Known to the Imperial Japanese army air force as the Hayabusa (peregrine falcon) and to the Allies by the reporting name ‘Oscar’, the Nakajima Ki-43 was the most advanced fighter available to the Imperial Japanese army air force in the opening phases of the Pacific War of World War II. As such the type came as a very considerable shock to the Allied air forces, which had deployed into these regions only the more obsolescent of its ‘modern’ monoplane fighters in the belief, apparently confirmed by intelligence data, that the Imperial Japanese army air force had not yet fielded a ‘modern’ monoplane fighter with retractable landing gear to replace the Nakajima Ki-27 with its fixed landing gear. The Ki-43 was encountered in large numbers over China, and in smaller but nonetheless significant numbers over the Philippines, Malaya, Burma, the Netherlands East Indies and New Guinea. In all these theatres the type proved to have the measure of Allied fighters in terms of agility and overall performance, but also revealed its lack of offensive firepower together with the lightweight and basically unprotected airframe that was incapable of absorbing much battle damage.
The origins of the type can be traced to 1937, when the Imperial Japanese army air force started the process of acquiring a successor to the Ki-27. In a break with its previous practice, the service decided not to proceed via the competitive design process, but in December 1937 issued to Nakajima the requirement for an advanced monoplane fighter providing agility at least equal to that of the Ki-27 and with the same armament of two 0.303 in (7.7 mm) machine guns, but also possessing markedly improved performance that included a maximum speed of 270 kt (311 mph; 500 km/h) at optimum altitude, climb to 16,405 ft (5000 m) in a time of no more than 5 minutes 0 seconds, and range of 432 nm (497 miles; 800 km).
Nakajima entrusted the task of designing the new fighter to a team under the supervision of Hideo Itokawa, and this team quickly evolved a cantilever low-wing monoplane of basically all-metal construction with fabric-covered control surfaces. The core of the structure was the semi-monocoque fuselage, which changed from a circular section at the front to an oval section farther to the rear in the area of the cockpit. This last was raised above the line of the fuselage and provided the pilot with very good fields of vision. The fuselage supported the plain tail unit and the stressed-skin wing, which was dihedralled, tapered in thickness and chord to its rounded tips, and carried outboard ailerons on its trailing edges. The airframe was completed by the landing gear, which was of the tailwheel type with wide-track main units that retracted inward into bays in the underside of the wing.
Construction of the three prototypes proceeded as rapidly as completion of the design, and the first prototype recorded its maiden flight on 12 December 1938 with the powerplant of one Nakajima Ha-25 Sakae (Army Type 99) air-cooled 14-cylinder two-row radial engine rated at 980 hp (731 kW) for take-off and 970 hp (723 kW) at 11,155 ft (3400 m), and driving a two-blade wooden propeller of the fixed-pitch type. The armament comprised two 0.303 in (7.7 mm) Type 89 fixed forward-firing machine guns in the upper part of the forward fuselage with synchronisation equipment to fire through the propeller disc. The second and third prototypes joined the flight test programme in February and March 1939, and all three aircraft were soon delivered to the Imperial Japanese army air force for official trials.
During their official trials the Ki-43 prototypes revealed their ability to meet the service’s performance requirements, but received criticism for their comparative lack of agility which, according to most service pilots, resulted from the weight of the retractable main landing gear arrangement. For a time the future of the Ki-43 was in the balance, but the Imperial Japanese army air force finally decided that despite the feelings of most service pilots, retractable landing gear represented one of the ways forward in fighter design.
Between November 1949 and September 1940, therefore, Nakajima built another 10 aircraft as service trials machines, and these differed from the prototypes mainly in having all-round vision canopies; additionally, the second of these aircraft was powered by an engine with a two-speed supercharger, the seventh had an armament of two 0.5 in (12.7 mm) Ho-103 machine guns for heavier offensive punch, the eighth introduced ‘butterfly’ combat flaps whose deployment increased control sensitivity and decreased the turning circle by providing more lift, the ninth had an Alclad-treated Duralumin skinning and cowling gills, and the tenth was based on the same structure as the ninth aeroplane but was powered by the Nakajima Ha-105 air-cooled 14-cylinder two-row radial engine rated at 1,100 hp (820 kW) for take-off, possessed an armament of two 0.5 in (12.7 mm) Ho-103 machine guns, and introduced a slimmer fuselage in combination with revised flying surfaces. The tenth service trials aeroplane was clearly an outstanding air-combat fighter and, with the ‘butterfly’ combat flaps of the eighth aeroplane and the original Ha-25 radial engine now driving a two-blade metal propeller of the two-pitch type, formed the basis of the production model. This was built as the Ki-43-Ia and entered service as the Army Type 1 Fighter Model 1A Hayabusa with an armament of two 0.303 in (7.7 mm) Type 89 machine guns and attachments under the inner parts of the wing for two drop tanks.
The Ki-43-Ia was soon supplanted in production by the Ki-43-Ib (Army Type 1 Fighter Model 1B Hayabusa) with the fixed forward-firing armament increased to one 0.5 in (12.7 mm) Type 1 (Ho-103) machine gun and one 0.303 in (7.7 mm) Type 89 machine gun, and by the Ki-43-Ic (Army Type 1 Fighter Model 1C Hayabusa) with the fixed forward-firing armament further strengthened to two 0.5 in (12.7 mm) Type 1 (Ho-103) machine guns.
The Ki-43-Ia may be taken as typical of the Ki-43-I series, and in features other than its powerplant and armament differed from the Ki-43-IIb in details such as its internal fuel capacity of 124.3 Imp gal (149.25 US gal; 565 litres), span of 37 ft 6.25 in (11.437 m) with area of 236.805 sq ft (22.00 m²), length of 28 ft 11.75 in (8.832 m), empty weight of 3,483 lb (1580 kg), normal take-off weight of 4,515 lb (2048 kg), maximum take-off weight of 5,695 lb (2583 kg), maximum speed of 267.5 kt (308 mph; 495 km/h) at 13,125 ft (4000 m), cruising speed of 173 kt (199 mph; 320 km/h) at 8,200 ft (2500 m), climb to 16,405 ft (5000 m) in 5 minutes 30 seconds, service ceiling of 38,500 ft (11750 m), and maximum range of 648 nm (746 miles; 1200 km) with drop tanks.
Production of the Ki-43-I series, all delivered by Nakajima between April 1941 and February 1943, was 716 aircraft.
The Ki-43-I soon acquired an excellent reputation in the Imperial Japanese army air force, and the reluctant admiration of its foes, who found themselves seldom able to keep this agile air-combat fighter in their sights for more than the shortest time. The Achilles’ heel of the Ki-43-I, as with most Japanese warplanes of this period early in the Pacific War, was its combination of a very light structure and lack of any protection for the pilot and fuel supply: Allied pilots therefore found that even a very short burst was sufficient to cause a structural failure, kill or wound the pilot, or set fire to the fuel supply.
The Allies’ lack of co-ordination meant that the type initially received two reporting names: in the CBI (China, Burma and India) theatre the Ki-43 was thought to be a development of the Ki-27 with retractable main landing gear units and was allocated the name ‘Jim’, while in the South-West Pacific Area the name ‘Oscar’ was given and then adopted universally after it had been appreciated that the Ki-43 was not a Ki-27 derivative.
From the beginning of the Ki-43 programme it had been planned that a more powerful engine would eventually be fitted, and soon after the Ki-43-I had entered service five aircraft were taken in hand for modification with the Nakajima Ha-115 (Army Type 1) air-cooled 14-cylinder two-row radial engine rated at 1,150 hp (857 kW) for take-off and 980 hp (731 kW) at 18,735 ft (5600 m) and driving a three-blade metal propeller of the constant-speed type. Completed between February and May 1942, these prototypes were followed by three pre-production aircraft constructed between June and August 1942 and, from November 1942, by the Ki-43-IIa production model that entered service with the designation Army Type 1 Fighter Model 2A Hayabusa.
The Ki-43-II series differed from the Ki-43-I series in a number of respects other than the powerplant, most notably the relocation of the supercharger air inlet from its original position under the cowling to a new position on the upper part of the cowling lip, the modification of the wing for reduced span and area for improved speed at low and medium altitudes, a slight heightening of the windscreen and cockpit canopy, the introduction of a new reflector gun sight, the revision of the underwing hardpoints for the carriage of 551 lb (250 kg) bombs as alternatives to drop tanks and, perhaps just as important as all of the above, a measure of protection in the form of armour plate for the pilot and self-sealing for the fuel tanks.
Production of the Ki-43-IIa was comparatively short-lived, for the type was soon supplanted by the Ki-43-IIb (Army Type 1 Fighter Model 2B Hayabusa) that in its first form was distinguishable only by changes to the carburettor air inlet, which up to this time had been located under the cowling but was now incorporated within the oil cooler that was changed from an annular type (located around the propeller shaft to the rear of the spinner) to a honeycomb unit under the engine cowling. Later changes in the Ki-43-IIb production standard included a relocation of the hardpoints to positions farther outboard under the wings to prevent the bombs from hitting the propeller in dive-bombing attacks, and the movement of the carburettor inlet/oil cooler installation to a position farther to the rear.
All these changed were standardised in the Ki-43-II Kai, which also introduced individual ejector exhausts for a measure of additional thrust, in place of the earlier collector ring arrangement. The model also introduced a number of airframe modifications designed to facilitate production and simplify field maintenance.
With a number of major commitments to the Imperial Japanese navy air force as well as the Imperial Japanese army air force, Nakajima was unable to satisfy demand for the Ki-43 series and the Imperial Japanese army air force therefore brought two other manufacturing sources into the Hayabusa programme. These were the Dai-Ichi Rikugun Kokusho (1st Army Air Arsenal) and Tachikawa Hikoki K.K., both located at Tachikawa. The army facility soon showed that it lacked the skills requirement for mass production of a fighter and was pulled out of the programme after delivering 49 examples of the Ki-43-IIa from Nakajima-supplied components, but Tachikawa proved altogether more successful and between May 1943 and August 1945 delivered 2,629 aircraft of various Hayabusa models to supplement Nakajima’s total of 3,208 aircraft (716 Ki-43-I and 2,492 Ki-43-II fighters).
The Ki-43-III was the main variant produced by Tachikawa, after Nakajima had delivered 10 prototype aircraft between May and August 1944. The Ki-43-IIIa was basically a development of the Ki-43-II Kai with the uprated powerplant of one Nakajima Ha-115-II engine rated at 1,190 hp (887 kW) for take-off and 950 hp (708 kW) at 20,340 ft (6200 m). The improved fighter entered service as the Army Type 1 Fighter Model 3A Hayabusa, and was dimensionally identical to the Ki-43-IIb. In respects other than its engine, the Ki-43-IIIa differed from the Ki-43-IIb in details such as its internal fuel capacity of 144 Imp gal (173 US gal; 655 litres) that could be supplemented by 92.4 Imp gal (111 US gal; 420 litres) carried in two 46.2 Imp gal (55.5 US gal; 210 litre) drop tanks, empty weight of 4,233 lb (1920 kg), normal take-off weight of 5,644 lb (2560 kg), maximum take-off weight of 6,746 lb (3060 kg), maximum speed of 311 kt (358 mph; 576 km/h) at 21,920 ft (6680 m), cruising speed of 239 kt (275 mph; 442 km/h) at 8,200 ft (2500 m), climb to 16,405 ft (5000 m) in 5 minutes 19 seconds, service ceiling of 37,400 ft (11400 m), typical range of 1,144 nm (1,317 miles; 2120 km) with standard fuel, and maximum range of 1,727 nm (1,990 miles; 3200 km) with drop tanks.
The only other variant of this fighter, which was numerically the most important type fielded by the Imperial Japanese army air force in the course of the Pacific War, was the Ki-43-IIIb. Built to the extent of only two prototypes, this was a dedicated interceptor with a fixed forward-firing armament of two 20 mm Ho-5 cannon and a powerplant of one Mitsubishi [Ha-33] 42 (Ha-112) air-cooled 14-cylinder two-row radial engine rated at 1,300 hp (969 kW) for take-off and 1,100 hp (820 kW) at 20,340 ft (6200 m).
The Ki-43-I was also operated in small numbers by the Thai air force during the Pacific War, and after the war the Ki-43 was also flown in small numbers by the air arm of the nationalist forces fighting a return of the Dutch imperial power to the Netherlands East Indies (now Indonesia), and by the French fighting the communist guerrilla forces in French Indo-China.
Nakajima Ki-43-IIb Hayabusa
Type: fighter and fighter-bomber
Accommodation: pilot in the enclosed cockpit
Fixed armament: two 0.5 in (12.7 mm) Ho-103 (Type 1) fixed forward-firing machine guns with 250 rounds per gun in the upper part of the forward fuselage with synchronisation equipment to fire through the propeller disc
Disposable armament: up to 1,102 lb (500 kg) of disposable stores carried on two hardpoints (both under the wing with each rated at 551 lb/250 kg), and generally comprising two 551 or 132 lb (250 or 60 kg) bombs
Equipment: standard communication and navigation equipment, plus a reflector gun sight
Powerplant: one Nakajima Ha-115 (Army Type 1) air-cooled 14-cylinder two-row radial piston engine rated at 1,150 hp (857 kW) for take-off, 1,100 hp (820 kW) at 9,350 ft (2850 m), and 980 hp (731 kW) at 18,735 ft (5600 m)
Internal fuel: 124.1 Imp gal (149 US gal; 564 litres)
External fuel: up to 88 Imp gal (105.7 US gal; 400 litres) in two 44 Imp gal (52.8 US gal; 200 litre) drop tanks
Dimensions: span 35 ft 6.75 in (10.84 m); area 230.37 sq ft (21.40 m²); length 29 ft 3.25 in (8.92 m); height 10 ft 8.75 in (3.27 m)
Weights: empty 4,211 lb (1910 kg); normal take-off 5,710 lb (2590 kg); maximum take-off 6,450 lb (2925 kg)
Performance: maximum level speed ‘clean’ 286 kt (329 mph; 530 km/h) at 13,125 ft (4000 m) declining to 251 kt (289 mph; 465 km/h) at sea level; cruising speed 237 kt (273 mph; 440 km/h) at 16,405 ft (5000 m); climb to 16,405 ft (5000 m) in 5 minutes 49 seconds; service ceiling 36,750 ft (11200 m); maximum range 1,727 nm (1,988 miles; 3200 km) with drop tanks; typical range 950 nm (1,094 miles; 1760 km) with standard fuel