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Kawanishi H11K Soku (Blue Sky)

Heavy Flying Boat / Transport Proposal [ 1946 ]

Proposed towards the end of World War 2, the large Kawanishi H11K Soku represented what would have become one of the larger flying boat entries of the conflict - a near-complete mockup was destroyed during an Allied air raid in 1945.

Authored By: Staff Writer | Last Edited: 08/24/2020 | Content ©www.MilitaryFactory.com | The following text is exclusive to this site.

Being an island superpower at the time of World War 2 (1939-1945), the Empire of Japan relied heavily on the capabilities of its flying boat fleet. As the reach of the Empire grew to threaten the American West coast, the Australian mainland, and many parts of Southeast Asia, this fleet of aircraft became evermore critical to the Japanese scope of operations in the Theater.

By this time in history, the Kawanishi concern was an established flying-boat-maker and claimed some of the more successful flying boat designs of the war - the H6K "Mavis" and the H8K "Emily" being two such examples. Before the close of the war in August of 1945, it tried its hand at what would have become the largest flying boat in Imperial Japanese Navy (IJN) service - the oversized H1K "Soku" ("Blue Sky").

Flying boats have always offered several key inherent strategic advantages during war, namely operational range and no restriction on airstrip length. Able to take-off from a water source, the aircraft type could be operated nearly anywhere that the IJN was committed to during the far-reaching conflict. For the new design, the aircraft would be of considerable size in an effort to haul as much war-making goods from Point A to Point B as possible - helping to reinforce Army positions across the Pacific and turn the tide of active battles elsewhere.

One key restriction placed on engineers was in extensive reliance on wood for metal had become a precious commodity in wartime, resource-strapped Japan. Designers returned with a dimensionally large flying boat showcasing a high-wing monoplane form that held outrigger pontoon legs (non-retractable). The fuselage was slab-sided with a deep profile, the flight deck (notable for its range of windows for optimal viewing out-of-the-cockpit) seated atop the frontal section and ahead of the wing mainplanes. The empennage was conventional, tapered to meet the base of the single vertical fin. Horizontal planes were feature at the base of the fin itself. As with other flying boats, the aircraft was given a boat-like hull for water landings and take-offs so no complicated wheeled undercarriage was required. The crew complement numbered five and local defense was to come from 3 x 13mm Type 2 machine guns.

Selected propulsion power was to be generated from 4 x Mitsubishi MK4Q "Kasei 22" (Ha-32-22) radial piston engines developing 1,850 horsepower each and driving four-bladed propeller units. These were seated (as pairs) along the wing leading edges in the usual way - given proper clearance from the spray of the salty ocean.

Internally, the aircraft would feature a smaller upper deck for the flight crew and a lower deck representing the cargo hold. A split-nose assembly gave the needed access to the deep hold which could ferry several military vehicles, some 80 combat-ready troopers, wounded (MEDEVAC role), or comparable cargo loads as needed.

For Kawanishi, the aircraft was designated as the "KX-8" and IJN authorities approved the design direction of the large, much-needed aircraft as the "H11K" - the initial production model to become the "H11K1". A full-scale mockup was ordered and this began construction on Shikoku.

Due to the worsening war situation for Japan heading in 1945, the H11K's mockup was not to be completed until later that spring and Kawanishi became evermore committed to fighter production for the ultimate defense of the Japanese mainland. The final blow for the giant flying boat arrived on April 1st, 1945 as Allied bombs rained down on the Kawanishi facility destroying the nearly-completed mockup. Since there was no time to resurrect the design and interceptors were in dire need, all work was ended on the H11K.

Projected performance for the flying boat included a maximum speed of 292 miles per hour, a cruise speed of 230mph, and an operational range out to 2,415 miles.©MilitaryFactory.com
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Service Year

Imperial Japan national flag graphic
Imperial Japan

Development Ended.


National flag of modern Japan Imperial Japan (cancelled)
(OPERATORS list includes past, present, and future operators when applicable)
Maritime / Navy
Land-based or shipborne capability for operating over-water in various maritime-related roles while supported by allied naval surface elements.
General transport functionality to move supplies/cargo or personnel (including wounded and VIP) over range.
Intelligence-Surveillance-Reconnaissance (ISR), Scout
Surveil ground targets / target areas to assess environmental threat levels, enemy strength, or enemy movement.
X-Plane (Developmental, Prototype, Technology Demonstrator)
Aircraft developed for the role of prototyping, technology demonstration, or research / data collection.

123.7 ft
(37.70 m)
157.4 ft
(47.97 m)
41.2 ft
(12.55 m)
Empty Wgt
58,213 lb
(26,405 kg)
100,421 lb
(45,550 kg)
Wgt Diff
+42,208 lb
(+19,145 kg)
(Showcased structural values pertain to the base Kawanishi H11K Soku (Blue Sky) production variant)
Installed: 4 x Mitsubishi MK4Q "Kasei 22" (Ha-32-22) 14-cylinder air-cooled radial piston engines developing 1,850 horsepower driving four-bladed propellers.
Max Speed
292 mph
(470 kph | 254 kts)
2,485 mi
(4,000 km | 7,408 nm)
855 ft/min
(261 m/min)

♦ MACH Regime (Sonic)
RANGES (MPH) Subsonic: <614mph | Transonic: 614-921 | Supersonic: 921-3836 | Hypersonic: 3836-7673 | Hi-Hypersonic: 7673-19180 | Reentry: >19030

(Showcased performance specifications pertain to the base Kawanishi H11K Soku (Blue Sky) production variant. Performance specifications showcased above are subject to environmental factors as well as aircraft configuration. Estimates are made when Real Data not available. Compare this aircraft entry against any other in our database or View aircraft by powerplant type)
3 x 13mm Type 2 Heavy Machine Guns (HMGs) for local defense.

For offensive operations, internal war load of torpedoes, depth charges, and conventional drop bombs. Otherwise cargo hold for transporting vehicles, wounded, combat infantry, or comparable cargo loads.

Supported Types

Graphical image of an aircraft medium machine gun
Graphical image of an aircraft heavy machine gun
Graphical image of an aircraft conventional drop bomb munition
Graphical image of an aircraft aerial torpedo
Graphical image of a naval depth charge

(Not all ordnance types may be represented in the showcase above)
Hardpoint Mountings: 0

H11K "Soku" - Base Series Designation; partial mockup was all that was had of the program.

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Images Gallery

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Image of the Kawanishi H11K Soku (Blue Sky)
Image from the Public Domain.

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