The Type 91 was an aerial torpedo of the Imperial Japanese Navy. It was in service from 1931 to 1945. It was used in naval battles in World War II and was specially developed for attacks on ships in shallow harbours. The Type 91 aerial torpedo had two unique characteristics. Firstly, it used wooden stabilizers attached to the tail fins which were shed upon water entry. Secondly, it engaged an angular acceleration control system to control rolling movements, which was very advanced for its time. This system made it possible to release the Type 91 not only at a cruising speed of 330 km/h (180 kn) at an altitude of 20 m (66 ft), but also in a power-glide torpedo-bombing run at the maximum speed of the Nakajima B5N 'Kate', 378 km/h (204 kn) The Type 91 torpedo was an 18 in (450 mm) diameter torpedo, similar in size to other nations. There were five models put into service, with high-explosive warheads weighing from 213.5 to 526.0 kg (471 to 1,160 lb) with effective ranges from 1,500 to 2,000 m (1,600 to 2,200 yd) at 42 kn (78 km/h). Since the Type 91 torpedo was the only practical aerial torpedo of the Imperial Japanese Navy, it was simply known as the Koku Gyorai or "aerial torpedo". Surface warships and submarines used other types of torpedoes, namely the Type 93 and Type 95 respectively, while the Type 97 torpedo was designed for use by midget submarines.
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