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Analysis and Observations on Fly Cast Video Footage

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Graeme H
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Re: Analysis and Observations on Fly Cast Video Footage

#11

Post by Graeme H » Sat Aug 10, 2019 5:01 am

Dirk le Roux wrote:
Fri Aug 09, 2019 9:29 pm
Graeme simply stood less square to the cast, facing the wall and casting across his chest. I am fairly convinced the cast was shot perpendicularly to Graeme's position relative to the wall (and the cast). Graeme can confirm. Regarding the velocity values, remember that this was a very laid-back kind of cast.
Yes, of course this was what I was doing. I set the camera up so that it's facing directly at the wall then cast parallel to the wall. Doing otherwise can lead to errors, similar to using the dynamic roll cast that Christopher Rownes makes to illustrate certain features of a cast. ;)

A dark wall is best, but they can be very hard to find.

While a measure on the ground at the distance of the fly line away from the camera would be nice, I find it easier it be very careful when marking the line and use the marks themselves as the "calibrations stick" in Tracker. Having marks on the ground is just as likely to be erroneous since the fly line may not be exactly over those marks during the casting sequence.

I need to mark up another line. My next one will be a bright yellow DT8. Using lines thinner than 8wt can make digitising the marks difficult (it's hard to see thin lines).

Cheers,
Graeme
FFi CCI

Dirk le Roux
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Re: Analysis and Observations on Fly Cast Video Footage

#12

Post by Dirk le Roux » Sat Aug 10, 2019 9:01 am

Graeme H wrote:
Sat Aug 10, 2019 5:01 am
While a measure on the ground at the distance of the fly line away from the camera would be nice, I find it easier it be very careful when marking the line and use the marks themselves as the "calibrations stick" in Tracker. Having marks on the ground is just as likely to be erroneous since the fly line may not be exactly over those marks during the casting sequence.
Hi Graeme

Good idea to use the line marks themselves to get an idea of distortion. Setting the distance between the two markers nearest the fly to 1m while that section was close to the centre, and then measuring the distance between the same markers when at extremities, I got this result:

calibration.jpg
The error appears to be 5 to 10%, which means a point at extremity will appear to travel 5 to 10% faster than one at the centre.

Regards,
Dirk

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Graeme H
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Re: Analysis and Observations on Fly Cast Video Footage

#13

Post by Graeme H » Sat Aug 10, 2019 9:38 am

I usually use 3 markers to reduce the error. That is, I get a three metre section as my calibration stick.
FFi CCI

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Graeme H
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Re: Analysis and Observations on Fly Cast Video Footage

#14

Post by Graeme H » Sat Aug 10, 2019 10:42 am

4 markers …
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Dirk le Roux
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Re: Analysis and Observations on Fly Cast Video Footage

#15

Post by Dirk le Roux » Sat Aug 10, 2019 11:05 am

Hi Graeme

That is what I did for calibrating my working files (mine was 3 markers, 2m). To get an idea of the distortion however, the trouble is to find a straight 2 or 3m length at the extremities. And then, the nearer-to-centre part of that longer length softens the effect of the distortion, being less far to the edge of field.

Crude as the 1m comparison may be, it does suggest that the distortion error of your footage is not dramatic.

Regards,
Dirk

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Re: Analysis and Observations on Fly Cast Video Footage

#16

Post by Dirk le Roux » Thu Aug 15, 2019 7:53 pm

I wanted to get to the forward cast but got interested first to look at the prequel to Graeme’s (normal cast) back-cast.

Near the end of roll-out from the previous forward cast, we see the line configuration looking reasonably straight with a fair-looking loop:

kick-over_1.jpg

The fly line tip went down quite fast:

kick-over_2.jpg

Kicking over and appearing to hinge around the initial dip before the loop:

kick-over_3.jpg

This sequence happened in 0.05 seconds, while the rod tip moved in the direction of the back-cast (to the right) by roughly an inch. This shows a compiled trace of the sequence:

kick-over_seq_1.jpg

With the kick-over motion came appreciable vertical displacement of the part of the line between the loop and the bump indicated by the arrow, compared with line nearer the rod tip.

The further history lasting to RSP can be seen in this fingerprint trace:

BC prequel trace_1.jpg


The dotted line is leader, at instances where I could distinguish its shape at all.

By the time the fly line end started moving appreciably more in the direction of the cast (at 1) rather than downward, the rod tip has moved about a fifth of the way to RSP. Here one can see illustrated quite well what Mark said in his Straight Lines Rule page: “Same deal with the fly line – it needs to be kept as straight as possible or part of the force you apply will be spent taking up slack or following curves in the line instead of moving the fly line in your intended direction.” Also, one can appreciate the change in slope this falling of the rear of the line brought about by the time that work on the whole length of the line could start in earnest.

It is quite difficult to follow any progression of the sometimes-growing-large bumps in this rear area during this period, though it appears that if there was propagation, it was in both directions, bumps crossing. Note also that the initially deep bump just at the fly line end did not propagate toward the fly until position 1 was reached. This confirms a lack of tension at the fly line end until that time.

As the rear end falls, line progressively “peels off” the dense ridge identified between the arrow and rod tip in the previous image. The rod tip’s initial motion was upward, and I suspect the angle between this motion and the initial ridge originated the previously discussed bump a. Likewise, it appears the rod tip’s domed path originated bump b.

Regards,
Dirk

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Re: Analysis and Observations on Fly Cast Video Footage

#17

Post by Paul Arden » Fri Aug 16, 2019 5:25 pm

Truly excellent Dirk! I shall endeavour to find somewhere suitable to film loops. I’d love to have both accuracy as well as distance loops analysed and contrasted. Maybe also some dynamic rolls and aerielised snaps.

Many thanks,
Paul
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Dirk le Roux
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Re: Analysis and Observations on Fly Cast Video Footage

#18

Post by Dirk le Roux » Fri Aug 16, 2019 11:11 pm

On to Graeme’s (normal cast) forward cast.

INITIAL CONDITIONS FOR THE FORWARD CAST:

• Line slope early during the stroke:
FC start slope.jpg

As the line was not straight, like it was (almost) at some point during the beginning of the back-cast stroke, I measured two ways of considering the start slope.

• Line configuration at RSP is a slim S-curve, with the bottom curve quite close to the fly end:
FC config.jpg

The rod tip position at RSP is closer to the highest tangent-to-the-horizon part of the stroke path than that of the back-cast. This suggests a more upward launch trajectory than the back-cast had.


TRACKED PATHS OF MARKERS DURING THE FORWARD CAST:

This image shows the tracked paths of all line markers from where line marks became identifiable to where the line appeared to be fully extended (fly was out of view):
FC markers.jpg

From where they could be tracked, paths once again mimic the configuration of line ahead at the instant, but stretched out:
paths vs configuration FC.jpg

As I was unable to trace the markers early on, I include the fly path and its velocity magnitude plot to provide an idea of speed history during the forward cast:
FC M 5 and fly velocity, BC fly V inset.jpg

The back-cast’s fly velocity magnitude plot is inset for comparison. Things overall appear to progress more smoothly with the forward cast than with the back-cast. Less “flutter” of the fly path can be observed here.

Where the back-cast fly speed went from 16.35m/s² around RSP to a maximum of 21.27m/s², the forward cast’s went from 19.44m/s² around RSP to 39.19m/s², at which point the fly fluff went out of view. A 100% fly speed increase.

Dirk le Roux
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Re: Analysis and Observations on Fly Cast Video Footage

#19

Post by Dirk le Roux » Fri Aug 16, 2019 11:16 pm

RISE AND FALL OF LINE MARKERS DURING THE FORWARD CAST:

Due to the incomplete possible traces, a full look at vertical acceleration from RSP, therefore markers’ overall “fall” history, could not be obtained for the forward cast.

Nevertheless, with the forward cast the same correspondence of up or down acceleration peaks to rounding bends or rod leg events can be seen:
FC M 5 acceleration vectors.jpg

Again, the highest magnitude (not Y) acceleration was at the long backwards-pointing vector, where the marker passed through the loop nose, and registered 336m/s² (33g).

We are afforded a look at markers vertical motion after they have joined the rod leg, as this forward cast was not influenced by the beginning of another back-cast, the rod tip after the stop having drifted forward and down, followed by an upward drift as the line was let to fall:
markers post-rod-leg-join.jpg

Once joined in the rod leg, markers show a downward and then an upward pulse. These pulses correspond with a wave travelling down the rod leg toward the loop, the downward pulse preceding the wave’s arrival and the upward pulse coinciding with markers turning the wave bump’s bottom. The image instant shows the rod leg wave bottom at the green marker (M 3). The grey dots in the markers’ Y acceleration plots and bold circles on the track paths correspond to this instant.

Arrows in the following image show where the rod leg bump intersected markers at various instants and its effect can be clearly be seen at the orange, yellow and green paths. The overall image instant is at the end of the rod tip forward drift, marking the beginning of upward drift, circles in the track paths once again indicating the position of markers at this time:
drift & wave points.jpg

We will return to this bump travelling down the rod leg.

Dirk le Roux
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Re: Analysis and Observations on Fly Cast Video Footage

#20

Post by Dirk le Roux » Fri Aug 16, 2019 11:31 pm

FORWARD CAST FINGERPRINT TRACE:

Here’s the forward cast’s fingerprint trace, starting from loop formation:
composite fc.jpg

Bumps like those discussed in the previous back-cast section can be seen, and their shape- and spatial path evolution show similar characteristics to those previously discussed. The loop nose path is down-up-down, influenced by curves and straights in the path of the fly leg behind.

An image showing, like the previous image of the back-cast, the same number of intervals either side of the instance (line position marked white) just before bump b started rising noticeably above its previous trajectory:
FC cross-over_2.jpg

Again, the change in trajectory coincides with the fly having rounded the turn associated with bump a. Again, the fly leg becomes inclined down toward to the loop as bump b rises, the loop nose path rising correspondingly, then descending once bump b have propagated out the line.


WAVE TRAVEL AND REFLECTION:

Do waves in the line always propagate toward the fly end? The following sequence, tracking bump e’s origins, suggests not:
rod leg wave.jpg

It is not clear where this bump originated. Logic suggests that due to its initial propagation towards the rod tip, it should have propagated from the fly leg and through the loop at around frame 3. Preceding 3 though, such a bump in the fly leg is not readily apparent. Once at the rod tip, bump e reflected and then propagated towards the loop.

As bump e still approaches the rod tip, we can see bump b (grey arrow) entering the view in frame 7 above. Once bump e had completed its reflection at the rod tip, these two waves travel vertically aligned up to where b enters the leader and cannot further be clearly distinguished:
bumps same v travel.jpg
bumps same v travel.jpg (30.52 KiB) Viewed 467 times
Their distance from the loop having remained the same, it follows that during this period these waves propagated through the line at the same velocity. Bump b during this period is clearly in front taper territory, which, recalling the wave speed equation v=\(\sqrt{\frac{tension}{linear \,mass}}\) suggests that the reduction in tension in the fly leg was proportional to the reduction in linear mass.

Regards,
Dirk

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