Main Discussion Area > Flight Bows
Working on a giant bow to beat Allen Case!
sleek:
The draw length will be what makes the difference. The longer the better.
willie:
--- Quote from: Badger on March 23, 2018, 06:32:38 pm ---
--- Quote from: avcase on March 23, 2018, 06:23:22 pm ---The giant bow should not be able to throw an arrow any faster than a regular size bow. Somewhere north of 200 fps with a reasonably sized arrow would be a major success. But the trajectory of the huge arrow from the huge bow should travel much farther than the same speed arrow at normal scale. Air resistance will be much less a factor on the big arrow trajectory.
Alan
--- End quote ---
Allen, that's what I was coming up with. Even the dry fire speed calculations were disappointing. The only way to make the giant bow short that I can think of is to have an extra long draw length. I was hoping for 500 fps plus and it doesn't look like I will even come close to that.
--- End quote ---
I took a few minutes to refine the previous calculation a bit. Accounting for a spectra string that has a breaking strength at 5 times draw weight, the arrow speed drops down 4%
The initial calc also assumed a 10% energy loss for hysteresis. I should mention that the sheet is estimating a max and min spread for kinetic energy and arrow speeds , perhaps giving a broad definition to what constitutes hysteresis? Nevertheless, I have to use a value of 60% energy loss to hysteresis, to bring a 10GPP arrow speed down to 226 FPS at the lowest estimate of it's speed, and an 80% loss for average speed. 5 GPP speeds are of course higher.
Perhaps a reasonable vibration loss for a "normal" bow is lumped in with hysteresis to create the hi/low spread, at any rate with the exponentially high energy storage, something else is not scaling very well. I would prefer to suspect that hysteresis is what doesn't scale, otherwise there might be some potentially limb shattering vibrations in a bow 10 time normal size.
Steve, can you refer me to your source of the formulas for the dryfire calcs? It would be interesting to see which method scales better.
Badger:
Willie, my estimates on speed were based on 100,000# stored energy at 60% efficient at 1/800 arrow weight of stored energy. From that I established a virtual mass not accounting for any additional hysteresis which I know I would have. I did all my figures on a scratch pad which I trashed but can easily do them again
willie:
I did find some examples of virtual mass calcs, let me see if I can duplicate your results, just for the sake of a learning exercise. what did you get for the fps?
125 lb arrow? if I understand you correctly?
Badger:
I don't remember now, I just remember I was disappointed
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