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## Covered topics /how it works

Moderator: Torsten

Dirk le Roux
Posts: 474
Joined: Thu Sep 19, 2013 5:09 pm
Location: Pretoria, South Africa

### Re: Covered topics /how it works

Before I place velocity history charts of various other casts, first the cast that had zero rod leg to start with, kept form and rolled over all the way (albeit with the rod leg having touched down toward the end).

The instant shown is likely just after the rod leg's guestimated (can't see the actual, due to line and grass colour) touch down.

Here is further food for analysing the haul/rod tip/triple haul check. Whether it is pull-back from just being held, pull-back from a shoot or restraining a shoot, the result with all is the rod leg velocity increases in the direction opposing the fly leg, increasing propagation/tangential/rotation speed of the loop. Evidently, the effect on fly velocity (and trajectory) is near immediate.

Cheers,
Dirk

Merlin
Posts: 1474
Joined: Wed Jan 09, 2013 8:12 pm
Location: France

### Re: Covered topics /how it works

Thanks Dirk

Let's see if I understand the graphics:
* from the beginning (time zero), the rod leg is held back which reduces its forward speed up to 0.4s of the record
* there is a rise in fly leg speed from 0.7s, and there is another rise from 1s likely (IMHO) because of the touch down: there is a reaction from the ground on the fly leg, I have to look again at the snap cast simulation I did in the past to check the effect.

That would mean a 300 ms delay in between the beginning of the check and the response from the fly leg if I do not fool myself with my (mis)understanding.

Am I correct?

Merlin
Fly rods are like women, they won't play if they're maltreated
Charles Ritz, A Flyfisher's Life

Merlin
Posts: 1474
Joined: Wed Jan 09, 2013 8:12 pm
Location: France

### Re: Covered topics /how it works

A1 there is a force at the loop front from momentum change that pulls the line
A2 momentum of the fly leg is the driving force of the loop
I seem to be the only one who thinks A1 is a sufficient answer and think that examples of line flight with no loop, non-tethered loops, and tethered loops should help to give enough data to resolve the question.
Gordy

I think A1 is not for laymen because there are hidden subtleties there. In fact there is a reference to an internal force which is applied to the fly leg (subsystem) if we consider it separated from the loop, and it is not said but there is a pull from the fly leg on the loop at the same time. It is line momentum that makes it move forward, and rod tip hold back which creates a loop.

Someone not accustomed to such details would say here is the “big wheel”, and A2 would allow another comment like here is the “big finger”. The driving force on the loop is the force at tip (tethered cast), the only external force.

In post #23 I did my best not to enter in this domain. I think anyone can cast a rope, hold back the end and see the rollover. There are several possible approaches, like mentioned by Dirk:
The tension at a propagating loop can be derived from more than one point of departure, and your momentum change one is as valid as working from centripetal acceleration, from wave behaviour or from energy conservation. The outcome relates the tension to the line's linear mass density and the square of the loop's tangential velocity, which also is the loop propagation speed or rate of the fly leg shortening and rod leg lengthening.
But this is really technical, and we could refer to energy for example to explain the behavior of the fly leg, energy is something familiar to people: the transfer of line from the fly leg to the rod leg which is held back implies that there is a concentration of energy in the fly leg. But at the same time air drag acting on the loop and the fly leg spoils the energy of that leg. Depending on conditions (e.g. loop size, line size, wind, etc.) the fly leg might accelerate or decelerate (especially at the end of the rollover) up to the end of its flight / rollover, this depends on the energy balance (concentration vs air drag). After that one can explain that there are other conditions (shoot, check) which can affect the rollover. One does not need to explicit tension in the energy equation of a tethered cast. If you want to introduce tension, that is a different story and it can also be done without referring explicitely to equations.

For laymen my recommandation is: no equation, no internal force, no change in momentum over time, no subsystem.

Merlin
Fly rods are like women, they won't play if they're maltreated
Charles Ritz, A Flyfisher's Life

Dirk le Roux
Posts: 474
Joined: Thu Sep 19, 2013 5:09 pm
Location: Pretoria, South Africa

### Re: Covered topics /how it works

Hi Merlin

Not quite.
Merlin wrote:
Thu Aug 13, 2020 8:31 am
* from the beginning (time zero), the rod leg is held back which reduces its forward speed up to 0.4s of the record
The rod leg decelerates in free flight and is never held back. In fact, it is in free flight before it is formed and germinated from the tip of a quarter circle segment semi loop (see #42).
That would mean a 300 ms delay in between the beginning of the check and the response from the fly leg if I do not fool myself with my (mis)understanding.
No, 0.7s is where the rod leg tip moves in front of the grass plane, where it became invisible. From there you can project in perspective from my feet to where the very tip must have touched grass. My estimate is at just before 0.9s. Between tip touch down and enough line in contact with grass to generate sufficient impeding friction must have taken another while, duration unknown.

All the best,
Dirk

Dirk le Roux
Posts: 474
Joined: Thu Sep 19, 2013 5:09 pm
Location: Pretoria, South Africa

### Re: Covered topics /how it works

Hi Merlin

I mostly agree with what you wrote, yet
Merlin wrote:
Thu Aug 13, 2020 9:07 am
The driving force on the loop is the force at tip (tethered cast), the only external force.
The force at the tip is not quite the only external force. What is the driving force once the cast becomes untethered, and the loop continues to propagate?

Further down. We can state loop tension less technically, like "The thicker the line rotating through the loop, the higher the tension there; the faster the fly leg becomes rod leg, so quadratically higher the tension". I have found in discussion with laymen, at least hereabouts, that they relate to force more easily than to energy.
For laymen my recommandation is: no equation, no internal force, no change in momentum over time, no subsystem.
Not including an equation shouldn't be too hard, excluding momentum change even, but for the rest, lofty, really lofty. Once you are able to formulate such a description for the simplest (tethered cast) situation, test if the same palette of concepts can explain this one:

All the best,
Dirk

gordonjudd
Posts: 1372
Joined: Sat Jan 19, 2013 11:36 pm
Location: Southern California

### Re: Covered topics /how it works

For laymen my recommendation is: no equation, no internal force, no change in momentum over time, no subsystem.
Merlin,
Doesn't that view support Paul's A2 answer that "momentum of the fly leg is the driving force of the loop"? To me that concept (though widely accepted) is wrong on two counts. Momentum does not remain the same as the fly leg shortens in a normal cast, and momentum is not a force.

I hope we agree that there has to a change of momentum to produce a positive acceleration force in a tethered cast. Constant momentum would violate the nominal conservation of energy principal.

Of course energy is not actually conserved either but can it be accounted for by taking the energy loss related to drag into account. That was done on the old board.

As to the source of that acceleration force I don't see that is coming from the rod tip. This might be somewhat of a chicken and egg problem, but I think the force comes from a either a change in the momentum of the moving mass going around the loop or because of the change in the momentum of the varying mass in the fly leg. Either way you get an rho_l*V_tangential$$^2$$ answer.

Do you disagree with this view about where the source of the positive acceleration force?
I see the tension at the top (or bottom) of the loop as resulting from the momentum change of the line going around the loop and the rod naturally provides a corresponding offsetting force at its end of the rod leg.

In the situation where the heavier rho_l of a sink tip line starts going around the loop the line tension at the bottom of the loop will increase due to its larger rho_l*v_tan2 tension. The rod deflection will of course increase due to that tension increase, but it is in response to not the cause of the higher tension.
Does your model of the flying loop end up with momentum being conserved since there are no outside forces acting on the line?

Gordy

James9118
Posts: 1239
Joined: Thu Jan 10, 2013 6:59 pm
Location: N.Wales

### Re: Covered topics /how it works

Why do you guys consistently ignore the rotational mass in the actual loop front, i.e. the angular momentum of the loop itself?

Merlin
Posts: 1474
Joined: Wed Jan 09, 2013 8:12 pm
Location: France

### Re: Covered topics /how it works

Dirk,
The force at the tip is not quite the only external force.
Absolutely correct, there is gravity, but apart from the morphing issues, it does not help understanding the rollover very much.
What is the driving force once the cast becomes untethered, and the loop continues to propagate?
Good question and I never talked about a driving (external) force in that case for good reasons I think.
Once you are able to formulate such a description for the simplest (tethered cast) situation, test if the same palette of concepts can explain this one:
I’ll do my best best Dirk

Gordy

Without rod tip (or hand path) there would be no loop shaped, it is not the line momentum which creates the loop, something must shape the loop first.
The rod deflection will of course increase due to that tension increase, but it is in response to not the cause of the higher tension.
Correct and chicken and egg issue: without rod tip, there is no tension at the free end of the rod leg.

I found a document written in 2013 (by me) on a small booklet as I was lazing on a beach in Indonesia. It gives the basis for calculating legs speeds with a loop and air drag for an untethered rollover. That will save my time for writing the next spreadsheet of a new (always simplified) model.

James
Why do you guys consistently ignore the rotational mass in the actual loop front, i.e. the angular momentum of the loop itself?
Good question indeed, and in my case it is because I hardly see how to manage the application of a torque on a flexible medium (line). Newton would say that the variation of angular momentum in time equals the sum of external torques. Any suggestion is welcomed. For the time being I use to split loop momentum in two (vertical and horizontal), as usual should I say.

Merlin
Fly rods are like women, they won't play if they're maltreated
Charles Ritz, A Flyfisher's Life

Dirk le Roux
Posts: 474
Joined: Thu Sep 19, 2013 5:09 pm
Location: Pretoria, South Africa

### Re: Covered topics /how it works

James9118 wrote:
Thu Aug 13, 2020 3:25 pm
Why do you guys consistently ignore the rotational mass in the actual loop front, i.e. the angular momentum of the loop itself?
Good question, James.

It is possibly a valid approach. To me, it is often a useful thought experiment to visualise throwing an invisible rotating hoop, a healthy initial rotation notionally corresponding to a robust, sustained rollout, and also considering the scenarios of it spinning (pull-back), rolling (tethered) or skidding (shoot).

The difficulty for me is that things become complicated when one considers interactions with the rest of the line. The loop is not a body on its own but a geometric dynamic/disturbance local in the flexible body, mass passing through it or vice versa. So you have transactions at its interfaces, beyond which motion is not angular, where I would need to jump to and from other terms. Further, there is the dependence of angular momentum on the moment of inertia, of which mass distribution in radius is a constituent, and we have seen that loop radius makes no difference regarding tension.

Merlin, as evidenced by your paragraph to Gordy, I am sure you have not forgotten the major external force called air drag. It may help you explain how a loop can be formed without hand or rod tip path assistance.

All the best,
Dirk

Merlin
Posts: 1474
Joined: Wed Jan 09, 2013 8:12 pm
Location: France

### Re: Covered topics /how it works

Air drag is included in the new drafted model for an untethered cast Dirk (perequisit for me)

Looking forward seeing a video showing a loop initially shaped without a tip or a hand, just air drag. There is no way my models can handle a reshaping of the loop by air drag, and I wonder if there is an existing one under use.

Now I have to go back and debug the beast.

Merlin
Fly rods are like women, they won't play if they're maltreated
Charles Ritz, A Flyfisher's Life