Angular Momentum

[Graeme H]

When we crack a whip, we pull back on "the rod leg" as if it's a snap cast. That action has angular momentum working throughout the "half oval", whereas a cast has AM working only in the "quarter oval". I'm sure you've noticed that it's hard (maybe impossible?) to crack the tippet on a fly rod without pullback at the right moment.

We were briefly discussing dolphin noses at the recent FFi get-together in Tumut. Nothing was resolved. I still have my ideas on why they occur (more linear momentum in the fly leg than the loop is removing) but I don't have any good experiments to test my theories against. To give you an answer, I think insufficient angular momentum might be the cause. Could be wrong though.

Cheers,
Graeme

[Paul Arden]

Hi Graeme,

If you remove the leader - or most of it - you don’t need to pull back to crack the whip. I think it’s only because the energy has been dissipated by the leader that pull-back near LSP might sometimes be necessary. In fact it’s extremely difficult not cracking the whip without a leader using our normal casting stroke.

Incidentally I tried whip cracking at Haysie’s a few years ago and found it difficult. Mostly because I was trying to use pull-back!

Cheers, Paul

[James9118]

Hi Graham,

Many thanks for that data, that tracker software looks great.

You know that changing the frame of reference in no way changes the physics or any derived or predicted results right? Yes, I know it appears like a fudge to get more material rotating, but what is being discussed is very loop centric so having a moving frame of reference based on the most forward part of the line makes sense, but ultimately doesn't change anything.

James

[Graeme H]

G'day James,

Yep, I know changing the frame of reference has no impact on the physics when it's done correctly. I've seen it done incorrectly a few times though, and the result is that some people think the loop is performing a magic act on the line.

I just don't see any need for it here. Focusing calculations on a place that puts every part of the system in motion just seems bonkers to me. That is, in that system, the caster, the fly leg, the upper part of the loop, the lower part of the loop, the rod leg and the air are ALL moving relative to the loop nose. All of their motions must be accounted for in any calculation.

In an earth frame of reference, the only part of the line that is moving though space is in the fly leg. Once anything in the fly leg hits the lowest part of the loop, it stops moving away from the caster. I don't know how that can be simplified any further. It's certainly easy to capture on video and analyse.

Cheers,
Graeme

[Paul Arden]

Once anything in the fly leg hits the lowest part of the loop, it stops moving away from the caster.

Not if it’s being shot of course.

James, you once told me that angular momentum was conserved. Does this mean that it is constant throughout the cast?

Cheers, Paul

[Graeme H]

Yes, of course Paul. I'm referring to the type of cast I videoed above.

For the sake of filling in the gaps here's a tracked version of the video Gordy posted:

[Torsten]

Paul:

James, you once told me that angular momentum was conserved. Does this mean that it is constant throughout the cast?

Conservation of angular momentum is valid for closed systems, but it's not for fly casting and would be completely wrong here.
(else the line would spin forever) Not even mechanical energy is conserved because of the non-conservative forces acting on the line (internal line friction, drag losses, skin friction etc.)

Graeme:
Nice videos. To derive something from that, actual measurements would be important (velocity history plots from various parts of the line).

Angular momentum (or rotational momentum) is usually a property to describe rigid bodies, but a fly line is flexible and so the application is more difficult. What you can say is something about the local rotational momentum at small segments of the fly line. It's difficult to compare angular with linear momentum (different units) - but you can compare the rotational kinetic energy with the linear kinetic energy.

Of course line segments at the loop front rotate and have thus a rotational kinetic energy - the question is the magnitude compared to the rest of the line. According to a paper from Jeff Kommers, the rotation plays only a smaller role - but it's something that can be measured by analysing the videos.

[John Waters]

Thanks Graeme, very much appreciated.

John

[Graeme H]

Graeme:
Nice videos. To derive something from that, actual measurements would be important (velocity history plots from various parts of the line).

Thanks Torsten. That sort of information is available within the tracked videos normally but in these ones, there are frames skipping. It doesn't like it when frames are duplicated or missing, but it is possible to construct pretty good velocity, acceleration, force graphs and so on when the videos behave nicely.

Cheers,
Graeme

[gordonjudd]

I've seen it done incorrectly a few times though, and the result is that some people think the loop is performing a magic act on the line.

Graeme,
What do you mean by magic act?

Is the line tension at the top and bottom of the loop that is produced by the change of momentum (in this case because of the direction change) magic or is it just the application of rotational dynamics?
Gordy