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Ki-61 "Hien"

Kawasaki Ki-61
  • Fighter
  • Kawasaki

Fighter Ki-61 "Hien" ("Swallow") was a unique machine among Japanese combat aircraft of the Second World War. It was equipped with the Na-40 inverted V-piston engine, which was a licensed version of the German Daimler-Benz DB-601A engine.

The first prototype Ki-61 was manufactured in December 1941 and successfully passed flight tests, showing good maneuverability and controllability in the air. True, in comparison with the Ki-43, the fighter had a rather high specific wing loading (146 kg/m²), but this shortcoming was fully compensated for by powerful weapons, protected gas tanks and armored vehicle protection. The maximum speed of the Ki-61 at an altitude of 6,000 m was 591 km/h. To conduct military tests, the Kawasaki company built 11 more prototypes, but even before their completion, an order was received for the serial production of the fighter.

The Ki-61 entered service with the Japanese army at the end of 1942 and, in accordance with the Japanese designation system, received the name "Type 3 Model I Army Fighter", as well as the name "Hien" (Swallow). By the end of the year, the plant in Kagamihara had produced 34 serial fighters in two versions: Ki-61-la (armed with two 12.7-mm synchronous fuselage machine guns No-103 and two wing 7.7-mm machine guns "type 89") and Ki- 61-Ib (four 12.7 mm No-103 machine guns in the wing and fuselage).

The Ki-61-I fighter began to enter service in February 1943; in total, about 1380 aircraft were manufactured in two versions, differing in their weapons. After that, another 1274 Ki-61 KAI fighters appeared with an elongated fuselage and various weapon combinations.

Further developments led to the creation of the Ki-61-II KAI high-altitude fighter, with the Na-140 engine. The new Hien turned out to be the only Japanese fighter capable of effectively operating at high altitude against the B-29 Superfortresses, but the low reliability of the engine reduced its combat value to a minimum.

A total of 374 such aircraft were built, again in two versions with different weapons, but only 99 of them were equipped with the Na-140 engine. Like the rest of the Japanese fighters, the Hien was soon unable to withstand its American opponents.

Ki-61-II Prototype in Akeno Fighter School 1943

Ki-61 'Hien' Specification
Ki-61 I b Ki-61 I KAI Ki-61 II KAI
Crew 1
Wing span, m 12.0
Wing area, m² 20
Length, m 8.75 8.94 9.16
1XPE Ha-40 Kawasaki, hp Ha-40/1175 Ha-40/1175 Ha-140/1500
Weight, kg:
Empty weight 2380 2640 2840
Maximum takeoff weight 5950 3470 3780
Speed, km/h maximum 592 560 610
at altitude 4860 4860 6000
Time to 5000m, min 5,5 7,0 6,0
Service ceiling, m 11000 10000 11000
Service range, km 600 580 600
12.7-mm machine guns 4 4 -
20-mm cannon - - 4
External bomb load, kg 500 500 500


Note: Ko, Otsu, Hei and Tei are the Japanese equivalents to a, b, c, d. Kai ('modified' or 'improved') was also used for some models of the Ki-61.

Kawasaki Ki-61
  • Ki-61 12 original prototypes.
  • Ki-61-I-Ko The first production version. This version had a fully retractable tailwheel and two 7.7 mm (0.303 in) Type 89 machine guns in the wings and two synchronized 12.7 mm (0.50 in) Ho-103 machine guns in the fuselage forward decking. The wings had racks outboard of the wheelwells, which were capable of carrying one 151 L (40 gal) drop tank or a light bomb. Weights: 2,238-2,950 kg (wing loading 147 kg/m²).
  • Ki-61-I-Otsu The second production fighter variant. As it was found the armament was too light against allied aircraft and the tailwheel retraction mechanism was unreliable, the aircraft was modified accordingly. Two 12.7 mm (.50 in) Ho-103 machine guns replaced the wing 7.7 mm (.303 in) machine guns, with modifications to the upper-wing bulges, and the tailwheel doors were removed and the tailwheel locked in the "down" position (although the mechanism was still intact). Max takeoff weight 3,130 kg.
  • Ki-61-I-Hei 800 German-made 20 mm Mauser MG 151/20 cannons and ammunition supplies were imported to Japan by submarine. The Hei was built in conjunction 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. The fuselage was longer (engine bulkhead forwarded of 19 cm, added a small fuel tank). Trial fittings found that these could be placed into the existing wing if the gun was laid on its side and a fairing was provided on the underside of the wing for clearance of the breech mechanism. 388 Ki-61s were so modified;[37] the first of these was completed in September 1943[37] or January 1944.[10] The last was completed in July 1944.[37] Max takeoff weight 3,470 kg, max speed 580 km/h at 5,000 m, time to that height was seven minutes.
  • Ki-61-I-Tei This machine featured two 12.7 mm (.50 in) Ho-103s in the modified (stronger) wings, provisions of external storage using fixed underwing pylons, and a non-retractable tail wheel. 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 (the same as with Hei version). The continued supply of MG 151's via submarine was not able to be guaranteed and the Ho-5 was ready. Several internal changes were also made. These included the simplification of several systems for increased dependability and ease of maintenance. The rear section of the fuselage was also made to be easily removable to further facilitate the ease of repair work.[38] There is now some doubt as to whether the Tei ("d") designation was used.[39] Ki-61-I-KAId Interceptor variant with 2 x 12.7 mm (.50 in) fuselage machine guns and 2 x 30 mm (1.18 in) wing cannon.
  • Ki-61-I- w.c.e.s. An experimental aircraft with a wing cooling evaporation system, modelled on that used by the Heinkel He 100. It was the fastest Ki-61 built, achieving 630 km/h (395 mph), and the last with a retractable tail wheel.
  • Ki-61-II Prototype with 10% greater wing area and a slightly different airfoil. An Ha-140 engine with 1,120 kW (1,500 hp) for takeoff was fitted; the cowling panels were redesigned and the supercharger air intake was longer. A redesigned windscreen incorporating an extra panel was mounted further forward. The transparency to the rear of the sliding canopy was redesigned to increase rearward visibility. The prototype was first flown in December 1943; flight trials showed that the new wing was unsatisfactory and only eight Ki-61-IIs were built.
  • The Ki-61-II was one of only very few Japanese aircraft able to reach the operational altitude of the B-29s raiding Japan with decent firepower. Subsequently, the majority of B-29s lost to Japanese fighters were shot down by the Ki-61-II. Due to the unreliability of the Ha-140 and the destruction of the Akashi plant by a B-29 bombing attack, the bulk of the Ki-61-II-KAIs built were airframes only, which were later converted to Ki-100-Is.; 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 (.50 in) 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. This version had a cut-down rear fuselage and a canopy design which was later used by the Ki-100-II.
  • A total of 3,159 Ki-61 were built.
Kawasaki Ki-61
Photo Description
Drawing Ki-61-Ib

Drawing Ki-61-Ib


The Ki-61, "Hien", Kawasaki.

Ki-61-Ib, April 1945

The Ki-61-Ib of the commander of 244-th sentay (regiment) of major T.Kobajasi, April 1945


  • "Japanese Aircraft of the Pacific War." /Francillon, Ph.D., Rene J./
  • "Warplanes of the Second World War" /Green, William./