Kawanishi H8K

Soon after the first three Kawanishi H6K2s entered service in January 1938, the Imperial Japanese Navy issued a development contract to the company for a new large flying-boat to serve as a replacement for that aircraft, appreciating that it would be two or three year before a prototype would emerge. The estimate was fairly accurate, the Kawanishi H8K1 prototype making a first flight on the last day of December 1940, and while like its predecessor it was powered by four engines, in other respects it differed considerably. The high-set cantilever monoplane wing tapered in both chord and thickness from wing root to wingtip and served also to mount strutted and braced underwing stabilising floats at about two-thirds span. The hull was more conventional, losing the graceful lines of the H6K, and mounted at its rear a tail unit with single fin and rudder. Power for the three prototypes and early production aircraft was provided by four Misubishi MK4A Kasei (Mars) 11 radial engines mounted in nacelles at the wing leading edge. Accommodation was provided for a crew of 10, and defensive armament comprised five 20-mm cannon in port and starboard blisters and in nose, dorsal and tail turrets, supplemented by three 7.7-mm (0.303-in) machine-guns in two side hatches and a ventral position. Comprehensive armour protection was provided and the bulk fuel tanks within the hull were partially self-sealing and incorporated a carbon dioxide fire-extinguishing system.

The H8K was, therefore, an advanced aircraft, and designed to a specification that called for performance superior to that of the Short Sunderland. Not surprisingly, Kawanishi was greatly disappointed with early tests which showed that the first of the H8K1 prototypes was dangerously unstable on the water. Modifications were begun immediately to rectify this situation, including an increase in hull depth of 1 ft 9 3/4 in (0.55 m). New tests showed considerable improvement, but the second and third prototypes introduced the deeper hull and additional less major hull modifications, as well as an enlarged vertical tail fin. Service trials conducted with the modified flying-boats showed acceptable water performance, still not as good as that of the H6K, but they demonstrated such marked improvement in flight characteristics that the navy had no hesitation in ordering the type into production in late 1941 under the designation Navy Type 2 Flying Boat, which acquired subsequently the Allied codename 'Emily'. The type remained in operational use until the end of the war, by which time construction of all versions totalled 167.

The initial H8K1 production version (16 built), which was the same as the second and third prototypes, was soon superseded by the major series version, the H8K2 (112 built) which introduced more powerful Mitsubishi MK4Q engines. a revised tail unit, more extensive armament and ASV radar. After being used for service trials, the original H8K1 prototype was given Mitsubishi MK4Q engines and converted for use in a transport role, then being redesignated H8K1-L; it was later developed as a production transport accommodating 29 to 64 passengers according to role, being powered by the MK4Q engines and having armament reduced to one 20-mm cannon and one 13-mm (0.51-in) machine-gun. Ordered into production as the Navy Type 2 Transport Flying-Boat Seiku (clear sky), the variant was built to the extent of 36 aircraft. Two early production examples were used as development aircraft for an improved version with retractable wingtip floats and a retractable dorsal turret, being redesignated H8K3; they were later tested with 1,825-hp (1361-kW) Mitsubishi MK4T-B Kasei 25b engines under the changed designation H8K4, but no production aircraft were built.

Early production aircraft entered service in 1942, the type's operational debut being made on the night of 4-5 March 1942 when two aircraft based at Watje Atoll in the Marshall Islands, some 2,300 miles (3700 km) east of Pearl Harbor, were despatched to make a bombing attack on Oahu Island. This operation involved refuelling from a submarine at French Frigate Shoals, and it seemed unjust that such an ambitious piece of planning was frustrated by heavy cloud cover over the target area. Nevertheless, H8Ks proved highly effective, and the type was deployed on bombing, reconnaissance and transport missions, its heavy defensive armament making it a formidable adversary.

Variants

H8K1 Prototype
    One experimental prototype and two evaluation aircraft.
H8K1 (Navy Flying Boat type 2, Model 11)
    First operative model of series, 14 built.
H8K1-L
    Redesignation of the first prototype, after it was converted into a transport aircraft.
H8K2 (Model 12)
    Version with more powerful engines and major armament, equipped with search radar, 120 built.
H8K2-L Seiku ("Clear Sky"),(Model 32)
    Transport version of H8K1. Armed examples were equipped with two 20 mm Type 99 cannons and transport capacity of 29-64 passengers (36).
H8K3 (Model 22)
    Experimental version, H8K2 modified. Equipped with retractable floats in wingtips, sliding hatch side gun locations in place of the blisters, and a retractable dorsal turret all in an effort to increase speed, 2 prototypes.
H8K4 (Model 23)
    H8K3 with different engines, 2 converted.

Specifications (Kawanishi H8K2)

General characteristics

    * Crew: 10
    * Length: 28.15 m (92 ft 4 in)
    * Wingspan: 38.00 m (124 ft 8 in)
    * Height: 9.15 m (30 ft)
    * Wing area: 160 m² (1,721 ft²)
    * Empty weight: 18,380 kg (40,436 lb)
    * Loaded weight: 24,500 kg (53,900 lb)
    * Max takeoff weight: 32,500 kg (71,500 lb)
    * Powerplant: 4× Mitsubishi Kasei 22 radial engines, 1,380 kW (1,850 hp) each

Performance

    * Maximum speed: 465 km/h (290 mph)
    * Range: 7,150 km (4,440 mi)
    * Service ceiling: 8,760 m (28,740 ft)
    * Rate of climb: 8.1 m/s (1,600 ft/min)
    * Wing loading: 153 kg/m² (31 lb/ft²)
    * Power/mass: 0.22 kW/kg (0.14 hp/lb)

Armament

    * Guns:
          * 5 × 20 mm Type 99 cannon (one each in bow, dorsal, and tail turrets, plus one each in two waist blisters)
          * 5 × 7.7 mm (.303 in) Type 97 machine guns in fuselage hatches
    * Bombs: 2 × 800 kg (1,764 lb) torpedoes or 1,000 kg (2,205 lb) of bombs or depth charges

Avionics

    * Radar: Type 3 Mark 6 Model 4 (Type H6)