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Lift force acting on a fly line

Moderator: Torsten

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gordonjudd
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Re: Lift force acting on a fly line

#261

Post by gordonjudd » Sun Jul 14, 2019 4:46 pm

You just can't apply the illustration in my first posting directly to video analysis, a inclination does not mean lift force. The goal was first to quantify the resulting lift force for certain cases to get an impression if it's significant and I've made a table for some cases.
Torsten,
I guess you and I are the only one that sees this experiment as providing a solid validation of the lift theory.
Image
Yes, at least we have a few data points and seem to be in the right range. Ideally we would measure line samples in a wind tunnel, but I'm guessing it's difficult with our resources?
Unlike Graeme's experiment with a sphere tied to a string where the tension in the string provided the opposing force to the x directed form drag on the sphere (that is a totally different experiment and thus the string tension plays an important role) this experiment is based on a torque balance on a distributed mass that has some length. In no way does Graeme's ball on a string experiment show that the lift on an inclined section of line going through the air comes from tension from the rod tip.

The example of an upward angle on the rod leg where the tension from the rod is pulling down on the line should have been enough evidence (remember it only takes one counter example to disprove a theory) to show that does happen.
Image

By putting the axis for the torque calculation at the tie point in the line, the tension in the line has no contribution to the torque since its moment arm is zero. Thus the torque balance is achieved when the +y directed force resulting from form drag on the string balances the downward force due to gravity and skin drag on the string. Thus this experiment shows that the drag theory fits with the actual results to within my ability to provide and measure the velocity of a uniform air-stream with the floor nozzle on a shop vacuum.
I'm wondering, if textured lines show different properties.
I am sure they would. Dimples on a baseball bat show it can be swung about 6-7% faster than a bat with a smooth surface. http://news.mit.edu/1994/bat-0302 I don't know how big the bumps (or dimples) would need to be and they would probably out dB a Sharkskin line.

Gordy

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Graeme H
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Re: Lift force acting on a fly line

#262

Post by Graeme H » Sun Jul 14, 2019 10:33 pm

Dirk le Roux wrote:
Sun Jul 14, 2019 2:50 pm
Hi Graeme

Can you turn on the velocity vectors for that same snapshot please?

Regards,
Dirk
Yep, I'll do that when I get home this afternoon. They'll be messy because picking markers on this line is difficult, especially in the fly leg.

Cheers,
Graeme
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Graeme H
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Re: Lift force acting on a fly line

#263

Post by Graeme H » Mon Jul 15, 2019 12:22 am

Hi Torsten,

Thanks for your response.
Torsten wrote:
Sun Jul 14, 2019 1:25 pm
... Note that you don't need a horizontal flow to generate a vertical force component (lift). E.g. I'd expect for the point marked with a red circle in #248 an upward force component as a result of form drag and skin friction.
That's interesting. The proposed attack angle (\(\alpha\)) at that point is well in excess of 20° there, maybe 45°. Can you explain why you think lift might be occurring there? Which direction and velocity do you think the line is traveling there? The tell tales here and the tracking data I've worked with show the line surrenders the vast majority of its forward movement at the loop nose. Or are you saying this because the line is moving downwards and there is resistance to movement in the vertical sense?

I can agree with the latter, but all the tell tails at the 34 second mark of the video also show form drag opposing the falling velocity. Most people would agree that a lifting force is not present there, wouldn't they? (Apart from tension ...)
You just can't apply the illustration in my first posting directly to video analysis, a inclination does not mean lift force.
Sorry Torsten, but you implied that to be the case in your first post of this thread with this sentence:
If you can spot a section of a fly line in a video, with an inclination and you know the velocity, you can compute the lift force.
Now maybe people have taken that too far and have read more into your post than you intended. I get that. However, we can directly measure the velocity and the inclination of the line and we can show that the line in the rod leg has no significant forward velocity with direct visual evidence from tell tails.
The goal was first to quantify the resulting lift force for certain cases to get an impression if it's significant and I've made a table for some cases.
Your goal may well be to quantify a lifting force for certain cases, but if we can't demonstrate those conditions even exist in a cast, why give people hope that there is a case for lift in a cast?

The paper you referenced in post #1 specifically says:
From a global point of view, the lift forces obtained by RANS CFD calculations are close to the experimental results
for inclinations lower than 5°
What we see from Tracking data indicates that as the line in the rod leg approaches \(\alpha\) angles of 0, the Vx approaches 0 too. An attack angle of less than 5° has a U value approaching zero, so Fn approaches zero. As you've noted, attack angles above 20°are not valid. That puts us in the first three lines of your table in Post #48, where your maximum lift force was 5% of the force of gravity.

The conditions discussed in the referenced paper are not honoured by line in the rod leg. If someone thinks they are, please explain where it exists in the cast and one of us can devise an experiment to prove those conditions are met (or not). The only place I can find those conditions is in the leading edge of the fly leg, but that would force the line down, wouldn't it?
Maybe is the roadmap pedantic, but I think that's better than doing hand-waving and we need patience, because doing these steps require some time.
Okay, well who is doing this work? Is it going to be peer reviewed? How long before the results are published?

Call it handwaving if you like, but at least we can see evidence of progress by doing experiments. So far we're up to 27 pages of discussions without any support for the proposed lifting force. Ockham's Razor suggests that we have 26 pages more than we need.

Cheers,
Graeme
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Graeme H
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Re: Lift force acting on a fly line

#264

Post by Graeme H » Mon Jul 15, 2019 12:53 am

gordonjudd wrote:
Sun Jul 14, 2019 4:46 pm
The example of an upward angle on the rod leg where the tension from the rod is pulling down on the line should have been enough evidence (remember it only takes one counter example to disprove a theory) to show that does happen.
Image
An excellent example Gordy. Well done on finding a picture illustrating tension in the rod leg. You don't need to rely on just one example though. I can give you thousands of examples where tension in the rod leg pulls the fly leg down at the loop nose. In addition to the one you've shown above, try this one from earlier in this thread.*

Tension vector.jpg
Tension vector.jpg (18.65 KiB) Viewed 57 times

Tension always acts in the direction of the fly line. If the line is hanging from something, it's pulling down on that object.

So what does Newton's Third Law of Motion suggest about that? I believe Newton would say "If tension in the string is pulling something down then something is pulling the string up."

If only we could discuss tension as a mechanism to keep the line suspended in the air, eh? Alas, we'll save that for another thread.

Christopher has superb tension in the rod leg. He produced it with high line speed and well focused direction of the fly line. By aiming the direction slightly up after the low back cast, his loop maintains a good height throughout the cast. If he had aimed even higher, the loop would have climbed even more and the rod leg would be even more inclined away from the rod tip. I don't see where aerodynamic lift applies in his cast though.

Cheers,
Graeme

* I can also give you an example where tension pulls the line up from the fly leg. That was split off to another thread though.
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Merlin
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Re: Lift force acting on a fly line

#265

Post by Merlin » Mon Jul 15, 2019 6:18 am

Paul

The example is there:
tails 2.JPG
tails 2.JPG (23.31 KiB) Viewed 31 times
Look at the white arrows and the direction of flow. Can anyone explain why there is a tail pointing down in the rod leg?
Did you notice that there is no form drag detected by tails around the top of the loop? is it due to the way there are attached to the line?

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

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Merlin
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Re: Lift force acting on a fly line

#266

Post by Merlin » Mon Jul 15, 2019 6:22 am

Grunde,

The first sentence refers to Graeme’s instruction video (N1). The second one refers to the way I name internal and external forces. For external forces, I try to use the word “force”, as far as I can (if I do not forget). Tension / compression are words I keep using for internal forces. These structural forces are the consequence of the effect of external forces. If I seat on a chair, I put the legs under compression, the acting external forces are my weight and the reaction from the ground. If I pull on both ends of a string, I create an internal tension within the string; the external forces are the pull from my arms.

Now speaking of the analysis of one of Graeme’s video by Dirk, let’s consider the fact that the nose of the loop of the FC has a down up motion. The question is to identify which external force (N1) is responsible for the up motion. For the downwards one, the usual suspect is gravity and maybe loop direction at start.

What about tension (as an internal force)? Is the following graphic some representation of it? There is no such external force at the place the vector is illustrated. Someone may conclude that the Big Wheel concept is back.
tails 3.JPG
tails 3.JPG (15.75 KiB) Viewed 28 times
Sometimes the behavior of the line looks strange, like for the snap cast for example. The trajectory of the loop slightly deviates from its original place (it moves sideways), and somehow surprisingly the loop rolls over and climbs in the air (there is no lift here). The external force responsible for that is the weight of the “ground leg”. Internal tension may vary because the rotation speed of the loop varies and also because the linear density of the line can change (e.g. leader entering the loop).

Coming back to the loop nose which starts climbing (1.88 s) before the line accelerates (2.05 s), what forces can cause this change? Gravity acting through the rod leg (increasing sag)? Force at rod tip? Or some drag forces (e.g. fly leg, eventually fly). One difficulty lies in the relative importance of these forces, and at the end of a cast, the winners are drag forces. I do not have an answer at this time.

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

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Graeme H
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Re: Lift force acting on a fly line

#267

Post by Graeme H » Mon Jul 15, 2019 6:34 am

Merlin wrote:
Mon Jul 15, 2019 6:18 am
Can anyone explain why there is a tail pointing down in the rod leg?
Yes.

The tell tails are not "massless". The line in the rod leg had stopped moving down and was beginning to move up again, but the tell tail you refer to was still dropping due to its own inertia after the line stopped its downwards vertical travel.

This is in the seconds 37 - 43 in the video.
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Graeme H
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Re: Lift force acting on a fly line

#268

Post by Graeme H » Mon Jul 15, 2019 6:54 am

Merlin wrote:
Mon Jul 15, 2019 6:18 am
Did you notice that there is no form drag detected by tails around the top of the loop? is it due to the way there are attached to the line?
Yeah, funny about that! :pirate: This is the crux of the problem for many people. If you think the loop has form drag due to its shape, you're always going to struggle with the question of "what is happening in the loop?". Being "loop centric" for all your casting experience is the root cause of this misunderstanding.

I will once again state this because people are not reading it properly: the line in the nose of the loop has lost virtually all of its forward velocity.

The line in the nose of the loop is travelling mostly down, so the tell-tail is nearly vertical.

The tell tail in the loop nose is horizontal only when line is being shot.

I have video of dozens of casts showing this to be the case. I have Tracker data that clearly shows this to be the case. If you want a "carpet bombing" of it, I'll create a long video showing cast after cast with vertical tell tails in the loop nose. Boring as bat shit, but maybe learning by rote is the only way?

Yes, it's a shock to find out the loop nose doesn't have the form drag you think it does, especially when all the perceived wisdom is saying that "sharp loops are aerodynamic". (Sharp loops are indicative of a well-focused line velocity vector, making them efficient for long casts.) It just doesn't work the way you probably think it does. Make your own videos if you don't believe mine. Like I said, this is all very repeatable science.

Cheers,
Graeme
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Dirk le Roux
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Re: Lift force acting on a fly line

#269

Post by Dirk le Roux » Mon Jul 15, 2019 7:14 am

Graeme H wrote:
Mon Jul 15, 2019 6:54 am

The tell tail in the loop nose is horizontal only when line is being shot.

I have video of dozens of casts showing this to be the case. I have Tracker data that clearly shows this to be the case. If you want a "carpet bombing" of it,
Hi Graeme

I would be very interested in a few (only :) ) clips of the line shot loop footage.

Regards,
Dirk

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Graeme H
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Re: Lift force acting on a fly line

#270

Post by Graeme H » Mon Jul 15, 2019 7:20 am

Merlin wrote:
Mon Jul 15, 2019 6:22 am
Coming back to the loop nose which starts climbing (1.88 s) before the line accelerates (2.05 s), what forces can cause this change? Gravity acting through the rod leg (increasing sag)? Force at rod tip? Or some drag forces (e.g. fly leg, eventually fly). One difficulty lies in the relative importance of these forces, and at the end of a cast, the winners are drag forces. I do not have an answer at this time.
Merlin,

The loop is where the fly leg becomes the rod leg. If the fly leg is going up, the loop goes up too.

Not every fly leg is perfectly straight in the vertical direction.

Hump.jpg
Very rough drawing ....
Hump.jpg (10.14 KiB) Viewed 21 times

As the line at A enters the loop, the loop maintains its height.

As the line at B enters the loop, the loop rises.

As the line at C enters the loop, the loop drops.

As the line at D enters the loop, the loop maintains its height.

The loop is not the line. It has no trajectory of its own. It's only the place where the fly leg is prevented from going forward.

Cheers,
Graeme
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