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

[Lasse Karlsson]

And happy to call the dolphin nose a wave, just like the tailing loop comes from a transverse wave, and mends are. Not convinced that the loop front is a wave though..

Yep, I get that Lasse. It's the only bend that travels along the line we can't call a wave, even though it can sit between a tail and a mend and travel at the same speed along the line as both of them at the same time.

Cheers,
Graeme

Cool, it's a bend caused by the medium traveling, the two others are traveling in the medium.

Now back to your lost tension cast that fails, do you have one that isn't caused by you altering the casting arc with minus 25ish degrees? As I say, I can get a cast to fail in the same way, but I can't get one to fail that has sufficient velocity to unroll before stepping/throwing the rod/dropping the line...

Cheers
Lasse

[Dirk le Roux]

Save for outlining a really nice-looking fish that morphs into something more Amoeba-like, I don't think you have removed tension from that cast, Graeme. A wave needs tension to propagate, and that fish loop still propagates.

The best way to remove as much as possible tension from the rod leg is to cast by hand and let go. Even then, if the air succeeds in peeling some rod leg from the line at the front, you start having tension again.

Or shock-release the rod leg so that it is sped up to fly leg velocity and simultaneously not fixed to something which is still moving about.

Cheers,
Dirk

[Torsten]

Graeme:

Interesting answer. What is the force causing the change of momentum at the loop front?

If there is a speed difference between fly and rod leg there exists a force too, because the slower moving bottom leg pulls at the front of the fly leg (and vice versa, Newton's third law) => tension, because tension is a "pulling force transmitted axially". If both legs move at the same speed, the tension would diminish. Seems to me plausible that the loop gets less or even unstable in this case.

[Merlin]

Before thinking about N3, pay attention to N1 for a variable mass system Torsten.

Merlin

[Graeme H]

Cool, it's a bend caused by the medium traveling, the two others are traveling in the medium.

Are you really saying the medium doesn't travel sideways when we place a mend in the line?

Now back to your lost tension cast that fails, do you have one that isn't caused by you altering the casting arc with minus 25ish degrees? As I say, I can get a cast to fail in the same way, but I can't get one to fail that has sufficient velocity to unroll before stepping/throwing the rod/dropping the line...

I'll find some other collapsed casts for you. I'm not going to bother running them through tracker though.

[Paul Arden]

Hi Graeme, I’ll post this here.

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.

So I understand what you are saying. The flycasting loop wave is actually travelling vertically upwards from the rod tip and at 90 degrees from the casting direction. So for you the loop straightening is it’s amplitude and the wave length is half the height of the loop.

I say half the height because a full wave cycle would be both back and forward casts. I suppose however that because the wave doesn’t travel vertically upwards and instead the loop straightens then I’m not sure how exactly to measure the wavelength? Is it twice the height of the loop above the rod tip when the fly passes the rod tip? I’m not sure. In fact I’m not sure how to measure the amplitude either because normally a wave rebounds on its own accord as it propagates and doesn’t need to be rethrown in order to start the process anew.

So it’s not a full transverse wave cycle that propagates as a transverse wave, that you mean, rather it’s half a transverse wave that finishes at loop straight? And then we start another half wave cycle with the next casting stroke and over and over again?

Sorry If I sound a bit confused, but I’m getting there right?

Thanks, Paul

[Graeme H]

Hi Graeme, I’ll post this here.

So I understand what you are saying. The flycasting loop wave is actually travelling vertically upwards from the rod tip and at 90 degrees from the casting direction. So for you the loop straightening is it’s amplitude and the wave length is half the height of the loop.

I say half the height because a full wave cycle would be both back and forward casts. I suppose however that because the wave doesn’t travel vertically upwards and instead the loop straightens then I’m not sure how exactly to measure the wavelength? Is it twice the height of the loop above the rod tip when the fly passes the rod tip? I’m not sure. In fact I’m not sure how to measure the amplitude either because normally a wave rebounds on its own accord as it propagates and doesn’t need to be rethrown in order to start the process anew.

So it’s not a full transverse wave cycle that propagates as a transverse wave, that you mean, rather it’s half a transverse wave that finishes at loop straight? And then we start another half wave cycle with the next casting stroke and over and over again?

Sorry If I sound a bit confused, but I’m getting there right?

Thanks, Paul

Yes, you seem to be following along so far.

Yes on the loop straightening being the amplitude. The exception is an underpowered cast in which the line fails to straighten - in this case the amplitude is the distance the loop landed away from us (in the same way the amplitude of a mend is the distance it deviates from the straight line to the target.)

No on the wavelength. Wavelength is calculated from the period of the wave and the speed of propagation of the wave, which itself must be calculated. Here are the diagrams of each (period and wavelength):

Screen Shot 2020-10-20 at 8.38.07 pm.png (58.79 KiB) Viewed 161 times

From this, we can see that the Wavelength = Period x Velocity (of the wave*)

The Period is the time between the start of each cycle, e.g. the completion of each forward cast.

The Velocity (of the wave) is the average velocity of a falling line during the cycle, calculated from the distance an object will fall during the Period divided by the period. (Minus the time it's being lifted by the rod ...)

The full wave cycle is when it returns to the place it started from and starts the next movement. For example, a pickup and lay down cast or a full false cast (back cast and front cast.) You could time the cycle by the time it takes for the fly to pass the caster on the front casts of each false cast it you like - it doesn't matter at which point in space the fly passes.

If you are only including a front cast OR a back cast, you can't calculate wavelength or period and you break the rule about the wave not transporting the medium. (Returning the fly to its starting position in space is the same as the going from peak to peak in the diagram above.)

I know this is all somewhat counterintuitive and I don't blame anyone for not grasping the concept. I certainly don't teach this sort of detail to any student but I do often tell them the loop is a wave and tension in the line is important to make the wave move along the line. They easily grasp that concept.

Cheers,
Graeme

* Note: the velocity of the wave is NOT the velocity of the line moving across the ground.

[Graeme H]

Hi Torsten,

I asked

"If there is no tension in the rod leg in these distance events, why is the rod leg not falling at the same rate we'd expect all objects on earth to fall at?

to which you replied:

Like above, forces acting against the gravity force such as drag force, change of momentum at the loop front etc.

I asked what causes the change in momentum, and your reply was:

If there is a speed difference between fly and rod leg there exists a force too, because the slower moving bottom leg pulls at the front of the fly leg (and vice versa, Newton's third law) => tension, because tension is a "pulling force transmitted axially".

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.

Can you clarify please?

[Graeme H]

Now back to your lost tension cast that fails, do you have one that isn't caused by you altering the casting arc with minus 25ish degrees? As I say, I can get a cast to fail in the same way, but I can't get one to fail that has sufficient velocity to unroll before stepping/throwing the rod/dropping the line...

Cheers
Lasse

Here are a bunch of casts. Not all of them have the loop failing to propagate but there are enough there to see what happens. We should be looking at the shape of loop itself in these casts.



I know the fly basically travels about as far, but this is about showing the loop (a wave) can't propagate through a string without tension. The fly leg carries on for a while due to inertia but loop itself loses that "angular momentum" that people are citing as an important factor. It just stops "spinning".

Cheers,
Graeme

[Paul Arden]

I’m a bit lost on the bit Graeme,

The Velocity (of the wave) is the average velocity of a falling line during the cycle, calculated from the distance an object will fall during the Period divided by the period. (Minus the time it's being lifted by the rod ...)

Does this mean that the wave is travelling towards Earth and not the sky?

Thanks,
Paul