It's easy to see that many of the people here are almost exclusively into archery, because the rifle nuts out there who are doing long distance shooting understand most of these concepts very well. As a certified gun nut, as well as an archery nut and pilot, I can tell you that the study of ballistics and aerodynamics has described most of these matters very well for many years. Because of archery's rather primitive heritage and low flight-performance characteristics, these matters have not been considered generally as important in archery until more recently. There are literally books written on these subjects, and I could go on and on for hours discussing them, but there are a few basic fundamentals.
First, a drawn bow stores potential energy which on release converts to kinetic energy. The energy stored in the bow at full draw depends on the design of the bow and the length of the pull. A more efficient bow design stores more energy, and a longer draw also stores more energy. It is not simply the peak draw weight of a bow that dictates the energy that is put into the bow or the energy that will be returned on release. That is why when looking at the draw length versus force curve on a bow that the area under the curve becomes so important. Of course, the bow must be efficient in returning that energy in order to propel an arrow faster. Compound bows generally store more energy for a given draw length and peak weight than stick bows and are more efficient at returning that energy to the arrow for a variety of reasons including higher initial pull weight, longer peak weight and more efficient acceleration of the arrow due to the lighter holding weight that increases quickly to the peak weight and maintains the higher weights over a longer period of time. Efficiency of a bow can be somewhat higher or lower depending on arrow weight, but generally speaking the kinetic energy of a lighter arrow will be about the same as that of a heavier arrow shot from the same bow by the same archer. Kinetic energy equals mass times velocity squared. That is different from momentum which is mass times velocity. Because velocity becomes much more important in the energy equation than mass, lighter arrows are much more dependent on velocity to "earn" their energy than are heavier arrows.
It is well known in aerodynamics and ballistics that drag increases logarithmically with speed. That is why some of our politicians are once again trying to lower the speed limit to save gasoline. A object, like a car or an arrow, that is going faster will be more affected by drag from the air around it than a slower moving object and will have a greater percentage decrease in velocity over a given period of time if no additional force is used (like a motor). So, the same object going faster will slow more quickly than when it starts off slower. The drag of the air converts the kinetic energy of the projectile into heat so that less energy is available. That is main reason that Swamp Pygmy has observed less penetration by an arrow at 30 yards than at 10 yards -- the energy in the arrow is less as the distance increases and the velocity decreases. At extreme velocity, the heat generated by the drag on a projectile in air can be impressive, like a meteor or the space shuttle becoming fiery hot on entering the earth's atmosphere. If there were no air to cause drag, an arrow would maintain its forward velocity until gravity accelerated its vertical velocity so that the arrow was pulled into the earth (actually they were pulled together, but the arrow's gravitational pull is so small that it can be ignored for all practical purposes).
The main purpose of the feathers is to guide the arrow by acting both as air foils and as drag devices. There can be some gyroscopic advantages, but even straight vanes will produce stable directional flight. Flu flu arrows are designed to have greater drag and slow an arrow even more quickly as a result. If the back of an arrow is trying to slow down faster than the front, the back end will stay at the rear and the front will stay at the front giving the arrow great directional stability but also slowing down the arrow. An unfletched arrow will be noticeably faster at 20 yards than a fletched arrow if the arrow flies straight, but the fletched arrow will be directionally much more stable.
A heavier arrow of the same length and diameter will have a higher sectional density, and, if the shape is the same, will have a higher ballistic coefficient. These terms will be very familiar to experienced rifle shooters. Sectional density relates very well to penetration of a more or less solid object, while ballistic coefficient describes the ease with which the projectile penetrates the air or some other low density fluid. If the shape of the projectile is exactly the same in every way, the higher the sectional density, the higher the ballistic coefficient. The higher the ballistic coefficient, the less the projectile will be affected by drag.
So, a projectile of the same shape that is heavier and slower will be less affected by drag than one that is lighter and slower. That is why a heavy arrow out of the same bow will not slow down as quickly as a lighter arrow. The bow will push the lighter arrow faster, but the heavy arrow will keep its speed up longer. Since the heavy arrow is going slower though, and the acceleration of the arrow towards the earth is constant for arrows of all weights and will bring both down to earth at the same time, the faster one will drop less at any given point as long as it is able to maintain a velocity advantage. That is why lighter arrows shoot flatter over typical shooting distances. That makes range estimation less critical and increases the odds of hitting the target as range increases unless you are very good at determining range and accommodating the arc of your arrow at various distances. The same types of problems are encountered with wind drift, but I won't go into that here.
When we start talking about penetration of a target, and specifically a game animal, other considerations are encountered. We all hear Dr. Ashby's theories and studies, but some of the reasoning is difficult to understand. The latest thing we have been hearing about is the single bevel issue. That all arises because when we use feathers, and especially when we use feathers with a helical twist, we impart spin to the arrow to help stabilize it. If the bevel of the broadhead is shaped to help maintain the spin of the arrow during its penetration into an animal, in the same direction as it was going during flight, less energy is lost and more energy can be retained for penetration. If the broadhead is shaped to stop the spin (double bevel) or reverse the spin (right hand single bevel broadhead on a left helical fletched arrow) substantial amounts of energy are used to stop the spin or reverse it and that energy is no longer available to aid in penetration. Also, the more energy that is lost in stabilizing the arrow in flight (because of excessive fletching or poorly tuned arrows) the less energy is available for penetration. Furthermore, if a poorly tuned arrow turns partly sideways and exposes more surface area in the air or at the target, the more energy will be expended on things other than driving the broadhead into the animal.
One of the other very important things to consider is how a broadhead (or a knife) cuts. One way is through micro tears from the rough edge that is present on even the most finely honed cutting surface. Under a microscope a knife edge has a very serrated appearance. These tiny "serrations" grab the tissue and tear it apart as the broadhead or knife cuts through the animal. For the most part, however, the ability to produce this penetrating cut arises from the EXTREMELY high pressure that is exerted at the very, very thin edge of the cutting tool. Because pressure is measured in units like pounds per square inch, just imagine how quickly the pressure will grow if the surface area is decreased dramatically. Think about how incredibly small the surface area is on the edge of a razor. That means the force is absolutely amazing when weight is applied to that surface. It is that same reason why needles can penetrate so much better than a flat piece of dowel. And even if you decrease the diameter of the piece of dowel by half, the pressure per square inch will be four times as high because area decreases by the square of the diameter. Think now of sharpening the dowel to a fine point and what that would do to the pressure. That is the idea of a nail and is the key to its ability to penetrate objects. The amount of force on the tiny amount of surface area on the point of the nail is tremendous. A razor edge is many times smaller than the point of a nail or a sharpened stick and also unzips the tissue along the length of a long, aerodynamic cutting edge, allowing the high drag shaft and fletching to follow a path of much less resistance after the hole has opened up. The main reason why a super sharp edge on a broadhead is so important is that it vastly increases the pressure on the cutting edge.
I know I have thrown out a lot of concepts here, but the main things to remember are the following:
1. Arrows that are weighted forward of center are inherently more stable because the front part wants to keep going forward due to the fact that drag affects it less than the lighter rear end of the arrow (the center of gravity is in front of the center of pressure).
2. Heavier arrows go slower but maintain their speed better, both in the air and in the target.
3. Lighter arrows go faster and have less drop at the same distance as a heavier arrow (one of the few advantages of a lighter arrow).
4. Fletching slows down an arrow, especially the rear part, so that it stays towards the rear and gives flight stability.
5. A well tuned arrow is more aerodynamic and will maintain its velocity and penetrate the target better.
6. A sharper broadhead edge will create far higher pressures at the target and will penetrate much better.
7. A bow is more efficient with arrows in a given range (arrows that are too heavy or too light can rob energy from the system).
Hopefully this may help someone understand why those in the know at Trad Gang (I am talking about people who are much more experienced than I am) recommend arrows on the somewhat heavy side that are well tuned and are tipped with extremely sharp cut on contact broadheads designed to work efficiently with the fletching (which should not be overdone with fletch that is far too big because it creates major drag). Also, because arrows do have quite a bit of drag and are not particularly high in energy levels (unlike large caliber rifle bullets fired at high velocities), distances from the archer to the animal should be kept as close as possible. Accuracy suffers too at distance, especially when sighting devices are not used, and accuracy is very important in hitting places that allow adequate penetration to vital organs that will result in quick deaths.
Sorry to be so wordy. Hope this helps someone. Maybe most of you already knew all this, and I certainly did not go into the detail this subject really deserves. I know Dr. Ashby has provided much better discussions of this than I have, but I wanted to approach it from a different viewpoint.
Allan
Topic: For those who decreased draw weight.......... (Read 3455 times)
