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Smoothness - and the use of the CA

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Paul Arden
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Post by Paul Arden »

Maybe you'll have it all worked out.

:laugh:
Have a great holiday Bob :cool:
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Merlin
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Post by Merlin »

Not sure how difficult it would be to get this with all your obi van engineering software
What do you assume the hand path looked like to come up with the predicted tip path in your modified 2-D model? Could you modify it to force a match in the two curves?


Hi Bernd and Gordy

The modeling can be done but for the time being it is lacking something: the effect of guides. The current models runs as if the line was attached to the tip, but in real life the line induces a supplementary bending moment due to its action on the guides, and this is at least one reason explaining the lack of fit in my graphic.

If you look after tails, then you have to model the hand path (included in the model), and this is an important parameter. I think that the cause of tails is a “jerk” as defined by Gordy: too much acceleration at the bad timing. It is not a question of linearity or non linearity of the acceleration, if I cast with a constant acceleration but it is too intense, then the tail will pop up.

The cast I tried to model is this one (Source Graig Spolek publication). You can see the hand path. I talked about that several times but when I see I get tails, I rotate the shoulder to modify my hand path and get better loops, without changing the way I accelerate.

Image

To model a tip trajectory I need several models: a large deflection model of the rod (meaning I need to know its design), and the multi motion casting model which describes the caster (arm, forearm, wrist). After simulating the rotation speed record (so I need it), I derive the force values acting on the tip in its own frame (as if the rod was in a standstill). Then I report them in the large deflection model, using different angle position of the rods during the simulated cast. This is how I move from 1D to 2D. Ideally, I should recalculate everything step by step, but it is out of reach for my patience.

Merlin
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gordonjudd
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Post by gordonjudd »

I am starting to think that over analyzing rod butt acceleration is a waste of time until we clean up the connection between it and line acceleration.
and
Maybe you'll have it all worked out.

Bob,
Maybe you do not understand what Grunde's and Merlin's SHO model is all about.

The input to that model is the angular acceleration applied at the butt of the rod and the output gives the deflection of the rod vs time (which provides the acceleration force to the line) for that input.

Merlin was the first to come up with that model several years ago, and it is a shame that most people still do not understand how it works. It is a simple 1-D model, but as you have seen from Merlin's posts it can give some very useful insights into the dynamics of casting.

Gordy
"Flyfishing: 200 years of tradition unencumbered by progress." Ralph Cutter
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Post by gordonjudd »

Then I report them in the large deflection model, using different angle position of the rods during the simulated cast. This is how I move from 1D to 2D. Ideally, I should recalculate everything step by step, but it is out of reach for my patience.

Merlin,
Is the effect on the bendform of a particular rod design for a given acceleration force and pull angle of the rod predicted using a static model?

Rather than worrying about the effect of the guides (most of that effect for all but the tip top is in the axial direction of the guides and thus won't do much) I think the big difference will be in including the inertial bending of the rod caused by its own mass. Dr Spolek estimated the inertial bending can be 30-40% of the bend we see, so that is a big effect.

Since you do not have the resistance from the line it is much easier to apply torque to the rod without a line so this picture overstates the bend that comes from inertial bending, but you can see it is quite large in this case.
Image

I think the inertial bending of the rod will also react much quicker to changes in the acceleration than does the deflection at the tip since that reaction gets smoothed by the lag in loaded frequency of the rod. I imagine there is some lag for inertial bending but I think it will have a much smaller lag time than does the tip.

Galerkin model to inertial bending here we come.

Gordy
"Flyfishing: 200 years of tradition unencumbered by progress." Ralph Cutter
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Merlin
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Post by Merlin »

Hi Gordy

In fact I am calculating the number of gs applied to the rod, so the bending moment due to the mass of the rod is taken into account. The calculation is not purely "static" but is somehow "dynamic". This is not the reason of the misfit, and that's why I am suspecting the effect of line in the guides.

Merlin
Fly rods are like women, they wont´play if they're maltreated.
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Eugene Moore
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Post by Eugene Moore »

Where the constant acceleration does promise the maximum work output with the minimum energy requirements, I don't believe it couples well with a variable load from the fly line and a flexible driving member from the rod. It is easily calculated which may make it promising for the math models, but does not take into account the finite torque available from the caster. It's also difficult to smoothly combine constant acceleration to a start and stop event in the cast. There must be time taken to rise to constant acceleration and time taken return to zero. If there is no time you have “jerk” or “shock” to a spring system with undesirable results.
I feel a constant torque would be more appropriate from the stand point of the caster with feedback from the rod deflection and the fly line loop unrolling from the previous back cast.
This would mean a lower acceleration as the fly line loop unfurls followed by an increasing acceleration as the rod deflects and the effective rod length decreases.
The data from previous posts on actual casts seems to support this approach.
As stated previously, we aren't capable of accurately feeling acceleration, but are capable of feeling the torque effect caused by the acceleration. This can be the mechanism for coordinating our output with feedback from the rod allowing the caster to change tempo with what is felt by the changing loads and still staying within the finite torque the caster can develop.
Eugene Moore
gordonjudd
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Post by gordonjudd »

I feel a constant torque would be more appropriate from the stand point of the caster with feedback from the rod deflection and the fly line loop unrolling from the previous back cast.

Eugene,
A good part of the torque comes from the deflection acceleration force on the line as discussed here.

Thus as the deflection of the rod varies the torque will need to increase in a non-linear fashion to maintain the angular acceleration rate of the butt of the rod. This means a non-uniform torque would be required to achieve a nominally constant angular acceleration of the butt of the rod. That was the case for the data Grunde measured for the Paradigm cast as shown in the total forcing torque applied at the butt of the rod (blue curve) below.
Image
You can see the total forcing torque applied at the butt of the rod was quite non-linear in that cast, even though Mathia's produced a fairly constant angular acceleration of the rod butt in that cast.

The data from previous posts on actual casts seems to support this approach.

Can you specify what data for an actual cast you are referring to that would imply you would want to apply a constant torque?

Because of the SHO response of the rod to forcing inputs applied at the butt, I think you know constant angular acceleration will produce a sinusoidal-like variation in the deflection of the rod.

Some of your comments might apply when using a stiff broomstick where there would be a a scalar relationship between the angular acceleration of the butt of the rod and the applied torque it took to get it, but that is not what happens when using a flexible lever like a fly rod.

Gordy
"Flyfishing: 200 years of tradition unencumbered by progress." Ralph Cutter
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Merlin
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Post by Merlin »

Hi Gordy

I checked the "line in guides" effect on the deflection in static conditions and high load: very small...

So the misfit of the model comes from another reason.

Missed again...

Merlin
Fly rods are like women, they wont´play if they're maltreated.
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Eugene Moore
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Post by Eugene Moore »

Most of the graphic representations shown here have a curved accelerating profile. Rather than working against the rod working with it may increase output velocity. With a constant acceleration output should be a straight line. It isn't and won't be due to the non-linear spring rate of the rod. With a constant acceleration velocity should also be a straight line. The increasing nature of the fly line load from the back cast coupled with the non-linear rod deflection won't allow straight line dynamics.
If the rod were rigid and the fly line load fixed and constant, the application of constant torque would result in constant acceleration. Since neither of these two are correct, constant acceleration may in fact be the worst choice for an acceleration profile.
Perhaps someday you can instruct me in why the mechanics for striking a golf ball with a club is similar in any aspect with throwing a cast with a fly rod.
Eugene Moore
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Post by gordonjudd »

Perhaps someday you can instruct me in why the mechanics for striking a golf ball with a club is similar in any aspect with throwing a cast with a fly rod.

Eugene,

Because the the physics of doing work in rotational dynamics are the same for both sports. The use of delayed rotation in distance casting is much the same as maintaining lag in a golf swing. That was the point I was trying to make post #32.

Thus applying a non-linear torque where the rotation is very small at the beginning and very large at the end will produce much more work energy than if a constant torque is applied over the same angle range as discussed here.

Here is the discussion of the relevant torque applied over angle mechanical work principle iin rotational dynamics as described in Wikipedia
Image

Rather than working against the rod working with it may increase output velocity.

What does working with or against the rod mean?

With a constant acceleration output should be a straight line. It isn't and won't be due to the non-linear spring rate of the rod.

What do you mean by output in that statement? Is it the output line velocity or the angular butt rotation velocity?

I don't think the non-linear spring rate of the rod has much effect for 10 meter fishing casts. A constant k spring will still produce a sinusoidal-like deflection curve when constant angular acceleration is applied to it. That is what Merlin's model is all about.

Gordy
"Flyfishing: 200 years of tradition unencumbered by progress." Ralph Cutter
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