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Lift force acting on a fly line

Moderator: Torsten

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Graeme H
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Re: Lift force acting on a fly line

#251

Post by Graeme H » Sun Jul 14, 2019 10:49 am

Paul Arden wrote:
Sun Jul 14, 2019 7:40 am
That's great Graeme! Is this the line you are bringing Snakehead fishing?
No, this is an old Rio Permit line I had lying around. I was worried putting nail knots in a $200 line might cost me too much ... :)

I'm bringing a SA AST Tarpon line.
FFi CCI

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Merlin
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Re: Lift force acting on a fly line

#252

Post by Merlin » Sun Jul 14, 2019 11:07 am

Nice cycloids Graeme, in line with theory.

Did I say “always”? No, I said “sometimes”, but you easily fall into traps I’m sorry, let’s be factual and not negate things which are not pleasing to see.
As I watch them go around the point of the loop, Merlin, they all are pointing straight up.
So let’s say the “most of the time” they are pointing straight up, you have a counterexample available. What does that means physically speaking?Iit means that the boundary between the loop and the rod leg is difficult to find, and this is reinforced by the following:
I think it means that there is some forward movement of the line in the rod leg.

IMHO, the rod leg is characterized by a zero horizontal speed, assuming the caster is not playing with the rod tip. Simple, isn’t it?
Did you know that dramatically increasing skin and form drag on a Rio Permit line doesn't add any lift to the cast either?)
Interesting finding Graeme, so can you suggest an explanation for the lift of the nose of your loop described by Dirk in the analysis of one of your valuable videos?

I am not a believer of an effect of the loop,for the fly leg, I am still skeptical.

Merlin
Fly rods are like women, they won't play if they're maltreated
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Graeme H
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Re: Lift force acting on a fly line

#253

Post by Graeme H » Sun Jul 14, 2019 11:38 am

I'll let Paul answer the questions you've asked him Merlin.

As for an explanation for the rising loop Dirk found: yes, I have an explanation. However, this thread is about lift. As Torsten said, discussions about tension should be placed in another thread.

Cheers,
Graeme
FFi CCI

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Merlin
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Re: Lift force acting on a fly line

#254

Post by Merlin » Sun Jul 14, 2019 11:55 am

Ok so let’s wait for that thread. But if I consider N laws, a change in trajectory must come from an external force. Tension is internal as far as I know.

Merlin
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Paul Arden
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Re: Lift force acting on a fly line

#255

Post by Paul Arden » Sun Jul 14, 2019 11:56 am

I think there is some forward movement because there is a small wave running through the rod leg. As that runs through a given point on the rod leg this will account for a little forward movement because the line becomes straighter.

Furthermore there is often some downward deflection of the loop. As this returns there will also be forward movement.

So even with a perfectly stable rod tip I think we will still see this phenomena.

Cheers, Paul
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Paul Arden
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Re: Lift force acting on a fly line

#256

Post by Paul Arden » Sun Jul 14, 2019 12:10 pm

Can you show me the loop in question please Merlin? The loops that I’ve seen travel upwards or even maintain height have come from a backcast that has an upward trajectory - which is most backcasts of course.

Cheers, Paul
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grunde
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Re: Lift force acting on a fly line

#257

Post by grunde » Sun Jul 14, 2019 12:41 pm

Merlin wrote:
Sun Jul 14, 2019 11:55 am
But if I consider N laws, a change in trajectory must come from an external force. Tension is internal as far as I know.

Merlin
I don’t get what you are trying to say here, could you please elaborate?

Cheers,
Grunde
"Essentially, all models are wrong, but some are useful."
George E. P. Box

Always question the assumptions!

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Torsten
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Re: Lift force acting on a fly line

#258

Post by Torsten » Sun Jul 14, 2019 1:25 pm

Graeme,

thanks for sharing that video, this is of course a great way to visualize U - the relative flow velocity to the line. You can also take two neighboring markers and draw in the normal vector to the fly line, this should look similar.
The video demonstrates that the underlying assumption of this thread is false because air is not blowing on the inclined section of line in the rod leg.
No it's not wrong, it's just an simplified example; U is the relative flow velocity. 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.

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.

For video analysis we need to generalize the model for a fly line segment first, that it works for all kinds of movements, like rotations too (step (3) in the roadmap). I have already a model for that, but need to check first before I'll write about it. With position and velocities of two markers you can compute then the resulting forces due to form drag and skin friction.

I've meant with step (2) verification of fly line segments. That means we measure the form drag and the skin friction of a piece of fly line and compute realistic values for the drag coefficients. If that's not possible we should at least do some plausibility checks and compare literature values. Maybe someone has access to a wind tunnel?

With (3) we can do video analysis, this requires a dataset of positions and velocities of every marker on the fly line. Ideally the markers should be closely spaced. With that we can compute direction and magnitude of the resulting forces of form drag / skin friction.

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.

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Re: Lift force acting on a fly line

#259

Post by Dirk le Roux » Sun Jul 14, 2019 2:50 pm

Graeme H wrote:
Sun Jul 14, 2019 10:23 am
These are the paths of the bits of wool. What was the theoretical path supposed to look like? Where is a tangent applicable?
Hi Graeme

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

Regards,
Dirk

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gordonjudd
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Re: Lift force acting on a fly line

#260

Post by gordonjudd » Sun Jul 14, 2019 4:12 pm

the relative flow velocity to the line. You can also take two neighboring markers and draw in the normal vector to the fly line, this should look similar.
Torsten,
I agree with that observation. Look at the direction of the velocity vector of the line marker and I would expect the tell-tail to point opposite to that velocity because the air drag is opposing that movement. Nifty experiment anyway. Thanks to Graeme for sharing it.

It would be interesting to see how where the tail-tell at the middle of an inclined section of line (i.e. different points from frame to frame) as that inclined section of line plows through the air. That is the direction that is relevant to this topic.
Can you turn on the velocity vectors for that same snapshot please?
That should clarify it.

Gordy

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