Whip effect

[gordonjudd]

The caster rotates the fly rod near the butt and this "rotation center" you refer to will not move upwards since it is hold by a constraint - like the grip of a whip.]

Tobias,
I think we are talking about two different "rotation centers" I am talking about the intatant center of rotation or instantaneous velocity center that includes the effects of both the translation and rotation of a rigid body (like the butt of the rod with little or no bend) on its velocity at a given point in time.

From a velocity standpoint the velocity of a point on the rod is equal to omega x r where omega is the angular velocity of the objects angular orientation change at a point in time and r is the distance from a point on the butt to the instantaneous rotation center.

Similarly for the KE of the object. In order for the kinetic energy of the moving object to be equal to KE=1/2*I*omega.^2 then the calculation of the objects moment of inertia (I) must take the distance from its c.g. to the instantaneous velocity center into account via the parallel axis theorem.

The caster rotates the fly rod near the butt

I don't think the caster rotates the rod about the grip. It could just as easily be rotated about the shoulder or the elbow if the angular velocity of those joint angles dominates the movement of the rod.

Thus the instantaneous velocity center will not be at the center of the axis of rotation when that axis center is free to move in space. Consequently the axis of rotation of a rod is not near the grip unless the angular velocity change of the shoulder and elbow are zero and the rotation comes from a fixed wrist location.

Chris Korich uses very little wrist rotation in his accuracy casting stroke, thus his instant velocity center never gets very close to his wrist. Similarly for Noel's casting robot. In that case the both the rotation axis and the instant velocity center was fixed at the stepper motor not at the point where the rigid linkage from the stepper motor were connected to the grip of the rod.

Just as an aside I don't see how your fit to the radius of curvature of the rod has anything to do with angular momentum.

The mass of the rod is not moving along those circle fits so how could those circles be related to angular momentum?

Gordy

[hshl]

Gordy,

From a velocity standpoint the velocity of a point on the rod is equal to omega x r where omega is the angular velocity of the objects angular orientation change at a point in time and r is the distance from a point on the butt to the instantaneous rotation center.

annex 2 of my investigations (page 53 below) gives an short explanation. Because the flexible fly rod has intrinsic "inner" degrees of freedom it is difficult to assign all rotating mass elements to only one conjointly center.

I don't think the caster rotates the rod about the grip. It could just as easily be rotated about the shoulder or the elbow if the angular velocity of those joint angles dominates the movement of the rod.

For the effect we're talking about it doesn't matter where the instantanous center of rotation is. My instantanous center of rotation is for sure below the butt as I use a little wrist too, but I do know good casters using more wrist and therefore there instantanous center of rotation is higher. However "near the butt" is not a defined point.

Cheers, Tobias

[gordonjudd]

Because the flexible fly rod has intrinsic "inner" degrees of freedom it is difficult to assign all rotating elements to only one conjointly center.

Tobias,
I agree with this 100% and suspect the velocity center of rotation for the upper tip section is much further away from its c.g. than it is for the butt where the center is around the elbow at MAV.

As noted in the Wikipedia article as the translation velocity increases the distance to the velocity rotation center get larger and larger. Since the translation velocity of an upper section is so much larger than the translation velocity of a section near the butt I would expect the rotation center for the upper sections to be located a sizeable distance away from the I.C. for the butt.

My instantaneous center of rotation is for sure below the butt as I use a little wrist too,

If you measured the instant center of the butt in your cast as I did for Chris I think you would find your I.C of rotation at MAV would be near your elbow not below the butt.

For the effect we're talking about it doesn't matter where the instantaneous center of rotation is

If the angular moment of a section of rod depends on the distance from its c.g. to its velocity center of rotation i.e. I=m*r.^2
I would think the change in the distance to the velocity center of rotation would have a big effect on how the angular momentum of the butt and an upper section of rod change with time.

If the vector for the angular momentum points 90 degrees out from its rotation center as shown below:


Then I would not be surprised if the angular momentum vector for a tip section of the rod at around RSP1 was well below the grip of the rod since its translation velocity is so large at that point.

Gordy

[VGB]

Gordy

I think we are talking about two different "rotation centers" I am talking about the intatant center of rotation or instantaneous velocity center that includes the effects of both the translation and rotation of a rigid body (like the butt of the rod with little or no bend) on its velocity at a given point in time.

Since a fly rod is not a rigid body, it cannot have a single instantaneous centre of rotation and I’m not clear what benefit you derive from your analysis of a section of the fly rod. It appears to me that Tobias paper is based upon a rotating reference frame and is a demonstration of principle rather than a numerical calculation.

One of the things I do like about the paper is that it has undergone a degree of verification and that’s quite a rare beast in this part of the board. It’s probably worth having a thread on verification and validation, so that there’s a common understanding of the processes that models undergo to demonstrate their validity.

Vince

[Merlin]

Tobias

Do not try to get a rule out of these attemps, the problem is rather tricky and what you have seen are examples, not universal truth.

I am working on the DH rod 2D casting model, and there is work to do to be able to split energies. If you look again at definitions you will see that these are conventions for leverage and even inertia, and not actual physics: how much leverage do you have at line launch?

I remember the discussion we had in Berlin with Franz Joseph, we do know that the technical issue is complex and that for the time being, no student volunteered to give a deep theoritical look at the inertial effect. I am just relying on Lagrangian equations, which is better than simple models (e.g. spring and marble). The link provided by Torsten is interesting, we can get the feel of what is behind, but we cannot yet match your approach with theory.

Merlin

[hshl]

Gordy,

If the vector for the angular momentum points 90 degrees out from its rotation center as shown below: [Image] Then I would not be surprised if the angular momentum vector for a tip section of the rod at around RSP1 was well below the grip of the rod since its translation velocity is so large at that point.

if you look on figure XIII you can see how Franz- Josef and I estimate the further motion of the "center of the rotating mass". There we wrote: "The center of rotation of the rotating mass moves back towards the rod grip during retraction / discharge". I think this supports your estimation. This "backmoving" center of rotation also describes the self deceleration process of the fly rod.

Franz- Josef already gave a short explanation about the redistribution in post #24 of the Thread Fly rod deflection. We strongly assume the whip effect contains a component of angular momentum and kinetic energy redistribution which moves similar lake a wave or a circular motion along the rod to the tip. It is coupled to the "dominated distribution" of angular momentum during the early phase of the cast but it is an independent component which can not easily be described by the overall angular momentum of the rod rotated at the grip. It is a bit similar as the effect that accelerates the tip of a whip:

It tries to clarify, that in contrast to a rigid fly rod the lower mass elements of a flexible fly rod have a significant higher angular velocity ω than the upper ones during the early phase of the cast, rotation respectively. This leads to a „butt dominated distribution“ of angular momentum L (L = J * ω; J = moment of inertia). Since this angular momentum distributed along the lower mass elements just can’t disappear due to the energy conservation theorem, it must be transferred towards the upper mass elements during the later phase of the fly cast.

There it forms a circular wave that finaly can accelerate small masses to high velocities like happening in the tip of a whip.

Hi Vince,

Since a fly rod is not a rigid body, it cannot have a single instantaneous centre of rotation and I’m not clear what benefit you derive from your analysis of a section of the fly rod. It appears to me that Tobias paper is based upon a rotating reference frame and is a demonstration of principle rather than a numerical calculation.

Yes, the calculations in my investigations should rather support a trend and as I explained they were only meaningfull in comparison with a rigid fly rod. Thanks.

Do not try to get a rule out of these attemps, the problem is rather tricky and what you have seen are examples, not universal truth. [...] I remember the discussion we had in Berlin with Franz Joseph, we do know that the technical issue is complex and that for the time being, no student volunteered to give a deep theoritical look at the inertial effect.

Absolutely agreed and I think I didn't let any daubt that my investigations are just another brick in the wall of the complex fly cast. Meanwhile I offered this "rotation center issue" to two universities (Berlin and Potsdam), but both rejected it due to its high complexity.

But as Gordy already mentioned, there are some questions left ...

Tobias

[Merlin]

Here is a graphic to illustrate the lack of realism of the estimate of the leverage effect as it is calculated today (with the actual lever arm and not the rigid one).Sorry for doubling the word effect in the graphic.

For the lightest carry, there would be more energy than in the actual case. That just cannot be and it is the result of a convention stating that we can split energies. The energy of the line is made of the one due to inertia and the one due to the spring displacement by the lever. You cannot really split that, you can only estimate the pure elastic part of it. This split is a misconception to me which induces us in error.

With the DH rod, there is a significant difference in elastic energy with or without the inertial effect. It is not a few percent here like in the SH rods case, and this might well result from the relatively low stiffness and the relatively high mass of the rod.

Merlin

[Paul Arden]

Hi gents,

What is the relationship between pull-back and the whip effect?

Cheers,
Paul

[Merlin]

Hi Paul

I dig out old runs I did 18 months ago, PB can give a little bit of extra forward speed thanks to the inertial effect, but this is rather small.

No PB case: 3.8 m/s due to "whip", max line launch speed 27.9 m/s
PB case: 4.6 m/s due to "whip", max line launch speed 29 m/s

The model cast is an average fishing one, not a 170.

Merlin