The Whip Effect

[gordonjudd]

In my video 'COAM' is not used at all.

Tobias,
Which video are you referring to?

COAM seems to be a central tenet in a number of your videos on Vimeo.


I would still like to understand how you are calculating the composite angular momentum of a flexible rod since the nominal (omega*r) velocity of different sections of the rod do not have a common omega value. The changing shape of the bent rod results in different parts of the rod having different omega values as shown in Merlin's diagram. In addition, because the rod is flexible the direction of the actual velocity of the moving mass is not 90 degrees to the different radius radius values as it would be in a rigid body.

Gordy

[hshl]

Gordy,

Which video are you referring to?

Of course to the video Paul refers to

COAM seems to be a central tenet in a number of your videos on Vimeo.

Yes, in my older videos I did. Didn't you read annex 3 of my investigations? So I don't know in which direction your statement should drive us ?!

In Annex 3 of my investigations Franz-Josef gave further explanations. To avoid misunderstandings we rather use the term 'redistribution' instead of 'COAM' .

Redistribution of AM is the better term as I used in the video Paul refers to.

And finally there is no chance to "crack the tip" of a fly rod by an inertial / whip effect.

Agreed, but a whip which is not used to reach supersonics contains the energy transfer property too, isn't it ?

Tobias

[Merlin]

Tobias

By which mechanism can some kinetic energy of a (stiff) rod can be transmitted to a soft line? Wave on a string? Impedance would be a problem I guess.

Merlin

[hshl]

By which mechanism can some kinetic energy of a (stiff) rod can be transmitted to a soft line? Wave on a string? Impedance would be a problem I guess.

I don't know Merlin,
my investigantions end up estimating the tip speed and with it the whip analogy too. As far as I know (from you) it is a very difficult issue. I think there is a kind of 'interruption' since the energy passes two mediums (fly rod -> fly line) with different properties.

Tobias

[Bernd Ziesche]

Hi everyone,
reading some of Professor Goriely's summaries about whip physics, the first thing coming to my mind always was, that we don't cast a whip, but a fly rod. Biggest difference for me seems to be, that we have a fly line (depending on a) it's length outside the tip, b) taper, c) density and d) velocity relative to rod plane/direction, not to forget about the fly and leader as well) providing all different resistances against the rod's tip. This and the fact, that we are simply not casting our fly rod in a closed system, but one coming with so many variables, makes me think, that we cannot just apply the physics of a typical whip here. Nor do I think we can quantify the level of "whip effect" (for me it would be more of a fly rod effect) in general.
It seems to me no one of you fine physicists yet can answer Paul's simple question in a simple way. This makes me think to have a point?

Was there ever made a calculation or even better a slow motion study of the increase in tip speed for straightening when only moving the rod itself?
Gordy your great video about the increased level of counterflex when not having the line attached comes to my mind!

About the "pirouette effect" there are great videos explaining that effect in general. Easy understandable. Only as far as I understand they all refer to a system, which is not too far away from a closed system (coming with very little drag at the connection to the outer world of that system). Since this is very different for the fly cast (in which we always hold the rod in our hands and though add significant force), it yet doesn't convince me, it to be very helpful trying to add this effect into the explanantion of a fly cast. I just don't see at what point we have conserved the energy, but constantly adding more energy to the rod.

So I am wondering, if anyone can explain the whip effect, the pirouette effect and how they apply in fly casting in an easy understandable and most of all correct way. I take it, a correct explanantion among physicists would be the one not creating any further arguments among you fine guys?

Still hard for me to understand, why not looking directly at what happens in the fly cast instead of trying to apply principles, that never were written to be applied in understanding the fly cast. Yet this seems very complicated to me. You please tell me, if I am wrong.

No offense here of course. Just having a bit of a feeling, that explaining the fly cast doesn't have to be this complex, does it?
Regards,
Bernd

[Merlin]

Hi Bernd

Interesting thinking.

To my knowledge, no one ever tried to apply the physics of the whip to a fly rod. Whip is used as an analogy, just like in the old times French people used to call fly fishing “whip fishing”, likely because the rod and the line look like a giant whip. The visual link is strong enough to allow some people considering that there can be a transfer of energy from the rod to the line comparable to the transfer of energy from the butt to the tip of a whip. However, there is little evidence, if any, of such a specific mechanism.

The fly rod is a driven harmonic oscillator and bad luck this is not simple machinery. If that was the case there would be little difficulty to design a fine rod. Simple casting models have been used to understand rod casting mechanisms, the first one to my knowledge was written by Steve Fry some 40 years ago. They have been compared to records, but even the best high resolution videos cannot unveil all mechanisms involved. These mechanisms have been known by about a century, and at that time one spoke of “momentum”, or inertia if you prefer. Simple models (from Steve, Grunde or my first 1D model) use a wrong description of a spring and that creates an inconsistency in terms of kinetic energy of the rod: it is as if two thirds of it had disappeared. Bringing back consistency (as described in “The physics of the overhead fly cast”), allows explaining the “momentum” aspect of the mechanisms. Part of the mass / MOI of a fly rod reacts to the acceleration / deceleration of the butt and allows decreasing or increasing tip speed. There is some wavy aspect associated to the KE of the rod as various parts of the rod are stopped at a slightly different moment, but this is not directly comparable to the physics of the whip.

I remember having carefully studied the video from Gordy that you mention, and it is indeed possible to simulate that. But IMHO, there is no mean to identify the inertial effect on tip speed from it, just as it is impossible to isolate the spring component of tip speed from that video. I shall have another try. The pirouette effect (reduction / increase of the MOI of a rod as the rod loads and unloads) does not allow identifying the inertial effect, as modeling angular momentum remains a dead end up until now (welcome to volunteers). It is another aspect of it, more visual but unfortunately not easy to explicit. To maximize the effect of rod mass, you have to design a very deep butt action rod. If that rod is slow enough, then you can get two distinct acceleration phases which was known at the end of the 19th century as the “double action” for some fly rods. If the rod is sufficiently fast, then these two acceleration phases merge and you cannot easily identify the inertial effect anymore.

I am not going to rewrite “Swing, spring and whip”; all explanations are already on paper. The momentum / inertial / whip mechanism is due to mass distribution along a fly rod and creates extra tip speed as we decelerate the rod butt; as well as it induces a backwards tip motion as the rod is accelerated forwards from a standstill. I cannot make the explanation simple or shorter.

Merlin

[Paul Arden]

If you think about it from a practical casting sense we have these three characteristics. Spring, Lever and Whip. Spring is bend and release. Lever is just a magnification of motion at the butt. Whip may indicate more feeling; ie working the rod.

How does line mass outside the tip affect the whip?

Is it possible that we can take one rod profile and look at the effect of different force inputs applied in relation to these three attributes?

Does their respective balance change as the input changes. Obviously more input means less leverage (pirouette). Does the whip have a linear output progression as we increase force? I remember years ago that Tim R. had a comp 5 rod that had two “flat spots” (or so it was written here). I can imagine something like that would have a profound impact on the whip output as the force input changes.

I think that the explanations are reasonably simple. Maybe we need to just talk about the whip. Ie how this energy travels up through the rod. Would you regard it as being a wave?

Thanks (sorry lots of questions )!
Paul

[Merlin]

No problem Paul. Most answers are in the Swing Spring & Whip paper, page 7 (evolution of energy shares with carry).

The inertial effect emphasises tails in the line, I forgot to mention this one. It has some wavey aspect as would say James, visible on KE curves, but I do not think that this kinetic energy is transmitted through a wave to the line, although I should not underestimate the unexpected for me.

Merlin

[gordonjudd]

I think that the explanations are reasonably simple. Maybe we need to just talk about the whip. I.e. how this energy travels up through the rod. Would you regard it as being a wave?

Paul,
I cannot see that there is a wave like energy flow from the rod to the line; so as generally is the case, simple understandings of complex topics like the so called "whip effect" (a bad descriptor IMHO) can lead to misunderstandings of what is going on.

I find the whip effect (or impact of the inertial mass of the rod) to be quite complicated and suspect that Merlin is the only one who has firm grasp of how it actually impacts line speed.

How does line mass outside the tip affect the whip?

I like the simplicity of Merlin's finger-spring-marble model (or Grunde's car-spring-brick model) of how the spring deflection of the rod reacts to the rotational acceleration of the rod butt and that in turns produces a given line speed by applying a force to the line over some distance. That model assumes the spring has no mass while the the much more complicated 2-D model takes the mass of the rod into account. The resulting line speed difference is on the order of 10% so it is not a big effect for today's graphite rods.

So I am wondering, if anyone can explain the whip effect, the pirouette effect and how they apply in fly casting in an easy understandable and most of all correct way.
and
No offense here of course. Just having a bit of a feeling, that explaining the fly cast doesn't have to be this complex, does it?

I think that Bernd's observations are on the mark.

I don't think there is pirouette effect since the rod system has outside torques being applied to both ends of it. It is not just the rod that is being rotated but the acceleration of the line must also be taken into account.

I think the whip effect accounts for the inertial mass of the rod and thus is interesting to people who want to understand another layer of the onion, but is not a big effect for today's light graphite rods.

Gordy

[Bernd Ziesche]

Thanks Merlin, thanks Gordy for your fine answers.

In regard of understanding the level of "whip" better, would it be help to produce a graphite rod coming in constant weight distribution (no taper)?
We could then bend both the tapered rod and the none tapered one and see how tip speed would increase during straigthening. To keep this simple, we might do without any weight connected to the tip. Might give further understanding about the taper-based whip effect? Also we could add some resistance.
Ok, I would have to think about how to imitate the squared increase of wind resistance relative to line speed during straightening.
If this is wrong thinking, let me know please.
Thanks,
Bernd