The Khotan bow and the P-effect

Kviljo · Post by **Kviljo** » Wed Feb 03, 2010 4:48 pm

Adam Karpowicz wrote:"as the flattening of the limb... "

?? There is no flattening.

Adam

Yes there are

I just did a small test with a plastic shoe-horn, and it does flatten when sufficiently bent. It does seem that the stiffer the material, the earlier the flattening sets in. Which seems to cohere with the modern stiffer materials in the ACS-bows, while the horn/sinew limbs would need to be bent further for the effect to set in.

Wether or not the effect would have a noticable influence on the draw in a bow with such curved limbs, is another question.

I do buy the explanation that the transversely curved limbs we see in the Khotan bow can be because of long-term stress from the poisson effect. But I'm not completely convinced it is the only possible explanation for the curved limbs.

I'll see if I can take some photos showing the flattening of the limbs.

Adam Karpowicz · Post by **Adam Karpowicz** » Thu Feb 04, 2010 12:28 am

Great. And while it flattens the force of bending goes down, doesn't it? That would be llike a compound bow! I would think this sound more like a kink in the concave limb rather than a controlled process. But who knows, maybe it is worth investigating in a real bow.

I was under the impression the ACS limbs were convex to increase the resistance to bending. This makes the limbs non-bending with the reduced mass. The stiffness of the tips is then maintained all through the draw. The flattening when the force drops would then be a failure, not an improvement.

Adam

Kviljo · Post by **Kviljo** » Thu Feb 04, 2010 6:55 am

Hmm, I might have misunderstood how the ACS-limbs work. I thought the curvature were "active". I have only read about them, and it is a while ago, though.

Yes, the bending resistance goes dramatically down once it flattens. It might require extremely well tillered and evenly transversely curved limbs to make sure all of the limbs kink over at the same time. And the limbs might even have to look more like a tube cut in half than the rather small curvature we see in the original bow.

I guess one way of testing wether or not this effect was applied in the Khotan bow, is to make one with flat limbs, and see if the limbs gain as much curvature as the original. If the original still has more curvature, it might have been built into it.

jack farrell · Post by **jack farrell** » Fri Feb 05, 2010 2:43 pm

Kviljo; Maybe you should try to put your hands on Andrew Halls articles from JSAA on Hunnic type bows. He reports on about 15 variations of this bow type, which includes the Khotan bow.

Kviljo · Post by **Kviljo** » Fri Feb 05, 2010 7:23 pm

Ahh, that sounds like good litterature! What is JSAA? - and do you have any advice as to where I could start to search for the articles?

jack farrell · Post by **jack farrell** » Fri Feb 05, 2010 7:40 pm

Journal of the Society of Archer Antiquaries. You might contact the author...member list.

Kviljo · Post by **Kviljo** » Fri Feb 05, 2010 9:11 pm

Oh, of course!

Thanks!

Tapio Manner · Post by **Tapio Manner** » Mon Feb 08, 2010 5:06 pm

The Poisson effect is a question of using the simple bending stiffness equation against the plate bending equation. The magnitude of the effect is E vs. E/(1-nu^2), the tendency is to reduce the effect by transverse bending. It can split simple wooden slats and cause some slight havoc on badly constructed limbs. The only time I have seen real problems is when we used some 600 GPa carbon fiber on some satellite structures. Otherwise it is more of a curiosity but of course should be known.

TLM