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Upward Force from Form Drag

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Merlin
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Upward Force from Form Drag

#81

Post by Merlin » Wed Feb 21, 2018 5:47 pm

I don’t really know Gordy, but if viscoelasticity is important, you need to appreciate both at the same time and I cannot tell which combination is important, and have no idea about that characteristic for lines.

Merlin
Fly rods are like women, they won't play if they're maltreated
Charles Ritz, A Flyfisher's Life

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James9118
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Upward Force from Form Drag

#82

Post by James9118 » Wed Feb 21, 2018 5:53 pm

Merlin wrote:I don’t really know Gordy, but if viscoelasticity is important, you need to appreciate both at the same time and I cannot tell which combination is important, and have no idea about that characteristic for lines.

Merlin
I can measure viscolelastic properties of materials in my lab - I have a DMA.

Shouldn't this be opened in a new DNL thread? Have we come to a consensus that lifting loops as per Gatti-Bono is a crock?

Gordy - are you going to bother answering my question?

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Graeme H
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Upward Force from Form Drag

#83

Post by Graeme H » Wed Feb 21, 2018 6:57 pm

gordonjudd wrote:
If you watch the end of the fly line in isolation in those casts, you can see the line itself does not accelerate after the initial "back" cast.
and
While he pulls the rod leg out from under it, the loop appears stationary, but once the line within the rod leg is no longer moving, the loop just resumes its journey to the right of screen.
Graeme,
I don't know if we are looking at the same part of the cast, but it appears to me the loop (and hence the fly velocity) does accelerate once the loop starts going forward again after the rod leg hits the water on his snap. The green dots in the overlay below track the front of the loop at two frame intervals as it is going to the right of the screen.
Image
It looks to me the space between the markers is increasing as the loop propagates to the right (it actually goes backward since the velocity of the rod leg is higher than the fly leg until the fly leg hits the water). That would indicate the velocity of the loop over the ground is accelerating.

Gordy
Hi Gordy,

We are definitely not looking at the same part of the line. I’ve already said which part of the cast I’m looking at.

You’re wrongly assuming the loop velocity is solely controlling the fly leg speed. The loop velocity is the average velocity of the rod leg and the fly leg. Fly leg velocity was determined by the rod tip speed during the back cast (to the right) and the rod leg speed is determined by the tip speed during the snap (to the left).

The loop stalls when the rod leg velocity to the left matches the fly leg velocity to the right (I.e. average velocity approaches zero) and indeed, the loop can move to the left if the rod leg velocity is greater than the fly leg velocity. Only the loop accelerates to the right as the rod leg velocity drops. The fly leg velocity was already established during the back cast and it does not change just because the loop stalls or accelerates.

When the rod leg velocity falls to zero, the loop begins to move towards our right again because the fly leg velocity has not changed.

The fact that the loop can move to the left while the fly continues traveling right should inform you that the loop velocity itself is not controlling fly leg velocity.

Can you put dots on the tip of the fly line? I’m traveling and can’t shoot a video with the marked line to show all of this in action.

Cheers,
Graeme
FFi CCI

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Merlin
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Upward Force from Form Drag

#84

Post by Merlin » Wed Feb 21, 2018 7:22 pm

Hi James

No problem to open another thread on DN.
You have the mean to document the problem and this is great. Do you feel comfortable in producing DN (e.g. using a board like Graeme) with a number of lines?
The record of unresolved disputes is rather large on this forum, and none of them brought something to the community, may I suggest to give it up?
Maybe you can organize some kind of testing that would bring some light on the influence of viscoelasticity of lines on DN shaping. I do not expect an explanation about the origin of the DN but some real world information.
By the way, do you have the video corresponding to your high launch angle competition cast, it seems that the inclined section of the loop is huge? The potential compensation of loop weight is highly dependent on its shape. It is possible for an inclined loop but very unlikely for a half circle one. There is nothing like a simple rule in fly casting.

Merlin
Fly rods are like women, they won't play if they're maltreated
Charles Ritz, A Flyfisher's Life

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guest
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Upward Force from Form Drag

#85

Post by guest » Thu Feb 22, 2018 7:27 am

Merlin

Can we have a thread on the flat earth theory? That’s another dispute that remains unresolved because one side is relying on evidence and the other on blind faith.

Regards

Vince
Bright but shite

IANACI - There’s no such thing as absolutes

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gordonjudd
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Upward Force from Form Drag

#86

Post by gordonjudd » Thu Feb 22, 2018 3:44 pm

You’re wrongly assuming the loop velocity is solely controlling the fly leg speed.
Graeme,
When the rod leg velocity is zero, (which it is once the rod leg is anchored by the water in the frame grabs above as I mentioned in my post), what is the relationship between the loop velocity and the fly leg velocity?

As you carefully explained the loop velocity over the ground is equal to .5*(v1+v2) where v1 is the positive velocity to the right of the fly leg and v2 is the positive velocity to the right of the rod leg. Thus when v2 is equal to zero, the fly leg velocity is equal to 2*v_loop. Consequently when the rod leg velocity is zero (no pull back, no shooting of line) there is a 2:1 relationship for the fly leg velocity over the ground compared to the loop velocity over the ground. Thus if the loop velocity is accelerating as shown in loop markers above, then the fly leg is accelerating as well.
The fly leg velocity was already established during the back cast and it does not change just because the loop stalls or accelerates.
Do you think that when the rod leg velocity is zero the fly leg velocity remains constant even though the loop velocity over the ground increases?

Gordy

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gordonjudd
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Upward Force from Form Drag

#87

Post by gordonjudd » Thu Feb 22, 2018 4:32 pm

No problem to open another thread on DN.
Merlin,
I am all for getting more data on what might cause the DN shape, but I suspect that another round of hand waving on its myriad causes won't get us any closer to understanding it.

James offer to measure some lines in a new DN thread is very welcome. I would just like to see what the Young's modulus might be for different lines in addition to getting an idea of their damping coefficients.
In your reference to line stiffness being a primary factor in forming a DN loop, do you think the DN is more likely to form with a stiffer line or one with a smaller stiffness such as yarn?
Based on Dr. McMillen's result for yarn and Morsie's observation about producing "camel humps" in an older DN thread
https://imageshack.com/a/img923/3757/w9GD3N.jpg
The line is a tropical line. I couldn't get the same result with a cold water line, (and the tapers were quite different). I thought a longer front taper would create more humps but not so.
I am thinking the DN shape is more likely to be observed in a less stiff line.

Gordy

TrevH
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Upward Force from Form Drag

#88

Post by TrevH » Thu Feb 22, 2018 11:48 pm

Should be interesting.

I suspect the relationship between both mass and stiffness affects the propensity to DN and I would like to see how that plays out.

Cheers
Trev

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Paul Arden
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Upward Force from Form Drag

#89

Post by Paul Arden » Fri Feb 23, 2018 2:49 am

Sorry I had to fish every opportunity in the past two weeks and then my smart phone took a dive. With regards upward force at the loop face is the conclusion that it doesn't exist or that it has negligible effect? If someone can sum up the argument in a nice way I can take it to Noel (which would be the right thing to do since he wrote the paper).

Many thanks,
Paul
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Merlin
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Upward Force from Form Drag

#90

Post by Merlin » Fri Feb 23, 2018 11:10 am

Hi Paul

I just sent you a mail, I think I have found where Noel overestimates the "lift" for a half circular loop. There is a "pi" constant missing when he evaluates the vertical acceleration. Correcting this error allows finding similar results to mine.

Merlin
Fly rods are like women, they won't play if they're maltreated
Charles Ritz, A Flyfisher's Life

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