page_title: Type 91 torpedo
section_title: Kyoban stabilizer plates
hierarchical_section_title: Type 91 torpedo / Main components (Type 91.rev.2) / Kyoban stabilizer plates
context_page_description: 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 aerodynamic 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 180 knots at an altitude of 20 m, but also in a power-glide torpedo-bombing run at the maximum speed of the Nakajima B5N or Kate, 204 knots.
  The Type 91 torpedo was 450 mm in diameter. There were five models put into service, with high explosive warheads weighing 213.5 kg to 526.0 kg and having effective ranges of 1,500 m to 2,000 m at 42 knots.
  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.
context_section_description: The Type 91's tail fins were fitted with Kyoban wooden aerodynamic stabilizer plates.  Introduced in 1936 these stabilized the torpedo in flight and helped to ensure the proper angle for water entry. The plates were designed to shear off on water entry, absorbing energy and preventing the torpedo from diving too deep. The aerodynamic wooden plates stabilized the torpedo in both the vertical and horizontal axes and provided drag to ensure the torpedo struck the water at or near the proper water entry angle despite the inevitable variations in drop altitudes and air speeds encountered in combat.  The structure was simple and worked well, as seen at Pearl Harbor, which was generally considered too shallow for aerial torpedoes prior to the attack.  The Kyoban was so effective the US Navy copied it for their Mark 13 torpedo after observing it in action at the Battle of the Coral Sea.
  Two versions of the Kyoban stabilizer were used: A box-shaped version for single-engine carrier-based torpedo-bombers Nakajima B5N and Nakajima B6N and a cross-shaped version For twin-engine land-based torpedo-bombers G3M, G4M, P1Y, and Ki-67.  The cross-shaped version used longer plates to lower drag resistance but needed more clearance under the fuselage. In the case of land-based torpedo-bomber aircraft, a plate was set inside the bomb bay to smooth the airflow, otherwise the vortex coming in the bomb bay would disturb the torpedo at release.
caption: Type 91 aerial torpedo, tail section and aerodynamic tail stabilizer plates
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