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Analysing loop propagation

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Merlin
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Re: Analysing loop propagation

#21

Post by Merlin » Wed Sep 16, 2020 12:55 pm

Graeme

Instead of starting by a transverse wave in a vertical line, make a snap vertically. Is the shaped loop coming from the snap a transverse wave then?

Merlin
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Paul Arden
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Re: Analysing loop propagation

#22

Post by Paul Arden » Fri Sep 18, 2020 4:21 pm

Sorry I was offline the last couple of days. I think everyone agrees it’s a wave, Graeme. It’s just that most have an issue with the use of Transverse.
transverse wave

a wave in which the medium vibrates at right angles to the direction of its propagation.
I’d be happy with fly loop wave, which would distinguish it from whip wave, however I Imagine that the physics are remarkably similar even if the loop is closed with the whip. Or am I mistaken?

Cheers, Paul
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James9118
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Re: Analysing loop propagation

#23

Post by James9118 » Fri Sep 18, 2020 6:33 pm

Paul Arden wrote:
Fri Sep 18, 2020 4:21 pm
I think everyone agrees it’s a wave

Cheers, Paul
I don't :D

You can call it what you want, it's just a name. However there seems to be a push to label it as a 'wave' in order to give it some sort of greater validity in a physics sense. You could call it a 'wibble' or 'Lord Tarquin Digby-Smythe' if you want, it doesn't change the physics involved in throwing a tethered piece of string though. Not that we can agree on the physics of throwing a tethered piece of string :D .

Cheers, James.

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Re: Analysing loop propagation

#24

Post by Paul Arden » Sat Sep 19, 2020 6:22 am

I find it quite fascinating how there is fundamental disagreement on the principles governing how a loop unrolls. James I know is momentum of the fly leg and angular momentum of the loop. Which at least I can understand :D

I’m not sure if it’s disagreement as to how it works or how best to quantify it. I’ve always thought that if you notice phenomenon that isn’t readily explained by the structure/s that one is using to analyse, then there is something missing from that approach. Of course we do simplify things in order to try to understand them. The matchstick man 5 Essentials are a really good example of that.

Anyway getting back to one of the thoughts behind this thread, I was wondering if we considered the loop front to be a wave could it help explain the existence of the dolphin nose? In other words as the loop wave travels through the fly line, does it displace the fly leg downwards at its leading edge.

That’s where my mind was wondering out here on the lake :D
I think we can at least agree that the Dolphin Nose is a wave? :???:

Thanks guys,
Paul
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Merlin
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Re: Analysing loop propagation

#25

Post by Merlin » Sat Sep 19, 2020 7:58 am

For the time being there is a disagreement on the name :D

To me a loop is a wave in the general sense, there are various types of waves and there is none that fits all situations. In publications authors use the word wave and it does not refer only to fly lines (e.g. chains). To me using the word wave does not imply a specific physical mechanism. There are general rules behind and which might bring confusion sometimes, an example is qualifying the loop as a transverse wave.

If I consider the particular case of a nice fly line loop travelling in space at constant speed, then tension is the same at both ends of the loop and not so surprisingly, the relationship between tension and wave speed is the same than the one established for transverse waves in strings. Can you then conclude that a fly line loop is a transverse wave? Not really if you look at the displacement of the medium. Why not keep things simple and just call that a wave?

Analyzing physically the behavior of waves bring us back to consider momentum variations (in other words Newton laws) for legs and loop (angular momentum). The last example I saw was in a paper on whip by Dr Terry Mc Millen. The analysis was made at the elementary stage (a piece of whip) and allowed estimating the particular behavior of a complete loop in a whip. I guess the same can be made for a fly line; we just need a knowledgeable academic volunteer.

The DN is another type of wave; some consider it as a soliton (solitary wave). How it is created, why it looks stable, I just don’t know.

Merlin
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Re: Analysing loop propagation

#26

Post by Graeme H » Sat Sep 19, 2020 9:10 am

I’m away at the moment. When I have a keyboard in front of me, I’ll answe more fully.
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Re: Analysing loop propagation

#27

Post by carlz » Sun Sep 20, 2020 4:32 pm

The mechanics of fly line movement is fascinating. At first appearance, it is simple. There are two forces acting on the line (my hand and gravity) How hard can it be?

But there are so many confounding factors. By tethering of the line to the rod tip, you loose the simple F=MA equation and have to think of this equation as F=dP/dT. The time derivitive of momentum, since the mass of the moving object (fly leg) is not constant.

Then throw in a flexible rod and anything you think you know breaks down. That and trying to figure out what set of coordinates to base your equations in is a challenge.

And I don't even know where to start with the angular momentum associated with the loop itself.

The videos are a great way to check any theories. They don't lie.

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Graeme H
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Re: Analysing loop propagation

#28

Post by Graeme H » Tue Sep 22, 2020 10:15 am

Merlin wrote:
Wed Sep 16, 2020 12:55 pm
Graeme

Instead of starting by a transverse wave in a vertical line, make a snap vertically. Is the shaped loop coming from the snap a transverse wave then?

Merlin
Yes, of course I think that, but that's not the question I'm asking here: Which rod movement in that video is the one that separates the preceding transverse waves from the one that produced the loop? And how doe that rod movwmwnt differ from those before it?
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Graeme H
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Re: Analysing loop propagation

#29

Post by Graeme H » Tue Sep 22, 2020 10:27 am

James9118 wrote:
Fri Sep 18, 2020 6:33 pm
I don't :D

You can call it what you want, it's just a name. However there seems to be a push to label it as a 'wave' in order to give it some sort of greater validity in a physics sense. You could call it a 'wibble' or 'Lord Tarquin Digby-Smythe' if you want, it doesn't change the physics involved in throwing a tethered piece of string though. Not that we can agree on the physics of throwing a tethered piece of string :D .

Cheers, James.
Actually James, I don't label it as such to win any obscure physics argument. I do it because it helps me improve my own casting and teach my students about casting.

I know that by altering the tension in the medium, I can control the behaviour of the wave a little more easily (e.g. controlling an underpowered curve cast or forming and maintaining the "sexy loop".)

I know that I can explain a morphing loop more easily if the loop is considered a wave.

I know I can explain snap casts to my students.

I know that I can explain the deleterious effects of excessive overhang if the loop is considered as a wave travelling through a medium.

I have some ideas on why dolphin noses occur but they can't be discussed with the people I respect (including yourself) who harbour a closed mind to the first step in that discussion.

I'm sure there are other "mysteries" that will be unravelled when discussions mature, but for time being, I'll let the arguments rage on about the things I've mentioned above. They don't bother me ...

Cheers,
Graeme
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Graeme H
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Re: Analysing loop propagation

#30

Post by Graeme H » Tue Sep 22, 2020 10:56 am

Merlin wrote:
Sat Sep 19, 2020 7:58 am
Can you then conclude that a fly line loop is a transverse wave? Not really if you look at the displacement of the medium. Why not keep things simple and just call that a wave?
Can you show me any transverse wave in which the medium is not displaced?

For example, in the video I posted above, a still at ~3 seconds clearly shows the medium is displaced to the right of the screen in relation to its original position.

Screen Shot 2020-09-22 at 6.31.15 pm.png
Nice loop, BTW ...

I've stated this before, but a transverse wave has the characteristic of the medium being displaced in a direction that is perpendicular to the direction of wave propagation. In this video, it's very easy to see the direction of propagation is vertical and the medium is displaced horizontally.

Where people are not fully considering the implications is that in a fly cast, the direction of propagation must be vertical BECAUSE the line is being displaced horizontally (therefore the propagation direction is 90˚ to that - i.e. vertical!). We miss that because the medium is not long enough to see the full extent of the wave length.

Here's a thought experiment for those willing to explore a bit further. (I will turn this into a real experiment when I can find the right location.):
  • What would a cast look like if I have 100m of fly line tethered at one end to the top of a 50m tall tower and at the other end, to my fly reel through a fly rod? If I "cast" the fly line in such a way that would make a classic loop, would the loop travel upwards through the medium after reaching the end of the slack line? (This is essentially the same experiment I've videoed herein but with the excitation point is located at the bottom of the string instead of the top of it.)
  • If I used the contraption that Lasse used to show two different rods casting a line simultaneously so that one line was cast in the regular way and the other was tethered to the top of the tower, how would each loop differ before the loop reached the end of the regular cast?
If you're following this far, you're possibly wondering where the vertical direction of wave propagation goes. That's where gravity is coming into play and why the loop seems to float: We are aiming the cast in such a way that the vertical aspect of wave propagation is countered by the acceleration of gravity, the two apparently cancelling each other out. The loop appears to have some sort of magical quality of lift or antigravity, when it's really only falling at a similar rate to the upward propagation of the wave.

But of course, we always ignore gravity because physics is easier that way ... :???:

Cheers,
Graeme
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