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Spanish experiment

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Spanish experiment

#21

Post by Unregistered » Fri Jan 25, 2013 7:45 am

Merlin wrote:Aitor, I'm lagging behind now, but I prepared that post and let you know about it:

We can set up two experimental conditions and get two opposite results. It is just a question of tuning (spring stiffness, masses, falling height).

I am trying again to explain what is behind the dynamics of fly rods. We all know there are a variety of them, built for different purposes (at least the range of weight they can cast). The basic principle to give speed to the line is to move a force at tip in the direction of the cast. Using a spring offers the opportunity to do so by comparison to a broomstick. But to be efficient, we have to define the main characteristic of the spring, its stiffness, in relation with the weight of line we cast. As I told above, I can choose a set of conditions that will make the broomstick at least as performing than a flexible lever (maximum line speed).

In the case of an absolutely rigid rod (broomstick), the force at tip is the direct consequence of the acceleration in rotation of the rod, and when the maximum rotation speed is achieved, the line is released. If we use a spring we change the force at tip. The usual vision is that the tip is lagging at start (little force on the line), then it accelerates drastically (the force at tip can be the double than the one in the broomstick case) and the force can travel over a long distance beyond the place where the rotation speed is maximum (during the deceleration of the butt). This is not for free because the corresponding energy is provided by the caster which has to move the rod and compress the spring.

The misconception is to think that the spring will compress until the final stop of the rod and that from this moment, the elastic energy contained in the spring is given back to the line. This may be the reason why so many people looked after the amount of energy stored in the spring as relevant of the casting efficiency. But this is not the way it works; it works because the caster can move a significant force over a long distance (by comparison to the rigid rod). This force varies in time and can be smaller or higher than in the case of the broomstick, but at the end of the day, if the system is properly tuned, the energy in the line is larger than the one obtained with the broomstick. The elastic energy in the rod is mainly converted in loop shaping and kinetic energy of the rod which causes the counter flex, and maybe a small part is directly converted in line energy.
Now let’s come back to the reason explaining why the cast can be more or less efficient depending on the various parameters which affect it. First point: the more energy one puts in a cast, the larger the rod deflection is. Second point: this is not true for speed; there is an optimum somewhere around 50% (+/- 10%) relative deflection depending on the non linearity of the rod.

There is an upper limit and a lower limit to rod stiffness. Since we need to move a force, this force needs to occur as the rod butt is rotating, either in acceleration or deceleration. If RSP is achieved before the end of rotation (stiff rod), then the line is launched before the end of rotation and energy has not been introduced into the system. If the rotation is stopped before the rod has completely unloaded, some force will not be moved and again we lose an opportunity to feed energy in the line.

So the designer’s job is at least to offer a rod that will react positively (the caster will be able to generate energy in a given space frame and time frame). He then has to tune its stiffness for the line one wants to cast. The energy is generated as the spring is loading and unloading during the butt rotation. The idealized situation is to get RSP as the rod is stopped, then energy is produced under optimum conditions (moving force). As the conditions of the cast are changing (line length), the caster has to adapt his motion to stay close to that optimum. Since the mechanical system is somehow tolerant with modern rods, there is some room for error without losing a significant amount of speed, but this is why no one tries to sell you a noodle or a broomstick for fishing.

I think you can realize why defining the condition of the experiment is crucial. As I said we could select conditions to fail demonstrating the way to use a spring properly. I worked on the “trebuchet” parameters to see what we could do. The main point today is to know the stiffness of the candidate rods. It would be nice to be able to demonstrate opposite results by tuning the system accordingly. I think this would avoid finding a good reason not to believe into the experiment. Now we have to define the criteria that will tell us the outcome of the experience. To me you need a high speed camera to measure the launching speed of the mass you intend to put at the tip level in the trebuchet experiment. Maybe the launching angle will be different; I think we can also record it with a camera.

I understand that Lasse has strated his own experience, so thanks to you Lasse if you can provide us with the parameters of your experiment (spring stiffness, masses, height of fall).

Merlin
Merlin,
I find this to be a very good and understandable summary of the whole issue. Thanks.
I will try to find time to pose some questions about it.

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Spanish experiment

#22

Post by Stoatstail50 » Fri Jan 25, 2013 9:25 am

Yes I think its a really neat summary too.

There are a couple of things that I would like to clear up though... :)
...and when the maximum rotation speed is achieved, the line is released.
Under normal casting conditions, can the line be released before the maximum rotation speed is reached ? basically, could the maximum rotation speed occur anytime prior to a deceleration, a reduction in torque ?
This is not for free because the corresponding energy is provided by the caster which has to move the rod and compress the spring.
I think that this has not been properly explained in the past and the impression has been given to those of us in the contemptible position of not having the training of you "foolish engineers", unintentionally I'm sure, that energy outputs could have been greater than the energy inputs. It is nice to see an explicit statement otherwise.
The misconception is to think that the spring will compress until the final stop of the rod and that from this moment, the elastic energy contained in the spring is given back to the line.
If you mean that misconception propounded by non-foolish non-engineers or elderly and ill educated casting instructors and their ignorant ilk, it is far more complex than that. You are in many respect preaching to the semi-converted on here I think. For me agree that the rod compresses until the maximum rotation speed is reached, assuming we mean the same thing by MRS. This is not at all the same as "the final stop of the rod". I don't believe that all the elastic energy contained in the spring is given back to the line, never have. How much may be a matter of debate but it a debate that I'm happy to sit back from.
First point: the more energy one puts in a cast, the larger the rod deflection is. Second point: this is not true for speed; there is an optimum somewhere around 50% (+/- 10%) relative deflection depending on the non linearity of the rod.
Hurrah....no idea about the percentages but, apart from that, totally agree....could you please explain what you mean by the non-linearity of the rod, just so that I can be sure we that I am thinking about it in the same way.
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Spanish experiment

#23

Post by Unregistered » Fri Jan 25, 2013 10:46 am

Stoatstail50 wrote:
This is not for free because the corresponding energy is provided by the caster which has to move the rod and compress the spring.
I think that this has not been properly explained in the past and the impression has been given to those of us in the contemptible position of not having the training of you "foolish engineers", unintentionally I'm sure, that energy outputs could have been greater than the energy inputs. It is nice to see an explicit statement otherwise.
Good post!
The impression of a bigger energy output than the energy input is the key issue here, and that impression remains unsolved.
Because that bigger energy input contributed by the caster can not be possible in the case of a spring/brick driven by a weight under the influence of gravity: the energy that this specific "caster" can apply is always exactly the same.

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Spanish experiment

#24

Post by TrevH » Fri Jan 25, 2013 1:35 pm

Aitor wrote: The impression of a bigger energy output than the energy input is the key issue here, and that impression remains unsolved.

Does anyone really believe you can get more energy out than you put in?! :???:

Cheers
Trev

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Spanish experiment

#25

Post by Stoatstail50 » Fri Jan 25, 2013 1:52 pm

No.
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Spanish experiment

#26

Post by gordonjudd » Fri Jan 25, 2013 4:15 pm

The impression of a bigger energy output than the energy input is the key issue here, and that impression remains unsolved.
Because that bigger energy input contributed by the caster can not be possible in the case of a spring/brick driven by a weight under the influence of gravity: the energy that this specific "caster" can apply is always exactly the same.
Aitor,
I don't know what energy numbers you are using to come away with the impression that Merlin's model is saying the output KE in the brick in your experiment is greater than initial PE of the lead mass above the ground.

Merlin pointed out that for his simulation using a lead mass of 10 grand a brick mass of 5 g the KE of the brick at the maximum release velocity was 33% with the string and %50 with the spring. Different mass ratios would produce different values assuming the spring constant of the spring was adjusted accordingly to get the maximum launch value.

For your example with a lead mass of 50 g and a brick mass of 10 g the ratio of the output KE to the input PE was 16% with the string (10/(10+50)) and 24% with an the optimum spring constant of .295 N/m.

In a situation with a larger lead mass and a relatively small brick mass to get closer to a constant acceleration input to the spring as is done in the car/spring/brick model the relative KE value with a string would be
KE=PE*(m_brick/(m_lead+m_brick))
and the ratio with the optimum spring would be somewhat larger.

But in no instance is Merlin's model saying there is more energy output in the brick than the initial PE value of the lead mass. As you know that is an impossibility.

Gordy

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Spanish experiment

#27

Post by gordonjudd » Fri Jan 25, 2013 4:55 pm

Ps. I'm quite intrigued that no one has taken the 5 minutes it takes to set up Aitor's experiment, but are happy to argue for several weeks about what might or might not happen.
Lasse,
It is one of the mysteries of board discussions, but I think Aitor's questions help everyone to understand what is going on. He is on firm ground when he questions the validity of a model, especially one that appears to be "voodoo" to some readers at Sexyloops.

Fortunately, it appears you did take the time to do the experiment, and from your description have shown results that exactly mirror Merlin's predictions. The fact that you see the brick mass hitting the ground further away from the end of the table indicates that it had a higher velocity with the spring than it did with the string. I would be scratching my head if the experiment did not turn out that way.

I imagine you are doing this indoors so you cannot take some high speed videos of your runs, but that would be worthwhile doing. I have found that in most cases "seeing is believing" although there is always the problem of having to believe in something before you can see it.

Thanks for taking the time for just running the experiment. It appears friction was not as big a problem as I thought it might be to show the difference between a string or spring driven acceleration force on the brick. As Merlin mentioned the peak force with an optimum spring can be nearly twice a big as it is for a string (or a broomstick in the case of casting). The theoretical force difference for Aitor's 10 and 50 g mass values is shown below.

In the case of the falling lead mass the distance starts out at 2 meters and ends up at 0 meters when it hits the ground. Thus time-wise, this plot is running from right to left and the maximum deflection in the spring (and hence its maximum force) is reach after the brick has moved about .7 m from its initial starting point.
Image
For those mass values the difference in the launch velocities was 5.7 m/s with the string and 7.1 m/s with the spring, so you can see the added acceleration force provided by the spring makes a big difference in the work energy that is applied to the brick.

Energy wise the PE of the .05 kg mass with a starting height of 2 meters was close to 1 J. Thus the output KE energy of the brick was about 17% of the input value with the string and about 25% with the spring. Without knowing the spring constant of your rubber band, I cannot come up with comparable values for your experiment.

Gordy

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Spanish experiment

#28

Post by Lasse Karlsson » Fri Jan 25, 2013 5:02 pm

Merlin wrote:Thanks Lasse

Can you try to estimate the "spring stiffness" by measuring a deflection of the elastic for a reasonable mass

Thanks,

Merlin
Hi

Elastic Weight

15 cm ------ 0 gram
15,4 ------- 7,5
15,8 -------- 15
16,9 -------- 30
19,4 -------- 60
29,0 ------- 120

I have the increments for increasing weights of 7,5 grams upto 135 grams.

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Lasse
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#29

Post by guest » Fri Jan 25, 2013 5:31 pm

He is on firm ground when he questions the validity of a model, especially one that appears to be "voodoo" to some readers at Sexyloops.
Gordy

It appears that your new persona is as big an arse as the old one.

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#30

Post by Lasse Karlsson » Fri Jan 25, 2013 6:41 pm

gordonjudd wrote:
Ps. I'm quite intrigued that no one has taken the 5 minutes it takes to set up Aitor's experiment, but are happy to argue for several weeks about what might or might not happen.
I imagine you are doing this indoors so you cannot take some high speed videos of your runs, but that would be worthwhile doing. I have found that in most cases "seeing is believing" although there is always the problem of having to believe in something before you can see it.


Gordy
Hi Gordy

Right now I have -12 outside, not much experimentation going on there...
I did it on my living room table, using two spinfishing lures as bricks...

[vimeo]58204382[/vimeo]

It's pretty similar to Grundes high speed video where he superimposed a broomstick on the cast.
Broomstick leads all the way until the flyrod whips past at the end.

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