Main Discussion Area > Flight Bows
Virtual Mass revisited
sleek:
--- Quote from: willie on September 26, 2020, 11:49:01 pm ---sleek,
the equation is from the other site. it shows the basics, but does not include all the ways tillering can be done poorly.
Here is the same bow with the strain increasing towards the tips. Is this what you mean? Whether any part of the limb bends first or last as it is drawn is an interesting idea, and might depend on string angles. How it returns to brace could be different, as the arrow is accelerating. maybe.. I think you would have to have a slo-mo camera to actually tell.
--- End quote ---
How it returns to brace IS and SHOULD be the entire focus and good of you to point that out. The way it returns to brace is ideally controlled by the arrow. The moment you let go of that string, the resistance of your fingers no longer is providing the force that keeps the bow in its drawn shape. Its now the arrows inertia ( at rest, it wants to stay at rest ) and the inertia of the limb ( other things too but too many to easily address ) that slow the rate at which the bow returns to brace. The rate it returns, and the orderly fashion in which it does so, has much to do with its efficiency. Too fast and the wave of energy transfers becomes sloppy, or dirty. The effect Steve mentions of Hysteresis also comes into effect here as well. Him coming up with the idea of time sensitive Hysteresis was a stroke of brilliant insight there.
Hysteresis slows the bows ability to provide power to its load, and becomes part of the load in all honesty. Id bet you could calculate it as such if you were to measure the difference in energy of two different weighted arrows. A heavier arrow is well know to carry more energy than a lighter one, because the it spends more time absorbing the bows energy, vs a lighter arrow. The difference in energy between a heavy arrow and light one could easily be a good indicator of amount Hysteresis in the bow.
Its also noteworthy that the Hysteresis in a bow continues to show up after the arrow is shot. As the limbs relax from the shot the string tension at brace increases. Steve mentioned this same thing earlier. Id like to take a scale ( tensiometer ) rigged to read string tension and see how a bows string tension increases after its been returned to brace, and time it as well.
As for your second drawn bow you show, I dont mean the tips should bend more, or even at all. Im a believer in stiff tips. I mean that the outer working limb ( be it tips or mod limb ) should bend first. One the string angle is increased on them as the bend back, the leverage advantage on them is decreased and they stop bending. The energy of the draw is then focused to the area behind that, and it bends until the string angle relative to it is is beyond any mechanical advantage to cause it to bend further before the wood behind it starts to flex. As it bends, the string angle at the wood closet to the tips becomes even greater loosing even more mechanical advantage, ensuring that the outer wood won't bend any further. I need to get some dial indicators and clamp a bow to a table to test and prove this is how it actually works.
willie:
--- Quote from: Badger on September 27, 2020, 12:21:33 am --- Willie that is pretty close but I like to keep outer limbs stiff to further reduce working limb area.
--- End quote ---
more like this?
Digital Caveman:
Kinetic energy = 1/2mass times speed squared. This means that if you double the speed of an object, you quadruple its energy. This means that a bow shooting twice as fast has four times the kinetic energy in its limbs.
Because heavier arrows cause bows to shoot slower, far less kinetic energy is sunk into accelerating the limbs, which means a larger percentage is put into the arrow.
This explains why light tips are so efficient. Because the limb tips are the fastest moving part of the bow, they take a significantly larger share of energy per Oz to accelerate than mid and inner limb areas.
The kinetic energy of the limbs the moment before departure is not available to the arrow. Instead, it is converted to limb vibration and handshock.
scp:
I'm reading the words, but not quite grasping what they are referring to.
Badger: I have always suspected that a tiller where the limb bent progressively from outer to inner could also help to control this vibration by simply returning to brace over a more extended period.
Sleek: So I tiller the working limb closer to the tips to bend first as I draw it the bow, and the inner limb to bend last. The inner limb is last to bend on draw but first to release its energy upon string release. This flows like a wave all the way to the tips allowing the most energy efficient delivery of energy possible.
To me, they appear to be saying the opposites. Am I wrong?
DC:
Didn't we decide that you can't get part of a limb to bend first? More or less, yes but not first or last. If you take any snape limb and move the tip 1/4" the inner part of the limb will move, maybe not much but it will move. Like the Parsons carriage. ;D
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