Kawasaki Ki-61

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Ki-61 Hien
Type	Fighter
Manufacturer	Kawasaki Aircraft (Kawasaki Heavy Industries)
Designed by	Takeo Doi and Shin Owada
Maiden flight	December 1941
Introduced	1943
Retired	1945
Primary users	IJA Air Force Chinese Nationalist Air Force
Number built	3,159
Variants	Kawasaki Ki-100

The Kawasaki Ki-61 Hien (飛燕, roughly "flying swallow") was a Japanese World War II fighter aircraft used by the Imperial Japanese Army Air Force. The Allied code name assigned by the United States War Department was "Tony". The Japanese Army designation was "Army Type 3 Fighter" (三式戦闘機).^[1] It was the only mass-produced Japanese fighter of the war to use a liquid-cooled inline "V" engine.

[edit] Design and development

A factory fresh Ki-61-I-Ko being ferried to either the 68th or 78th Sentai in New Guinea; it has yet to receive the distinctive dark green "palm leaf" pattern which was usually applied by front-line units in the Southern theatre, nor does it have the usual black anti-glare panel forward of the cockpit.

In all of its versions, the Ki-61 Hien was a single-seat, single-engine fighter made almost entirely from metal alloys; only the control surfaces were fabric-covered. It was ruggedly constructed, and was relatively heavy for a Japanese fighter. The aft part of the triangular cross-section fuselage was long for a fighter powered by the DB-601 engine (or one of its copies), and featured a type of "razorback" dorsal structure behind the cockpit. The engine was in the nose, together with a pair of 12.7 mm caliber Ho-103 machine guns. The Ho-103 was a light weapon for its caliber (around 23 kg) and fired a light shell, but this was compensated for by its rapid rate of fire. The ammunition capacity was limited, having only around 250 rounds for each weapon. An unusual feature of the Ki-61 was that the engine bearers were constructed as an integral part of the forward fuselage, with the cowling side panels being fixed. For servicing or replacement the top and bottom cowling panels only could be removed. The windshield was armoured together with a 13 mm armoured steel plate in the pilot's seat. Fuel capacity was around 550 l (145 gal) allowing for a long range (over 1,000 km (540 nm) without external tanks). The wings were quite large in area, giving a low wing loading. They housed another pair of machine guns or cannons, as well as the undercarriage gear when raised.

Takeo Doi and Shin Owada designed the Ki-61 Hien in parallel with the Kawasaki Ki-60 interceptor. Created around the Ha-40 engine (a development of the Kawasaki V12 engine, itself a license-built version of the German Daimler-Benz DB 601A), the Ki-61 improved on the Ki-60 design by incorporating a revised wing and various streamlining and weight-saving measures. The aircraft first flew in December 1941. Although test pilots were enthusiastic about its self-sealing fuel tanks, upgraded armament, and good dive performance, the wing loading of 146.3 kg/m² (30 lb/ft²) was considered excessive by Japanese standards. (The Ki-43-Ia's 92.6 kg/m² (19 lb/ft²) was considered borderline.^[2])

To address these concerns, Kawasaki staged a fly-off between two Ki-61 prototypes and the Ki-43-II, pre-production Ki-44-I, LaGG-3 (flown to Manchuria by a defector), Bf 109E-3, and a captured P-40E Warhawk. The Ki-61 proved the fastest of all the aircraft and was inferior only to the Ki-43 in maneuverability.^[3][4][5][6]

The Ki-61 was the last of the DB-601-powered fighters and it was soon overshadowed by fighters with more powerful engines. By the time it first flew in December 1941 – only one year after the Macchi's first flight and three years after the Bf 109E – the DB-601 was already underpowered compared to the new 1,500 hp inline or 2,000 hp radial engines being developed (and already nearing the mass-production stage) to power the next generation of combat aircraft: the P-47, Fw 190 and Bf 109 G. Moreover, the inline Ha-40 engine proved to be an unreliable powerplant.^{[7] [8]}

The DB-601 engine on which the Hien was based was designed with very critical tolerance limits, and in the Ha-40 Japanese technicians developed a lighter version (by roughly 30 kg) that required even higher tolerances. Reaching these levels proved to be a "stretch" for Japan's aviation manufacturing capabilities, which was further complicated by the variable quality of the materials, fuel, and the lubricants needed to run this sensitive, high-performance engine smoothly. The Japanese equivalent of the more powerful DB-605 engine was the Ha-140, which was fitted onto the Type 3 to produce the Ki-61-II high-altitude interceptor.^[7]

Compared to the Ki-61-I, the Ki-61-II had 10% greater wing area, more armor, and – with the Kawasaki Ha-140 engine – an increased power of 1,120 kW (1,500 hp). After overcoming initial fuselage and wing stability problems, the new interceptor reverted to the original wing and was put into service as the Ki-61-II-KAI. However, the Ha-140 engine had reliability problems of its own which were never fully resolved, and around half of the first batch of engines delivered were returned to the factory to be re-built. Shortly after, a US bombing raid on 19 January 1945 destroyed the engine factory in Akashi, Hyōgo, and 275 Ki-61-II-KAI airframes without engines were converted to use the Mitsubishi Ha-112-II radial engine, resulting in the Ki-100. While the Ha-112 solved the problems encountered with the Ha-140, the new engine still had a weakness: the lack of power at altitude, which diminished its ability to intercept high-flying B-29s relative to the Ki-61-II.^[7]

[edit] Operational history

Unusual wartime photo of a captured Ki-61 being tested by the USAAF

The new Ki-61 Hien fighters entered service with a special training unit, the 23rd Chutai, and entered combat for first time in spring 1943 during the New Guinea campaign. Initially, due to its unusual appearance for a Japanese fighter, the Allies believed it to be of German or Italian origin, possibly a license-built Bf 109; the Italian-like appearance led to its code name of "Tony".^[9][10]

The first Sentai (wing) fully equipped with the Hien was the 68th in Wewak, New Guinea, followed by the 78th Sentai stationed at Rabaul. Both units were sent into a difficult theatre where jungles and adverse weather conditions, coupled with a lack of spares, quickly undermined the efficiency of both men and machines; this was especially the case for new-design aircraft, which are particularly prone to teething problems, as the Ki-61s were. Initially, this campaign went successfully for the Japanese Army Air Force (JAAF), but when the Allies re-organized and enhanced the combat capabilities of their air forces, they gained the upper hand against the JAAF.^[8]

High aircraft losses were experienced in some incidents during this campaign. For example, while in transit between Truk and Rabaul, the 78th lost 18 of its 30 Ki-61s. Other units were involved and sometimes, even more unfortunate: only two of (possibly) 24 Nakajima Ki-49s reached Rabaul in June 1943. Almost all of the modern Japanese aircraft engines, especially the Ki-61's liquid-cooled engines, suffered a disastrous series of failures and ongoing problems,^[8] which resulted in the obsolescent Ki-43 forming the bulk of the JAAF's fighter capability. At the end of the campaign, nearly 2,000 Japanese aircraft had been lost in continuous air attacks from up to 200 Allied aircraft at one time (among them, around half were B-24s and B-25s armed with fragmentation bombs^[8] After the Japanese retreat, over 340 aircraft wrecks were later found at Hollandia.^[8]

Even with these problems, there was a general Allied concern regarding this new fighter:

The Hien entered combat in the spring of 1943 in the New Guinea war zone, covering [mainland] 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. ... General George Kenney [Allied air forces commander in the Southwest Pacific] found his P-40 Warhawks completely outclassed, and begged for more P-38 Lightnings to counter the threat of the new enemy fighter.^[11]

The Ki-61 was also utilised in Southeast Asia, Okinawa, China and as an interceptor during US bombing raids over Japanese home islands, including against B-29 Superfortresses. The Ki-61 was notable for many reasons: initially identified as of either German or Italian origin, these aircraft were capable of matching Allied aircraft such as the P-40 in speed, and as evaluation had already showed, were superior in almost every respect. However, the armament of the early Hien was lighter, but still sufficient for most purposes. Of the allied fighters encountered at the beginning of World War II, only the P-38 was measurably superior.^[12] The Ki-61 carried a great deal of fuel, but due to having self-sealing fuel tanks, it did not have the reputation for being "easily flammable" as were many other Japanese aircraft.^[7]

Due to the additional weight, the Ki-61's performance and agility suffered when its armament was increased, but it still remained capable with a 580 km/h (313 kt) maximum speed. The cannon armament was essentially needed to counter the Allied bombers, which proved to be difficult to shoot down with only 12.7 mm machine guns. The empty and maximum weights for the Ki-61 prototype (2 x 12.7 mm + 2 x 7.7 mm) were 2,238 kg (4,934 lb) and 2,950 kg (6,504 lb), respectively; for the Ki-61-I basic (4 x 12.7 mm) 3,130 kg (6,900 lb); and for the Ki-61-KAI (2 x 12.7 mm + 2 x 20 mm), 2,630 kg (5,798 lb) and 3,470 kg (6,750 lb).^[7]

A number of Ki-61s were also used in Tokkotai (kamikaze) missions launched toward the end of the war. The Ki-61 was delivered to 15th Sentai (groups/wings), as well as some individual Chutaicho (junior operational commanders) in other Sentai, and even to operational training units in the JAAF. The aircraft was largely trouble-free in service except for the liquid-cooled engine which tended to overheat when idling on the ground and suffered from oil circulation and bearing problems.^[13]

[edit] Ki-61 Special Attack Unit

An ex-244 Sentai, 2nd Chutai Ki-61 found and photographed at Chōfu airbase by USAAF personnel in 1946. Photographer: more than likely James P Gallagher of the USAAF.^[14]

The tactic of using aircraft to ram American B-29s was nothing new in 1944. The first recorded ramming by a Ki-61 occurred in late August, during a raid when B-29s from Chinese airfields were sent to bomb the steel factories at Yahata, enlisted pilot Shigeo Nobe of the 4th Sentai intentionally sliced his aircraft into a B-29. Other attacks of this nature followed, as a result individual pilots determined it was a quite practical way to destroy B-29s.^[15]

On 7 November 1944, the officer commanding the 10th Hiko Shidan made ramming a matter of policy by forming ramming attack flights specifically to oppose the B-29s at high altitude. The aircraft were stripped of their fuselage armament and protective systems in order to attain the required altitudes. Although the term "Kamikaze" is often used to refer to the pilots undertaking these attacks, the word was not used by the Japanese military.^[16]

The units assigned to the 10th included the 244th Hiko Sentai, then commanded by Captain Takashi Fujita who organised a ramming flight named "Hagakure-Tai" ("Special Attack Unit"), which was composed of three sections: the 1st Chutai "Soyokaze", 2nd Chutai "Toppu", and the 3rd Chutai known as "Mikazuki".

First Lt. Toru Shinomiya was selected to lead the attack unit, he would became famous by ramming an American B-29 and living to tell the tale. Shinomiya attacked the B-29 on 3 December 1944, and brought himself and his damaged aircraft home; Shinomaya's Ki-61, which had lost most of the port outer wing, was subsequently put on display in a major department store in Tokyo. He would eventually lose his life as a Tokkotai pilot in the battle for Okinawa. Another 244th pilot, Corporal Masao Itagaki, performed a similar feat on the same occasion, but had to parachute from his damaged fighter. A third pilot, Sergeant Nakano, of the Hagakure-Tai of the 244th rammed another B-29 and crash-landed his stripped-down Ki-61 in a field. These three pilots were the first recipients of the Bukosho, Japan's equivalent to the Victoria Cross or Medal of Honor, which had been inaugurated on 7 December 1944 as an Imperial Edict by Emperor Hirohito (there are 89 known recipients, most of whom fought and scored against B-29s.)^[17][18] Sergeant Shigeru Kuroishikawa was another distinguished member in the unit.

The existence of the ramming unit had been kept confidential until then, but it was officially disclosed in the combat results announcement and officially named "Shinten Seiku Tai" ("Heart of Heaven Intercept Unit") by the Defense GHQ.

But these pilots gained no reprieve and despite their successes they were obligated to continue these deadly and dangerous ramming tactics until they were killed or else wounded so badly that they could no longer fly. They were regarded as doomed men and were celebrated among the ranks of those who were going to certain death as Tokkotai (kamikaze) pilots.^[19]

Some other Ki-61 pilots also became well-renowned, among whom was Major Teruhiko Kobayshi who was credited by some with a dozen victories mostly due to conventional attacks against B-29s.^[20]

[edit] Ki-61 units

The Hikosentai, usually referred to as Sentai, was the basic operational unit of the IJAAF, composed of three or more Chutai (companies or squadrons). A Sentai had 27 to 49 aircraft, with each Chutai having 16 aircraft and pilots plus a maintenance and repair unit. Several sentai had other units under their operational control, most notably the Hagakure-Tai ("Special Attack Units") of 244 Sentai. By 1944, with the depredations of Allied attacks on supply lines and airfields, as well as the loss of pilots and aircraft through combat attrition and accidents, few sentai were able to operate at full strength.

Source: Sakaida, Henry. Japanese Army Air Force Aces 1937-45.^[24]

[edit] Variants

Ki-61

12 original prototypes.

Ki-61-I-Ko

The first production version. This version had a fully retractable tail wheel and two 2 x 7.7 mm (0.303 in) Type 89 machine guns in the wings and two synchronized 2 x 12.7 mm (0.50 in)Ho-103 machine guns in the fuselage forward decking. The wings had racks outboard of the wheelbays, which were capable of carrying one 40 gallon drop tank or a light bomb.

Ki-61-I-Otsu

The second production fighter variant. As it was found the armament was too light against allied aircraft and the tail wheel retraction mechanism was unreliable, the plane was modified accordingly. Two 12.7 mm Ho-103 heavy machine guns replaced the wing 7.7 mm machine guns, with modifications to the upper-wing bulges, and the tail wheel well doors were removed and the tailwheel locked in the "down" position (although the mechanism was still intact).

Ki-61-I-Hei

A number of German made Mauser MG 151/20, 20 mm cannons were imported to Japan by submarine. The Hei was built in conjuction with the Otsu variant on the Kawasaki production lines but some "conversion kits" were directly sent to New Guinea. In this variant, the wing machine guns were replaced by Mauser cannons with associated modifications.

Ki-61-I-Tei

Featured reinforced wings. The forward fuselage was elongated by 190 mm (7.5 in) just after the exhaust line and forward of the windscreen to make room for the installation of Japanese 20 mm Ho-5 cannon in the fuselage decking. In the wings the 12.7 mm machine guns returned in a similar arrangement as found on the Otsu. This version also featured a lightened structure, revised rear fuselage, and a fixed tail wheel. ^[25]

Ki-61-I-KAId

Interceptor variant with 2 x 12.7 mm fuselage machine guns and 2 x 30 mm wing cannon.

Ki-61-II

Prototype with 10% greater wing area and a Ha-140 engine with 1,120 kW (1,500 hp) for takeoff; first flight December 1943; eight built.

Ki-61-II-KAI

Pre-production version which reverted back to the Ki-61-I-Tei wing, a 220 mm (8.7 in) fuselage stretch, enlarged rudder, and Ha-140 engine; 30 built.

Ki-61-II-KAIa

Armed with 2 x 12.7 mm machine guns in the wings and 2 x 20 mm cannon in the fuselage.

Ki-61-II-KAIb

Armed with 4 x 20 mm cannon.

Ki-61-III

One prototype only.

A total of 3,159 Ki-61 were built.^[26]

[edit] Operators

 China

• Chinese Nationalist Air Force

Operated some captured aircraft

 China

• People's Liberation Army Air Force

Also operated some captured aircraft

 Indonesia

• In 1945, Indonesian People's Security Force (IPSF) (Indonesian pro-independence guerrillas) captured a small number of aircraft at numerous Japanese air bases, including Bugis Air Base in Malang (repatriated 18 September 1945). Most aircraft were destroyed in military conflicts between the Netherlands and the newly proclaimed-Republic of Indonesia during the Indonesian National Revolution of 1945–1949.^[27]

 Japan

• Imperial Japanese Army Air Force

[edit] Specifications (Ki-61-I-KAIc)

Data from The Great Book of Fighters^[28]

General characteristics

• Crew: One
• Length: 8.94 m (29 ft 4 in)
• Wingspan: 12.00 m (39 ft 4 in)
• Height: 3.70 m (12 ft 2 in)
• Wing area: 20.00 m² (215.28 ft²)
• Airfoil: NACA 2R 16 wing root, NACA 24009 tip

• Internal fuel capacity: 550 l (145.2 US gal)
• External fuel capacity: 2 x 200 l (53.8 US gal) drop tanks

• Empty weight: 2,630 kg (5,800 lb)
• Loaded weight: 3,470 kg (7,650 lb)
• Powerplant: 1× Kawasaki Ha-40 liquid-cooled inverted V-12, 875 kW (1,175 hp)

Performance

• Maximum speed: 580 km/h (360 mph) at 5,000 m (16,405 ft)
• Range: 580 km (360 mi)
• Service ceiling 11,600 m (38,100 ft)
• Rate of climb: 15.2 m/s (2,983 ft/min)
• Wing loading: 173.5 kg/m² (35.5 lb/ft²)
• Power/mass: 0.25 kW/kg (0.15 hp/lb)

• Time to altitude: 7.0 min to 5,000 m (16,405 ft)

Armament

• 2x 20 mm Ho-5 cannon, 120 rounds/gun
• 2x 12.7 mm (0.50 in) Ho-103 machine guns, 200 rounds/gun
• 2x 250 kg (550 lb) bombs

[edit] See also

Related development

• Kawasaki Ki-100
  

Comparable aircraft

• Heinkel He 100
• Macchi C.202
• Messerschmitt Bf 109
• LaGG-3
  

Related lists

• List of military aircraft of Japan
  

[edit] References

[edit] Notes

[edit] Bibliography

[edit] External links

• KI-61 article on vectorsite (public domain)
• Article on Ki-61-II-Kai production No.5017, still in existenceRetrieved: 7 June 2008.
• Japanese aircraft
• USN report on captured Ki-61 (pdf file)Retrieved: 11 April 2008
• Website on Kamikaze from Japanese and American perspectivesRetrieved: 12 April 2008
• Japanese website dedicated to the Tokkotai JAAF and JNAFRetrieved: 7 June 2008