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Topic: Angular Acceleration in Distance Casting, Do distance casters use contant accelera

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Post Number: 11

WetWading

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Posted on: Feb. 23 2009,10:04

(gordonjudd @ Feb. 22 2009,23:00)

QUOTE

I'd be useful if Paul could confirm is his cast was hauled.

If Paul can hold 85 feet of line in the air without hauling, then he has to be the only person in the world that can.

Gordy

I've little doubt Paul was hauling, but surely anyone who could cast 100 ft without hauling can carry 85+.

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Post Number: 12

Paul Arden

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Posted on: Feb. 23 2009,10:51

Skip to the previous post in this topic.

Hi guys, yes this was a hauled cast, 120 MED on a 6TCR - and I know it was a good one because I called it A_1!

I also took a video at the time but I have no idea where it is at the moment. I was planning on comparing the two for an overlay but couldn't work out how to do it :glare:

I'll have a look for the film today. I'm having some technical problems with the palm screen but it would be good to do it again.

I also noticed that the acceleration is non-linear. Which also means delayed, right?

Cheers, Paul

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Post Number: 13

Magnus

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Posted on: Feb. 23 2009,11:05

Paul

If you could ask Gordon to explain what the log/log plot means it would save me the embarrassment

Gordon will, I hope, be working up something on a cast with a very good 'smoothness' - constant acceleration we think. The acceleration of the rod in your cast is non-linear as I understand it - that's what's delaying the rotation in a manner of speaking. If the acceleration was constant (ie uniform) the slope of the velocity/time graph would be a straight.

Can you recall the carry for that cast and do you happen to know the mass of the line at that length?

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Post Number: 14

Paul Arden

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Posted on: Feb. 23 2009,11:15

No sorry the carry wasn't measured but I imagine 86-90ft. I have problems casting smoothly with the analyzer attached on the forward cast, because the analyzer doesn't allow for free rotation on the cradle grip. Which may be why I was getting complete stops in one direction and incomplete in the other.

How does the analyzer distinguish between forward "creep" and effective casting stroke on the cast to determine casting arc?

Cheers, Paul

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Post Number: 15

Magnus

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Posted on: Feb. 23 2009,12:38

Hi Paul

Hmmm - that's really a question for Noel or Bruce.

We're working with the opened prb files. If you graph those its pretty clear when sustained angular acceleration starts. For your trace you can see the rod wobbling up and down as you trot to and fro

This is the trace from the start of the pause leading up to that peak - you can see how the values go up and down and the rod goes up and down - then you launch into sustained rotation.

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Post Number: 16

gordonjudd

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Posted on: Feb. 23 2009,14:16

QUOTE

If you could ask Gordon to explain what the log/log plot means it would save me the embarrassment

Magnus,
No dumb questions over here. That is how we are all going to learning something about casting.

On the face of it, using a log-log plot to find the exponential factor for something that varies as a function of time raised to some power say
Phi=a*t.^power
Where Phi=butt phase angle
a is some coefficient
t is the independent time variable
power is the exponential factor for the time variation

Is straightforward. If you plot that relationship on a log-log plot then the equation effectively becomes
log(phi)=log(a) + power*log(t).

Which is in the form of a polynomial fit like
y=const + slope*x

Thus the slope of the butt phase data vs time on a log-log plot should show the exponential factor for how the butt phase is linked to some power of time.

Consequently if you plot a polynomial function like y=x.^3 on a log-log plot in excel and you use a ruler to measure the slope you will find the slope on the paper (don’t use the log values to get values but use the physical length of the delta y and the delta x values you get with the ruler) is 3.

Back in the days when I used to take videos of casters and then fit the resulting butt phase angles vs time variations to Robson’s model I thought this would be a neat way to see what the best fit exponential factor would be.

However, in actual application I soon realized that you don’t know when to set the t=0 relative time point to get an accurate exponential power number. It turns out the resulting slope of the log-log plot is very sensitive to where you decide set the zero time point. If you move it back earlier in time the exponential factor will increase and if set it later in time the exponential factor will decrease.

Consequently I cannot say with certainty that the exponential power for the butt phase angle in Paul’s cast was 5.5 as is shown on that the log-log plot. That is just the way it came out using the t=0 reference time that I had used for his data. I did not play around changing the assumed zero time reference point to see how sensitive the exponential factor would be.

If you had video, then you could independently set the t=0 point to the frame where the hand was starting to come forward. With that additional constraint, then the resulting exponential power factor derived from the log-log plot could be a reasonably good measure of the delay in the rotation. The longer the delay before the rod started to rotate the higher the exponential factor would be with an independent way to set the t=0 value.

Without the independent video time reference I can’t say with certainty the exponential power factor for the angular acceleration in this cast is equal to 3.5. Prior to seeing Paul’s data most of the data I was seeing had a linear increase in the acceleration curve.

In rotational mechanics, the derivative of the angular acceleration is called “jerk”. (Who says engineers don’t have a sense of humor in picking that name.) An example of that is shown in the figure below. In this plot the angular velocity is in green and the angular acceleration is show in blue.

I use a linear fit to the acceleration curve to come up a best fit value for the jerk expecting it to be linear. The linear fit to find the nominal jerk value is shown in the red curve below. However in Paul’s case the slope of the acceleration curve varies in some nonlinear fashion (power of three maybe), so the jerk number does not mean much.

Who would have thought such a simple question could turn into such a long winded answer? :;):

Gordy

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Post Number: 17

gordonjudd

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Posted on: Feb. 23 2009,15:00

QUOTE

I also noticed that the acceleration is non-linear. Which also means delayed, right?

Paul,

As noted above if you had an independent way to establish the t=0 time point (video would be a good way to do it) then I think you would get a very good correlation between the amount of delay before you really started to turn over the rod (what Magnus called “then you launch into sustained rotation”) and the exponential slope factor from a best fit to the measured angular velocity from the casting analyzer.

The more delay you have the higher the exponential power needs to be to get the required rod turnover in a shorter time period. It would also probably give an indication of the how strong your shoulder, elbow, and especially wrist need to be to provide the moment required.

Have you tried moving your feet to get an even longer stroke? Theoretically that should provide more energy into the line, but the timing required might be difficult to do in a way that would maximize the applied energy.

Gordy

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Post Number: 18

Marc LaMouche

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Posted on: Feb. 23 2009,15:46

(gordonjudd @ Feb. 23 2009,16:00)

QUOTE

Have you tried moving your feet to get an even longer stroke?

ain't just the feet that are movin' around, Gordy

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Post Number: 19

gordonjudd

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Posted on: Feb. 23 2009,16:30

QUOTE

ain't just the feet that are movin' around, Gordy

Now that is what I am talking about in taking a step forward and reaching with the arm to extend the path length.

I think this type of move on the forward cast combined with a step backward on the backcast to get the ultimate path length is something to strive for.

QUOTE

I've little doubt Paul was hauling, but surely anyone who could cast 100 ft without hauling can carry 85+.

Thanks. I stand corrected and have learned something. Mostly that I need to stop using the word "only" in a sentence.

Gordy

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Post Number: 20

Paul Arden

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Posted on: Feb. 23 2009,17:18

Thanks Gordy, this is not the cast in question, but this is the stroke: http://www.sexyloops.com/movies/170/

I don't really use the wrist in the way you probably do, I sort of flop it on the backcast and cradle grip on the forward. I first saw Rick do this and decided I had to learn it (because he was casting further than me!).

I do move my feet. It's hard to use the "closed stance" without the step. We really need to sync casting to analyzer to understand what's going on. A couple of things bother me, one is that it's very difficult to keep the analyzer in plane with the cast, especially on the long stroke, and the other is that the analyzer inhibits rod rotation of the cradle grip.

Cheers, Paul

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168 replies since Feb. 23 2009,02:15

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