Japan’s Forgotten Flying Boats Pioneer

In the early 20th century, Japan explored the potential of flying boats for its naval forces. As an island nation, flying boats offered strategic advantages, but the country initially lacked facilities and expertise to produce them on a large scale. To address this, the Hiro Naval Arsenal was established in August 1920, becoming Japan’s first dedicated aircraft repair and manufacturing facility for naval use.

At that time, Japan’s other naval aircraft factories, located at Yokosuka and Sasebo, primarily served as repair facilities and were not designed for full-scale aircraft production. The new Hirosho factory, completed in October 1921, filled this gap. Its first project involved producing a licensed version of the British-designed Felixstowe F.5, a twin-engine flying boat regarded as one of the best of its kind during World War I.

Felixstowe F.5: The Start of Production

The Felixstowe F.5 was introduced to Japan with assistance from engineers at Short Brothers, one of the companies responsible for its design. Partially assembled F.5s were shipped to Hiro, allowing Japanese engineers to gain experience and develop the skills needed for independent production. By late 1921, Hiro began full-scale production of the aircraft. The initial models were equipped with imported Rolls-Royce Eagle engines, each generating 360 horsepower. Later, Hiro began producing 400-horsepower Lorraine engines under license.

In 1925, Hiro engineers experimented with upgrading the F.5 by installing more powerful 450-horsepower Lorraine engines. These modified aircraft were designated as the F.1 and F.2 models. Despite these enhancements, the airframe design was considered outdated by the mid-1920s. While the modified F.5 variants were not adopted for service, they provided valuable experience for future projects.

Experimenting with All-Metal Flying Boats

As Japan’s aviation industry matured, attention turned to all-metal aircraft construction. In the early 1920s, the Japanese Navy collaborated with Germany’s Rohrbach company to study advanced construction techniques. Japanese engineers and officers visited Rohrbach’s facilities to learn about these innovations. By 1925, components for an all-metal flying boat based on Rohrbach’s designs were shipped to Japan for evaluation.

Hiro’s engineers created a prototype modeled on the Rohrbach R1, which was tested extensively for its structural and flight characteristics. Around the same time, a rivalry emerged between Hiro and Mitsubishi to become the Navy’s preferred flying boat manufacturer. Mitsubishi established a facility in Berlin, the Mitsubishi-Rohrbach Company, to produce components for their experimental designs. In response, Hiro developed its own prototype, designated the R3.

The Hiro R3: Lessons Learned

The Hiro R3 represented a blend of German and Japanese engineering philosophies. Powered by two 450-horsepower Lorraine engines, the R3 incorporated several design changes to distinguish it from the original Rohrbach concept. It featured rounded wingtips, a larger hull, and redesigned wing floats to improve takeoff performance. Despite these improvements, the R3 struggled with weight issues that reduced its climb rate and takeoff capacity. Additionally, it performed poorly in rough seas. These shortcomings prevented the R3 from entering production, but the project provided valuable experience that influenced Japanese aircraft development for years to come.

Transition to Indigenous Designs

By the mid-1920s, the Japanese Navy decided it was time to move beyond license-built aircraft and develop an original flying boat design tailored to its specific needs. Using insights gained from the Felixstowe F.5 and the R3, Hiro’s engineers began work on a new project. The resulting aircraft, completed in 1927, was officially adopted as the Type 15-1 Flying Boat, more commonly known as the H1H1.

The H1H1 retained some design elements from the F.5, such as its wooden hull, but introduced new features, including a lightweight single-bay biplane wing structure. Early production models used bench-type aileron balancers, which were later replaced with more effective horn-balanced ailerons. The aircraft was powered by 450-horsepower Lorraine engines, the same type used in the R3. Production initially took place at the Aichi factory in 1927 and later moved to the Hiro facility in 1929 after the completion of the F.5’s production run.

Advancements with the H1H2

As production of the H1H1 progressed, Hiro experimented with an upgraded version, designated the H1H2. This model featured an all-metal hull reinforced with external stiffeners. Although the H1H2 was slightly slower than the H1H1 due to its increased weight, it was significantly more durable and performed better in rough water conditions. These improvements demonstrated Hiro’s growing expertise in aircraft design and manufacturing.