Wind Tunnel Update

I got the new improved windtunnel running and the tried my reference 9.5" nock test shaft. It started off stable then started to wobble and ended up with the nock end swinging round in circles almost touching the wall of the tube... Hmmm?
I slipped a piece of slit pipe down inside the flared section and the stabilised! I took noted the test figures and the figure for the drag was exactly the same as previous test.
Oh well it looks like the fancy wooden flared inlet will be coming off!

Wind Tunnel

I'm now on the mk2 wind tunnel :-)
The mk1 proved the principle and gave some useful figure for drag. It needed some baffles to help achieve linear air flow as the rotating fan created a vortex, which ironically held the arrow steady in the centre but made it spin V rapidly.
Mk1 arrow wind tunnel

I've reverted to the 65mm drainpipe for the mk 2. I've made a grid of thin sheet metal (from the back of a old scrapped oven) to smooth the airflow, this is pushed up into a cardboard tube. The plastic drainpipe fits down inside the cardboard tube with the slight gap between two blocked off with draught excluder. This arrangement gives a nice clean interface between the grid and allows me to try longer pipes or other arrangements.

I've made a flared wooden inlet for the top end of the pipe, as a further possible improvement, mostly just for the fun of doing it... dunno if it will make any difference.

As a test I made up some 9.5" lengths of 3/8" Ash arrows shaft with different nock styles the other end of the shaft was tapered and had a thread glued to it to allow the shaft to dangle into the pipe.
The results were nicely measurable as were as you might expect with the tapered nock having less drag and the one cut square having most.

Lockdown Ideas and Distractions

Arrrghhh... no pictures!
I've been doing more videos on my Youtube channel "Del Cat"... (it's not monetized, no damned adverts to wait through). I've had some good reaction and even done one video in response to a specific question.

I've been toying with the idea of making a wind tunnel to measure the drag of arrows. It would be a vertical pipe with a fan at the bottom drawing the air through. The arrow would be suspended from its point by a fine thread which would connect via a lever which pressed down onto my digital grain/gram scales. Switching on the airflow should in theory increase the apparent weight of the arrow as the airflow pulls it down due to the drag.
The main problem is getting a fan with sufficient airflow, but then I thought of my dust extractor!
It's spec says 183 cubic metres per hour air flow. Now it's just a matter of some arithmetic to convert that into feet per second (fps) of air flow through a 60mm drain pipe. Unfortunately it comes out to about 52fps which is much slower than a flight arrow (~200fps or more).
On closer examination of the fan assembly from the dust extractor I found the air inlet is only 30mm diameter which is half the diameter of my drain pipe and thus 1/4 of its cross sectional area... ha, now if I make the pipe 30mm diameter, that would increase the air speed to about 200fps.
There are nice commercial dust extractor fans which shift 100 cubic metres an hour, but they are about £90 , rather big and probably noisy... not worth it for something I'll prob' only ever use once... not enough room in the garage for a decent wind tunnel really.... I could doubtless fill a large barn with stuff given half a chance.
Of course this is all rough calculation back of an envelope stuff (other writing surfaces are also available, although backs of fag packets have now been largely superseded).
There are many problems with wind tunnel design, even a crude one, but there is tons of stuff online for anyone interested. One obvious problem is that as the pipe diameter gets smaller the arrow can hit the side and the diameter of the arrow starts to become a bigger factor in blocking the airflow than its genuine drag. Still it may provide useful comparisons between similar diameter arrows with different tapers, points or fletchings... ah, that gives me an idea... a relatively short vesion could just test fletch configurations, drag vs area of fletch!

Anyhow, no rush to leap in doing it yet as I have another distraction... I bought myself a blues harmonica! The rest of the family are V musical but I'm not, I can sing, but if someone harmonises I get immediately phase locked into their notes!
I'd been given one as a kid one Christmas, and we had to call out who had given us what present so that we could all write our "thank you letters " when the excitement had died down. When my Dad asked who gave me the mouthorgan, I gleefully replied "'Our Monica!" (It was from Uncle Morris and Aunty Monica)
I had no idea how to play it and of course in those days, if you wanted to learn it would be from a book or a record if you were lucky. Nowadays with the wonders of Youtube it's easy to find a tutorial that suits your aspirations. Within a few hours of the blues harp arriving (see I already have the lingo!) I could make a reasonable sound and even managed a rather lumpy jam with my Son on his guitar. It took me a good 20 minutes to get the grin off my face.

I hope you are all finding stuff to do, keeping safe and refraining from drinking disinfectant.

Needle Bow II

I had a skinny offcut of Yew which I patched up to make it thick enough in the middle and then glued on a scrap of Ash as a backing, it's only 55" long and no wider than my thumb at the widest, the upper limb is even narrower.
I just did it for the fun of it and even had to overlay a patch on the side of the tip to correct the string line due to some waggle at the end of the limb. It's about 33# at 27" and shoots very nicely being so narrow. I did a video of the whole process from glued up to being shot:-
https://www.youtube.com/watch?v=juag-r3zlD8&t=77s

Ash Backed Yew

 I've made an Ash Backed Yew for myself I was aiming for 80# @ 32" but I may be a whisker under as I was using some scruffy offcuts of Yew glued up as billets. Once the billets were Z spliced together and the Ash back glued on (with about 1" of reflex at the tips) I could see it was a tad thin near the centre so I glued on a strip of similar Yew about 4mm thick, which bought it up to just about enough thickness. It made it a bit tricky to tiller as the added on section was a bit stiff at the very centre but still left a slightly thin area where it faded out. I made a video of the whole tillering process as someone had been asking about it.
Del's Tedious Tillering Video
I haven't actually got round to making a string for it yet so it's not been shot... mind I can only shoot it at 10 yards at the moment.

Whilst sorting my Yew and sawing out the various billets I created a skinny Yew off cut, (top left pic) once again, this had a rather thin area, so I added a patch (top right pic) and then glued the back on so that it was over the patch, leaving a nice clean belly.The back was a similar Ash off-cut making a rather thin 55" bow, this has been a good quick tillering exercise, with most of the cleaning up and wood removal being done on the belt sander! I'll edit a video of that later on, it'll probably just about make 35#@27" and will be called Needle Bow II.

The Corona virus lockdown hasn't been too bad for us as we're luck enough to have a fair amount of room, a reasonable sized garden and stuff to keep us busy... I must admit I was sufficiently bored at one point to resort to cleaning the cooker hood, I also cleaned the car which is something I usually do about twice a year. I really feel for those stuck in small flats with young kids and no garden. We live in strange times.

I couldn't resist adding the pic of the Alconet flowers, it's rather invasive, but the bees love it. Small flowers but very cheerful, there is a lot of blue in the garden at the mo', vast swathes of Forget-me-not and the Rosemary has it's dusky blue flowers too

Crossbow Prod Details

One of my online friends asked me about the poundage obtainable from some 20mm wide glass laminated crossbow prod of say 36" long
Of course it depends on plenty of factors... is it a one piece prod or individual limbs which have several advantages but some disadvantages.
Individual limbs can be adjusted to angle back or forward with tapered shims and will allow a centre shot, the downside however is that you sacrifice some working limb length in the mounting clamps.

Anyway I said I'd provide some pics and dimensions which should allow him to do some back of an envelope calculations to get some sort of ball part figure.
One thing to note is the nock which has a substantial horn overlay and is bound with carbon fibre soaked in epoxy, this is needed to stop the string cutting into the limb tip and splitting down it.
Dimensions:-
Overall nock to nock 37.5"
width x thickness at root of limb 44 x 8.3mm
    "              "        "       mid limb 37.5 x 6.8mm
    "              "        "       tip            25.6 x 5.6mm
Brace height 3" power stroke 12" draw weight ~90#

20mm wide glass seems a bit narrow, sow maybe two pieces could be used side by side to give 40mm , obviously that's easier to say than to do as glue ups are notoriously slippery, but maybe two bits could be joined edge to edge with a few tiny spots of epoxy to hold them together?

All this sharing of info seems to have increased during the Covid19 lockdown as people are communicating more. I did a You tube video for one bloke giving hime more details of my belt drive conversion on my mini mill.
Any how keep busy one and all and try to stay safe and sane!

Crossbow Bolts and Warbow Arrows

There's been a lot of interweb activity due to Coronavirus lockdown, much of it has been an irritating waste of space, but there have been some sensible discussions.

One chap was questioning if warbow arrows were sorted in spine in any way... some people chipped in asking why "random arrows" would be all put in a barrel together. Of course that sort of comment is weighted to support his own position... the arrows were not "random" they were made to some sort of specification (albeit, maybe not a written one) in that they would have been suitable for warbows, say 3/8 - 1/2" shafts, big long fletchings (3 per arrow), heavy head, between 28 and 32" long. We know from documentary evidence that the shafts and heads were not necessarily assembled where they were made, so sheaves of shafts and barrels of heads would be sent up to the Tower of London. maybe one bloke cut all of his shafts the same length, maybe they were trimmed to length when they were made into arrows.
The point is we shouldn't jump to conclusions. What we do know is that measuring the spine of an arrow shaft by hanging weights on it and doing arithmetic is a modern idea. Now that doesn't mean that a native making arrows for his bow didn't flex 'em to select those he liked best or flew best, but, spine wasn't actually "measured"...
Ah, but does it even matter?... The bloke who asked the question had no real experience, and was just quoting what he'd read (sigh). So, as well as stating my experience I thought I'd better back it up with an actual experiment.
Does the spine of a Warbow arrow matter? Well we know they fly pretty well out of heavy warbows, but what happens if you shoot one out of a 70#@32" bow? It should theoretically be over stiff in terms of static spine*. Then if we shoot a 5/16" 30"arrow of 40-45 spine from the same bow what happens? Does the sky fall in?
Well the heavy arrow hits the boss square on at 10 yards having straightened out already due to the heavy head and big fletchings. The light arrow is still flexing as it hits at 10 yards, but they both fly in a perfectly usable manner. (see pic)

To keep me busy I've been turning up some 3/8" points for my heavy 275# repro "Light Sporting Crossbow" It uses up some odd Ash shafts I have lying around and some of the feathers I picked up the other year. I'll post some pics and video when I test them in the crossbow.
I've finished one bolt and tested it from the crossbow. It flew nicely at 10 yards, I also checked the spin by lobbing it up in the air underarm and watch it spin as it fell back down. The 2 fletchings are slightly offset to enhance their natural curve to induce spin. You can see on the pic of the fletching jig I have stuck a bit of white card under the front edge of the clip that holds the fletch, that just lifts it a tad.


* For those who don't understand the difference between static spine and dynamic spine.
Static spine is simply how the shaft flexes when a weigh is hung from it's centre... but when it is shot from a bow there are other factors like the weight of the head... if the head is very light, there is very little to stop the arrow accelerating, so there is little force bending it. With a heavy head, there is lots of inertia, so rather than accelerate forwards the arrow will flex.
Taking this to the limit, and a 50 spine arrow with a 15 grain head can be shot from a warbow. Conversely an infinitely heavy head (which is simulated when you over draw and jam the point of the arrow into the belly of the bow) will cause the same 50 spine shaft to explode! The length of shaft also effect dynamic spine...if you pick up a 3 foot length of scaffold pole and give it a shake it feels rigid. If you pick up a 20 foot length holding it at the centre and jerk you hand up and down you can feel it flexing.