Tried And Tested Pt.1

TRIED AND TESTED

A History of the Yokosuka Naval Technical Air Arsenal         

Part 1 Kaigun Kokusho (1932-1939)

The following article first appeared in the issue number #7 of our magazine "Arawasi International", Sept-Dec 2007. It was a collective effort of the editors of the magazine at the time and was planned as a series. Unfortunately, for various reasons it was never completed. We hope to do so on this blog.

Perhaps because they handled the less glamourous aspects of aviation—from research, aircraft and engine manufacture and distribution to repairs and modifications—the IJN’s four Naval Air Arsenals have tended to receive scant attention from aviation historians. But some idea of the importance of these establishments to the war effort can be gauged by the 31,700-strong workforce present at all branches at the end of the Pacific War.

These civilian and military personnel represented the last members of an organization that had not only gone head to head against major aircraft manufacturers for IJN contracts, but also provided those manufacturers with extensive technical assistance. These close ties were mirrored in many military aircraft designs retaining the “Y” identifier—for the key Naval Technical Air Arsenal at Yokosuka, Kanagawa Prefecture—even though production was contracted out to aircraft manufacturers in the civil sector.

The Naval Air Headquarters had assumed responsibility for the management of all naval aviation projects at the time of its inception on April 1, 1927. Five years later, on April 1, 1932, the Kaigun Kôkûshô (Naval Air Arsenal) that formed at Yokosuka came under the Naval Air Headquarters’ umbrella.

Another reason for the dearth of coverage, apart from material only available in Japanese, has been the confusion caused by the changing, shortened forms of the Japanese names that were bestowed on the Yokosuka site. Often the activities of other sites have been erroneously attributed to or described under the catch-all Yokoshô (the shortened Japanese for Yokosuka Naval Air Arsenal) banner, although this was merely one location. Steeped in naval arsenal history since 1869, Yokosuka was to be part of a Naval Air Arsenal setup only early in the organization’s history.

The evaluation of new aircraft was the remit of the Yokosuka Kôkûtai, which was collocated with operational units at nearby Oppama air base. Yokosuka had had aspects of aviation tacked on to its naval arsenal activities since 1913 and its reputation as a research and design establishment dated back to 1917, when it manufactured the first operational naval aircraft designed in Japan.

Yokosuka, 1932-style, acted as the hub for aircraft manufacture and ordnance, aero-engines and flight testing, in collaboration with the Kasumigaura Branch of the Naval Technical Research Institute. Design teams from the Hiro Arsenal were transferred to work under one roof at the Kôkûshô, as it came to be called.

Under its first commander, Rear Admiral Edahara Yurikazu, the Kôkûshô was organized into seven departments:

1. Sômubu – Administrative Affairs Department

2. Kagakubu – Science Department (research into aircraft components and performance; aircraft testing)

3. Hikôkibu – Aircraft Department (aircraft design; component research; aircraft building and maintenance; armament design)

4. Hatsudôkibu – Engine Department (aircraft engine design; research and testing of fuel and lubricants; design, manufacture and maintenance of engines)

5. Heikibu – Weapons Department (research and testing of aircraft weapons)

6. Hikôjikenbu – Aircraft Testing Department (aircraft flight testing and research; test the performance and basic flying characteristics of new aircraft immediately after their delivery from the manufacturers; accident investigation). 

7. Imubu – Medical Affairs Department

This was to remain unchanged until April 1, 1939, when the organization’s title was changed to the Kaigun Kôkû-Gijutsushô (Naval Technical Air Arsenal, or Kûgishô), the name it was to retain until just a few months before the end of hostilities in 1945.

Industry Links (1): Mitsubishi

The relationship between the Kôkûshô and industry became closer as the 1930s progressed and Japan became enmeshed in a conflict with China, although it was not until July 7, 1937, that IJNAF units were to be involved in a major action.   

In 1932, the year in which the Kôkûshô was established, the Naval Aircraft Establishment was organized and what turned out to be the overly ambitious 7-Shi equipment acquisition programme was initiated. The IJN’s requirements for a series of aircraft ranging from carrier-based fighters and bombers to reconnaissance seaplanes were to remain unfulfilled, as the Kawanishi E7K1 reconnaissance seaplane was the only design placed in quantity production and the Hiro G2H1 attack bomber was produced in limited numbers.

The efforts that were put into avoiding a repeat of this situation gave renewed vigour to the Japanese aviation industry. So much so that just two years later the 9-Shi (1934) programme resulted in several famous Japanese aircraft entering production, including the Mitsubishi A5M series of carrier-based fighters, later allocated the Allied codename Claude, and the Mitsubishi G3M (Nell) attack bomber. The availability in numbers of these two types in particular permitted the Japanese military to pursue its military ambitions when the Sino-Japanese War flared up in earnest in July 1937.

Tail markings

Most Mitsubishi aircraft delivered to the Kûgishô received a tail marking comprising the following:

1. Katakana “コ” (KO) denoting Kôkûshô

2. First two digits of the aircraft’s short designation

3. A number that either showed the number of prototype (1st, 2nd, 3rd, etc.) or the last, usually two, digits of the aircraft’s serial number

Exceptions to the above rule are seen below in the B5M1 “KO-361” and the F1M2 “KO-S21.” The significance of these markings and the reason why these particular aircraft received these tail marking is unknown.

Mitsubishi Type 0 Observation Seaplane (F1M2, Pete)

One of the first F1M2 prototypes, 1938. Note the twin-blade propeller and the absence of spinner.

(Photos: Encyclopedia Vol. 1)

Mitsubishi Type 97 Carrier Attack Bomber (B5M1, Mabel)

One of the two B5M1 prototypes, which were put through comparative trials with a Nakajima design for nine months from February 1937.

(Photo: Arawasi)

Mitsubishi J2M1 Raiden (Jack), s/n 706

Sixth prototype of the 14-Shi interceptor fighter that was with the Aircraft Testing Dept. at Oppama air base, Kanagawa Prefecture, July 1942.

There are no known photos of the first five prototypes. Note the shape of the cowling and the canopy, which gave the impression of a racer and was not actually liked by pilots accustomed to the A6M Zero-sen. The aircraft was overall light Mitsubishi gray (hairyokushoku). Top test pilot Kofukuda and 381st Kôkûtai veterans remember this aircraft as being overall orange, so perhaps it was over-painted at some stage.

(Photos: Encyclopedia Vol. 1)

For more on the particular "Raiden", check HERE.

Mitsubishi J2M3 Raiden-kai (Jack), s/n 3034

Test production (34th prototype) at Oppama air base, late 1943.

J2M3s had a thick bulletproof windshield and the main fuel tank of the fuselage was protected by rubber sheeting combined with layers of rubber sheet and sponge. The radio antenna was raked forward to make the line longer and improve the quality of the reception.

(Photos: Encyclopedia Vol. 1)

(Photos: FAOW#61)

For more on the particular "Raiden", check HERE.

Mitsubishi K7M1

The second of only two prototypes built in 1938, both of which were assigned to the Yokosuka Kôkûtai for liaison and transport duties.

(Photo: Encyclopedia Vol. 1)

Mitsubishi Type 1 Land-Based Attack Bomber (G4M3, Betty)

The overall-orange third prototype G4M3 after delivery to Kûgishô, February 1944. “G4” is the shortened aircraft designation and “33” is the last two digits of the serial number (all G4M3 prototypes received their tail number in the 30s).

The main differences between the G4M2 and G4M3 were: self-sealing wing fuel tanks; different tail gun position; higher-angled horizontal tail surfaces; the wing-root fillet and airframe differences. However, this early G4M3 prototype has exactly the same fuselage as a G4M2 and only has the G4M3’s wing-root fillet.

(Photos: Encyclopedia Vol. 1)

Mitsubishi G6M1 Land-Based Trainer (Betty)

The aircraft was originally conceived by the IJN to carry augmented armament to protect Betty formations. Nevertheless, performance dropped significantly and in the end it was used as an unarmed crew trainer from late 1940. 

The overall paint scheme (NMF) is standard pre-war style, which was discontinued from February 1941. The tail was painted red to make the aircraft more visible in the event of an emergency landing on water.

(Photo: Encyclopedia Vol. 1)

Mitsubishi A7M "Reppu" (Sam)

The third A7M2 with Mitsubishi Ha-43 engine. It was originally the second A7M1 with Nakajima NK9K Homare engine.

(Photo: NARA via Arawasi)