In this new article we are going to tackle several aspects of another interesting physical phenomenon generated by a katana, the tachikaze. Literally meaning "the Sword Wind", it is the peculiar whistling sound a katana makes when a proper cut is made along its relative cutting plane. In a katana with bo-hi (the longitudinal side indent originally created to lighten the blade without compromising its integrity and impact resistance) the tachikaze will be a much stronger sound than in one without; this is due to a complex acoustic and aerodinamic phenomenon that generates small air vortexes or disturbances when the edge of the sword is moving on the correct trajectory, as this interesting article from Scientific American explained in very technical detail. Incidentally, this is also the way in which the sword will move through space and, eventually, through a solid object (our opponent, for example) in the most efficient way, by concentrating all of its force on the smallest point of impact (the edge). To better understand how this is achieved, we have to check the below picture: In the picture you can see how, given any position in space for the katana, exists a circular "cutting plane" originating from its edge and of which the sword itself is the radius. In a correct cut the edge of the sword and the circle originating by the katana as a radius in that particular plane are going in the same direction. This will cause the correct "tachikaze" noise and it will tell the user that that cut would have actually cut its target. However, this also means that if the edge of the sword is offset, even slightly, from the circular cutting plane the katana is moving on during the cut, there will be no tachikaze and the sword would not cut, either properly (imagine entering the opponents body superficially and getting bent and stuck) or at all (bouncing back). This is exemplified, in an exaggerated way, by the next drawing: As you can see, the orientation of the edge must be straight relative to the cutting plane of the sword (represented here still by the dotted lines) for it to be cutting efficiently and produce the tachikaze. This is why, in competitions and in higher level exams, as well as in our everyday practice, a good importance is given to reach the correct tachikaze sound; while this is always secondary to formal or execution/body control mistakes, it's the only true feedback we have that we are properly doing Iaido, cutting our enemy (within or not), as opposed to just emptily and soullessly waving a metal object through air!
1 Comment
Jun Bryan Penaranda Beatingo
6/12/2021 12:50:12 pm
This is a highly i formative article about tachikaze. Thank you so much for writing this.
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