Flight Bow Experiments

 I haven't posted for ages, but the experiments with my Osage flight bow needed documenting for my own sanity.
The bow was originally somewhere between 70-80# shooting a 27" flight arrow (measured to the BELLY of the bow). Having overdrawn and stuck the point of an arrow into the belly a tip overlay sheared off. I repaired it, but didn't have the bottle to shoot it again.
So... I reworked it down to about 60# @26" ( I didn't want to draw it further on the tiller).
My PB had been 341 yards, and now the re-worked bow shoots 320yards (using the same arrows).
I've eased off the arrow pass and done a slight sideways correction at the arrow pass with heat (The bow was curved to the archers right away from the arrow pass).
High speed video at 1000 frames per second shows good arrow flight, and I thought adding an extended shelf to give 3" of over draw and therefore a shorter lighter more aerodynamic arrow might get it shooting further. Video shows the arrows rotating and tipping over at about 45 degrees after a few yards of flight, so there is a lot of tuning work to do. 
https://www.youtube.com/watch?v=YAIq9j6HsfE
Shooting with the shooting machine has it's own problems, clamping the bow too hard can cause misalignment of the string. The papier mache extended shelf shows some promise.

The other problem is arrow mass and spine, the best performing arrows seem to be about 280-300 grain, lighter was giving more speed but less distance in live trials on the field. I haven't got good speed figures using the overdraw shelf and chrono yet. Maybe the arrow isn't flying true through the chrono, I'm also not utilising the full overdraw as I'm short drawing with the machine, for fear of over stressing the wood.

Another question, is the lower limb a tad weak?

Hickory Warbow

The bow is from a 3x2" length of Hickory lumber. I was aiming for 120#, but due to my second rough out being a bit over enthusiastic I ended up with a weak point meaning it would only be 100#.

The belly is heavily heat treated, which helps it get to 100# (at 31") it performs smoothly and better than expected according to JT, who actually wants me to make him one at 120#.
One of the other chaps from the club, Innis, wants an 80# elb, and is happy for me to re-work the 100# as it is still fresh (only had 4 arrows shot from it), so all in all, that's a win-win situation.
The real point of this post is to make note of some dimensions as a reference:-

Overall length (nock to nock) 75.5"
Centre line width             34.6mm    thick 26.1mm
12" along (lower limb)    32.9                    19.3
24"                                   29.2                    18.9
36"                                   16.5                    13.4

Those figure probably don't meet the 5/8 thickness/width rule for an ELb, but once re-worked it will be fine.
To reduce it from 100# to 80# could theoretically be achieved by reducing the width to 80% but that would probably make the tips too narrow, however I can reduce the width a bit and then round the belly to make it more ELB and less AFB.
The build and test shot videos are all on my Youtube channel, here's the link to the test shooting:-
https://www.youtube.com/watch?v=zZrILwkx3sg

I've since re-worked it as an 80# elb mostly by reducing the width and slightly rounding the belly without removing th heat treating from the middle of the belly at all.

Hickory Kid's Longbow (aide memoire)

 I recently got some nice rough sawn Hickory from an old mate of mine (he'd bought up a load from someone who was shutting up shop many years ago), I hadn't done anything with it yet, then my mate JT asked if I had a light weight bow to suit an 11yr old lad (about 20-25# and 25"draw).
It seemed like a good opportunity to saw out a slat of Hickory and give it a try.
Unfortunately I didn't take any pics as I was in a rush to hand it over to JT who was going to finish sanding it and fit a leather grip.
I took it to the club and actually roved with it! I had to treat most marks as a par 2! But I was happy to be shooting something light as my left elbow has been giving me gyp (tennis elbow), I even tried it shooting one of JT's warbow arrows which flew surprisingly far!
Anyhow I got JT to take some measurements for future reference as I'm thinking of maybe trying a 35# Hickory flight bow.

Length 55" nock to nock, 25# @ 24" 
Bow width and thickness at; Nock: width: 10.5mm, depth 9.0mm Mid: width 20.3mm, depth11.2mm Ctr-width: 22.3mm, depth 17.3mm

Good Day at the Club

 I went to our Sunday rove, despite a stinking cold. I had a Yew stick bow to hand over, to replace a smashed 35# Yew primitive. I also collected the smashed one for a post mortem (I couldn't see any problem, other than it being in two pieces!). The stick bow, performed beautifully with arrows flying out with not even a hint of waggle.
I was given a couple of bottles of wine by another guy as a thank you for making him the "Robust Yew Primitive" with which he won his last 3D field shoot.
I wasn't up to shooting having had a flu jab and covid jab in my upper arms the day before, but I had a good natter with the guys, a tot of medicinal rum and I walked out to the first mark with them.
The weather was lovely and a breath of fresh air did me a power of good, knackered by the time I got home though!

Out of interest, I sawed a cross section of the failed bow, near where it broke. It shows rather nicely how deep the heat treating penetrates, how I'd maintained a fairly even thickness of sapwood, despite the heart/sap boundary not following a ring.

Arrow Speed Testing

 I finally got round to testing the reworked shoot through flight bow, (a Hickory Mollegabet, with heat treated Yew belly). It did 216 fps, but jarred my hand and elbow terribly. Investigation confirmed my theory that most hand shock is caused by an uncomfortable grip. Obviously the light flight arrow and high speed is a big factor too.
I've done some reshaping of the grip and ordered some 3mm neoprene foam sheet to pad out the grip, before re-testing. I think there is still a good 2 inches of draw to be had, so I'm optimistic about its performance.
Video of the speed testing here:-
https://www.youtube.com/watch?v=uICsUspyvO0
Video of the bow here:-
https://www.youtube.com/watch?v=tlnfIAbGhH0

More Boo Yew Ipe

 Ah well, my calculations/estimates seem to be a bit too pessimistic, but better too strong than too weak! It's too stiff even after reducing the Ipe a little and rounding the belly. It was about 50# at 12" unbraced.
It was also trying to flip over sideways as it was rather thick compared to its width, also being unbraced and only pulling a few inches doesn't help.
I don't want to make the Ipe too thin, so I've carefully run it through the bandsaw down the middle of the thickest lamination and re-glues it. That takes about 1.5mm from the thickness.
Hopefully once it's finished I'll be able to work out some sort of fiddle factor for boo/Yew/Ipe.

Boo Yew Ipe Flight ELB draw weight estimation

I'm trying to build a flight bow, but the belly lamination will be an Ipe slat which is 4.5 mm thick and this doesn't leave a lot of material for tillering. 
So I'm tillering it as Boo Yew stave and trying to estimate how the extra 4.5 mm of Ipe will increase the draw weight. Just for ease of calculation I'm assuming the Ipe will be the same as the Yew (it is stronger in compression, but I'm just trying to get an estimate.

Currently the bow is 40# @ 17.5" which correlates to 61.7 @ 27" draw (short draw flight bow).
Mid limb thickness is 16mm. 16 cubed is 4096 (stiffness being proportional to the cube of the thickness)

If I add 4.5mm to the thickness this gives 20.5 which when cubed is 8615.
8615/4096 = 2.1 so adding 4.5mm would give 2.1 times the draw weight! 129#
Hmm, looks like I need to take more off the belly before glue up.
Trying to calculate an appropriate draw weight and length without the Ipe to give the desired final result with the Ipe is beyond me, so it's down to the old successive approximation. Keep doing a bit and re-calculating.
BTW. Target draw weight is somewhere between 60 and 80#, so I have some margin to play with.

I've done some more work, but this is all rather guesswork. Adding 4.5 mm (I'm calling it 4 to allow for finishing) won't give the same effect at the tips as at mid limb. Say the tip is 12mm thick an extra 4mm is making it 33% thicker whereas near the grip where it is say 20mm thick it's only adding 20%. I s'pose taking the mid limb thickness isn't too bad.
I've currently got it drawing 19" at 35# with a 15mm thickness mid limb, which is giving a finished estimate of 101# . Still a bit high, if I can get the estimate down to below 90, I think I'll go for glue up.