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

[Graeme H]

Well you are definitely not making life any simpler, Graeme

So let’s recap. Fly to bridge, the transfer wave travels up. Got it. It’s obviously slowed by gravity.
Line on floor. Not a transverse wave. Got that!
Loop in air. It’s a transverse wave that propagates to earth. Is this the same for an inverted loop? I assume so.
What about a side cast? Is this also a transverse wave? And is that also propagating to Earth?

I’ll have to think about the mend but I’m going for a Zwift ride now.

Cheers, Paul

The wave going upwards when the line is tied to the bridge is not slowed by gravity because there is no transfer of mass upwards. If anything, it will propagate faster as it rises due to the increase in line tension with height. (The line at the top is holding up all the line below it while the line at the bottom is supporting nothing.)

Loop in the air/inverted loop/side cast are all transverse waves propagating towards earth.

Cheers,
Graeme

[Paul Arden]

If anything, it will propagate faster as it rises due to the increase in line tension with height.

Yes that’s true; my mistake.

So for you basically gravity is the reason for casting loops to be transverse waves.

Sometimes I cast off mountain and cliff tops. There I can get some pretty exciting up-draughts, in fact if it’s strong enough the line will lift upwards instead of fall down. Would that mean the loop becomes a transverse wave in the upwards direction?

Cheers, Paul

[Dirk le Roux]

Wave propagation direction is not determined by the rod tip. It's the direction of tension in the line.

Hi Graeme

Could you please explain this a bit more? So I can relate to the concept at starters level, please use the simple examples of the classical pulse in a horizontal string, both sides tethered, and that of a rope hanging from a bridge? With the latter, for pulses generated from both ends.

Thanks,
Dirk

[Merlin]

Sometimes I cast off mountain and cliff tops. There I can get some pretty exciting up-draughts, in fact if it’s strong enough the line will lift upwards instead of fall down.

Paul

Next time you are in those conditions try an horizontal loop and see if it veers. Skin drag on the fly leg contributes to lifting the whole line by a moment effect. This is why an inverted loop is more difficult to cast since the moment of skin drag forces drives the line downwards in that case. This effect has been demonstrated (and scientifically documented) for a string shooter.

Merlin

[Paul Arden]

That’s a really interesting one, Daniel. The roof of my boat is 10 feet off the water and also a good height to play with carry. I’ve certainly looked for this veer but I find it completely impossible to tell from my angle (and there are other potential causes too of course - eg position of the backcast or a slight breeze).

However what I don’t understand is, if the face of the loop does lift, then how it can lift the entire fly leg behind it? What I have seen sometimes with vertical loops is the appearance of the front of the loop lifting above the fly leg, which may indicate this upward force. I’ve certainly seen collisions in slow motion that have been caused this way.

Personally I do think there may well be a force there that helps offset gravity. I know that Graeme and others talk about climbing trajectories keeping the loop airborne but I also can and often do throw high trajectory backcast that straightens at an angle above the horizontal, and then I cast aiming downwards, below the horizontal, on the forward cast.

When I watch these loops unroll they appear to largely hold flight trajectory better than a line in free fall. A great test would be to fire a long forward cast below the horizontal with a line in free fall beside it. This is very difficult to achieve with a long line of course, but I would expect the free fall line to pass the loop, even though the loop has been thrown downwards. (That’s not physics theory that’s put me there; that’s simply hours and hours of casting off cliffs!).

You know, when I used to cast off the cliff in NZ I could carry 105’ of line in the air for two, three or more minutes at a time, shaping loops. I would often fire the forward loop down well below the horizontal. There is an appearance to loop propagation, and in particular how it holds its flight path, that trajectory explanations don’t fully explain, for me anyway.

I would love to do that 90’ carry video off a log as a 100’ off a mountain or a cliff top. It’s impossible to film this stuff on my own but my wife is back soon (her resort shuts for the monsoon). At the very least we should be able to get something off the boat roof.

Cheers, Paul

[Graeme H]

Wave propagation direction is not determined by the rod tip. It's the direction of tension in the line.

Hi Graeme

Could you please explain this a bit more? So I can relate to the concept at starters level, please use the simple examples of the classical pulse in a horizontal string, both sides tethered, and that of a rope hanging from a bridge? With the latter, for pulses generated from both ends.

Thanks,
Dirk

Hi Dirk,

This very easy explanation might help. He's done it so I won't repeat it.





Regarding the bridge wave propagation speed: A wave in a string held at the top will slow down as it reaches the bottom (all things being equal) because the tension in the string is highest at the top of a hanging string. At the bottom, it approaches zero.

The opposite happens when the wave is generated at the bottom of the bridge.

Cheers,
Graeme

[Merlin]

Paul

I do not speak of the loop, but of the fly leg. There is some weight compensation at the loop level thanks to skin drag. Let’s put this aside and look at the whole picture. The fly line has some stiffness which is reinforced if I can say so by internal tension. Forces acting on the line (drag, gravity) create moments which tend to make the whole line rotate around the rod tip (we stay in tethered conditions for sake of simplicity).

drw1.JPG (13.48 KiB) Viewed 1437 times

At the beginning of the cast the overall moment tends to lift the line which is traveling on a “ballistic” trajectory. This moment effect is not the dominant one, it is the ballistic effect which drives the final trajectory, but the moment effect can change the trajectory a little. The nose of the loop can then be lifted a little.

drw2.JPG (13.94 KiB) Viewed 1437 times

As the line is rolling over the position of forces regarding to rod tip can change and at some point drag and gravity moments can compensate each other.

drw3.JPG (13.72 KiB) Viewed 1437 times

At the end of the cast all moments tend to rotate the line downwards from its ballistic trajectory. The nose of the loop can then move down.
The ballistic trajectory itself may be made tighter if the fly leg accelerates and more domed if the fly leg decelerates.

drw4.JPG (12.99 KiB) Viewed 1437 times

In the case of an inverted loop all moments can contribute to a downwards rotation which makes such casting unpleasant.

Merlin

[Graeme H]

Sometimes I cast off mountain and cliff tops. There I can get some pretty exciting up-draughts, in fact if it’s strong enough the line will lift upwards instead of fall down. Would that mean the loop becomes a transverse wave in the upwards direction?

I would say so. If I ever get to play in one of those indoor sky diving places, I'll take a fly rod and my camera.

Do you know any skydivers who also fly fish?

Cheers,
Graeme

[Dirk le Roux]

This very easy explanation might help. He's done it so I won't repeat it.

and

Regarding the bridge wave propagation speed: A wave in a string held at the top will slow down as it reaches the bottom (all things being equal) because the tension in the string is highest at the top of a hanging string. At the bottom, it approaches zero.

The opposite happens when the wave is generated at the bottom of the bridge.

Graeme, I don't understand how from those explanations you relate propagation direction to the direction of tension in the line (which is bi-directional). I read "Wave propagation direction is not determined by the rod tip. It's [determined by] the direction of tension in the line". Or did I misunderstand?

[Graeme H]

Sorry Dirk. I assumed it was self evident.

Peg a string at one end and move the other end and the wave travels towards the peg. The lecturer in the video moves his end and the wave travels towards the student.

Yes, tension is bi-directional, but the wave travels away from the impulse. Put the impulse in the middle of the string and the wave travels away from the impulse in both directions. Since we are only moving one end of the string, the wave moves away from us through the medium once it's formed.

Do you have an example that contradicts that?

Cheers,
Graeme