Coam again

[Walter]

I think the issue is one of absolutes. It has to be one or the other. I think it’s more of a phased thing. At the start of the cast fly leg momentum is everything. Once the loop forms there is a transition from one to the other. At some point the fly leg has less mass than the loop and the loop becomes the driving force. At the end of the cast when fly leg length is zero and we have final loop roll over the loop is totally in control and we see the fly kick over as it goes around the loop.

There are also initial conditions to consider which is why I always like to look at assumptions. For example, if your cast doesn’t have enough momentum to fully unroll then the later phases never happen. So, if I was talking about a case where the line doesn’t fully unroll I would include that in initial assumptions.

As someone pointed out the loop has am. Without am there is no loop because am creates tension and the balance between tension and centripetal forces is why a spinning wheel of line is round (like a lariat). Just as a side thought, the am is not evenly distributed around the loop that could be one reason why semicircular loops aren’t stable…

I can understand why Vince doesn’t think the falling chain applies because of how gravity is acting on the loop but I want to go through the analyses on line before I agree. One thing I see in all of the analyses is that the narrower the loop the greater the increased acceleration. I think that’s significant. Another common area in all of the analyses is that they attribute the extra acceleration to conservation. So if we could somehow do the falling chain and turn off gravity in the middle of the experiment would the chain still experience some amount of acceleration or would it suddenly revert to falling at constant speed?

As I said earlier, we don’t have a good idea of what happens in the loop. It’s nothing to be embarrassed by because if you read the scholarly papers about the falling chain most of them point out that there are conflicting views of what happens there as well.

[Walter]

Apologies if this is naive but I’ll learn by asking.

Why can’t a loop be considered just the transition zone between compression and tension?
The fly leg is colliding against the slowing line in the loop, which would produce compression in the medium of the line. The rod leg is pulling on the loop front moving away from it which should produce tension. The medium between must be in transition between the two. Because that medium has stiffness the transition must be gradual rather than sharp. That gradual transition will be oriented (I’d think) in a curve with neither compression or tension at the apex of the loop.
George

Not naive at all and I can’t give you a satisfactory explanation one way or the other on that.

I know Vince and others have talked about a push me pull you concept like you described. I’ve thought same way as well but I have a slightly different view that I’ll write up for discussion when I have time.

It would be tempting to say that the caster gives the loop it’s initial am and loop shape and then the combination of loop, fly leg and rod leg do exactly what you describe. But if you’ve seen Gordy’s video of casting into a wall the point of contact with the wall and fly leg moves upward until the fly leg works its way up past the top of the wall, then a loop forms and the fly leg/loop carry on their own way. The loop gets created without the caster being involved. How do we get from fly line moving along without a loop to having the initial loop form without the caster manipulating the rod tip to make that initial loop?

[George C]

Thanks Walter
I had included the same observation about the video of the loop crashing and reforming but deleted it to not confuse the issue. Here is a little video I just made.
How is the behavior of the tape different from a fly loop?
<iframe title="vimeo-player" src="" width="640" height="360" frameborder="0" allowfullscreen></iframe>

[Walter]

George,

I don’t think it is any different but you’ve already created a “well formed” loop through your your actions just like you do when casting a fly line. What happens if you strip out 10 or 20 feet of tape and then push it against an object or 10 or 20 feet away? It will probably form a loop but it will probably lack direction.

Or if you start with a loop of radius zero?

I’m probably just confusing the issue at this point.

[Paul Arden]

Just as a side thought, the am is not evenly distributed around the loop that could be one reason why semicircular loops aren’t stable…

I’ve always been impressed by how remarkably stable the nature of loops seems to be. Even multiple loops of different diameters in the same cast. It’s those round semi-circular loops that can turn into tight top pointed V loops.

I’m not surprised that they are so “controversial” amongst physicists; they are almost magical. Loops can appear to defy physics.

I said earlier, we don’t have a good idea of what happens in the loop. It’s nothing to be embarrassed by because if you read the scholarly papers about the falling chain most of them point out that there are conflicting views of what happens there as well.

I think we should be further ahead in our agreed collective understanding. That we are not, I think, is my fault. If I had stayed studying Engineering at university – and not gone fishing in NZ – I think we would be. But then again Sexyloops probably wouldn’t exist.

Perhaps we should make a list of topics that everyone actually agrees about and write up the physics involved? At least then the more casual reader would get the impression that not everything flycasting physics is controversial?

Cheers, Paul

[Merlin]

Why can’t a loop be considered just the transition zone between compression and tension?

Hi George

Because there is no compression in the line, depending on conditions the tension at loop nose might be lower than tensions in the ends of the loop. Compression is another red herring I’m afraid.

Merlin

[Merlin]

Hi Walter

I do not know where you are going with this thread (anything to do with CoAM?), but to me it brings more confusion than understanding. Although there is tension pulling on the fly leg, it just influences the variation of speed of that leg, which flies on its original momentum. That tension is not what drives the fly leg.

The tension in the rod leg at tip level is not always necessary for a rollover, or we could not shoot any line. It is again a problem of AM, in particular for pullback or snap.

The important point to consider is the loop with both its horizontal momentum and angular momentum which determine tensions at the top and at the bottom of the loop. The fly leg is the engine and the loop may be considered as a transmission device fitted with brakes (drag forces).

Merlin

[Walter]

Daniel,

I’m sorry if the purpose of this thread confuses you but thanks for participating. Coam is part of it it’s about determining how force applied by the rod tip actually affects the speed/velocity of the fly leg. We have a lot of conflicting views in my opinion and none of them are really satisfactory.

WRT to loop turnover there is tension in the rod leg even when shooting line. The change in momentum of the rod leg as the rod leg increases in mass is a measure of that. Other forces acting on the rod leg include drag from air and rod guides as well as gravity. I wanted to stick to the tethered cast for simplicity and then consider the other forces later.

You could consider the momentum in the loop as the generator of tension in the legs or you could consider the am as a measure of tension in the legs.

The transmission analogy is also interesting to me because I see it as a continuously variable transmission rather than just a fixed gear set. That continuously variable nature compensates a for a lot of variations that can happen in a cast.

[Walter]

I think we should be further ahead in our agreed collective understanding. That we are not, I think, is my fault. If I had stayed studying Engineering at university – and not gone fishing in NZ – I think we would be.

Truthfully, I don’t think it would have made much difference for the complex issues.

[Walter]

This might interest some of you.

I took a bit of a different approach with this. I considered the KE of both the rod and fly legs in a tethered cast. You may say that the rod leg has no velocity, therefore it's KE is always zero. But the center of mass of the rod leg is moving. The calculations for KE in both the rod and fly legs are based on the velocity of the center of mass for both legs.

For this model:
- level line - linear density of 0.001 kg/m
- initial fly leg length - 20 m
- speed of the fly leg (not the speed of the center of mass but the speed of a point on the fly leg) 10 m/s for the full duration
- radius of loop is zero (not realistic I know but it's a model)
- Y axis represents KE
- X axis is time times 10 plus 1 - each step is 0.1 sec.

Here is the result. I am not going to make any comments about it. I'll let other people comment about it first.
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