Continuing to be nosey… I Coldn’t help reading the articles on knot strength.
I spent a long time tying knots, not a great variation, in quantity. The emphasis was not on the ability of the knot to withstand " X, Y or Z ? physical punishment during use . The important point for ME was when the cord BROKE. The knot was the LOCK…not the KEY.
Parachute packing requires cord to break at a specific point in its strain window. If it dosn’t break then someone hits the deck.
Nobody brought one of my chutes back…saying it didn’t work.
Back to where we started…It’s not just what type of cord…are all test samples the same…is the tension the same. There are many other factors to consider.
Just a few more spanners in the works I can?t see an empirical answer to the question " which knot is better than an other ? " there are to many unknown’s: Storage, handling, contamination, life span and remaining life span. If you tighten a stretcher harness to quickly it reacts differently to one tensioned at a slower rate.
A wet rope ties differently to a dry one ?..? same principle ?
However I do realise the need is to understand what something is all about. After all the end of the day I’m an engineer…if I can’t see how it works…I don’t understand it…therefore I need to know how it works. It’s a vicious circle.
??! Please expand on what of parachute packaging depends upon the breakage
of some knot–I have a book on parachutes ([i]The Parachute Manual) and don’t
recall any mention of using some knot for the sake of purposefully weakening the line.
And what was the knot selection for this?
There IS such consideration given in the USA-Maine on long-/ground-lines for lobster
“trawls”, such that if snagged by open-mouth-feeding & endangered Right whales
the line is ensured of rupture at a load deemed not too harmful for the whale. (A
better tact is now taken in having boyantly neutral (not fully “sinking”, which risks
entanglement w/surface stuff) longlines.)
I have spent the day considering my comments regarding parachute packing. So much so that I have made contact with the Central Training School for my past trade. Asking for assistance to clear up the weakness in my comment.
Until then I will try and offer a clearer picture of my comment. Unfortunately I can only talk from memory, another reason for seeking assistance from an authority.
The parachute in question is an Airborne Forces static line deployment parachute. This parachute would be used at a lower deployment altitude than a sporting static line deployment parachute. Therefore it does not, I?m lead to believe, include the subject: ?the Mouth Lock Tie ?
The mouth lock tie is a length of cord passed around the parachute rigging lines and tied together. Cord and knot construction are unknown. It is tied just below the top of the rigging lines and below the canopy periphery.
As best as I remember this tie was in place to speed up canopy deployment. The first phase of deployment is the air stream enters the restricted canopy inflating the available space. At this time the mouth lock tie is still in place. When the restricted canopy can not inflate any further the mouth lock tie or the cord breaks. This allows the canopy to inflate completely at speed. I?m now wondering which does break first ? ~ underlined above
I?m also beginning to wonder if there is an additional reason for this tie. Would it restrict or stop a canopy Roman Candling ? failing to inflate because the rigging lines twist themselves up. If I remember correctly.
Dan?.I hope this helps. Its just made me more curious for answers.
Wow! That is interesting: a binding (?) whose most important quality is to break (fail) when a specified force is reached. I wonder if readers know other examples where a rigging is designed and tied to fail.
You have stepped right onto the hot coal that has been dogging any progress in this area since members started discussing it.
Because there are so many ‘what abouts’ and ‘what ifs’, this thing has escaped being brought to the bench because it would require a huge budget and a full research team in order to do it justice – and that just ‘ain’t gonna happen’. We stand on the edge of this swamp and stare at its complexity – The ultimate Gordian Knot and the question – “where to start?”
Alchemists must have felt a similar awe when they stared at the complexities of the world in reaction. Yet the world of ‘Chemistry’ has been trodden flat and nailed into our science books. To understand the science of real knots I believe we must start to tread some pathways of understanding into our field.
That is why I am proposing we tread out possibly the simplest path of all, a ranking of comparative weakness for the knots and their variants that we know, in one single type of cord, and why, through a rigorous testing protocol, we should nail every conceivable variable as we find them (we can always go back and vary them to see how the path wiggles). This is why it is most important is to have a Peer Group who will vet the dressing of every knot before it is tested. Only then will we know exactly what was tested under the name of (say) The Double Loop Larkshead.
Along the way of making that thin path, we might discover some logic to the organisation, or we might discover nothing. Either way, we will discover new questions (some you have highlighted without even marking out the line) that can perhaps be followed in their own greatly simplified manner. Gradually we should be able to tread flat the whole of our field and progressively write it into our own science books.
I think ‘Weak Links’ are often used where a limit to the applied force is required. My first experience of these was on my first hang gliding lesson – a ‘buddy tandem’ session, towed up by a micro-lite. The big ‘two man’ hang glider was sat onto a small wheel base for takeoff and clipped onto the back of the micro-lite. As we started to roll, a wheel found a hole and with a twang, the line parted. It rattled me a bit, the gear had broken and we hadn’t even got off the ground!! The instructor explained that it was just the weak link, designed to break if we hit an obstacle so as not to damage the airframe of the hang glider. The micro-lite came back around and the two cables were joined with a new piece of thin string taken from a packet labeled ‘Weak Link Cord’ and off we went without further interruption into my very first and very cold hang glider flight. Later on, flying solo, we were launched by a winch which again had the little bit of ‘weak link’ string just a few feet ahead of the airframe link pin.
The strange thing is that I never gave them another thought until today. I had always thought that the much thinner string was much weaker than the tow lines and that the string just broke when its breaking strain was reached – but of course I was wrong !!
The string did not break, the knot did, and what I know now about knots, dependent upon the knot used, that break point could vary by well over 200% and those knots could have been anything.
Does anybody know how these weak links are calibrated and supposed to be tied?
Just a daft idea?one thing I am good at?daft ideas.
Financing research into knot strength and testing.
Ever considered finding a University Graduate interested in engineering that may be able to design a machine or rig suitable for testing cord and knots. A mathematics? graduate interested in the kind of forces knots may be exposed too. You may even want to consider the need for a statistician. One final consideration could be a robotics engineer to design or write the machine?s programme. The last suggestion would rule out objectivity on behalf of the individual vetting the knot prior to testing. Is there a reason why a robot can not tie a knot. CNC machining and milling has been around a few years now: 0.05mm tolerance is close enough for most engineering needs these days.
I?m fairly sure sponsoring a student would be cheaper, for want of a better word, than going it alone. It could be a ? win win ? situation?student graduation and the guild gets the knowledge and accolade for sponsorship. Who knows you may even get them hooked a new skill.
Whilst I?m here?I had planned to present another example of a cords use for its breaking point. However I can?t get back into the web site which I had intended to use as a link. Can someone see if the following site works for them. I keep being told my computer is not set up for a connection port to www.http I was there earlier
Otherwise I wouldn?t be able to tell you where to go?.no offence intended. Nor would I presume to being here.
If or when you get there you are looking a sub menu ? aircraft interior ?. Once there the subject I?m describing is the forth photograph down?the yellow thing. The post regarding the ? yellow thing ? will follow in a few minutes?.I hope.
Q. 4 Derek S please elaborate on your final question?..How ?
The following is another example of where a cord is expected to break. This time it?s in my specialist area. The previous example was not my strongest subject.
First you need to click on the following link I NOW CAN NOT GET and scroll down to the forth photograph. On the left there?s a large yellow oblong. It?s a ten-man liferaft strapped into the aircraft.
The view you are looking at is of the top of the liferaft container, which is constructed from four flaps of waterproof fabric. The base is made of GRP Fiberglas.
The side flaps, long flaps, have webbing beckets sew into their leading edge. When the liferaft is stowed and the container is fully closed the two sets of beckets meet.
The method used to keep this container closed is as follows: A single length of 47lb nylon cord is tied to one of the end beckets. The knot is a bowline with the flying end served with a self- adhesive white cotton fabric tape. The cord is then feed through all the beckets, keeping as much tension on as possible. The final tie is 5 ? 7 half hitches tied to the last becket and served as before. Once this is in place and secure the temporary tie is carefully removed and the container left to rest. Temporary tie 450lb red para cord. All cordage is referred to by its breaking strain. I presume it?s the same today.
When the liferaft is inflated, in anger or for training , it breaks the 47lb nylon cord. The knots stay in place and generally have a few centimetres of cord still attached at their attachment point. Hence the reason I claim to have used cords that are expected to break.
Finally each photograph of the interior of the aircraft has at least one piece of equipment I was responsible for. I suppose in some sense it was my Ditty Bag.
" Weak Link " the term in Derek Smith’s last reply hit a cord.
There is a piece of equipment used on Seach and Rescue Helicopters that had a cord desigined to break at a specific point. It was documented as a deliberate " Weak Link ".
Another thing that has just come back into my memory 99% of the knots / ties I used were served by: wrapping with a specific adhesive tape, whipped with a common whipping or stitched through.
By served I mean the " Flying Ends… as I was taught ".
I learned all my knotting skills and uses EIGHT YEARS before this guild came into existance. My scouting experienceses were in reallity… just to get a badge…a status symbol.
Up until a few weeks ago I didn’t know the guild existed. My couriosity in narrow boat fenders and their construction now appeears to have dropped me right in it. Do I regret it ? …ask me if anyone ever pushes the right button !
I suspect that it is more accurate to say that the string broke in the knot. My informal testing is to attach both ends of a length of fishing line (e.g. 50# braided Dacron) to carabiners and pull (or jerk). The break always occurs near where the standing end enters the knot. I will need to dig out a magnifying glass to be sure, I suspect the break is usually close to where the standing end makes its first tight bend.
In any case, if the typical knot weakens the line by 50%, and the line is 47lb test, then the knotted line would be expected to break around 25lb. For many applications it may not matter whether it breaks at 20 or at 50. If greater precision is required, it may be better to use a manufactured ‘weak link’, such as engineered piece of plastic.