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Line Taper - Does it speed up or slow down the fly

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gordonjudd
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Line Taper - Does it speed up or slow down the fly

Post by gordonjudd » Tue Jul 06, 2010 1:36 pm

This question came up in the pre-load thread, and I think it merits a short thread of its own so we can answer a very basic question about how the taper of the line impacts the loop propagation speed.

Here is the disconnect regarding the effects of tapering a fly line that was buried in the pre-load thread:

The propagation of the loop formed in a fly cast (what I will call the “fly wave”) is often equated to what goes on in a whip. For example the Virtual Flycasting site says:
In his book Modern Fly Lines Bruce Richards states that,” The relationship between the whip and the fly line is obvious."


However, Lingard's graph shows the effect of tapering the end of a fly line will slow down the fly velocity as compared to the velocity at the end of the loop roll out with a level line. This implies the impact of the taper is to slow down the velocity of the fly so you get a more controlled turn over of the loop at the end of the cast.

In contrast, for the wave that Goriely analyzed in his whip wave paper. he shows the end of the whip is tapered to speed up the turn over of the whip and make it easier to crack.

How could the same effect (tapering the line) have the opposite effect in the fly wave as compared to the whip wave? Is Lingard wrong when he calculates that tapering the line will slow down the turn over of the fly?

If someone has a copy of Bruce Richards Modern Fly Line book maybe they can share what he has to say on the subject.

I think this discussion on line taper came from Bruce's Book but aside from this reference
The primary purpose of the front taper of the fly line is to allow proper delivery of the fly and leader. The taper from the belly of the line to the tip acts to reduce the mass of the line. As the loop of any fly line travels through the air, the mass of the moving part of the line decreases because that part becomes shorter. In tapered lines it decreases even more because the line becomes smaller towards the tip. This increases acceleration, resulting in greater wind resistance and greater energy dissipation, and therefore a more delicate delivery.


I could not find anything specific however about the impact of the taper on the turn over velocity of the fly.

To me this quote is a bit contradictory in that it says the purpose of the taper is to get a proper delivery of the fly but then it says "the taper will increase the acceleration of the fly" which would cause it to speed up not slow down.

I was hoping for more enlightenment on the subject and found some excellent observations and answers to questions on the effect of line taper by Bruce Richards here.

He says:
Now, about the taper. As the top leg gets shorter it gets lighter which causes the acceleration. If the line is tapered also it gets lighter even faster as it gets shorter.
.....
I know this may seem a little hard to believe, but check with a physicist, I have, it is correct.


I do not know who he talked to when he says a tapered line will get faster as the line density that goes around the loop decreases. However, it must not have been Lingard because that is just the opposite effect on the fly velocity that Lingard predicts.

Maybe even the experts are confused about what is going on with taper. I know I am.

Lingard says it will slow down the fly, Goriely says it will speed up the tip speed of a whip, and the above link on taper seems to be saying both.

What is going on here?

Gordy
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Post by Aitor » Tue Jul 06, 2010 2:22 pm

When presenting the fly the difference between a long/thin leader and a short/thick one tells something about the taper slowing things down.
A beginner managing to loose his fly on the backcast while hearing a cracking sound tells that there can be an acceleration of the fly leg also, even if the line is tapered.
Aitor is not like us, he is Spanish, and therefore completely mad.
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, Paul

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Post by gordonjudd » Tue Jul 06, 2010 2:47 pm

When presenting the fly the difference between a long/thin leader and a short/thick one tells something about the taper slowing things down.


Aitor,

Does that fact make you think that Lingard has the correct result on the effect of taper?

Gordy
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Bobinmich
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Post by Bobinmich » Tue Jul 06, 2010 3:15 pm

IMHO Bruce was okay right up to the last sentence. It would seem to me the decreasing energy available by the reduction of mass (taper) would be enough to slow the presentation if the aero of the fly remained unchanged (which it does). I like to think of it as casting into a pillow - not literally of course - but in concept.

I used to think that taper on a floating line was more sales hype rather than fact. I still think much of it is. But Bruce explained that radical variences from the norm can make significant differences in presentation, i.e. long small equals soft for a little fly but won't turn over a hex.

But then, I don't know anywhere near as much as Bruce.

Bob

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Post by Aitor » Tue Jul 06, 2010 4:25 pm

Gordy,

I can't argue about Lingard or Goriely studies, but what I see in the real world of fishing is that sometimes the fly seems to decelerate and sometimes seems to accelerate.

That quote you have mentioned is from page 68 of Bruce's book and it summarizes pretty well what he explains about this subject on that chapter. This quote is from page 65:

To understand fly casting and how fly lines work, it is essential to understand that kinetic energy, the energy of motion, is conserved.
Aitor is not like us, he is Spanish, and therefore completely mad.
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, Paul

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Post by White Hunter » Tue Jul 06, 2010 9:30 pm

Hi Gordy

I would have thought both lines will speed up, but the tapered one faster, for energy to be conserved, as the mass of the line in motion is decreasing, the only way for the initial forces to remain in balance, it to accelerate what remains of the ever shortening fly leg as time goes on.

The paper by Mosser and Buchmann illustrated this well, "with much help from Grunde alongside this, I slowly got my head around this conservation of energy law".

The tapering of a line will however see that air drag will eventually counteract acceleration of the line, leader and fly, and we want that to use that to our advantage in order to make a correct "presentation".

When you think about it, it's the final tip diameter that is the kicker here, and many fly lines have been made with fantastic head profiles, yet have been spoilt by the very front end.....

What about if we have a line .9mm at the end of the belly-start of the front taper, and is tapered down by only .15mm over say 50 feet.

And a second line again starts at .9mm at the same place but tapers down to .4mm over a distance of 10 feet..

Which one would present more gently assuming the same leader.......... The long "readily advertised" presentation taper...?

Lee

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Post by Aitor » Tue Jul 06, 2010 10:15 pm

What I don't get is the relationship between kinetic energy conservation and momentum conservation.
To my knowledge kinetic energy conservation has some exceptions while momentum conservation hasn't any.
Aitor is not like us, he is Spanish, and therefore completely mad.
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, Paul

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Post by White Hunter » Tue Jul 06, 2010 10:42 pm

Ok

Can someone now make me some glasses which not only measure tension in the line in shades of red and amber, but also with wind resistance lines acting on the line, loop and fly?, maybe in say ..........Green :cool:
I like to think of it as casting into a pillow - not literally of course - but in concept


Brilliant.....!! ;)

Cheers

Lee

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Post by gordonjudd » Wed Jul 07, 2010 1:15 am

I would have thought both lines will speed up, but the tapered one faster, for energy to be conserved, as the mass of the line in motion is decreasing, the only way for the initial forces to remain in balance, it to accelerate what remains of the ever shortening fly leg as time goes on.


Lee,

That is what I would have thought as well until I saw Lingard's velocity profile that compares the velocity history of a tapered line with a level line.

He shows the level line does just what you describe, but for the tapered line he notes:

Both lines are retarded in the early part of there trajectory because the aerodynamic drag outweighs the effects of decreasing mass. A distinct acceleration phase is encountered later in the flight which, in the case of the level line, is sustained to the end. The double tapered line, however, attains a sharp velocity maximum and then decelerated finally as the effect of the taper is felt over the last few meters of the cast.


Hendry's analysis comes up with the same result as Lingard, so maybe a fly wave and a whip wave have different characteristics. Paul seems to have discovered that fact when he found he could not crack a whip as easily as a flyline.

Which one would present more gently assuming the same leader.......... The long "readily advertised" presentation taper...?


A forty foot difference in length makes for a much more skin drag so I don't know how that would come out.

However, if they had the same tapered length, then Lingard would say the one with the lower linear mass density (the skinner line) would have the lower velocity.

Gordy
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Post by Merlin » Wed Jul 07, 2010 7:41 pm

Gordy,

Hum… a very difficult domain for models. Past Michigan State University studies indicate that everything can happen: deceleration, deceleration followed by acceleration, acceleration. Loop size is all important, line size too. For a given loop size there are “frontiers” for line behavior, going along with the three cases described above.

The leader is a problem itself: long? Short? You can see a difference between 7 feet, 9 feet, 12 feet leaders, not taking in account the 20 feet or more leaders used over here to fool big trout with a nymph. Most of the time (I am cautious), leaders decelerate the fly for presentation. But there is no rule for fishing, you know.
Merlin
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Post by gordonjudd » Wed Jul 07, 2010 8:21 pm

Hum… a very difficult domain for models. Past Michigan State University studies indicate that everything can happen:


Merlin,

Thanks for your comments, as you are the one person I was hoping would understand why the acceleration forces in a fly wave are different than in a whip wave and that is why Lingard's and Goriely's results of the taper effects are different.

For some reason Gatti-Bono and Perkins used a very high estimate for the skin drag in their Sports Engineering paper on "Comparison of Numerical and Analytical Solutions for Fly Casting Dynamics".

They used a tangential drag coefficient of .075 in that paper, even though their other papers use a value of .015 that is similar to the one Lingard used in his analysis. As a result their figure 7. shows the fly velocity as continually decreasing over the cast, rather than the nominal constant value with a speed up right at then end as predicted by Lingard.

However, I think their formulation of the equations for the energy conservation involved are the same and they conclude the taper in the line will slow down the fly velocity as described below.

Image

Thus their result is very similar to Lingard's curve, and show the consequence of tapering the line is to slow down the velocity of the fly rather than speed it up. This the opposite result that you would expect by equating the fly wave to the physics of the whip wave that Goriely analyzed in his inscrutable paper.

Gordy
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Post by Aitor » Wed Jul 07, 2010 9:52 pm

Gordy,

I can't discuss the papers on the physics of this issue, but when a beginner makes a crack sound in his back cast and looses his fly I think that the fly leg has accelerated.
Aitor is not like us, he is Spanish, and therefore completely mad.
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, Paul

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Post by gordonjudd » Thu Jul 08, 2010 1:06 am

I can't discuss the papers on the physics of this issue, but when a beginner makes a crack sound in his back cast and looses his fly I think that the fly leg has accelerated.


Aitor,

It certainly has accelerated and the acceleration force (some of which was probably from added tension in line due to starting forward too early) was no doubt larger than the drag forces so the velocity continued to speed up and crack off the fly.

And Lingard's theory says it would be even more likely to crack if you had a short frayed bit of leader (most whip crackers are around 15 cm long) attached to a higher density level line and also started to accelerate the rod forward before the loop had rolled out.

The papers analyzing the effects of taper in a fly line say the taper will slow down the speed of the fly, not speed it up as it does in a whip. When Bruce says:

The primary purpose of the front taper of the fly line is to allow proper delivery of the fly and leader.


I believe it, and Lingard's analysis says it does so by slowing down the velocity of the fly as the tapered line turns over.

I think you can see the difference even if you do not think it is true. It is hard not to accept the traditional wisdom that, "The relationship between the whip and the fly line is obvious."

It is more likely the difference the the propagation of a fly wave and a whip wave are not so obvious unless you have tried cracking a whip with a fly wave. Check out what Paul has to say about his experience.

Gordy
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Post by Aitor » Thu Jul 08, 2010 1:57 am

gordonjudd wrote:When Bruce says:
The primary purpose of the front taper of the fly line is to allow proper delivery of the fly and leader.
I believe it, and Lingard's analysis says it does so by slowing down the velocity of the fly as the tapered line turns over.
And I believe it also. But everyday experience says that with the same taper you can get a subtle presentation of the fly due to the slowing down and a cracking sound that results in a lost fly.
Some days ago I was fishing a dry fly with a 5 m long leader making soft presentations in very flat water; when inadvertently lost my fly in a branch behind me, the next back cast surprised me with a loud cracking sound: the lack of a fly at the end of the tippet made a huge difference.
Aitor is not like us, he is Spanish, and therefore completely mad.
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, Paul

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Post by gordonjudd » Thu Jul 08, 2010 7:02 am

the lack of a fly at the end of the tippet made a huge difference.


Aitor,

An excellent observation. There are little or no drag losses coming from the line as the loop turns over since the moving part of the line at that point is so short.

Consequently the drag from the fly is the dominating deceleration force at the end of the cast. No fly, no deceleration, and thus it is much easier to exceed the speed of sound at the end of the leader.

Gordy
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