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The practical side of considering loops as waves: Teaching and Self Improvement

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Graeme H
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Re: The practical side of considering loops as waves: Teaching and Self Improvement

#101

Post by Graeme H » Tue Oct 20, 2020 9:16 pm

We propagate it upwards and gravity pulls it downwards. Ideally, the two are balanced perfectly and it falls at the same rate that it is propagated upwards.

If we aim the cast too high (propagate it too far upwards) the cast finishes well above the target and the line falls with lots of slack. If we aim it too low, it crashes into the water.

Do it just right and you will carry 90' of line indefinitely. Have a look at your "practice on the log" video: you'll see the line is always going slightly upwards when you launch but by the time you're ready for the next cast, it has fallen to just the right height, allowing you to aim that next cast upwards too.

This is why I keep saying that we cannot ignore gravity when considering this. It is the force against which we are providing the tension through the rod tip. The wave propagates in the orientation of tension and oscillates perpendicularly to the tension orientation.

When you wiggle the line on the floor, you take gravity away and lose the tension force.

Cheers,
Graeme
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Paul Arden
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Re: The practical side of considering loops as waves: Teaching and Self Improvement

#102

Post by Paul Arden » Wed Oct 21, 2020 3:22 am

Hi Graeme, sorry I’m wasn’t talking about trajectory, I meant the vertical propagation of the wave. Since it’s not the casting direction I assumed it propagates upwards (trying to measure its length/speed. Previously you said it was the rate that it fell minus being held by the rod tip. That’s why I was confused.

I do see it as an issue because normally a transverse wave maintains its length as it passes through the medium (and repeats itself), where as in a fly cast the loop height ultimately goes to zero and only appears once.

I don’t think you can call a PUALD a full wave cycle because the back and forward casts are independent of each other. For example the backcast could be a wide loop and the forward cast tight. So I think it’s always two half waves, and I right?

Thanks,
Paul
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Graeme H
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Re: The practical side of considering loops as waves: Teaching and Self Improvement

#103

Post by Graeme H » Wed Oct 21, 2020 11:36 am

Hi Paul,

Sorry for the delay (working for a living sucks) and thanks for your questions. They have made me get some things clear in my mind to remove some ambiguity.

The wave propagates vertically in the direction of gravity and we cast with an upward trajectory to counter that propagation direction.

I know I've posted this video a few times, but it does demonstrate the wave propagating in the direction of gravity (tension) and the wave oscillating horizontally. If I were able to progressively aim the rod tip up a little, the wave would propagate downwards more slowly until eventually it would cause a collision (as it does in the second part of the video, where the line is on the ground and gravity /tension is removed).



Transverse waves in a string are normally examined with string that is "fixed" at each end. Oscillating one end produces a familiar wave. Releasing one end does not create a new type of wave just because the end is now free to move. But the behaviour of the medium does change, especially as the amplitude of the wave approaches and exceeds the length of the medium.

Regarding the PUALD, the asymmetric lobes of the wave (its shapes after each oscillation) don't negate its character as a wave any more than saying a mend not a transverse wave because it only moves one direction before returning. (I think the roll cast would be a better example to use as an objection/exception, but even that one returns the fly to its starting position. We still use the rod to oscillate the line approximately horizontally.)

Cheers,
Graeme
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Paul Arden
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Re: The practical side of considering loops as waves: Teaching and Self Improvement

#104

Post by Paul Arden » Wed Oct 21, 2020 1:28 pm

Hi Graeme,

It leads to the question that why does a wave that propagates in the downward direction not travel faster that the force of gravity when cast at a trajectory below the horizontal? Some of the below PUALD casts straighten at or above the horizontal on the backcast and are angled below it on the forward cast.

[media] [/media]

Also what I don’t understand is why the wave shape is inverted when compared to waves in a string when viewed from the perspective of propagation?

Thanks, Paul
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Torsten
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Re: The practical side of considering loops as waves: Teaching and Self Improvement

#105

Post by Torsten » Wed Oct 21, 2020 9:22 pm

Graeme, from the other thread,
Mends must be a constant source of frustration for you. The idea of a mend is to transport the line away from where it would otherwise land. Place a wave in the line and transport it sideways.
Not really frustration because I'm looking at these topics pretty much emotionless. The wave is one concept of many, I'd consider it for some cases like the mend, but the loop itself is not a wave for me. The loop is just a unique shape on it's own when the fly line is unrolling.
But for you, a wave can't transport the medium, so there is a paradox.
(Mechanical) waves usually do not transport matter, this in my opinion a major difference. And if the fly line is the medium, it is transported, yes it's a paradox and makes no sense to me.
I've said it a hundred times, but once again, the direction of of the wave path is perpendicular to the oscillation. Since we observe oscillation in the horizontal direction, the wave path is vertical.
Oh, I see you've got some of it already. You don't seem to be grasping the full concept though.
I don't see really a vertical propagation happening for an ordinary loop. My impression is that almost nobody here is grasping your concept.
In other words, you seem to be saying that when tension is removed from the rod leg, the force that causes the change of momentum at the loop front is tension.
I'm not sure if we have the same understanding if the concepts "force" and "tension". For the fly line it's different compared to a guitar string for instance. For the guitar is a string mounted between pegs and you can adjust the tension. Tension is just a axially transmitted pulling force, you can measure it along the string - and in this case the tension along the string is constant and adjustable by the pegs.
The fly line however is at least at one end free, thus without acceleration or external forces no tension. When the line unrolls, the redirected mass at the front of the loop is responsible for a force there. Combined with some other external forces tension is created and tapers down toward the fly. If top/bottom leg both move at the same velocity, the loop does not unroll and thus no acceleration / force by unrolling of the loop. I'll see if I can visualize this, if I can find some time at the weekend.

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Paul Arden
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Re: The practical side of considering loops as waves: Teaching and Self Improvement

#106

Post by Paul Arden » Thu Oct 22, 2020 12:42 pm

I’m happy to go into it deeper and see where it ends up, for me that’s the only way to resolve any matter really. I know when I throw inverted loops they are particularly poor fliers. If they were to propagate upwards, in this case, then I think they would be easier to control. So a transverse wave being applied to a fly cast in the vertical sense would have to propagate upwards with a normal (not inverted) overhead cast. However with side casts I don’t see them propagating in any horizontal direction. So it would have to be a non-propagating half transverse wave? What would that make it?

Cheers, Paul
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Graeme H
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Re: The practical side of considering loops as waves: Teaching and Self Improvement

#107

Post by Graeme H » Sat Oct 24, 2020 12:34 am

Hi Paul,

A transverse wave in a string propagates in the direction of tension. If you hold the fly vertically above you, the wave to propagate upwards. If you hold that fly to the north, the wave propagates to the north.

Pretend you're standing on the bridge at Belum with your line hanging down. The force of tension in the line will be produced by gravity and your rod tip (Newton's 3rd Law). Any sideways movement of the rod tip will produce a transverse wave that propagates from the tip in the direction of that tension (which is downwards) until it reaches the fly. No matter how hard you move the rod sideways, gravity always acts to pull the line downwards.

Regarding the side cast question: The orientation of the wave oscillation does not change the propagation direction of the wave itself. Consider a mend to the right versus a bucket mend: both mends travel away from you towards the loop (the direction of the tension force) even though one wave is vertical and the other is horizontal.

In the same way, a vertically oriented loop and a side cast loop both have a downwards wave propagation direction.

Cheers,
Graeme
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Paul Arden
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Re: The practical side of considering loops as waves: Teaching and Self Improvement

#108

Post by Paul Arden » Sat Oct 24, 2020 5:52 am

That doesn’t help me. :D The tension at the fly is zero.
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Dirk le Roux
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Re: The practical side of considering loops as waves: Teaching and Self Improvement

#109

Post by Dirk le Roux » Sat Oct 24, 2020 6:16 am

Paul Arden wrote:
Sat Oct 24, 2020 5:52 am
The tension at the fly is zero.
No, it's not.

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Graeme H
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Re: The practical side of considering loops as waves: Teaching and Self Improvement

#110

Post by Graeme H » Sat Oct 24, 2020 7:03 am

Dirk le Roux wrote:
Sat Oct 24, 2020 6:16 am
Paul Arden wrote:
Sat Oct 24, 2020 5:52 am
The tension at the fly is zero.
No, it's not.
Yep, it is not zero.

Pinch the tippet 2" above the fly. Does the fly float somewhere around your fingers or does it hang 2" below them? If it hangs 2" below them, there is something stopping the fly falling under the influence of gravity. That something is tension in the tippet.
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