritsu Dai Shijugo Chutai (Independent Squadron) in Japan in February 1943. This was the equivalent of a British Operational Training Unit, and was used to give pilots experience with the new fighter. The first operational unit to receive the new fighter was the 68th Sendai, in March 1943, following in the next month by the 78th Sendai.

The overall Japanese plan for the war had been to conquer a large empire in order to push the fighting as far away from the Home Islands as possible. The hope was that the Allies could either be held at the outer edge of the Empire, or be forced to pay such a high cost to conquer this line of strongholds that they would come to terms. This meant that both the Japanese army and navy were forced to deploy most of their forces at a long distance from Japan, and would ultimately allow the Allies to chose where they attacked, bypassing many strong Japanese positions. It also meant that the Ki-61 would make its combat debut in an unexpected location - New Guinea.

New Guinea

The first Ki-61 units were allocated to the defence of the Southern Front. Here the Japanese held most of New Guinea, although their attempts to gain a foothold on the south coast of that vast island, facing Australia, ended in failure. It was clear that the Allies would soon go onto the offensive on New Guinea, and so in April 1943 the 68th and 78th Sendais began the move south to the major Japanese base at Rabaul.

Both units suffered high losses during the move to Rabaul. The Japanese Army Air Force was used to operating over China or other large land areas, and had little experience of flying long missions over water. The 78th Sendai was taken to Truk on aircraft carriers, before flying on to Rabaul. On the first delivery flight 18 out of 30 aircraft were lost when they got off course, while on another flight a problem with the wing-mounted ferry tanks meant that a large number of aircraft ran out of fuel. The 68th Sendai, which began to move in May, made the entire trip in the air, flying from the Ryukyu Islands to Formosa, the Philippines and along the northern coast of New Guinea, also losing large numbers of aircraft on the way. By the end of May the two units had reached Rabaul, but they were already under-strength (with around 30 aircraft each, instead of the 54 they were meant to have) and had lost some experienced pilots before even reaching combat.

The two units were part of the 4th Air Army, and moved to New Guinea in July-August 1943. They soon began involved in some one of the more prolonged and intense air battles of the Pacific War, with the two sides based comparatively close to each other, on the opposite sides of the mountain ranges in the centre of New Guinea. Large numbers of Japanese aircraft were destroyed on the ground by repeated Allied air strikes, but when the Ki-61 did get into the air it caused the Allies some problems. The Ki-61 was better armoured and more robust than the Nakajima Ki-43 or Japanese Naval fighters, could out-dive the P-38s, P-39s and P-40s in use with the American air force in the area, and could out-manoeuvre any Allied fighter until the P-51 Mustang and F6F Hellcat entered service.

Despite these limited successes the Japanese Army Air Force suffered very heavily on New Guinea. The jungle conditions caused as many problems for the Japanese as for the Allies, and vast numbers of Ki-61s were lost on the ground and in the air. By the time the Japanese withdrew in the spring of 1944 the Japanese had lost many of their most experienced pilots, along with large numbers of their best ground crew. Only 5% of the Army pilots with more than 300 hours flying time survived the fighting on New Guinea.

The Philippines

After the disasters in New Guinea the Ki-61 next saw combat in large numbers during the defence of the Philippines. The 17th and 19th Sentais were based around Manila from September-October 1944, and were joined by the 33rd Sendai from Malaya and the 18th and 55th Sendais after the American invasion began in October. Once again the Ki-61 units were overwhelmed, with the 55th Sendai losing all of its pilots by the end of November!

Formosa and Okinawa

The 19th, 37th and 105th Sentais were posted to Formosa and Okinawa, while the 59th Sentai took part in the defence of Okinawa. Once again the Japanese units suffered heavy losses.

Defence of Japan

The Ki-61 was used in large numbers in an attempt to defend the Japanese home islands. Japan was split into Easter, Middle and Western Defence Sectors. The 18th, 23rd, 28th, 53rd and 244th Sendais were posted to the Eastern sector, which included Tokyo. The 17th, 55th and 56th Sendais were posted to the Middle Sector, and the 59th and later 56th Sendais defended the Western Sector.  

The Ki-61-I could only just reach the operating altitude of the B-29s, and even then only by having its weight reduced as much as possible. Most units thus struggled to inflict any damage on the Americans, although the 244th Sendai was an exception, proving itself to be a very dangerous opponent. The Ki-61 also used in an increasingly large number of suicidal ramming attacks against B-29s.

The Ki-61-II was a more capable aircraft, and could operate at the B-29's cruising altitude, but it never appeared in large numbers. It also entered service at about the same time as Allied fighter aircraft began to appear over Japan for the first time, with both US naval fighters and P-51 Mustangs from Iwo Jima entering the fray from February 1945.

Although the Japanese Army Air Force was suffering heavy losses over Japan, so were the B-29s. In March 1945 the high altitude day bombing campaign was replaced with low-level night raids, abandoning the idea of precision attacks. US losses dropped dramatically, and the Ki-61s were left to face Hellcats, Corsairs and P-51s. Although the Ki-61 could sometimes perform well against these aircraft, its pilots were almost always massively outnumbers and losses were very heavy. The radial-powered Ki-100, produced by fitting a radial engine to the airframe of the Ki-61-II performed a little better, but was never present in sufficient numbers to make any difference.

Variants

Ki-61

Twelve prototypes were produced with the designation Ki-61.

Ki-61-Ia

The Ki-61-Ia was the first production version of the Hien, and carried the same guns as the prototype, with two 7.7mm machine guns in the wings and two 12.7mm guns in the nose. Later aircraft were modified to carry imported 20mm Mauser MG 151 cannon in the wings, mounted on their sides.

Ki-61-Ib

The Ki-61-Ib was the second production version of the aircraft, and was armed with four 12.7mm machine guns, two in the wings and two in the upper fuselage. As with the Ia, some were later modified to carry 20mm Mauser cannon in the wings.

Ki-61-I KAIc

The Ki-61-I KAIc was the most common version of the fighter. It featured a modified fuselage (thus the KAI for Kaizo, or modified), and was armed with two 20mm Ho-5 cannon. The new fuselage was 20cm longer, and had a detachable rear section, with a fixed tail wheel replacing the earlier retractable version. The wings were stronger, allowing pylons to be added outside the undercarriage. These allowed the aircraft to carry either drop tanks or two 550lb bombs. The KAIc was armed with two 12.7mm machine guns in the wings while the two cannon were placed in upper fuselage. Production of the KAIc averaged over 100 aircraft per month from November 1943, peaking at 354 aircraft in July 1944. It was produced alongside the Ki-61-Ib until August 1944, and production ended in January 1945.

Ki-61-I KAId

The Ki-61-I KAId was a specialised anti-bomber version of the KAIc, produced in small numbers late in 1944. It reversed the armament of the KAIc, with 12.7mm machine guns in the fuselage and 30mm Ho-105 cannon in the wings.

Ki-61-II

The Ki-61-II was designed around the Kawasaki Ha-140, a more powerful but less reliable development of the Ha-40. The first prototype -II was completed in August 1943, and had a modified cockpit and a larger wing. Eight prototypes of the -II were produced, but the new wing was a failure.

Ki-61-II KAI

The Ki-61-II KAI was produced in response to the failure of the -II. Takeo Doi reverted to the standard Ki-61 wing, made the fuselage 22cm longer and increased the size of the rudder. The first Ki-61-II KAI made its maiden flight in April 1944 and was a great success. It could reach 16,450ft in six minutes, had a top speed of 379mph and could operate at the 30,000ft cruising altitude of the B-29 bomber. It was ordered into production as the Army Type 3 Fighter Model 2.

Ki-61-II KAIa

The Ki-61-II KAIa was the first production version of the -II, and carried the same guns as the Ki-61-I KAIc, with two 20mm cannon in the fuselage and two 12.7mm machine guns in the wings.

Ki-61-II KAIb

The Ki-61-II KAIb was similar to the KAIa, but was armed with four 20mm cannon, two in the fuselage and two in the wings. A total of 374 airframes of the two models were completed, but only 99 of them received their Ha-140 engines. On 19 January 1945 B-29 bombers from the US 20th Air Force destroyed the Akashi engine factory, ending production of the Ha-140 and leaving 275 fuselages without engines. Kawasaki responded by installed the Mitsubishi Ha-112-II double-row air-cooled radial engine into the airframes to produce the Kawasaki Ki-100 Army Type 5 Fighter.

Ki-61-III

The Ki-61-III was a single prototype of an improved version of the aircraft with a cut-down rear fuselage and an bubble canopy designed to improve the pilot's view (the same change was made to many Allied fighters).

Stats (Ki-61-I KAIc)
Engine: Kawasaki Ha-40 V-12 piston engine
Power: 1,175hp
Crew: 1
Wing span: 39ft 4.5in
Length: 29ft 4.25in
Height: 12ft 1.75in
Empty Weight: 5,789lb
Maximum take-off Weight: 7,650lb
Max Speed: 366mph at 13,980ft
Cruising Speed:
Service Ceiling: 32,810ft
Range: 1,181 miles
Armament: two 12.7mm machine guns and two 20mm Ho-5 cannon in wings
Bomb-load: Two drop tanks or two 551lb bombs under the wings

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How to cite this article: Rickard, J (2 June 2011), Kawasaki Ki-61 Hien (Swallow) 'Tony', http://www.historyofwar.org/articles/weapons_kawasaki_ki-61.html

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