The Hiro navy arsenal performed the trial production of F-5 flying boat, R-3 flying boat, the 15 type flying boat, and the 89 type flying boat, and became the factory which had technology and experience about a flying boat most abundantly. And the trial production of the ambitious three-engine large-sized flying boat was started in the spring of 1930. This flying boat was a great work which should also be called all the first metal large-sized machines that Japanese designed.
The generalization person of design was lieutenant colonel Misao Wada, the design specialist person was major Jun Okamura.
The wing adopted Wagner style web beam box type structure of the Rohrbach flying boat, and the body was the structure into which the structure of the Southampton flying boat and the 89 type flying boat was developed.
In order to determine the form of the body in the design of this flying boat, a tank examination was carried out under the shipbuilding major general Yuzuru Hiraga(battle ship Yamato designer) of a navy technical research center.
The main characteristics of this flying boat were as follows.
1. This flying boat was the first three-engine type, and level flight was possible for it even if which one engine stopped.
2. The linear portion of wings was only a centre wing and the outer wing was a dynamically rational tapered wing.
The engineer was able to pass along the inside of the centre wing and was able to go under each engine.
3. Seaworthiness were fully taken into consideration.
4. The machine gun seat was installed in the nose of an airplane, the side part, and the caudal portion, and there was almost no dead angle.
5. Bomb loadage was 1 ton for the first time in the navy.
The trial production of No. 1 was completed in the Hiro navy arsenal in 1931, and was transported by Yokosuka by air, and the long-term test flight was carried out by Major Daisan Nakajima test pilot.
Although it was decided once that the name of a flying boat would be the 90 type 1go flying boat (H3H1), directional stability was insufficient and there was a problem also in the steering force of an elevator.
Moreover, poor cooling of the enigne occurred.
Three repair was carried out by these problems.
Prototype was called type 1 and after repair aircraft were called type 2, type 3 and type 4 respectively.
An auxiliary vertical stabilizer was added to a horizontal stabilizer of type 3 and type 4 aircraft.
For the setting angle of a horizontal stabilizer, the type of mounting of the support was changed.
Moreover, the radiator was moved to the rear of engine Nasser.
Propellers were also exchanged.
This flying boat was used as a test bed of 14 cylinders air cooling double row radial shape Mitsubishi Shinten 950hp engines, and the last as a test prototype was decorated in 1933.
Source : ENCYCLOPEDIA OF JAPANESE AIRCRAFT 1900-1945, VOL.3 KAWANISHI AND HIROSHO