Conservation Of Angular Momentum (COAM)

[Graeme H]

Hi Paul,

If the fly leg is travelling faster than I can create slack in the rod leg (e.g. in a distance cast) then I can’t create slack faster than the fly leg retightens the line. Slack line is never established.

I’m staying away from waves here, so discussion about loop stability can wait. Let’s first see if COAM exists in sufficient magnitude to have any impact

Have you considered the question of tight loops necessarily having less AM than wide loops yet?

Cheers,
Graeme.

[Stoatstail50]

I thought it had been agreed that, in the real world cast, coam doesn’t exist.

A quick comment before analyzing your post in details: COAM does not exist since there are external forces acting on the loop,

[Paul Arden]

Hi Graeme,

Generally speaking it’s difficult to cast wide round loops at high speed. It would certainly put a different spin onto why initially wide 170 loops lead to longer casts.

Out and about. Back later!

Cheers, Paul

[Graeme H]

I thought it had been agreed that, in the real world cast, coam doesn’t exist.

I thought so too Mark, and yet it is still managing to find its way into discussions here.

I’m interested to find out why conservation of COAM is occurring in SL.

Cheers, Graeme

[Stoatstail50]

Because if it’s not happening in actual casting, it’s the only place where the theory can be conserved ?

[Merlin]

But once again Daniel, thanks for your diagram. It came at the right time for me to grasp what you've been trying to tell me.

You are welcomed Graeme

However, I disagree with AM having any significant contribution to the cast.

Agree, the “engine” of the cast is the fly leg momentum, not the AM of the loop for sure. However, AM is important for understanding the behavior of the loop and of the line as it rolls over.

A couple of times it has been stated that the external forces on the loop affecting AM are friction and gravity. I haven't seen mention of tension but I may be wrong there. However, I believe it is the most important external force.

You missed my post#8 which contains the wrong estimates of AM:

Consequently there is no COAM here, it increases because there is external torques applied to the loop (drag, mass with gravity, tensions), this is a loop mass effect.

In the meantime Gordy has made another analysis of the tracker data from the video where the loop morphs, and this time it gives 14% AM difference in between the “floating line” and the “sinking line “loops. So there is a rather small difference, but an increase in AM during the cast, which is rather limited. If one is not a purist, one could say there is nearly COAM.

By the way I see that you use fly leg speed for calculating AM, one must use the tangential speed (rotation in the loop frame, it is half the fly leg speed for a tethered cast), there is zero speed at the bottom in the earth frame. You are not alone to miss the point.

If angular momentum is conserved or destroyed, does it matter?

If it is destroyed it means either the loop has disappeared (no MOI) or that there is no more tangential speed, so no fly leg speed, finally no line rollover, but the cause does not come from the loop itself, it comes from tensions at its ends.
If it is conserved, or something like that, it might explain the behavior of the loop due to various parameters like line density for example. Things are not so obvious here, I am currently struggling to simulate the cast of a level line made of floating and sinking parts, a nightmare for the time being.

Merlin

PS: thanks to share your views about DN, to me it remains a locked-in wave trapped by the loop (mentioned by academics studying the string shooter), an UFO. I think there is an old thread about DN somewhere, many things were reviewed including solitons.

[VGB]

You missed my post#8 which contains the wrong estimates of AM

How are you measuring from the centre of rotation Daniel? Gordy seemed unclear about how to determine it at Post 31.

Regards

Vince

PS I started in the wavy DN camp before I moved out. Nobody could ever point at the movement that caused the wave

[Graeme H]

A couple of times it has been stated that the external forces on the loop affecting AM are friction and gravity. I haven't seen mention of tension but I may be wrong there. However, I believe it is the most important external force.

You missed my post#8 which contains the wrong estimates of AM:

Consequently there is no COAM here, it increases because there is external torques applied to the loop (drag, mass with gravity, tensions), this is a loop mass effect.

Yes, I'm sorry I missed that. I was indeed wrong when stating it hadn't been mentioned.

By the way I see that you use fly leg speed for calculating AM, one must use the tangential speed (rotation in the loop frame, it is half the fly leg speed for a tethered cast), there is zero speed at the bottom in the earth frame. You are not alone to miss the point.

Yes, sorry about that. I thought I had switched to the loop frame of reference when I read the graph. It should have been 5m/s (so even less AM ) It's not that I missed the point, but chose the wrong graph ...

If angular momentum is conserved or destroyed, does it matter?

If it is destroyed it means either the loop has disappeared (no MOI) or that there is no more tangential speed, so no fly leg speed, finally no line rollover, but the cause does not come from the loop itself, it comes from tensions at its ends.

I don't think it will be destroyed, but it is of such little consequence that it can be ignored, no matter how big we make it.

Would you care to comment on the implications that tiny loops have much less AM than big loops when the fly leg speed is the same? That is, the better the loop, the smaller the AM ...

PS: thanks to share your views about DN, to me it remains a locked-in wave trapped by the loop (mentioned by academics studying the string shooter), an UFO. I think there is an old thread about DN somewhere, many things were reviewed including solitons.

Sorry, no, not in this thread. That's a deviation from the subject of the thread and reintroduces waves into the discussion (I'm trying discipline here ... ). None of you guys want to hear that, so I'll keep my thoughts to myself. (But yes, that's the gist of it - not trapped but accompanying it.)

[Graeme H]

You missed my post#8 which contains the wrong estimates of AM

How are you measuring from the centre of rotation Daniel? Gordy seemed unclear about how to determine it at Post 31.

Regards

Vince

PS I started in the wavy DN camp before I moved out. Nobody could ever point at the movement that caused the wave

I'm just using the "instantaneous AM" here. at the point in the cast I had captured, those were the measurements. I know they'll change over time , but It doesn't matter if they do, IMHO. Conserved (or even existent) or not, AM seems like a red herring in the cast.

Look really carefully at the rod tip motion JUST at loop formation. That's where it starts and amplifies as the cast progresses.

[James9118]

I thought it had been agreed that, in the real world cast, coam doesn’t exist.

A quick comment before analyzing your post in details: COAM does not exist since there are external forces acting on the loop,

One of the issues with this type of discussion is the desire to over simplify things. Unfortunately the detailed physics of what's happening in a fly cast is quite complicated, and just getting down to one simple hypothesis is going to be littered with oversights (or just downright wrong).

Saying COAM doesn't exist because external forces act is a fallacy. Of course it exists (why wouldn't it - are fly lines magically removed from this principle?), but it needs to be viewed in conjunction with the other external forces. Yes, AM comes and goes within a cast, from line straight at the back to line straight at the front, but the understanding how the AM is created and destroyed is the same as understanding the role of COAM in the first place.

I really liked Merlin's quote above - the engine of the fly cast is the fly-leg momentum, however understanding the AM helps understand the roll out.

On a practical note - I once did a test where I cast a ST27 line with and without backing attached (I think I wrote a FP about it). My longest cast on the day was one without backing - i.e. a no attachment back to the rod, with a nice clean turnover. I think Lasse has had similar results.

James