The Nakajima Ki-84 Hayate (gale) was designed as successor to the Nakajima Ki-43 Hayabusa, and was known to the Allies by the reporting name ‘Frank’. The type was one of the best fighters available to the Imperial Japanese army air force in the closing stages of the Pacific War of World War II, and its high level of basic capability was boosted by the fact that, unlike several other high-performance Japanese fighters, it was available in useful numbers and without the teething problems that affected many of these other types. The Hayate was feared by Allied aircrews as it was well protected, well armed, fast and manoeuvrable, all features that were admirably demonstrated as this fighter gave a good account of itself in the desperate battles over the Philippines, the Ryukyu Islands and the Japanese home islands. Japan’s faith in the Ki-84 is emphasised by the fact that at the close of the war the type was in production in underground factories with a planned rate of 200 aircraft per month.
The Ki-43 was just entering combat service as the air headquarters of the Imperial Japanese army ordered Nakajima to start work on the creation of a successor type satisfying a specification that demanded a long-range fighter of the general-purpose type with a maximum speed of at least 345.5 kt (398 mph; 640 km/h) and preferably 367 kt (422.5 mph; 680 km/h), an endurance of 90 minutes at combat power at a radius of 216 nm (248.5 miles; 400 km), a wing loading of not more than 348 lb/sq ft (170 kg/m²) with a wing area of between 204.52 and 226.05 sq ft (19.00 and 21.00 m²), the powerplant of one Nakajima Ha-45 (Army Type 4) air-cooled 18-cylinder two-row radial engine, a fixed forward-firing armament of two 20 mm Ho-5 cannon and two 0.5 in (12.7 mm) Type 1 (Ho-103) machine guns, armour protection and self-sealing fuel tankage.
The task of designing of what emerged as the Ki-84 was entrusted early in 1942 to a team under the leadership of T. Koyama as project engineer, and reached a first culmination in March 1943 with the completion of the first of two prototypes. The Ki-84 was a conventional low-wing cantilever monoplane of basically all-metal stressed-skin construction with fabric-covered control surfaces and the powerplant of one Nakajima Ha-45 Model 11 engine rated at 1,800 hp (1342 kW) and driving a four-blade metal propeller of the constant-speed type. Like Koyama’s earlier Ki-62 and Ki-63 projects, the Ki-84 was characterised by the location of its horizontal tail surface slightly ahead of the vertical tail surface. The muzzles of the Ho-103 machine guns, which were installed above the engine in the forward fuselage, protruded from the engine cowling, and the Ho-5 cannon were mounted in the leading edges of the dihedralled wing, which was tapered in thickness and chord to its rounded tips, outboard of the pivot points for the inward-retracting units of the fully retractable tailwheel landing gear. All three landing gear units were fully covered by doors when retracted, virtually the full span of the wing’s trailing edges was occupied by outboard ailerons and inboard flaps of the ‘butterfly’ type with a combat setting to enhance agility, provision was made under the fuselage centreline for a single drop tank, and a large exhaust collector was mounted on each side of the engine behind the cowling gills.
The Ki-84-a first prototype recorded its maiden flight at Ojima airfield in April 1943, and was joined in the manufacturer’s flight test programme by the second prototype during June of the same year. The flight test programme moved forward swiftly and without major problem, and the two prototypes, together with some of the first machines of a pre-production and service test batch of 83 Ki-84-b and Ki-84-c aircraft, were delivered to the Tachikawa Army Air Arsenal for service tests. By now fully cognisant of the importance of speed and protection as a vital adjunct of combat survivability, the pilots of the Imperial Japanese army air force commented enthusiastically on the new fighter’s performance even though the maximum speed was below that specified. During its flight test programme the Ki-84 reached a maximum speed of 337 kt (388 mph; 624 km/h), climbed to 16,405 ft (5000 m) in 6 minutes 26 seconds, and attained a service ceiling of 40,680 ft (12400 m), thereby revealing the best performance of any Japanese fighter suitable for immediate production.
The 83 aircraft of the first service trials batch, completed between August 1943 and March 1944, revealed a number of differences between them in minor details but, progressively, fuselage changes were incorporated to ease production, and the shape and area of the vertical tail surface were modified to improve control on take-off as the aeroplane was found to suffer from propeller-induced torque. Some of the aircraft were fitted also with two under-wing racks, for bombs up to 551 lb (250 kg) in weight, in addition to the fuselage centreline hardpoint used to carry a drop tank, while still other aircraft were used to test partially retractable ski landing gear.
The first unit to fly the Ki-84, even before the Hayate was placed in quantity production, was an experimental chutai (squadron) which conducted intensive service tests in Japan under operational conditions, starting in October 1943. In view of the promising results of these tests, the Ki-84 was accepted for full service operation, and Nakajima was ordered to begin mass production of the new warplane as the Army Type 4 Fighter Model 1A Hayate (short designation Ki-84-Ia).
The manufacture of a second pre-production batch, in this instance of 42 aircraft, had been started in April 1944, and production aircraft were built alongside these machines. Both types were fitted with individual ejector exhaust stacks providing some thrust augmentation, and the centreline hardpoint under the fuselage was deleted, each of the two under-wing hardpoints being adapted to carry either a 44 Imp gal (52.8 US gal; 200 litre) drop tank or a 511 lb (250 kg) bomb.
In March 1944, the experimental chutai which had undertaken the Ki-84’s service trials was disbanded and most of its personnel transferred to the 22nd Sentai (wing). Equipped with Hayate fighters off the definitive production line, this unit moved to China where it was pitted against the US 14th Army Air Force. During the five weeks preceding its later transfer to the Philippines, the 22nd Sentai did much to establish the Ki-84-Ia as a formidable warplane possessing most of the virtues and few of the vices of earlier Japanese fighters, and in the process comparing favourably with the best Allied fighters. In the Philippines, where the aeroplane received the Allied reporting name ‘Frank’, 10 sentais fought desperately in an effort to halt the Allied offensive. Japan had lost the strategic initiative, however, and under the exacting conditions then prevailing the outnumbered Hayate fighters suffered from failures of fuel pressure and hydraulic systems and from weak main landing gear units.
In Japan, Nakajima tried to boost the production rate by inaugurating a second manufacturing line at its Utsonomiya plant and, despite the lack of skilled workers and limited supplies of aluminium and special steel alloy, delivered 373 examples of the Ki-84 during December 1944, the highest monthly production rate of any aeroplane for the Imperial Japanese army air force. Construction of the Hayate was also assigned to Mansyu Hikoki Seizo K.K. which, starting in the spring of 1945, built 95 examples of the Ki-84 in its Harbin plant in Manchuria. The production version of the Ki-84-I was powered by the Ha-45 (Army Type 4) engine rated at 1,900 hp (1416.5 kW), early production machines using the Model 11 or Model 12 variants of this engine with take-off ratings of 1,800 and 1,825 hp (1342 and 1361 kW) respectively, and late production aircraft receiving the Model 21 engine rated at 1,990 hp (1416.5 kW), in each case driving a four-blade propeller of the constant-speed type.
The Ki-84-Ia was followed into production by the Army Type 4 Fighter Model 1B Hayate (Ki-84-Ib) that differed in having the two 0.5 in (12.7 mm) machine guns over the engine replaced by two 20 mm Ho-5 cannon to give this improved fighter the heavyweight fixed forward-firing armament of four 20 mm cannon. A few examples of the Army Type 4 Model 1C Hayate (Ki-84-Ic) were also completed to a specialised bomber-destroyer standard with the fixed forward-firing armament of two 20 mm Ho-5 synchronised cannon over the engine and two 30 mm Ho-105 cannon in the leading edges of the wing.
The sudden loss of fuel pressure was a constant source of difficulty, the problem not being solved until the introduction of Ha-45 Model 23 engine, a unit rated at 1,900 hp (1416.5 kW) and developed as a version of the Ha-45 Model 21 with a low-pressure fuel-injection system. Then the whole of the Ki-84 manufacturing effort was slowed by the destruction of Nakajima’s engine factory at Musashi by a bombing raid by the US 20th Army Air Force: production of the Ha-45 engine was transferred to an underground factory at Asakawa and a new plant at Hamamatsu, but never again reached its previous peak.
By this stage in World War II, the Japanese were beginning to suffer from an increasing shortage of strategic materials of all forms, and in an effort to reduce demands on the nation’s dwindling supply of aluminium, Nakajima developed the Hayate Kai incorporating a rear fuselage, wing tips and a number of fittings of wooden construction, all manufactured in a shadow factory at Tanuma. Powered by the Model 21, Model 23 or Model 25 variants of the Ha-45 engine, this type was known as the Ki-84-II in Nakajima’s company nomenclature but, depending on the armament fitted, retained the Ki-84-Ib or Ki-84-Ic designations in the official army nomenclature.
Deliveries of the Ki-84-I and Ki-84-II in their production forms totalled 3,288 aircraft by Nakajima between April 1944 and August 1945, while Mansyu contributed another 94 Ki-84-I aircraft in 1945, as noted above.
The Ki-84-III was schemed as a high-altitude version, powered by the Ha-45 Ru engine with a turbocharger mounted in the lower part of the fuselage, but this variant was still on the drawing board when World War II ended. Plans were also made for a two-seat version with dual controls for the advanced training and conversion roles, but none was built. Instead some two-seat examples of the Ki-84-I, without dual controls, were produced in the field by removing the back armour plating and radio equipment to provide the volume needed for the installation of a ‘jump’ seat behind the pilot’s seat.
Although the Hayate Kai represented a step in the right direction in terms of conservation of strategic light alloys, it was felt that further saving of aluminium would result from a complete redesign of the airframe, and the Japanese ministry of munitions allocated to Tachikawa Hikoki K.K. the task of designing an all-wood version of the Ki-84. Three airframes for what was designated as the Ki-106 were built for Tachikawa by Ohji Koku K.K. (Prince Aircraft Co. Ltd.) at Ebetsu on Hokkaido island in 1945, and these machines were completed by Tachikawa with the powerplant of one Ha-45 Model 21 engine rated at 1,990 hp (1484 kW). The Ki-106 was characterised by an enlarged vertical tail surface and the notably smooth external finish obtained by applying a thick coat of lacquer over the plywood skin. During flight trials the skin bonding had to be strengthened, but the flight test programme from July 1945 proved satisfactory in all other respects. The fixed forward-firing armament comprised four 20 mm Ho-5 cannon in the first Ki-106 but, to save weight, this was reduced to just two cannon in the second and third aircraft. Tachikawa were working on a weight-reduction programme when the end of the war brought the Ki-106 project to a halt.
The data for the Ki-106 included a span of 36 ft 10.5 in (11.24 m) with area of 226.04 sq ft (21.00 m²), length of 32 ft 7.75 in (9.95 m), height of 11 ft 9.25 in (3.59 m), empty weight of 6,499 lb (2948 kg), normal take-off weight of 8,598 lb (3900 kg), maximum speed of 334.5 kt (385 mph; 620 km/h) at 21,000 ft (6400 m), climb to 16,405 ft (5000 m) in 5 minutes 0 seconds, service ceiling 36,090 ft (11000 m), and range of 432 nm (497 miles; 800 km) plus 1 hour 30 minutes of combat.
Several versions of the Ki-84 were designed during the last year of the war. These included the Ki-84N high-altitude interceptor with the powerplant of one Nakajima Ha-44 Model 13 (Ha-219) radial engine rated at 2,500 hp (1864 kW) and the wing area increased to 249.19 sq ft (23.15 m²). This was in the initial design phase at the time of the war’s end and was scheduled to go into production as the Ki-117. The Ki-84P was another projected high-altitude version also powered by the Ha-44 Model 13 engine but with a wing increased in area to 263.72 sq ft (24.50 m²), though this project was then abandoned in favour of a less ambitious Ki-84R using a standard Ki-84-I airframe married to the Ha-45 Model 44 engine rated at 2,000 hp (1491 kW) and fitted with a mechanical two-stage three-speed supercharger.
The Ki-113 was a variant of the Ki-84-Ib designed in the autumn of 1944 with the Ha-45 Model 21 engine but a greater proportion of steel in its structure: carbon steel for the cockpit section, ribs and bulkheads, and steel sheet skinning. A Ki-113 prototype was completed in early 1945, but was not flown as it was decidedly overweight.
The Ki-116 was the last variant of the Hayate to enter flight trials as a prototype created out of the fourth Mansyu-built Ki-84-I adapted with the Mitsubishi Ha-33 Model 62 (Ha-112-11) air-cooled 14-cylinder two-row radial engine rated at 1,500 hp (1118.5 kW) and driving a three-blade propeller borrowed from a Mitsubishi Ki-46-III. The conversion was very successful, a reduction of some 926 lb (420 kg) in empty weight being achieved, and the variant was undergoing tests at the time of Japan’s final defeat. The data for the Ki-116 included a span of 36 ft 10.5 in (11.24 m) with area of 226.04 sq ft (21.00 m²), height of 11 ft 4 in (3.45 m), empty weight of 4,938 lb (2240 kg), and normal take-off weight of 7,039 lb (3193 kg).
Nakajima Ki-84-Ia Hayate
Type: fighter and fighter-bomber
Accommodation: pilot in the enclosed cockpit
Fixed armament: two 20 mm Ho-5 fixed forward-firing cannon with 150 rounds per gun in the leading edges of the wing, and two 0.5 in (12.7 mm) Ho-103 (Type 1) fixed forward-firing machine guns with 350 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 unit rated at 551 lb/250 kg), and generally comprising two 551, 220 or 66 lb (250, 100 or 30 kg) bombs
Equipment: standard communication and navigation equipment, plus a reflector gun sight
Powerplant: one Nakajima Ha-45 (Army Type 4) Model 23 air-cooled 18-cylinder two-row radial piston engine rated at 1,900 hp (1417 kW) for take-off and 1,670 hp (1245 kW) at 4,725 ft (1440 m)
Internal fuel: 162.8 Imp gal (195.5 US gal; 740 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 36 ft 10.5 in (11.24 m); area 226.04 sq ft (21.00 m ); length 32 ft 6.5 in (9.92 m); height 11 ft 1.25 in (3.385 m)
Weights: empty 5,864 lb (3890 kg); normal take-off 7,955 lb (3613 kg); maximum take-off 8,576 lb (3890 kg) normal and 9,193 lb (4170 kg) overload
Performance: maximum level speed ‘clean’ 340 kt (392 mph; 631 km/h) at 20,080 ft (6120 m) declining to 282 kt (325 mph; 523 km/h) at sea level; cruising speed, maximum 240.5 kt (277 mph; 445 km/h) at optimum altitude and 205 kt (236 mph; 380 km/h) at 13,125 ft (4000 m); initial climb rate 3,789 ft (1155 m) per minute; climb to 16,405 ft (5000 m) in 5 minutes 54 seconds; service ceiling 34,450 ft (10500 m); maximum range 1,170 nm (1,347 miles; 2168 km) with drop tanks; typical range 914.5 nm (1,053 miles; 1695 km) with standard fuel