What makes Snap Casts (Interesting)

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What makes Snap Casts (Interesting)

Post Number:#1  Postby Geenomad » Sat Apr 07, 2018 2:46 am

Thought it would be better to start a new thread rather than attempting to divert the Upward Force from Drag “debate”.

Whenever I make or watch a snap cast I am intrigued by how much line movement bang is generated for the rod movement buck. Noticed it again watching Graeme's vid in the other thread. (Fellow SL member Andrew Connell and I talked about this yesterday when we got together for a cast and a chat so I know I’m not the only one who has noticed how really interesting snap casts are.)

Why is that so? Alternatively, what fundamental principles of (all) casting are being demonstrated?

My answer is that/the Straight Lines Rule. Two parts – a) cast in straight lines and b) with straight lines. Yeah, I know, that’s an ideal but it’s one worth aspiring to. It’s a relative universe in which Newtonian physics apply. Apologies for the repetition. :D

a) In a snap cast lots of line can be moved when the rod tip tows the fly line in two opposing directions but in both cases in nicely straight lines (relative to both the vertical and horizontal planes).

In both movements Force is being applied with vectoral simplicity or as I prefer, purity.

b) Line tension aids both loop propagation and vectoral purity. Snap casts don’t work at all without line tension.

Efficient casting requires vectoral purity and complete propagation of the loop.

Other than the Force(s) we apply, gravity and drag oppose us.

Cheers
Mark
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What makes Snap Casts (Interesting)

Post Number:#2  Postby Paul Arden » Sat Apr 07, 2018 7:54 am

Hi Mark,

Sorry that the old Board is still down because there are many great discussions on there. As I may or may not understand it, the principal reason that the snapcast is so effective is because of the large removal of mass from the fly leg and due to conservation of energy (not momentum - never use COM on this Board by the way because you will be shot down in flames :p however Conservaton of Energy is allowed and generally approved).

What I would really like to know however is how the Double Snap works. Maybe the Engineers-Physics guys can explain that one. :)

Thanks, Paul
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What makes Snap Casts (Interesting)

Post Number:#3  Postby Dirk le Roux » Sun Apr 15, 2018 9:24 pm

Hi Mark

Hopefully this, and on the next few posts due to the limitation on number of images per post, will be of interest.

First off - BIG thank you to Graeme for sharing his footage, introducing me to Tracker and tips on using it. This is a marvelous free tool!

I analysed Graeme’s second to last snap cast (once again, thank you Graeme!) from the video he placed in post #189 of the Upward Force from Form Drag thread.

A few notes on the analysis set-up:
  • As most of the line motion occurs along the axis shown by the fly line in the first image below, I set the Y-axis parallel to that. Y velocities therefore coincide with this slanted axis.
  • I cannot vouch that time calibration is spot on correct (and therefore the velocities and acceleration values shown). Graeme recorded the original footage at 240 fps and assured me that is the original file, so that is the value I set for the analysis.
  • One can set tracker at different footage frame step intervals advancing to each new position for marking. The smaller the frame step, the more data points, which would enable analysing more smaller movements such as the rapid velocity change around the loop. Due to quality of the footage (not that it’s bad, impressive what can be done with an iPhone) and associated pixel jumps to track small intervals, the results are noisy, and it is time consuming. I played around with different settings and found that, with this footage, noise reduces to an acceptable level at steps of each 5 frames.
Initial conditions:
  • The rod tip, with line following suit, moved upward very slowly before Graeme started the downward stroke. That meant a slight bit of initial upwards momentum, so fly line motion did not start at dead zero.
  • The fly line used is a Rio Versitip #8 marked with black sections at 1m intervals.

1a - all paths.jpg
Traced paths

The image above shows the paths traced and analysed on this footage so far (I exclude from this discussion the extreme bottom right pink and blue-white paths, as I will later discuss those focused on the studying the loop’s ‘levitation’). The rod tip is at the point in time where the downward stroke started. Tying in with your remark, Mark, a lot of straight lines shown by these line paths!


1b - RT + LF paths.jpg
Rod tip and loop front paths

The image above shows the rod tip and loop front paths. Interesting on the loop front path is that the climb path after its ‘freeze’ closely follows the previously downward going one. You can see where the loop front down and up paths start to slightly diverge about halfway between its split with the rod tip path and the loop freeze position.


3 - M 3 a=g.jpg
Initial fly leg markers, Rod tip maximum acceleration point

Showing the rod tip path and paths of 1m interval markers until, or to just before, their fly leg path meets the loop. Plots for the Y velocity and Y acceleration of the rod tip and that of Marker 3 (green) are shown. I excluded the rod leg marker paths from this part because I wanted to investigate what happens to the fly leg in the initial stages, and the rapid earth frame accelerations and velocities once markers join the rod leg, when included in the graph plots, ‘flattens’ the fly leg portion of plots and make it difficult to clearly see changes along fly leg progression.

The yellow path of Marker 1 is close to the rod tip, so the loop catches up to it very quickly. It tracks roughly a parabola apex before the marker flushes down the rod leg. The light blue Marker 2 path’s motion is more like that of Markers 3 (green) and 4 (orange) further down, but the loop meets Marker 2 rather soon so its motion until disappearing down the rod leg is abbreviated. I will share its plots in the subsequent post (three image limitation).

The rod tip position shown coincides with the instant the following can be seen on the Y velocity and Y acceleration plots:
  • Rod tip Y acceleration is at its maximum here, after which it starts regressing.
  • Before this instant, all fly leg markers decelerate from the bit of upward momentum they had. Their acceleration at this point is zero, where after they are accelerated upwards.
So rod tip acceleration from here on is visibly impeded (by what?) and the net zero Y acceleration in the fly leg at this point means its upward acceleration vector here equals the downward acceleration by gravity vector, here a-g=0. Air drag at the current velocities is negligible. What net force could be responsible for the net upward acceleration of the fly leg?

1. Pixies (includes magic)
2. Rocket behaviour
3. Tension at the loop

Some acknowledge tension at the loop but state it is so small in a ‘normal’ cast to only net accelerate the very last bit of fly leg’s lightweight end, leader and fly close to final turnover. It is also said that its effect can be seen in snap casts, ascribed to a high rod leg velocity in the opposite than fly leg velocity direction.

The very short rod leg's velocity measured above (measured by rod tip velocity at that instant being close to the -Y direction) is only about -13m/s at the a-g=0 instant and the fly leg velocity at that instant is close to zero. There may be some fps to m/s error here, but the suspected accuracy of the time scale is supported by the measured max downward Y acceleration of the falling fly leg markers approaching -10m/s² the further down the marker is situated (like Marker 4). So let’s say rod leg velocity is even -15m/s vs fly leg velocity of 0m/s, 17m/s vs 0m/s? That’s pushing it.

Continued below...
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What makes Snap Casts (Interesting)

Post Number:#4  Postby Dirk le Roux » Sun Apr 15, 2018 9:30 pm

Here is the similar plot for Marker 2 as promised:
2 - M 2 FL a=g.jpg
Marker 2 velocity and acceleration plots

and here the plot for Marker 4:
4 - M 4 a=g.jpg
Marker 2 velocity and acceleration plots


As could be seen, all fly leg markers exhibit their velocity reversal and zero acceleration point at or very near (step size influence here?) the same instant and rod tip position shown in the previous post for Marker 3.

Continued below…
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What makes Snap Casts (Interesting)

Post Number:#5  Postby Dirk le Roux » Sun Apr 15, 2018 9:44 pm

Further interesting to me is the rod tip’s acceleration reversal point (zero acceleration) occurring at this instant:

7 - M 3 FL @ RT decel.jpg
Rod tip acceleration zero, Marker 3 acceleration plateau

and

8 - M 4 FL @ RT decel.jpg
Rod tip acceleration zero, Marker 4 acceleration plateau


And whereas the rod tip’s acceleration reversal point coincides with the threshold of an acceleration plateau in markers 3 and 4, it precedes the threshold point of the loop’s acceleration plateau by one step:

Inked5 - RT + LF decel_LI.jpg
Rod tip acceleration past zero, Loop front plateau

This time step difference may be due to coarse steps used and requires calling for the magnification glass some time in future.

What is also interesting to note here is the loop front’s continued positive acceleration hereafter. This may be ascribed to line taper, with mass in the fly leg being less than that in the rod leg.

I plan to present an analysis of leg velocities in the loop tracking frame and of the dynamics around the loop’s levitation later this week, if there is any interest shown.

Regards, Dirk
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What makes Snap Casts (Interesting)

Post Number:#6  Postby VGB » Sun Apr 15, 2018 10:01 pm

Hi Dirk

I’ve tried to do a qualitative analysis on Aitors original version of this cast but it’s very difficult to do on secondhand video, you cannot work out the effects of the compression algorithms

I don’t ascribe to pixies or rocket behaviour, it’s not good for the lines plastic coating and tension at the loop is a result of external forces being applied to the line, so I find the following statement a bit odd:

Some acknowledge tension at the loop but state it is so small in a ‘normal’ cast to only net accelerate the very last bit of fly leg’s lightweight end, leader and fly close to final turnover. It is also said that its effect can be seen in snap casts, ascribed to a high rod leg velocity in the opposite than fly leg velocity direction.].


Some say that tension at the loop is a result of external forces and state that pullback on the rod leg and certain tapers mean that generalisations should not be applied due to the amount of variables in a cast. Also some say that v-fly is not a fixed ratio to v-loop as is shown in current models. They also say that during pull back, you can also see the loop motion relative to the direction of the cast slow down.

This is the point that really interests me and I look forward to your continuing analysis:

What is also interesting to note here is the loop front’s continued positive acceleration hereafter.


Regards

Vince
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What makes Snap Casts (Interesting)

Post Number:#7  Postby VGB » Sun Apr 15, 2018 10:52 pm

So rod tip acceleration from here on is visibly impeded (by what?)


Dirk

I think that the rod tip acceleration is impeded by the combined inertia of the fly leg and rod tip. I think it’s another occurrence of the rod tip going backwards when we first accelerate the rod.

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What makes Snap Casts (Interesting)

Post Number:#8  Postby Geenomad » Sun Apr 15, 2018 11:37 pm

Dirk le Roux wrote:Hi Mark

Hopefully this, and on the next few posts due to the limitation on number of images per post, will be of interest.


I hope so too Dirk. The role of line tension in loop propagation is interesting. Any potential role in loop travel would be even more interesting for me.

From the ghost of John Clarke who sadly left us just over a year ago..... Caught up with him at last night's séance.

As for levitation, the crowd is palpably hushed. At half time in this particular farnarkling contest the pixies have been sent off - no one really expected those selfish bastards to do anything for anyone else anyway. Divine intervention is back behind the halfway line and in retreat. Not likely to score from there. Meantime the attention of the crowd has been drawn to other matters. Curious and still more curious. Hang on, what's that? Did someone here mention casting? :D

Cheers
Mark
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What makes Snap Casts (Interesting)

Post Number:#9  Postby Graeme H » Sun Apr 15, 2018 11:59 pm

Dirk le Roux wrote:I plan to present an analysis of leg velocities in the loop tracking frame and of the dynamics around the loop’s levitation later this week, if there is any interest shown.

Regards, Dirk

Nice work there Dirk. Please continue: I'm interested.

Cheers,
Graeme
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What makes Snap Casts (Interesting)

Post Number:#10  Postby Graeme H » Mon Apr 16, 2018 12:13 am

I'm interested to see slightly positive rod tip acceleration at the highlighted frame in the last image posted above. The rod is still bent, indicating there is still some downward force, even if it's not full power. The graph indicates the tip is about to stop accelerating in the opposite direction to the force I'm applying, so its downward velocity is constant at that exact point. I guess it's a function of the image frame chosen at that very point, because the next point on the plot has positive acceleration (rod tip slowing.)

Nice work.

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