Friday, 3 August 2012

Oregon Yew

I've been busy decorating for the last week, but I've finished now and musing as to what to pick up next. I'd promised a guy I'd try to do a primitive for him. He's quite interested in the Hornbeam one I recently finished so I looked it over, cleaned up the arrow pass a bit to make it closer to centre shot and tried a couple of arrows. It felt good and the second shot skewered the 2x2" scrap of white pvc foam I was aiming at from 10 yards.
I set up the chronometer and it registered a respectable 139fps.
I looked at some Yew I have lying around and then though i should check the Oregon Yew again, the bug holes are still there, but a couple of skinny billets might make a longbow, I'm thinking maybe a flight bow with a slightly more 'elliptical' tiller.
I'm a bit ambivalent about the term 'elliptical' as I'm not sure exactly what is meant, does it mean simply a section of an ellipse, or I suspect it may mean an two arcs of a circle but separated by a stiff handle section of about 15", or is it just a slightly whip ended bow?
The geometry of a bow is quite complex.
If you take two rigid limbs (say 25" or so long) pivoted at the centre joined with a string and allow them to hinge back say 45 degrees, the string will come back say 20". If you now re draw the same set up with a rigid section of 15" between the two limbs making the bow longer. You now find that as the limbs come back to 45 degrees the string now draws back further!
So, in this simplified model, if the limb tips return to their rest position in the same time as they did in the first example the arrow will be travelling further in the same time, or, as we sometimes call it, faster!
Anyhow I've started marking up the two billets for splicing together.
Doubtless some pics later in the weekend.
Hmmm. Just noticed the 'explain more' so I tried to draw a diagram to show my simplified model of a bow and how a riser section effect the draw. Drat, it seemed to show the opposite conclusion! Arrrggghh, so I'm going to have to pore over the maths/geometry for some time, which isn't my fave thing, but having done it once it's wound me up!

No comments:

Post a Comment