Near constant acceleration.

[Paul Arden]

Started a new thread...

[Frank LoPresti]

That's understandable given the lack of clarity in your argument.

[Paul Arden]

Do you want me to press the issue?

[Frank LoPresti]

You have been for 5 years and your still back at square one

[Paul Arden]

How do you teach someone to throw a tight loop?

[Frank LoPresti]

So this is your idea of pressing the issue ? Pray tell, what's the point. I've heard 'em all already. Got anything new ?

[Bobinmich]

Frank,

Paul and I had a discussion on this a year or so ago. I had stated that casters tend towards constant acceleration. I was coming from stream casting and from analyzing what I try to do on a short to medium cast. Paul was much more rounded in his casting experience and was of the opinion that acceleration should be increasing throughout the cast. I don't think I ever convinced Paul, but in my opinion, we are both right. How, you ask? Here's what then.


In a medium range cast or a short cast with a limber rod, you tend to load the rod then try to complete the body of the cast by applying a constant force (torque) to the rod as it increases in rotational velocity. Kinda like flipping a big spider off the end of your rod into a bush without droppin it in your lap. I looked at video's and saw a reasonably constant rod deflection throughout the body of the cast (see the FFA training videos). This hints at constant force ergo constant acceleration. I also looked at a number of Gordon's plots and saw some velocity curves (vs time) that you could reasonably fit with a constantly increasing but straight line. Straight line would mean constant acceleration (v=at).
I also saw many CA curves where rotational velocity increased linearly.

But, on the other hand, I saw most longer distance casts to have an increasing acceleration. This is just the way it comes out when you are going for the max.

I also know that what you don't want is a decrease in the acceleration during the cast i.e. strong at first and getting weaker towards the stop. This is the stuff tailing loops are made of and you can, in fact, produce that infamous symtom, a concave arc path.

I don't think a person can intentionally produce a constant acceleration cast. They just can't sense enough variables to produce it. Besides that, acceleration is a calculated quantity and you don't have a computer in your head. But I do think that softer, shorter distance casts tend towards constant acceleration while longer, higher energy casts tend towards smoothly increasing acceleration.

I don't know how you could explain any of that to a student. I don't think it has any meaning for them, only for we physics mavents. I'd stick to flicking the big spider off the end of their rod into a bush and see where that goes.

Godspeed,

Bob

[Frank LoPresti]

I also know what you don't want is a decrease in the acceleration during the cast i.e. strong force at first and getting weaker towards the stop.

Hey Bob,

Sure I'm with ya on that. I also argee that no one can intentionally cast with constant acceleration, it kinda just happens or you develop a feel for what it's like and go from there. That angular acceleration increases from the start of the CS is the given. And just as you would not want the acceleration of the cast to decrease, going from a stronger force to a weaker force leading to the stop, you would not want to start out the cast with a sustained weak angular rotational force either and then add a strong angular force to that slower/weaker sustained angular force.

Frank

[gordonjudd]

I also looked at a number of Gordon's plots and saw some velocity curves (vs time) that you could reasonably fit with a constantly increasing but straight line.

Bob,
Depending on the caster that may be true for shorter casts. Bruce Richards "Tight Loop with Little Energy" is an excellent example of a cast made with near constant angular acceleration of the rod butt.

I looked at video's and saw a reasonably constant rod deflection throughout the body of the cast

However, because the rod acts like a forced harmonic oscillator (Grunde's car/spring/brick model) constant angular acceleration and deceleration of the rod would not be expected to produce constant deflection of the rod
"throughout the body of the cast."

This plot shows the expected (blue curve) vs the measured (red curve) perpendicular rod deflection in one of Grunde's slow motion videos. Even though Mathias applied near constant acceleration in this cast the rod deflection was never "reasonably constant" as shown below.

Grunde's model says the deflection should vary throughout a cast made with constant acceleration, and his measurements of an actual cast shows that is what you get in practice as well.

Gordy

[Tom]

Hi Gordy.
What exactly is constant deflection of the rod?
Which motion video from Grunde is related
to your plot?

Tom.

[gordonjudd]

What exactly is constant deflection of the rod?

Tom,

With constant deflection the bend in the rod (and hence the perpendicular tip deflection) would not change during the cast. Consequently the deflection vs time curve would just be a horizontal line with some fixed deflection value rather than having the constantly-varying sinusoidal-like shape shown above.

Which motion video from Grunde is related
to your plot?

The measured deflection curve is from the cast made with the Paradigm rod. You can download the video at Jason Borger's site
here.

We owe Grunde a big thanks for taking and digitizing all of the data from these high speed videos that show the details of how the rod and line interact in a 10 meter cast.

Gordy

[Bobinmich]

Tom,
The only place you may see rods tend to hold a constant deflection over part of the cast is on very short casts (20 or 30 feet) with soft rods. The Grunde casts are longer (45 to 50 feet with a 10 foot leader and some rod extension). This causes a longer casting arc and cosine effects become much greater.

You would not see it if the casters style was mostly wrist rotation forcing a wider arc. In fact, the longer the arc, the more sinusoidal looking the rod deflection becomes. It's kinda related to instant center. If you keep a straight wrist like they try to teach in the FFF videos and you are making shorter casts, the rod deflection becomes more constant because that is what you need to do to get a constant force on the line.

But there is nothing really constant in a fly cast anyway and assuming anything from one casters cast and one video is risky. There is tremendous variation from caster to caster, cast to cast, rod to rod, line to line, distance to distance, etc.

None of this has any meaning at all to a beginner. It would only confuse him or her. Telling someone to use constant acceleration or constantly increasing acceleration or sinusoidal acceleration would be meaningless when they don't even know the difference between acceleration and velocity.

Godspeed,

Bob

[Merlin]

Bob,

I follow you on the effect of the short cast; it changes the shape of the rod deflection curve to a significantly flatter one. The main cause is not the arc however, it is mainly the speed of the tackle, which is much faster than for a 50 feet cast, because of the smaller mass of line outside the guides.

I simulated also with my casting model (spring & marble, similar to Grunde’s car & brick) a higher share of wrist rotation, and get another type of flattering for the shape of the curve. So this matches again your observation.

For the model, a larger arc just increases the amplitude of deflection, not the shape of its curve versus time.

Thanks Bob, this is something I did not pay attention to before.

Merlin

[Bobinmich]

Merlin,

Does your spring and marble model use, either as a prime input or a calculated result, the actual input, in terms of force or moment, on the caster's hand side of the model? I have always been of the opinion that a proper model should address these quantities so that we can have a window into what the caster is actually feeling when he/she is making a cast.

I have a model that is somewhat incomplete in its scope but does use the caster's hand as the primary input. It is very difficult to use and not ready for publication, but it yields some insight as to what the caster feels as he casts. I just wish I had some time to devote to completing it.

Bob

[Tom]

Thanks Gordy.
Bob,what is cosine effect?

Tom.