Japanese military aircraft designation systems
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Japanese military aircraft, especially of the Imperial (pre-1945) period, are rather difficult to keep track of by their Japanese designations, primarily because multiple designation systems were in use by each armed service. This is what led to the Allies' use of codenames during World War II, and these codenames are still better known in English-language texts than the real Japanese names for the aircraft. Articles on Japanese aircraft in Wikipedia are titled by the manufacturer and short designation code.
Thus, the following may prove helpful. The confusion is not so much that any of the designation schemes are difficult, but that a number of different schemes were simultaneously in use.
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Imperial Japanese Navy
The Imperial Japanese Navy used several different aircraft designation systems simultaneously. Between 1931 and 1945, aircraft had Shi numbers designating the specification they were designed to. They also had a long form of Type and Model Number system used between 1920 and 1943, a short designation system akin to that of the United States Navy in use between the late 1920s and 1945, a system of popular names introduced to replace type numbers from 1943 through 1945, and finally a SADP (Service Airplane Designation System) number used from 1939 onward.
Specification Shi numbers
Japanese Navy specifications from 1931 were given an experimental, or Shi number, based on the year of the Emperor's reign the specification was issued in. Since multiple specifications could be issued in a year, the number was disambiguated with the aircraft purpose.
During the period this designation system was in use, the Emperor in question was Hirohito, the Showa Emperor, thus the years of Showa were those used, which began in 1926.
Thus, the Mitsubishi Zero was designed to meet the 1937 specification called 12-shi carrier fighter.
Long Type and Model Number system
After 1920, aircraft types were given a type number based on the last two digits of the year as counted from the mythical founding of Japan in 660BC by Emperor Jinmu. Added to this was a brief description of the aircraft's function. The Mitsubishi Zero was so-called because entered service in 1940 which was the Japanese year 2600, thus it was designated Type 0 Carrier Fighter.
Model numbers were added to show subtypes. By the late 1930s these were two digits, the first being airframe revisions, the second engine revisions.
The system was abandoned in 1943, when it was decided that it gave away too much information about the aircraft.
Short system
In the late 1920s a short designation scheme was adopted, which was strikingly similar to the United States Navy's 1922-64 system. This scheme used a letter or two letters to designate a type of aircraft, a number to indicate the number in series of that type of aircraft, and finally a letter to designate the manufacturer. Unlike the US Navy system, the Japanese system did not have a different number series for each manufacturer; also, the number 1 was not omitted.
Thus, the Zero's type in this designation system was A6M, which meant the sixth type of carrier fighter under this designation system, and that it was built by Mitsubishi.
Variants were indicated by an additional number at the end; repurposing an aircraft was indicated by a dash and then the new type letter.
Sometimes two aircraft were ordered from different manufacturers to the same specification at the same time, generally as insurance against the primary design not working out. In this case, the same series number was used for both.
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Popular names
After July 1943, names were given to Navy aircraft instead of type designations. These names were given according to a scheme based on the aircraft's role.
- Fighters: Weather and meteorological names
- Carrier fighters and seaplane fighters: Wind names usually ending with pu or fu (風)
- Interceptors: Lightning names ending in den (電)
- Night fighters: Light names ending in ko (光)
- Attack planes: Mountains names ending in zan (山)
- Bombers: Star or constellation names usually ending in sei (星)
- Patrol: Sea or ocean names ending in kai (海)
- Reconnaissance: Cloud names ending in un (雲)
- Trainers: Trees, plants and flowers
- Transports: Sky names ending in ku (空)
- Miscellaneous: Landscape names
Two special cases were the Ohka ("Cherry blossom") and the Kikka ("Orange blossom"). Both types were designated "special attackers", meaning kamikaze weapons - named for the fruit trees in the gardens of the Imperial Palace.
Imperial Japanese Army
The Japanese Imperial Army used a straightforward system based on year of service and type, nearly identical to the Navy's long type and model number system. This system was used from 1927. The "ki", or airframe designation was also used and became prominent in later years.
Long Type and Model Number system
The first part of the designation was a two-digit type number based on the Japanese year in which the aircraft entered service. A minor exception was the year 1940 (2600), for which the type number 100 rather than zero was used. This was followed by a description of the aircraft's function. If there were two or more aircraft with the same type and function, the latter was enhanced to further differentiate them. An example is the Type 2 single-seat fighter (the Nakajima Ki-44) and the Type 2 two-seat fighter (Kawasaki Ki-45).
Major modifications (such as a different engine) were indicated with a subtype number, officially in kanji but often in Roman numerals. Small-scale modifications (such as armament) are indicated with a Japanese ordinal (甲,乙,丙), or "kai"(改) if the modification was large but not enough for a new type number.
Short designation ("Ki" number)
The "Ki" (キ) airframe designation indicates the project number (written in Arabic numerals), and was assigned in sequence to all projects regardless of manufacturer or type. It was originally used only when the aircraft was under development, but towards the end of the war became the standard designation even for operational aircraft.
Popular names
Popular names such as "Hayabusa" (the Nakajima Ki-43)were not part of the official designation.