Warplanes of Japan: Kawasaki Ki-100

Kawasaki Ki-100

(IJAAF Photo)

Kawasaki Ki-100-I-Ko Army Fighter Type 5 Mark 1a of 59th Sentai 2nd Chutai in August 1945 in Japan.

The Kawasaki Ki-100 (キ100) is a single-seat single-engine monoplane fighter aircraft usedby the Imperial Japanese Army Air Service (IJAAS) during World War II. TheJapanese Army designation was "Type 5 Fighter" (五式戦闘機, Go-shiki sentouki, or abbreviated as Goshikisen). It was not assigned an Allied code name.

In early 1945, 275 Ki-100s were modified from Ki-61s as an emergency measure toaccept a 14-cylinder Mitsubishi Ha-112-II radial engine in place of theoriginal Kawasaki Ha-40 inverted V-12 inline engine, resulting in one of thebest interceptors used by the Army during the war. It combined excellent powerand maneuverability,[2] and from the first operational missions in March 1945until the end of the war, it performed better than most IJAAS fighters[3]against both United States Army Air Forces B-29 Superfortress bombers and P-51Mustang fighters, as well as U.S. Navy F6F Hellcat carrier fighters.

A newly built variant, the Ki-100-Ib, was produced with a cut down rearfuselage during the last months of the war which equipped five home defencesentai. High-altitude performance was further improved with the final variant,the Ki-100-II, however only three of these were produced before the war endedand this final variant never saw operational service.

Design and development
The Ki-100 was a stressed-skin cantilever low-wing single-seat enclosed-cockpitradial engine monoplane fighter with retractable undercarriage. Controlsurfaces were fabric covered.

In mid-1944, the Ki-61 was one of the best fighters of the Imperial JapaneseArmy Air Service (IJAAS). It was also the only production Japanese fighter tohave an inline powerplant, the V-12 Kawasaki Ha-40, a Japanese adaptation ofthe German Daimler-Benz DB 601 engine, as well as one of the first withfactory-installed armor and self-sealing fuel tanks. It also had a respectableperformance, in line with contemporary American designs, with speed and rate ofclimb emphasized instead of manoeuvrability and range. It was an effectivedesign, but suffered from engine shortages and reliability problems. Theseproblems led to the development of an improved model, the Ki-61-II (laterKi-61-II-KAI), powered by the improved 1,120 kW (1,500 hp) Kawasaki Ha-140inverted V-12 engine, which was heavier than the Ha-40 it replaced. Maximumspeed increased from 590 to 610 km/h (370 to 380 mph) and aside from the rateof climb, general performance was improved as well. However, it never performedas expected due to continued quality control problems with the engine, whilefar fewer engines were produced than required.

At this point of the war, the IJAAS was in desperate need of effectiveinterceptors to stop bombing raids over the Japanese mainland, so in October1944 a decision was made to use the 1,120 kW (1,500 hp) Mitsubishi Ha-112-II(Kinsei ["Venus"] 60 series), a 14-cylinder, two-row radial engine.The need for a new engine became urgent on 19 January 1945, when a bombing raiddestroyed the Ha-140 production plant,[5] leaving 275 otherwise complete Ki-61sengineless.

The Mitsubishi Ha-112-II was 54 kg (119 lb) lighter than the Ha-140 anddeveloped the same power but with much greater reliability. Three Ki-61-II-KAIswere modified to carry this engine as prototypes. Chief engineer Takeo Doi withtwo other engineers redesigned the Ki-61 airframe to accept the new engine.Their solution was to use a second skin to form a fairing riveted to thefuselage to smooth out the airflow behind the cooling flaps and multipleexhaust stubs of the new engine cowling. As this engine was lighter, they wereable to remove the lead counterweight in the tail that balanced out the heavierHa-140 engine.

The new model was flown for the first time on 1 February 1945. Without the needfor the heavy coolant radiator and other fittings required for a liquid-cooledengine, the Ki-100 was 329 kg (725 lb) lighter than the Ki-61-II, reducing thewing loading from 189 kg/m2 (38.8 lb/ft2) to 175 kg/m2 (35.8 lb/ft2). This hada positive effect on the flight characteristics, enhancing landing and takeoffqualities as well as improving manoeuvrability and reducing the turningradius.[6] During March and April 1945, experienced instructors from the AkenoArmy Flying School flew the Ki-100 in extensive tests against the Ki-84, whichwas the best IJAAS fighter then in operational service. Their conclusions werethat, given pilots of equal experience, the Ki-100 would always win in combat.

The flight characteristics of the plane surpassed the Hien's in all but maximumspeed, which was reduced by 29 km/h (18 mph) by the larger cross sectional areaof the radial engine, and the model was ordered into production as theGoshikisen (Go=five; shiki=type; sentoki=fighter) or Army Fighter Type 5. Thecompany's designation for it was Ki-100-I-Ko. All Ki-100-I-Ko were convertedfrom existing Ki-61-II Kai and Ki-61-III airframes. The integral engine mountand cowling was cut off and a tubular steel engine mount was bolted to thefirewall. Some redundant fittings from the liquid-cooled engine, such as theradiator shutter actuator, were left in place. The first 271 aircraft with theoriginal faired rear fuselage were rolled out of the factory between March andJune 1945.

In contrast to the unreliable engines used by the Kawanishi N1K-J, KawasakiKi-61 and Nakajima Ki-84 that were keeping many of these aircraft grounded, thenew engine was much more reliable. Although its maximum speed in level flightwas a bit slow for 1945, the Ki-100 could dive with North American P-51Mustangs and maintain speed after pullout, unlike most Japanese fighters.

Two remaining problems continued to hamper Japanese fighters towards the end ofthe war, these being unreliable electrical systems and poor radio equipment,and while the latter was never resolved, the Ki-100's electrical system wasless of a problem than with other types.

The armament remained the same as was used on the Ki-61, which was twocowl-mounted 20 mm (0.79 in) calibre Ho-5 cannons, with 200 rounds per guncomplemented by two wing-mounted 12.7 mm (0.50 in) Ho-103 machine guns with 250rounds per gun.

The Ki-100-I-Otsu were newly built as such, rather than being conversions, witha cut-down rear fuselage and improved canopy and 118 were produced from Maythrough to the end of July 1945. This version also featured a modified oilcooler under the engine in a more streamlined fairing.

The Ki-100-II was fitted with a turbocharged water-methanol injected engine forimproved high-altitude performance, mainly to improve interception capabilitiesagainst the Boeing B-29 Superfortresses, but only three prototypes were built,and none was used operationally.[9] Due to a lack of space no intercooler wasinstalled; however, performance was still enhanced above 8,000 m (26,000 ft).

The Ki-100 made its combat debut on the night of 9 March 1945, and suffered itsfirst loss a month later on the night of 7 April 1945, when a single Ki-100 ofthe 18th Sentai was downed by a B-29 Superfortress after "attacking theformation again and again". Allied aircrews soon realised that they werefacing a formidable new fighter.[11] Although far fewer Ki-100s were availablethan Nakajima Ki-84s, it was an important fighter in the Army's inventory. Awell-handled Ki-100 was able to outmanoeuvre any American fighter, includingthe P-51D Mustangs and Republic P-47N Thunderbolts which escorted the B-29sover Japan, and was comparable in speed, especially at medium altitudes. TheKi-100 was a tough opponent in the hands of an experienced pilot. The Ki-100along with the Army's Nakajima Ki-84 and the Navy's Kawanishi N1K-J were equalto the latest Allied types in the final year of the Pacific War.

Army fighter units equipped with this model included the 5th, 17th, 18th, 20th,59th, 111th, 112th, 200th and 244th Sentai, and the 81st Independent FighterCompany. Pilots were trained at the Akeno and Hitachi (Mito) Army FlyingSchools. Many Akeno and Hitachi instructors were from operational units andbetween training sorties they also flew combat missions, making the most of thefew fighters that were operational, but these wings were only partiallyre-equipped.

111th Sentai Ki-100-I-Otsu
During interception of high-flying B-29 Superfortresses (before the B-29sswitched to low-level missions) the new fighters struggled as the engine'sperformance still dropped off at high altitudes. The most effective strategyagainst the B-29 Superfortress remained the exceedingly dangerous head-onattack, which left the fighter relatively stationary in the sights of thebomber's defensive guns, making it an easy target. In this type of combat, theNavy's Mitsubishi J2M Raiden remained superior.

111th Sentai Ki-100s intercepted B-29 Superfortresses attacking Kobe on 5 June1945, claiming six bombers shot down and five 'probables'. The Americans recordeda loss of nine B-29s, including those downed by Ki-100s over the target area.Ki-100s of the same unit joined by 244th Sentai Ki-84s in a large-scaleengagement against 21st Fighter Group and 506th Fighter Group P-51 Mustangsover Nagoya Bay on 16 July 1945. The Ki-100 pilots claimed six P-51s and inreturn, five Ki-100s were lost, with three pilots killed, although Americanrecords only show one loss.

On 25 July 1945, 18 244th Sentai Ki-100 fighters clashed with 10 VF-31 GrummanF6F Hellcats from the light aircraft carrier USS Belleau Wood in an air battlewhere the Ki-100 pilots claimed 12 victories with two losses. Claims andcounter-claims for this action remain contentious. The Americans claimed twoKi-100s and admitted to losing two VF-31 F6F-5 Hellcats. These include a Ki-100and a Hellcat which collided, killing both pilots.

After the bombing of the Kagamigahara plant and the slow deliveries ofcomponents by satellite plants, the production rate of the Ki-100 declined, andbetween May and July, only 12 were delivered. Bombing ended production withonly 118 of the Army Type 5 Fighter Model 1b having been delivered.

The final flights made by the Imperial Japanese Army Air Service were made bytwo Ki-100s ferried from Komachi to Yokosuka, where they were handed over tothe United States, who then shipped them back to the US for evaluation.

Variants
Ki-100 (prototypes): Kawasaki Ki-61 II KAI modified with a radial engine. 3converted from Ki-61s.
Ki-100-I-Ko: initial production variant, modified from Ki-61 II KAI. 271converted from Ki-61s.
Ki-100-I-Otsu: improved canopy and cut down rear fuselage. 118 built.
Ki-100-II (prototypes): powered with a 1,120 kW (1,500 hp) Mitsubishi Ha-112-IIRu fitted with a turbocharger. 3 built.
Army Fighter Type 5 Mark Ia: IJA designation for Ki-100-I-Ko
Army Fighter Type 5 Mark Ib: IJA designation for Ki-100-I-Otsu. (Wikipedia)

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(IJAAF Photo)

Kawasaki Ki-100-1b Type 5 fighter.

(SDASM Archives Photo)

5th Sentai Kawasaki Ki-100-I-Otsu Army Fighter Type 5 Mark 1b, showing this version's cut down rear fuselage.

Kawasaki Ki-100-1b Type 5 fighter. Four were shipped to the USA, Ki-100-1b designated USAAF FE-312 was scrapped at Park Ridge, ca. 1950, Ki-100-1b (Serial No. 13012), FE-314 was broken up at Patterson AFB in 1959, FE-315 was scrapped, and FE-317 was scrapped at Park Ridge ca. 1950.  One was shipped to the UK.

(USN Photo)

Kawasaki Ki-100-II. The fighter's turbocharger on the underside is revealed by the offset oil cooler, and additional intake on other side.

(Alan Wilson Photo)

Kawasaki Ki-100-1b Type 5 fighter, RAF Museum Cosford, England. This is the only surviving example. It was one of 24 found at Tan Son Nhut Airfield in what was then French Indo-China. It was briefly made airworthy, but soon suffered a wheels-up landing after which it did not fly again. It was shipped to the UK during 1946 and was then stored at various locations for the Air Historical Branch, making occasional static appearances. In 1985 she moved to RAF St Athan and was returned to ground running condition, but the collection was dissolved in 1989 and the Ki-100 joined the Cosford Aerospace Museum. In 2003 it underwent further restoration before going on display in the ‘Milestones of Flight’ Hall at Hendon. She remained there until 2011 when she came back to Cosford and is currently on display in the ‘War in the Air’ hangar. RAF Museum, Cosford, Shropshire, UK.

(Aldo Bidini Photo)

Kawasaki Ki-100-1b Type 5 fighter, RAF Museum Cosford, England.

(Max Smith Photo)

Kawasaki Ki-100-1b Type 5 fighter, RAF Museum Cosford, England.

(Murgatroyd49 Photo)

Kawasaki Ki-100-1b Type 5 fighter, RAF Museum Cosford, England.