Working on a giant bow to beat Allen Case!

[willie]

  So arrow weight of 1/1000 of stored energy is what the testing will be using.

I am curious what draw weight you will tiller the 20 footer to,  in order to make the efficiency comparison?

[Badger]

   I will do it by physical dimension but I believe it goes up 4 times while the stored energy goes up 8 times. I want to see how well this holds up when actually built as opposed to theory. I built a 25 footer for the Da Vinci show and it came our pretty much exactly how I planned weight wise.

[Selfbowman]

Aluminum flag pole for arrow. Lol or a tele pole. Arvin

[willie]

interesting scaling ratios, Steve. I plugged a 64" by 1.25"wide bendy handled  bow into a spreadsheet and scaled up times 5 to 26'8" x 6.25" wide x 3" thick

for the same strain,
the poundage went from 40 to 1000   or  40 x 5 x 5
the stored energy went from  36 to 4500  or    36 x 5 x 5 x 5
and the arrow speed at 10 gpp from 169 to 357

the scaling of stored energy seems rather impressive, and an arrow of 1/1000 SE works out to  4.5 lbs or 31.5 gpp 
so if I plug a 4.5 lb arrow back into the calculator at that gpp, the fps drops back to  223

[Badger]

  Willie, I am glad you did that, I used my primitive math skills to try and estimate speeds and was somewhat disappointed and about ready to throw in the towel. At 357 fps I am much more encouraged. Can you blow that up 1 more time and see what you get, say 52 ft

[willie]

I can, but wouldn't you rather have the numbers for your current design length, with the  20 stiff handle? what wood are you currently considering. etc ?? can furnish a few more specs?

[Badger]

  Yes, I would prefer that. I used a virtual mass method for making a speed calculation and the dry fire speeds were much lower than I had hoped for. 

[sleek]

IF 10gpp is the way to standardize all bows against each other, then arrow speed should be average.

( Im saying it wont)

But, with such a long draw this is gonna show what I have been saying for years, and hopwfully get rid of the 10 gpp rule. The draw length must be calculated into the formula when figuring out arrow weight.

Steve, this is not news to you, but I really hope this project reinforces this point of mine.

[Badger]

  I base grains per pound on stored energy and not peek draw force. A bow drawing 10,000# but storing 100,000# energy because of the long draw is radically different than a 50# bow storing 48# energy. Grains per pound really doesn't work well on giant bows or miniature bows either for that matter. It does work well for normal size normal draw length bows.

[willie]

Yes, I would prefer that

Ok, I will try to "draw" the bow from scratch in woodbears spreadsheet,  what are your revised specs and dimensions?

Grains per pound really doesn't work well on giant bows or miniature bows either for that matter.

If I understand you correctly, you are saying efficiencies do not scale well?  Besides the problem of finding quality staves for giant limbs, like Alan mentioned, what other reasons come to mind that might be the cause of lesser efficiency?

[avcase]

If I understand you correctly, you are saying efficiencies do not scale well?  Besides the problem of finding quality staves for giant limbs, like Alan mentioned, what other reasons come to mind that might be the cause of lesser efficiency?
[/quote]

Finding a decent string will be a challenge. Standard rope materials will probably be much more elastic or heavy unless you want to spring for a very expensive spectra rope.

Alan

[Selfbowman]

Yes Alan that's a lot of sinew ! Steel cable would be saver with out stretch. Don't know about ship rope. Still watching. Arvin

[Badger]

  Spectra rope for sure, expensive but available. Once I get a solid plan down with some drawings I can see about how much sponsorship I can get.
  White oak in 12 ft long straight grained pieces can be found. Maybe scale back to 60 ft, use a 15 ft rigid center. Da Vinci drawing was an 80 ft long crossbow. I think It will require engineering all the way. A good idea for a show would be to put on a mock siege. All the pieces carried it in on wagons and then assembled by the soldiers on sight.  Would have to come up with a good use for a giant bow in a siege. Fire rate would be extremely slow.

[willie]

Scaled back to 60 foot is still impressive!   A nice graphic like similar to this will go a long way for getting folks onboard.

http://hexus.net/media/uploaded/2016/12/c8cf1d80-6528-4afa-a44c-ebb9927ae7c1.jpg

A series of test bow possibilities to consider would to be to build a 3', 6' and 12' bow of the same design. you could probably get all three from the same board. I think if carefully tillered to the scaled draw weights, a lot could be learned about how well you can scale the efficiency.

[Badger]

Scaled back to 60 foot is still impressive!   A nice graphic like similar to this will go a long way for getting folks onboard.

http://hexus.net/media/uploaded/2016/12/c8cf1d80-6528-4afa-a44c-ebb9927ae7c1.jpg

A series of test bow possibilities to consider would to be to build a 3', 6' and 12' bow of the same design. you could probably get all three from the same board. I think if carefully tillered to the scaled draw weights, a lot could be learned about how well you can scale the efficiency.

  60 ft might be more realistic, unless we can come up with a different construction method, I wonder if a limb could employ airplane wing technology.