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bow shape
In archery, the shape of the bow is usually taken to be the view from the side. It is the product of the complex relationship of material stresses, designed by a bowyer. This shape, viewing the limbs, is designed to take into account the construction materials, the performance required, and the intended use of the bow.
There are many different kinds of bow shapes. However, most fall into three main categories: straight, recurve and compound. Straight and recurve are considered traditional bows. If a limb is 'straight' its effective length remains the same as the bow is drawn. That is, the string goes directly to the nock in the strung (braced) position. The materials must withstand these stresses, store the energy, and rapidly give back that energy efficiently. Many bows, especially traditional self bows, are made approximately straight in side-view profile. Longbows as used by English Archers in the Middle Ages at such battles as Crecy and Agincourt were straight limb bows. A recurve bow has tips that curve away from the archer when the bow is strung. By definition, the difference between recurve and other bows is that the string touches a section of the limb when the bow is strung. Recurve bows made out of composite materials were used by, among other groups, the Persians, Scythians, Hyksos, Magyars, Huns, Turks, Mongols, and Chinese.
== Design factors ==
If a limb is 'straight' its effective length remains the same as the bow is drawn. That is, the string goes directly to the nock in the strung (braced) position. When the limb is recurved (tip of limb away from the archer), the string touches the limb before it gets to the nock. The effective length of the limb, as the draw commences, is therefore shorter. However, as the bow is drawn, the recurve 'unwinds', the limb becomes effectively longer, and the mechanical advantage of the archer increases. Counter to this, stresses are building up in the materials of the limbs. The belly of the bow (nearest the archer) is in compression, the back (furthest away from the archer) is in tension, and the line between is in shear.
The materials must withstand these stresses, store the energy, and rapidly give back that energy efficiently. The amount of energy stored is determined by the stresses withstood and the shape of the limb, from the unstrung position to strung (consider as pre-stressed), then de-formed further to full draw as the recurve unwinds. These basic principles of changing mechanical advantage, to efficiently store more energy, and deliver it to accelerate the arrow, were clearly understood in antiquity, as shown by the examples that follow.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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