Bow Design Fig 12 – Nocking point indicator
no bowyer has taken this up. [Fig 13] An arrow is a lifting body and flies significantly further than a ballistic trajectory would predict. Many archers use special fletchings, or just angle normal ones, to make the arrows spin. Historically, goose feather fletchings would also cause arrows to spin because feathers have natural lift characteristics. Since opposite wings have opposite lift, all fletchings were made from right wing feathers. (Left wing feathers became quills.) Any gyroscopic effect (mimicking a rifled bullet) is said to straighten the arrow. The lateral vibration gives the arrow a higher effective inertia and the estimated initial spin speed is typically 2000rpm. As the vibration dies away, the inertia drops and the spin speed increases to perhaps 2500rpm before the arrow hits a target at 70m (the Olympic distance) just over a second later. However, the details of how the spin interacts with the lateral vibration are unexplored; and a spinning cylinder is vulnerable to side-winds, which change the lift characteristics, so the efficacy of arrow spin must be regarded as unproven.
Fletchings are usually placed in three positions at the tail end of an arrow. This makes sense for stabilising a long gliding missile. However, the initial vibration of the arrow adds another factor. As the tail of the arrow wags from side to side, high speed filming shows the arrow veering slightly left and right as it leaves the bow. Moving the fletchings forward a few inches, so that they are near the node point, reduces this veering effect. There is a secondary effect (felt subjectively) that the bow is steadier in the hand. This may arise from a dampening interaction between rear mounted fletchings and the lateral vibration of the string. With the fletchings further forward, there is less dampening and thus less twitch caused by the initial lateral motion of the string. (This is a personal observation not common among archers.) Possible alternatives to traditional fletchings, such as ‘golf ball’ dimples, have yet to be investigated.
Modern bowstrings are usually made from synthetic materials. Dacron is less stiff and mainly used for longbows and less expensive recurves, where it is more forgiving. Higher performance strings, usually using materials such as ultra-high molecular-weight polythene, UHMWPE, are sold under various trade names. (This very strong, stiff fibre is also used, for example, in tyre reinforcement
Archery was revived in Victorian times and became a popular pastime in country houses for both men and women
Figure 13 – Effect of changing the structure round the bow window
and bullet proof vests.) The advantage is that thinner, lighter strings can be used. Since strings have to be accelerated along with the arrow, energy efficiency is improved by reducing aerodynamic drag, the energy stored in stretching the string, and the mass to be accelerated.
The changing string angles, as the bow is drawn, give an increasing mechanical advantage. Thus, counter-intuitively, the tension in the string reduces as the stress in the bow rises. The peak string static load is before it is drawn and dynamically when it straightens while being shot. At this latter point the ends of the limbs are effectively stopped dead by the stiff string, but the main part of the limb has momentum and continues to move forward. This initiates a vibration mode which travels inward to the riser, then back out again several times before fading away. Over time, this can be enough to damage the bow through fatigue. Bow failure is usually due to de-lamination of a limb or
occasionally a riser will break at the handle, where the bending stresses are highest.
A secondary effect can be seen if a recurve bow is put into a tensile test machine. [Fig 14] On drawing the bow, the riser and limbs are increasing in bending stress, but the string is reducing in tension. Unloading has the opposite effects. The combined result is to produce a ‘spring’ assembly with very little overall hysteresis. [Fig 15] The exchange of energy between the string and the limbs and riser is complex for the relatively static case of a tensile test. Add the dynamics of shooting the bow and the situation becomes really complicated. The recurve bow has never been analytically modelled in detail. Recurve bows were tested to find their efficiency in terms of the kinetic energy of the arrow compared with the work done by the archer. Wood and fibreglass bows were around 55% efficient compared with 70% for a good quality carbon-fibre limb and riser combination.
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