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 Mark13 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. Steering mechanism There are three separate steering systems: The full steering system: The vertical rudder system steers the torpedo left or right by switching the rudders to one of full-right, neutral or full-left in response to signals from the gyroscope. This system responds relatively slowly to deviations from the correct running direction. The proportional steering system: The horizontal rudder system changes the angle of the rudders to cause the torpedo to run at a deeper or shallower depth in response to signals from the depth meter. This system has a moderately rapid response to deviations from the appropriate running depth. The angular velocity steering system: The two roll-rudders switch to one of full-up, neutral or full-down in response to signals from the anti-rolling controller. When the controller detects the torpedo returning to the centre position, the system counter-steers the roll rudders in opposite directions. This system has a rapid response to deviations in running direction. The three systems operate simultaneously to maintain the appropriate direction, depth and attitude of the torpedo while running. Production sites The Type 91 was researched and developed at Yokosuka Naval Arsenal in Kanagawa Prefecture.