Bücker Bü 131 Jungmann Kyūshū K9W1 Momiji

In the late 1930s the Imperial Japanese Navy was looking for a replacement for its ageing Type 3 land-based primary trainer. According to Japanese Wikipedia, in 1938 the Navy tested a Bü 131 Jungmann that a private trading company had bought; History of War says that Bücker demonstrated a Jungmann and a Jungmeister in Japan that year and that the Navy bought the Jungmann. The trials went well: good performance, pleasant handling and low manufacturing and maintenance costs. In 1939 twenty more were bought for further evaluation as the Navy Experimental Type Bu trainer. In Navy service they carried the designation KXBü1, and at least one wore either naval grey or RLM 63 delivery colour, depending on the source.

The Navy was reluctant to rely on foreign designs before committing to a licence. History of War records that it asked Hitachi and Watanabe for a trainer similar to the Jungmann yet different enough not to count as a copy. Both firms produced prototypes in 1941, Watanabe building one monoplane and one biplane, but none could match the German original, so a Japanese representative went to Germany to buy a production licence. Japanese Wikipedia dates to June 1940 the Navy's order to the Watanabe ironworks (Watanabe Tekkōsho, later Kyūshū Hikōki) to put the Bü 131 into domestic production; Watanabe obtained manufacturing rights from Bücker and completed the first prototype in August 1941.

Seventeen prototypes and pre-production aircraft followed. Production began in 1942, before official adoption: History of War dates entry into series production as the K9W1, Navy Type 2 Primary Trainer Model 11, to August 1942. Formal adoption came in June 1943, and under the system of kanji names introduced that year the aircraft became the Kōyō (or Momiji, "maple") Model 11. The Allies gave it the code name "Cypress". When Watanabe changed its name to Kyūshū it kept the manufacturer letter W, which is why the aircraft is usually called the Kyūshū K9W1.

The K9W1 hardly departed from the original: a fabric-covered steel-tube fuselage (partly metal-skinned), fabric-covered wooden wings, Japanese instruments and interior equipment and, above all, the Hitachi Hatsukaze 11 (GK4A), an air-cooled inverted four-cylinder inline engine rated by the sources at 105 or 110 hp. It flew so well that some thought it too capable for a primary trainer. It had two weaknesses: it was so light that turbulence tossed it about, and its engine ran somewhat unevenly and vibrated heavily. Japanese Wikipedia gives a length of 6.62 m, a span of 7.37 m, a height of 2.67 m, an empty weight of 407.2 kg, a normal weight of 638.5 kg, a maximum speed of 182 km/h and a range of 641 km, with a crew of two and no armament.

Production figures disagree. Japanese Wikipedia and the Japanese website on Combined Fleet aircraft give 277: 217 by Watanabe (prototypes included) and 60 by Hitachi. History of War, the English and German Wikipedias and Dave's Warbirds give about 339, the figure History of War considers most likely. According to Japanese Wikipedia, production was halted in 1943: as front-line aircraft grew ever more demanding, a primary trainer of this kind lost its purpose.

The K9W1 was the Navy's standard primary trainer and was still in service in 1945. Aviation of Japan documents two aircraft of the 381st Kōkūtai at Tebrau, Malaya, in 1945, painted dark green over orange-yellow and carrying tail codes B2-20 and B2-21; at least one of them was test-flown by the RAF, brought back to the United Kingdom and kept at RAF Wroughton until it was scrapped in 1957 after being damaged in a fire. The Momiji's success prompted the Army to adopt the design, but instead of sharing the Navy's line it ordered its own version from Kokusai, the Ki-86, of which 1,037 were built.