Lift force acting on a fly line

[Paul Arden]

So Chris’ loops have a steeper V? How do the speeds compare? I’m guessing mine have considerably higher speed so that should really work in my favour.

Personally I think the biggest difference is that Chris is throwing a Roll Cast.

I think that the best way to break this deadlock will be two lines cast on one rod. One with a starting angle at 45 degrees below the rod tip and the other with the line held horizontally. At least then we can see if there might be other influences at play.

Cheers, Paul

[Torsten]

Guys, you're too hasty. As I've written, I'm still at small scale - a section a of the fly line. We need to get the basics right before going on and do detailed analysis. Also this thread is about the lift force first; I think anything about tension should be put into a separate thread.

I have the following rough roadmap in my mind:

(1) Investigate theory of (thin) cylinder aerodynamics, determine forces for typical inclinations, velocities seen in fly casting.

(2) Verify, validate the results of (1) by doing experiments with fly line samples.

(3) Extend (1) that it's applicable for rotations too, the result is a prediction model for resulting force magnitudes and directions with given nodal velocities for a segment of the fly line.

(4) Apply the prediction model to video analysis and to determine the significance at various parts of the loop.

(5) Do full fly line experiments to validate the plausibility of (4).

(6) Integrate the knowledge into a simulation model.

(7) Draw conclusions.

I think step (1) is not yet fully finalized, theory needs to be reviewed for special cases e.g. low inclination angles.

[gordonjudd]

that it's applicable for rotations too

Torsten,
What do you mean by rotations? Are you talking about the lift force you would expect for segments of the line going around the loop (i.e. rotating so the incident angle of the form drag changes)?

(2) Verify, validate the results of (1) by doing experiments with fly line samples.

Would this qualify as showing the tilt angle required to have the lift from form drag offset the weight of gravity for a section of line?

There is some question where the pressure center is to determine the moment arm for the upward form drag force but I think it shows your equation is very much in the ballpark of what you get in the wind gauge experiment.
Gordy

[Merlin]

Gordy

Any time an object is cast above horizontal, there is an extra delay by comparison to a free fall, and it is pretty difficult to separate this effect from a potential delay due to an aerodynamic delaying effect.

Merlin

[Torsten]

Hi Gordy,

Torsten,
What do you mean by rotations? Are you talking about the lift force you would expect for segments of the line going around the loop (i.e. rotating so the incident angle of the form drag changes)?

Yes, a single resultant force would be only for translations correct; imagine a piece of fly line rotating around its axis. If you would just integrate the distributed force, the result = 0; but realistic are two forces or better moments slowing down the rotation.

Would this qualify as showing the tilt angle required to have the lift from form drag offset the weight of gravity for a section of line?

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? I'm wondering, if textured lines show different properties.

[gordonjudd]

Any time an object is cast above horizontal, there is an extra delay by comparison to a free fall, and it is pretty difficult to separate this effect from a potential delay due to an aerodynamic delaying effect

Merlin,
I would think that would depend on how well you could measured the initial y velocity of the loop right after it is formed at MCF. In the Rowen's cast the initial launch angle of the loop was measured in cSwing as being around 2 degrees.

Owing to the pointing angle error of the camera the actual angle was probably less since the line on the water also was tilted at 2 degrees as discussed previously.

The initial x-velocity was around 25 m/s. Thus the initial y velocity would be 25*tand(2)=.9 m/s. To play it safe I used an initial y velocity for the line and rock of 2 m/s. However that initial y velocity of 2m/s is so small that it has very little effect the fall times of the line and rock.

There is a y momentum change in the fly leg at the beginning of the roll out that I would think add some positive y force at the top of the loop. That leg quickly reaches equilibrium tilt angle of around 4 degrees. That would produce a y momentum change of sind(4)*rho_l*25.^2which would be around .04 N for a 5wt line so it could contribute some lifting force at the top of the loop. That lift from the y momentum change is more significant than I thought so it could well be another lifting force in addition to the lifting force on the inclined section of line that Torsten' table says would be around .75*.0175=.013 N.

Regardless of the sources of lift the rod leg hits the water about .5 seconds after the horizontal line does. To me that is a significant difference and shows that something (and I don't think it is pixies) is offsetting the effect of gravity on the fall of the roll out of the loop.

Gordy

[gordonjudd]

Yes, a single resultant force would be only for translations correct; imagine a piece of fly line rotating around its axis. If you would just integrate the distributed force, the result = 0; but realistic are two forces or better moments slowing down the rotation.

Thorsten,
Do think doing the drag calculations in a moving frame where there is no messy rotation effects would make that calculation unnecessary?
Gordy

[Walter]

I think step (1) is not yet fully finalized, theory needs to be reviewed for special cases e.g. low inclination angles.

Which is why I think the javelin model is valid at this stage.

I like your approach of verifying some of the papers that have been referenced contentiously for so long on the board. Hopefully this will provide a basis that is more or less agreed on before moving forward.

One of the issues I’ve had with the board in the past is the tendency of people wanting to run before they know how to crawl.

Best of luck in this endeavour.

[Graeme H]

I have the following rough roadmap in my mind:
...

Good luck with it Torsten.

[Graeme H]

I've still not been satisfied that the original premise of this whole thread is valid in real life. The contention from Torsten's original post is that an inclined section of the fly line has air flow of U directed horizontally at it, and that that airflow retards the fall of the line due to a lifting force opposing gravity.

We can have a closer look at the question by examining a section of a fly line:


Let's say we have an section of a fly line in a air flow with the velocity U with the inclination angle alpha. Due to the form drag an upward component of this force - the lift force FL - is acting on the section of the fly line.

The assumption underpinning this whole thread is that the air does indeed flow like this. We have no validation for that assumption within the question posed. It has been presented as a given and we should then calculate the forces at work:

I'll leave it to you as an exercise to compute the lift force and compare it to the gravity.

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. It's here also a question of significance and this can be determined.

My post immediately following that stated \(V_x\) is approximately zero in the rod leg, thus U is also approximately zero, and the proposed force is of no significance. Well, that started a shitstorm!

Twenty four pages in, we've been politely asked to cool it and consider a plan of action to sort it out, as follows (my emphasis):

Guys, you're too hasty. As I've written, I'm still at small scale - a section a of the fly line. We need to get the basics right before going on and do detailed analysis. Also this thread is about the lift force first; I think anything about tension should be put into a separate thread.

I have the following rough roadmap in my mind:

(1) Investigate theory of (thin) cylinder aerodynamics, determine forces for typical inclinations, velocities seen in fly casting.

(2) Verify, validate the results of (1) by doing experiments with fly line samples.
.....
(7) Draw conclusions.

I think step (1) is not yet fully finalized, theory needs to be reviewed for special cases e.g. low inclination angles.

I certainly agree that we need to get the basics right before going on. We must validate the underlying assumption of horizontal airflow before even beginning the investigation of theory of thin cylinder aerodynamics (TCA). If the theory doesn't apply, there is no reason to continue with the proposed steps 1 - 7.

For TCA to apply, we need to first show that air is flowing across the fly line as per "Flow U" this diagram:




If that is the case, a "tell-tail" tied to the fly line will have an orientation similar to this:

Screen Shot 2019-07-14 at 11.42.29 am.png (19.01 KiB) Viewed 3012 times

It cannot have an orientation like this:

Screen Shot 2019-07-14 at 11.42.43 am.png (19.98 KiB) Viewed 3012 times

So, as you might expect, I've made a video. I used 10lb nylon to nail knot 5 cm lengths of 8 ply acrylic "wool" every 22 cm on a Rio Permit WF10F line and filmed casts with my iPhone at 240 fps.

The video clearly shows the tell-tails are nearly vertical in the lower leading edge of the loop (rod leg). It also shows the tell-tails are closely aligned to the fly leg direction in the upper leading edge of the loop (fly leg).

Vertical Tell-Tails

Screen Shot 2019-07-14 at 12.05.51 pm.png (169.77 KiB) Viewed 3012 times

This would indicate that the upper leading edge of the loop (the fly leg) experiences relative horizontal air flow and is the location in a fly cast where the underlying assumption for this thread might be valid. If FN can be acting anywhere on the fly line, it will be assisting gravity by pushing the fly leg down in a cast with a vertical loop.

The form drag that the rod leg experiences is that which any falling body would experience. It's the drag that develops as a body's vertical velocity increases and would eventually plateau as its "terminal velocity". Since we can clearly see that the rod leg is not falling beyond the loop, it has not reached terminal velocity.

Torsten's first post has this statement as the basis for the thread:

The title of Noel Perkins/Gatti Bono's paper can be misleading, lift means not that the fly line is actually lifting but rather the lift force due form/skin drag, which may slow down the downfall for sections of the fly line. (Not that the line is actually lifting, or only for cases where this force would exceed the gravity)

It's essentially saying "the force of air blowing on an inclined line retards the fall of that line".

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.

Here's the whole video:





Cheers,
Graeme