Author Topic: airplane tie-down knots  (Read 24644 times)

knot4u

  • Sr. Member
  • *****
  • Posts: 1076
Re: airplane tie-down knots
« Reply #30 on: November 06, 2012, 09:13:35 PM »
quote knot4u
All due respect to pilots, but wtf is that?

so, i think you can plainly see why i don't like the way pilots tie down their airplanes.  with any luck, i'll be able to actually teach pilots how to tie a trucker's hitch, but i have little faith they will learn it well.  a versitackle, being a few steps more complicated, would be even harder to teach a pilot, as most of them are already incapable of higher knotting.  also, to tie a good trucker hitch, i need a good loop to teach, that is easy to learn and even if they fail miserable tying it, it will still be secure.  any ideas?

The video provided by Sweeney is about the easiest way to tie a Trucker Hitch.

http://youtu.be/1qinEa2PN7A

To me, it looks easier than the example knots you provided that you say pilots use.  The guy in that video explains the whole thing clearly in one minute, and the video is not too short either.  As Sweeney said, an airplane tie-down is an application where the Overhand Noose in a Trucker Hitch is acceptable because the user is not cranking down hard to generate high tension.

I otherwise do not like an Overhand Noose in a Trucker Hitch because of its tendency to bind up under high tension. Also, a slip loop at that location is less secure than a fixed loop.
« Last Edit: November 06, 2012, 09:34:51 PM by knot4u »

Luca

  • Sr. Member
  • *****
  • Posts: 375
Re: airplane tie-down knots
« Reply #31 on: November 07, 2012, 12:05:02 AM »
Hi X1,



   In a "series of Half hitches", only the last one is a genuine, proper half hitch. All the others are nipping loops, because both their legs are loaded ...
   Now, my point is that "two Half hitches" ( i.e., one nipping loop and one half hitch ) are better than a doubled one - and four "Half hitches" ( i.e., three nipping loops and one half hitch ) are better than two doubled ones ( i.e., one double nipping loop and one doubled half hitch, like the one shown in the picture of kd8eeh )
   One may ask : Why then we use double nipping loops, and doubled half hitches ? I believe that this has to do with other things ...


Yes,OK,but talking about the(practice)contingency regarding of when someone runs(moment after moment) the knot of we're talking about,in my opinion , one runs(of course, unaware of better solutions) this"doubling "of the(first) Half hitch because maybe someone merely asked himself:"How can I prevent it from tipping over?"Because, I agree with what you write, but until  will not run  another Half hitch during the execution of the knot(moment after moment), the last executed at that point will be a half hitch, and not a nipping loop, this also applies especially for the first, that is not "closed "(remains a no tightened loop)like the others which follow,and that, again, in the contingency of the moment, is what holds all the load that one decide (or find himself) to apply before it is realized the second.

knot4u

  • Sr. Member
  • *****
  • Posts: 1076
Re: airplane tie-down knots
« Reply #32 on: November 07, 2012, 01:05:26 AM »
Yes,OK,but talking about the(practice)contingency regarding of when someone runs(moment after moment) the knot of we're talking about,in my opinion , one runs(of course, unaware of better solutions) this"doubling "of the(first) Half hitch because maybe someone merely asked himself:"How can I prevent it from tipping over?"Because, I agree with what you write, but until  will not run  another Half hitch during the execution of the knot(moment after moment), the last executed at that point will be a half hitch, and not a nipping loop, this also applies especially for the first, that is not "closed "(remains a no tightened loop)like the others which follow,and that, again, in the contingency of the moment, is what holds all the load that one decide (or find himself) to apply before it is realized the second.

Drop a few periods please. That's difficult to read.

kd8eeh

  • Exp. Member
  • ****
  • Posts: 159
Re: airplane tie-down knots
« Reply #33 on: November 07, 2012, 01:19:41 AM »
are you using swipe?

X1

  • Inactive
  • Sr. Member
  • *
  • Posts: 1200
Re: airplane tie-down knots
« Reply #34 on: November 07, 2012, 01:49:29 AM »
the last ... at that point will be a half hitch, and not a nipping loop

   Noope !  :)  The last knot would be a Half hitch, iff it will remain the last, and only during the time it remains the last - iff it will remain tied around the penetrating main line without any further involvement of its tail. If we are going to tie another, second half hitch after/below this first one, around the same line, we have to use the tail of the first as standing end of the second, so this tail will not remain a tail any more... Moreover, to tie this second hitch, in a way that the first ex-half hitch ( and now nipping loop) and the second half hitch work in tandem, and grip the main line together, the ex-tail of the first ( and now standing end of the second ) should be loaded - otherwise the second half hitch will be just a loose bight wrapped around the line...There is no point to call a bight around the main line "a half hitch", if both its legs are unloaded !
   There is nothing strange about a knot being something at one point, and being transformed onto something else later on. At a given instance, there is only one proper half hitch in a " series of half hitches",  the last one. The others are nipping loops. It does not matter what they were before this instance - now they are nipping loops, because both their ends are loaded - and this series of nipping loops is closed by a half hitch. 
   When we see a segment of a rope, we can not see, but we can know if it is tensioned or not - and we should call it with its proper name.
   There are also some indications/signs that can reveal its true character. A proper half hitch is able to absorb the tensile forces acting on its loaded end, because one part of it is "riding" on another part - and this other part, that is squeezed underneath the "riding" part, in between the "riding turn" and the penetrating main line, can not slip. At a nipping loop, the two legs can be crossed, but none of them absorbs the 100% of the tensile forces acting on the other leg - both are tensioned, and the direct consequence of this bilateral tension is the shrinking of the diameter of the loop and the nipping of the penetrating main line.
« Last Edit: November 07, 2012, 01:56:23 AM by X1 »

Luca

  • Sr. Member
  • *****
  • Posts: 375
Re: airplane tie-down knots
« Reply #35 on: November 07, 2012, 04:17:25 PM »
Hi X1,

the last ... at that point will be a half hitch, and not a nipping loop

   Noope !  :)  The last knot would be a Half hitch, iff it will remain the last, and only during the time it remains the last - iff it will remain tied around the penetrating main line without any further involvement of its tail. If we are going to tie another, second half hitch after/below this first one, around the same line, we have to use the tail of the first as standing end of the second, so this tail will not remain a tail any more... Moreover, to tie this second hitch, in a way that the first ex-half hitch ( and now nipping loop) and the second half hitch work in tandem, and grip the main line together, the ex-tail of the first ( and now standing end of the second ) should be loaded - otherwise the second half hitch will be just a loose bight wrapped around the line...There is no point to call a bight around the main line "a half hitch", if both its legs are unloaded !
....

But(even if my above words seem to contradict) I totally agree with all this!(Indeed, perhaps one could talk if even  the last performs the function of a Half hitch in the structure as is presented ..)and I do not discuss that, looking at the finished structure shown by  kd8eeh, the "doubling" of the first Half hitch(shape)appears to be superfluous;my speech,in the end, was a conjecture about  the practical reason because of which it is however made during the construction of this structure.So,you're right, because I would have been more specific, I would have to speak not of "Half Hitch", but of"Half hitch in shape but not in function" ,given that it is pulled also by the tail, and not only by the SPart;but I think that it is precisely for this reason that, being the first, is in danger of tipping over when it is pulled by the tail,so maybe someone has thought:"let's avoid this risk!",then maybe decided to apply this "doubling".
In short, the gist of my argument is this:a single Half hitch (tied around a ring or pole)is likely to tip over if it is pulled by the tail before it is tight around the object.

                                                                                                            Bye!

P.S.I apologize to everyone for not writing well my previous post (and many other posts I've written), unfortunately my writing in English is based on Google translator and it is not always easy for me to express myself in an appropriate manner, and therefore not I have nothing else to say except thank you all for your patience!

Knot_Easy

  • New Member
  • *
  • Posts: 9
Re: airplane tie-down knots
« Reply #36 on: November 07, 2012, 04:35:00 PM »
Something that has been overlooked in this thread is that aircraft sit on an undercarriage that, with varying degrees of stiffness, allow a huge range of movement.  Apart from making sure there is no slack,  it is almost irrelevant how much tension is put on the rope because the plane will always be able to bounce on its tyres.
Certified conventional aircraft have to be able to withstand a landing (without damage and at maximum weight)  at a  400 ft / minute descent rate.  This is the equivalent of a free fall drop of 4 metres (12') - not something you actually want to experience.

What this means is you will probably pull the wings off long before you completely compress the undercarriage.

Also too much tension, even with the plane at rest, can cause damage due to temperature variation.  This affects not just the pressure in the tyres but in some aircraft the pressure in the oleo struts that make up the landing gear.

So all the tension that is needed can easily be applied with one hand and any sort of hitch or loop.  Therefore the most important consideration is the knot?s resistance to jerking loads.

I have never had any problems with a bowline and a round turn and two half hitches.


roo

  • Sr. Member
  • *****
  • Posts: 1838
    • The Notable Knot Index
Re: airplane tie-down knots
« Reply #37 on: November 07, 2012, 05:33:24 PM »
Something that has been overlooked in this thread is that aircraft sit on an undercarriage that, with varying degrees of stiffness, allow a huge range of movement.  Apart from making sure there is no slack,  it is almost irrelevant how much tension is put on the rope because the plane will always be able to bounce on its tyres.
Certified conventional aircraft have to be able to withstand a landing (without damage and at maximum weight)  at a  400 ft / minute descent rate.  This is the equivalent of a free fall drop of 4 metres (12') - not something you actually want to experience.

What this means is you will probably pull the wings off long before you completely compress the undercarriage.

Also too much tension, even with the plane at rest, can cause damage due to temperature variation.  This affects not just the pressure in the tyres but in some aircraft the pressure in the oleo struts that make up the landing gear.

So all the tension that is needed can easily be applied with one hand and any sort of hitch or loop.  Therefore the most important consideration is the knot?s resistance to jerking loads.
.

Compression of the landing gear and pretension of the rope immensely helps absorb energy.  The energy such a pretensioned elastic system absorbs corresponds to the area under the Force (y) versus displacement (x) curve.

When you have pretension, the curve starts out higher, and can immediately absorb energy that otherwise would be accumulated into dangerous kinetic energy.

When you don't have pretension, the area under the curve starts out very small and can't absorb as much energy as quickly.  With slack, the energy absorption rate is zero for the entire range of the slack and really accumulates dangerous kinetic energy.

Try punching yourself:
1) With your right fist held tightly against your stomach with no gap.
2) With your right fist held lightly against your stomach with no gap.
3) With a one or two inch gap between your fist and your stomach (if you dare).

The landing gear and your tiedown should be a fairly elastic system such that changes due to temperature cause minimal changes in force.  Having an elastic aspect to the system also prevents force spikes due deflection, while pretension helps absorb large amounts of energy quickly.

I'd hate to see more people loosely tying down their loads or even leaving slack (! :o) as a result of this thread.  The state of affairs in load binding is already so poor nowadays that I hate following people on the highways with tied down loads.
« Last Edit: May 05, 2014, 03:55:34 AM by roo »
If you wish to add a troll to your ignore list, click "Profile" then "Buddies/Ignore List".


Dan_Lehman

  • Sr. Member
  • *****
  • Posts: 3903
Re: airplane tie-down knots
« Reply #38 on: November 07, 2012, 06:04:29 PM »
This discussion is losing sight of fundamental things : what is needed
for the aircraft.  We've read aircraft-official recommendations for having
some small amount of slack in the tie-downs, lest they result in some
damage to the structure.  Now, apparently we don't know how this comes
to be (the damage).

We've also see some *official* suggestions for tie-down material,
which amounts mostly to using a synthetic line to avoid rotting
and to have what is regarded as adequate strength (of about 3_000#).


Roo I think goes wrong (& I)  in thinking that slack will lead
much increased impacts : I'll now surmise that the wing(s)
won't rise so quickly as to be shocked on coming taught with
the lines, but that they have some overall resistance to slow
the rise; and that this amount of movement and involvement
of the entire wing is what is wanted BEFORE the line tension
figures significantly!?  --my conjecture.  (Which then leans
hard on how "1 inch" & "some" can be universally correct
across various aircraft --not to mention tie-down material?!)


As I've stated previously, it still seems a problem to recommend
a specific slack amount ("1 inch", or similar) and yet not to specify
the nature of the line ("static", low-elongation, moderate- to
high-elongation).  I surmise that the plot of forces upon given
windage and various lines will look different, and maybe could
be modeled by keeping the line constant but varying the slack
distance or pre-tension amounts.  But the so-far-seen official
recommendations don't include all of these factors.  (E.g., that
an elastic, laid-nylon line would not put too much stress on
the wing on the initial rise but then come into strength later,
adequately?!)

Gordon's Knotting-in-the-Wild observations area awaited; but we
can keep in mind those of my own re cleat hitching, and what sort
of mess of that I found (which is admittedly a task affecting mostly
ease of doing/undoing, not of attachment security).  Already, from
some of the on-line sources we've seen exposed above, we can
see different methods used.


--dl*
====
« Last Edit: November 09, 2012, 07:03:08 PM by Dan_Lehman »

roo

  • Sr. Member
  • *****
  • Posts: 1838
    • The Notable Knot Index
Re: airplane tie-down knots
« Reply #39 on: November 07, 2012, 06:33:26 PM »
Roo I think goes wrong (& I)  in thinking that slack will lead
much increased impacts :
Slack likely won't be a problem in low to moderate wind conditions.  I'm concerned about high wind or hurricane conditions where there is a sudden onset of a high energy gust.  That energy has to go somewhere.  Some of it will be taken by the decompression of the landing gear (more precisely an increase in potential energy [mgh]), but any energy in excess of this will be converted into kinetic energy in the plane.

By the way, I remember seeing a similar 1 inch rule in some transport manual, but 1 inch of slack wasn't called for.  Rather, they would gauge pretension in the wire rope by making sure the line couldn't deflect sideways more than 1 inch under 200 lbs of force (like plucking a harp).
« Last Edit: November 07, 2012, 06:59:29 PM by roo »
If you wish to add a troll to your ignore list, click "Profile" then "Buddies/Ignore List".


Sweeney

  • Global Moderator
  • Sr. Member
  • *****
  • Posts: 975
Re: airplane tie-down knots
« Reply #40 on: November 07, 2012, 07:23:03 PM »
Having read a number of technical specs (most of which went over my head I admit) I think I'm beginning to understand the problem.

Aircraft wings have to be able to take 2 loadings - a positive flying load load when flying normally and an inverted flying load when upside down. For normal small aircraft (this is hugely different for acrobatic planes) the maximum permitted inverted flying load is less than half the normal load. If tie down lines are too taut then this load capacity can be exceeded because all of the force is pulling the wing in the inverted direction via the tie down and structural damage can result. Allowing slack lets the plane rise slightly and only when this upward movement is restricted by the tie down is there an inverted flight load but by now a good deal of the wind lift has been taken up by the aircraft weight - normal flight load. I think that makes sense. What is clear is that this is not the same as tying down eg cargo where no movement is wanted.

Barry

roo

  • Sr. Member
  • *****
  • Posts: 1838
    • The Notable Knot Index
Re: airplane tie-down knots
« Reply #41 on: November 07, 2012, 07:33:04 PM »
Having read a number of technical specs (most of which went over my head I admit) I think I'm beginning to understand the problem.

Aircraft wings have to be able to take 2 loadings - a positive flying load load when flying normally and an inverted flying load when upside down. For normal small aircraft (this is hugely different for acrobatic planes) the maximum permitted inverted flying load is less than half the normal load. If tie down lines are too taut then this load capacity can be exceeded because all of the force is pulling the wing in the inverted direction via the tie down and structural damage can result. Allowing slack lets the plane rise slightly and only when this upward movement is restricted by the tie down is there an inverted flight load but by now a good deal of the wind lift has been taken up by the aircraft weight - normal flight load. I think that makes sense. What is clear is that this is not the same as tying down eg cargo where no movement is wanted.

Barry
This argument can be used the same way to argue for pretension.  As wind load increases, the inverted flight load is relieved by the normal flight load.  As long as your initial preload is within what the wings can take, all is good.

In the pretension scenario, you have the added benefit of avoiding impact loading.
« Last Edit: November 07, 2012, 07:42:45 PM by roo »
If you wish to add a troll to your ignore list, click "Profile" then "Buddies/Ignore List".


Sweeney

  • Global Moderator
  • Sr. Member
  • *****
  • Posts: 975
Re: airplane tie-down knots
« Reply #42 on: November 07, 2012, 08:19:35 PM »
This argument can be used the same way to argue for pretension.  As wind load increases, the inverted flight load is relieved by the normal flight load.  As long as your initial preload is within what the wings can take, all is good.

In the pretension scenario, you have the added benefit of avoiding impact loading.

I disagree. If the line is pretensioned then the lift caused by the wind which would normally allow the aircraft to rise subjecting the wing to a normal flying load as it lifts the aircraft is turned into an inverted flying load because the tie down is stopping any upward movement of the aircraft and in effect pulling the wing down from the outset - the weight of the aircraft does not have any bearing on this, only the force of the lift generated against the tie down rope matters.

Impact loading is unlikely to be as important simply because of the aircraft weight - a very powerful wind sufficient to cause an impact load of any size is likely to damage the wing in any event (pilots are advised to add boards to the wings if a high wind is expected - these disrupt the airflow and reduce lift).

Barry

roo

  • Sr. Member
  • *****
  • Posts: 1838
    • The Notable Knot Index
Re: airplane tie-down knots
« Reply #43 on: November 07, 2012, 08:36:14 PM »
If the line is pretensioned then the lift caused by the wind which would normally allow the aircraft to rise subjecting the wing to a normal flying load as it lifts the aircraft is turned into an inverted flying load because the tie down is stopping any upward movement of the aircraft and in effect pulling the wing down from the outset - the weight of the aircraft does not have any bearing on this, only the force of the lift generated against the tie down rope matters.
The wings generate the lift, not the body of the plane.  When the wings pull up they negate the inverted flight load while increasing the restraint tension.

The only exception would be if the tie downs were placed near the tips of the wings, which doesn't seem too common for the small craft we've been discussing.

http://www.j3-cub.com/forum/f79/tie-down-knot-17647/
« Last Edit: May 05, 2014, 03:58:18 AM by roo »
If you wish to add a troll to your ignore list, click "Profile" then "Buddies/Ignore List".


Fairlead

  • Sr. Member
  • *****
  • Posts: 334
  • IGKT Member since 1984 - IGKT Librarian
Re: airplane tie-down knots
« Reply #44 on: November 07, 2012, 09:12:40 PM »
Report on my visit to my flying club yesterday:
First I spoke to our recently retired Chief Engineer - he was adamant that tiedowns should NOT be taut and that is the main reason for using ground lines (14/16mm braid on braid pegged out along each aircraft parking line) to allow for pilots who like to yank their lines tight.  This applies to both high and low wing aircraft. 
The knots I found tying down mostly low wing aircraft  were tied in lengths of all sorts of rope, (none of which were more than 6/8 feet long) were predominantly various combinations of HALF HITCHES - though I did find two Fisherman's Bends, three clove hitches (around the ground lines). One bowline used as an eye to pass the working end through to pull down and secured with two Half Hitches.
So all in all quite disappointing.
Now back to the reasons NOT to use the Trucker's Hitch or Versitackle - Apart from the fact that both would be hauled down taut (see Engineers comments above) the Versitackle uses at least three times more rope (x2), that rope is weakened by two knots and chaffed by the action of pulling on the end to tighten it (how many times could you use this before having to replace it!).  The trucker's hitch (if made using the bellringers knot) needs to be under constant tension which is unlikely as the aircraft is likely to move forward or back slightly in the wind.
Finally - anyone who leaves an aircraft out in a hurricane deserves all he gets!

Gordon