Grain weight....
How much an arrow weighs does two main things to its flight. The most simple thing it does is it affects how fast the arrow will travel once shot. Lighter arrows travel faster ( up to a point... diminishing returns at a point ). That was the simple part to explain. The hard part gets its own paragraph.
Its difficult for me to explain, but it is a simple thing. Grain affects the arrows flight depending on where the grains are placed. If the spine ( how stiff the arrow is ) of an arrow is good for a 50lb pull bow with a 125 grain arrow head, and you take that head and replace it with a 200 grain head, your arrow will be underspined ( too weak ). Why the difference? The arrow has one job, To push the head to the target. If the head weighs to much, the shaft, under sudden acceleration of the bow, will flex too much as the head will want to stay still ( inertia ). If the shaft flexes to much, the flight will be a bad one and the arrow will land to the right ( somebody correct me if I am wrong here ) of the mark. Likewise, if the arrow head is too light in grains, the shaft will not flex enough in flight and it will land to the left of its mark. ( Center shot bows are more forgiving. ) That helps ( I hope ) explain the affects of grain on flight.
It also affects penetration. This is a hotly debated topic by many on just how much it is affected, but I dont know of any who say it isnt at all. A heavier arrow travels slower out the same bow than a lighter arrow. Some say they want the arrow in the target faster, others want it in there deeper. It boils down to a personal preference honestly. But the science behind it is inertia again. More weight, more penetration. To give you an extreme example, look at a ping pong ball and a rock of equal size. Rock travels 100 mph, Ping pong ball, 200. Please, you wanna throw one at me, I will take the ping pong ball please. The biggest argument on this issue is where to draw the line. I personally hate the 10grains per pound of arrow weight everybody likes to cite. I think it is more than needed. I prefer closer to 7. I like my arrow to travel fast, but there is still plenty of energy for penetration. Of course, the deer doesn't care how fast the arrow hit, dead is dead, no matter how fast the arrow flies.
Last part on this also gets its own paragraph, only because the one above is really longer than I like to read myself. Lets look briefly at energy distribution on impact. Arrow flex when shot. We all know that. But they flex even more on impact. Its a reversal of the shot. On release, the head wants to stay still, and the shaft bends on release pushing it away. On impact the head wants to slow down, but the tail of the shaft wants to keep moving, so it again bends. Only this time, that bending is wasted energy. You want as much of that shaft ( and its energy ) in line with the line of penetration. You can only stiffen the spine so much before you are no longer accurate, or just have crappy flight ( which incidentally means often times, an arrow that strikes the target at an angle, again, you want energy in line with the line of penetration ). So, the answer is to give as little weight at the tail of the shaft as possible, and as much forward as possible. You do this by tapering your shaft. Narrow end get the nock, getting wider ( pyramid style ) to the head, which gets the fat end. That way, on impact, very little energy ( E=M( mass ) x A ( acceleration, in this case, deceleration )) is wasted. With less mass in the tail of the shaft, you have less energy wasted, and more energy going into the penetration. That tail will whip allot less on impact also, which the tail whipping itself is wasted energy.
So as a recap to this book I just wrote ( I hope its was at least as informative as it was dull ) you want the spine stiff enough for the bows poundage AND the arrow head weight. You want your total arrow weight to be enough to carry the energy of the bow fast enough to suit you and maintain effective penetration. And I just explained my reason for my bias towards tapered shafts. But again, its a preference, regular dowel shafts work great too.
