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corvi
corivax
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October 2008
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corvi [userpic]
Atlatl!

I was playing with gfish's atlatl (a gift from the wondrous keen vixyish), and it is neat. I will now proceed to gush about it. :)


Here is the atlatl. There's a hook at one end, that you put into a notch in the dart, and at the other a weighted handle and loops to put fingers through.


Here is the atlatl loaded with one of the sixty-three inch darts, ready for throwing. The notch at the base of the dart fits onto the hook, and you use your thumb and forefinger to make "guides" so the dart stays parallel to the atlatl.



Then you swoosh it up overhand, and off goes the dart. Basically, the atlatl makes the lever of your arm longer, so you can apply more force, the same way you can hit a ball harder with a tennis raquet than you can throw one. Swoosh! (Picture of gfish, right after a throw.)

This analogy only works, though, if you regularly use a haunted tennis raquet.

See, archeologists had been digging up atlatls for years - they're found on every continent early man got to, a precursor to bow and arrow. We knew they were for spear-throwing, so people made spears out of good sturdy wood and launched them around a bit, and has such horrible results they decided the only way you could get any accuracy was if you'd been trained since you could walk to use one. Nobody knew how the Aztecs could launch darts that punched holes - front and back - through steel conquistador armor.

But things go very, very differently if you make your darts from, say, dogwood, something that bends. You load the dart onto the hook, and when you start to push the atlatl forward, you start a wave in the dart, like a ripple traveling across a pond. The ripple moves all the way down to the end of the dart, and echoes off the arrowhead, like sound, like a long low wavery C. And then the ripple rushes back towards the feathered end, your hand and the atlatl, but since you're just now nearing the top of your swing, you apply even more force, more music to it. So it echoes again, higher and louder, and bounces off the arrowhead one more time, and back to the fletching, but this time, if your timing is perfect, you're at the end of your swing; you aren't putting any more force into it to bounce the ripple back again.

So instead it jumps off the atlatl. The first time I got all the timing and motion right, I was astounded - it felt like the dart just drew a breath and came alive, a long and copper-striped snake. I didn't push it out into the air; it just flew.

The wierdest part, I think, is the aerodynamics. If you shoot a regular arrow, it "cuts" its way through the air, and the air flowing above it gets tangled up into eddies and turbulence. That's why we put feathers for fletching on arrows - it keeps the arrow's linear motion solid enough that the eddies don't just spin it uselessly end over end. Instead, if the arrow drifts a little away from horizontal, the eddies just push it back onto course. This stability is very important in archery; it's what keeps arrows flying straight.

When you're shooting a five-foot flexible dart instead of a foot-long arrow, you get the same effect, sort of. If you mess up and shoot too low, the entire dart writhes and curls in the air, pulls itself horizontal. If you fire too low, you can watch it actually bend and fly upwards. (This is incredibly spooky the first time it happens, I assure you. One wishes to grab the universe by its lapels, shake it a bit, and demand one's regular physics back.)

But, so incredibly cool! Ammunition that hums in your fingers like a wind through grass, uncurls and flies itself from the atlatl, and curves and changes direction in midair!

Oh, and can be thrown nine hundred feet. Or punch through steel armor. Twice. Or, when thrown very gently by a newbie, split wood at a great distance. This is one of gfish's throws:



So. Very. Cool. Squee!

Please note, this post does not mention a certain aluminum-edged cube, even once. This reference is too obllique to count. Nyah. :)

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Comments

ooooh neat neat neat neat!

*smile* that is all. 's too late at night for anything more coherent.

I haven't heard about an atlatl since 7th grade history when my teacher was sharing the above-referenced in-and-out-of-armor-on-a-tree-stump story, and he did such an awful job of explaining how it worked that it never really made sense to me.

The harmonics of the flex wave traveling on the spear, though, is fantastic. Thanks for sharing! Now I want to try...

Depending on your age, we might not have known how they worked yet. An enterprising engineering student named Bob Perkins was the guy who figured out that using a flexible dart quadrupled range and power, and he didn't work it out until the eighties sometime.

If you happen to find yourself in Seattle, look myself or gfish up; I'm sure he'd let you fool around with his.

interesting. on atlatl.com, mr perkins talks about the mechanics of the atlatl in such vague and mystical terms that if i hadn't been referred to him by you i would suspect he was making it all up. "the number pi is found for optimum performance in the Relationship between Atlatl length and dart length"?

Many of Perkin's ideas are generally accepted at this point. Others are still debated by people like Dick Baugh, another engineer-turned-atlatl-maker, who's done some interesting computer modeling. My objection to Baugh's computer modeling, as I understand it, is that he tends to model absolutely perfect throws on the human's part, and I think it'd be folly to design a weapon assuming that.

I don't know if the ideal ratio is pi (though I like to think it is), but it does seem to be very near 3.1.

Perkins has some papers that are much more interesting than his 'give me lots of money for a mysterious magical atlatl' website.

are his papers online?

(i assumed from "the number pi is found in the relationship" that he meant it was some rational multiple of pi, which is, you know, not really all that shocking. 1 pi is at least a bit entertaining, though.)

I'm pretty sure I learned about the atlatl the year after the iron curtain came down--I'm a young'n. The teacher didn't say anything about flexible darts, only normal spears... looking at atlatl.com, however, there's a lot to interest the physics and engineering backgrounds :-)

If I'm ever in your corner of the country, I'll look you up. I spend most of my life about twenty minutes from GSFC, however... drop me a line if you'll be in the area at some point. (I figure a NASA robiticist might have reason to visit there every so often.)

(Anonymous)

that atlatl is bob perkin's work it is his warrior atlatl i could reconize his work anywhere look it up at atlatl.com

What a great explanation! I'd never known that an atlatl was anything more than a lever arm.

yeah, me neither. neat!

does this make the exact length and stiffness of the dart fairly important, or can you learn to adjust your throw for them a bit?

Yes, construction has to be fairly exact. The dart:atlatl length ratio is, ideally, pi, which just fills me with glee. :)

The springiness of the atlatl itself, important for reflecting the wave once, is tuned with by the weight of the stone tied onto it.



Another thing that matters is the weight of the head of the dart, for bouncing the wave. The Smithsonian was doing a dig at an atlatl construction site in North Dakota and found three dart points, two made from chert and one from flint. The team was surprised to notice the flint head was a very different shape than the other two, and more thana centimeter shorter, until they weighed all three points and found less than an ounce's difference in weight between them.

Honestly, I do not think I myself, with all the aid of digital calipers and a well-stocked machine shop, could make one. :)

Erk. That should be "less than a tenth an ounce's weight".

(Anonymous)

Really? you couldn't make one with calipers and a well-stocked machine shop? Civilizations have been making the atlatl for thousands of years with rudimentary tools (flint and stone). Only early North Americans have used the weighting stone to "tune" an atlatl. The only real reason to precisely tune this weapon, is for competition purposes. If ones intention is to "fool around" with a fun and primitive hunting tool, or learn to hunt with it, such precision is not necessary.

yay! ancient weapons and traveling waves. these are a few of my fav-or-ite things! (i'll be good and won't sing.)

This is incredibly spooky the first time it happens, I assure you. One wishes to grab the universe by its lapels, shake it a bit, and demand one's regular physics back.
hee! well put.

unfortunately many ancient weapons make good can openers. i've read a charming story of a different flavor of javelin that had a bad tendency to pierce both shield and armor, and pin guys to ground while they were standing upright. :P

Wow, that's amazing. I feel the glee of new knowledge. :)

Wow, that’s awesome! I first heard about atlatls at a museum (Dixon Mounds in Illinois), but I’m pretty sure nobody knew at the time how they worked; it certainly wasn’t reflected in the exhibits about them, and I remember a staff member mentioning to a tour group that nobody recent had been able to do anything useful with one. I remember being very impressed with them at the time.

(I was also very impressed with the excavated gravesite. That’s since been closed as a public exhibit, due to laws about treating native gravesites respectfully, and I definitely understand the motivation, but it was an amazing experience as a child to see a hundred or so skeletons of all ages, sexes, and degrees of health, uncovered but otherwise as they had been buried. The exhibition building was built on top of the excavated gravesite, so you were indoors but looking down on the excavated mound.)

I recently saw a musuem exhibit in the Icelandic National Museum of three Viking-age graves, original bones in original positions with original artifacts (and a horse skeleton too), carefully arranged in sand in transparent boxes in the floor. It was stunning.

That's really cool. I'm imagining I kind of "whap" sound as it hits something.

Also note this comment does not in any way refer to extended periods of freedom from contact forces.

ohmy... shiny!

of course, this reminds me of my lust for a trebuchet...

That's really, really cool. I tried making them a lot as a child and they didn't work worth beans. Now I know why. That's awesome. Next time I'm out, I'd love to try it.

Thanks for the very accessible explanation of the physics behind this thing.

Very cool. Archery actually depends on the arrow being flexible also--although I don't think it is anywhere near as critical. In archery, the arrow actually bends around the bow.

Is the spear in the pictures aluminum?

-B.

Yeah, it's made from two standard hunting shafts cut down and fitted together. You can get wood ones, but they cost a lot more.

It's funny - I did a lot of archery before realizing that arrows wobble in the air. I'd somehow imagined they followed their trajectory perfectly.