Loop Dynamics

[Walter]

Okay,

Here's what I get with the axes switched using the simple model I showed earlier

angle vs speed 2.jpg (33.75 KiB) Viewed 393 times

[Paul Arden]

I’m not sure I like the sound of a vomit comet. I think I’ll be fishing that day

[Merlin]

Hi Walter

Following James’ suggestion to test a fly line as a falling chain, I read again a number of papers on this subject and found interesting information from academics like Terry Mc Millen (Gordy knows him) who reviewed a number of conflicting approaches of the problem. I took inspiration from those papers to write something.

An infinite line speed in vacuum is only achievable if the line is just folded, with a « zero » loop size (or very very small). It is not possible to be in that condition as long as there is a sizeable loop. It means that in practice one would never see high speed level if a test was possible in a vacuum chamber.

I took a simplified approach to illustrate that, by calculating an estimate of the maximum vertical speed of line tip as it is going to kick back (same situation as for a falling chain). See the graphic below for illustrations:

Loop in vacuum.JPG (40.85 KiB) Viewed 355 times

To simulate the kickback I assumed that the loop would reverse itself (dotted curve). I used one of the data set which you defined (rholine = 0.001 kg/m; 10 meters of line; 20 m/s initial speed) to calculate the values for different loop sizes:

tip vertical speed.JPG (22.62 KiB) Viewed 355 times

With the smallest loop (10 cm diameter), the vertical tip speed approaches 300 m/s. That is consistent with the fact that the folded line with “zero” loop is the necessary condition to get an infinite speed. For normal values of loop size, the maximum vertical speed should not exceed 120 m/s for such a level line.

Since such an experiment is not possible (we have no means), we only have simple models to get an idea of what is going on. I do not think that using a falling chain type of experiment would be satisfying, there is little energy involved and measurements need to be accurate to detect something like the potential effect of viscosity of lines. Given the estimates made on stored elastic energy in a line, there is little chance to detect that, the amount of energy involved is too small IMHO. I look forward reading other opinions with a minimum of technical support.

Merlin

[Torsten]

Hi Paul,

Infinite speed… must be some huge (infinite?) forces involved. The “boing” would be quite something.

just to address this quote - infinite speed (@zero mass!) is a mathematical artifact, would happen only if you simplify your model to a fly and rod leg only, the loop has zero radius, the line zero diameter, zero bending stiffness, no losses, fly leg, rod leg constrained to move along a straight line - a very unrealistic scenario. The forces are even for this case small (ask Gordy for the famous tension equation) - you know conservation of energy and such ..
Even with a vacuum chamber you couldn't replicate that. The maximum speed would be finite already when the loop has a radius greater zero.

Gordy, Merlin elastic energy (for stretch) well argued but you should check also bending stiffness/losses.

Greetings,
Torsten

[VGB]

The first that occurs to me is the "all of the energy is concentrated in the fly leg" approach. It ignores the energy in the loop.

It’s a way of speech Walter.

In the context of past discussions, it’s not a figure of speech. Spolek and Lingard neglected the energy in the loop, Robson used a rectangular loop. Dr Gatti-Bono attributed the difference in outcomes between analytical and numerical fly casting models as being due to the use of simplifying assumptions in the analytical models.

Would neglecting the energy in the loop also simulate having a very small loop?

Vince

[Merlin]

I do not neglect loop energy in my models.

To answer Torsten question about bending stiffness, I made an estimate for the same conditions than the elastic energy and found 0.0022 J. Peanuts again. Would a dynamic condition give 100 times that figure?

Merlin

[Walter]

Hi Paul,

Infinite speed… must be some huge (infinite?) forces involved. The “boing” would be quite something.

just to address this quote - infinite speed (@zero mass!) is a mathematical artifact, would happen only if you simplify your model to a fly and rod leg only, the loop has zero radius, the line zero diameter, zero bending stiffness, no losses, fly leg, rod leg constrained to move along a straight line - a very unrealistic scenario. The forces are even for this case small (ask Gordy for the famous tension equation) - you know conservation of energy and such ..
Even with a vacuum chamber you couldn't replicate that. The maximum speed would be finite already when the loop has a radius greater zero.

Gordy, Merlin elastic energy (for stretch) well argued but you should check also bending stiffness/losses.

Greetings,
Torsten

Torsten, excellent summary! I would also add that the mathematical artifact only happens in the tethered cast. I'm sure there are other factors as well.

What I think is important to note is that many of the scholarly papers can be very misleading to the average person because they fail to stress the importance of their simplifying assumptions and they often do not put things into context for the average person.

GB writes a paper with the conclusion that there is lift in the loop and that certain loop shapes have 4 times the lift of a semicircular loop. At no point is that placed into context regarding what it means in real life and now we have generation of casting instructors (and even some of the more scientifically minded) that accept the idea of flying fly lines in spite of all the evidence to the contrary and the fact that it is based on a very limited set of conditions, some of which do not exist in real life. Putting it into context, the amount of lift in a semicircular loop makes an almost undetectable difference in the cast and wrt the wedge-shaped loop we can conclude that 4 times nothing is nothing.

Other papers begin with the simplifying assumption that energy concentration on the fly leg leads to incredibly high speeds at the end of the cast unless we include drag in our analysis and, again, it's taken out of context by many people. We know that under certain conditions the fly leg will accelerate but, realistically, what are the conditions and how much acceleration can we expect if we take away drag?

There is a bit of tyranny associated with a modeling only approach to fly casting. They are interesting and I personally enjoy seeing the models but, in my opinion, we need to put more context around the papers so the rest of us can understand what they really mean, and we need to make sure they truly reflect real life. For many of us their value is in how they potentially lead to a better understanding in order to improve casting styles and equipment design.

I think there is still a lot of useful mileage in this topic that I hope to address some of it in the next few days.

Walter

[VGB]

I do not neglect loop energy in my models.

I didn’t say you did. Spolek and Lingard did though.

To answer Torsten question about bending stiffness, I made an estimate for the same conditions than the elastic energy and found 0.0022 J. Peanuts again. Would a dynamic condition give 100 times that figure?

Hopefully, James will be able to find out about the dynamic condition.

Vince

[Stoatstail50]

For many of us their value is in how they potentially lead to a better understanding in order to improve casting styles and equipment design.

Deeply puzzled….in what way have these models improved casting styles Walter ?

Edit. Even potentially ?

[Merlin]

I didn’t say you did. Spolek and Lingard did though.

Which papers are you referring to Vince? For Lingard, he takes the loop on board in his paper “Notes on the aerodynamics of a flyline” (1987). For Spolek, I have not yet identified the paper on fly line flight in my files. I have to dig again.

Merlin