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Paul Arden wrote: ↑Sat Jun 15, 2019 9:56 am
James, you once told me that angular momentum was conserved. Does this mean that it is constant throughout the cast?
Cheers, Paul
Hi Paul,
Obviously at some point the line lands straight thus all angular momentum must have been lost at this point, so the answer has to be no. The history of the angular momentum through the cast (prior to landing straight) is likely to be complex, and at this point in time I'd have to say 'I don't know'. (My gut feeling says it reduces due to losses)
Conservation of angular momentum is valid for closed systems, but it's not for fly casting and would be completely wrong here.
Torsten,
Agree completely and think the same thing holds for applying the conservation of linear momentum concept to explain the speed up of the loop as it rolls out on a cast.
Even with that in mind I think the conservation of angular momentum helps to explain why the loop diameter reduces when the heavier linear mass momentum of a sink tip line enters the loop as shown below for a Tenny 450 line.
I would expect the loop diameter to get smaller for a line with a negative taper in the diameter of its belly section (such as the MED 5) as it propagates for the same reason.
Torsten wrote: ↑Sat Jun 15, 2019 10:57 am
Conservation of angular momentum is valid for closed systems, but it's not for fly casting and would be completely wrong here.
Torsten, so by the same reasoning you'd dismiss hypotheses based on conservation of linear momentum then, i.e. most of the published papers on fly casting?
I'm sort of with you on this (although from previous threads I'm surprised at your statement). I've said for a long time that the losses are so crucial that they can't just be chucked in at the end to explain why a model doesn't match reality.
gordonjudd wrote: ↑Sat Jun 15, 2019 7:30 pm
Even with that in mind I think the conservation of angular momentum helps to explain why the loop diameter reduces when the heavier linear mass momentum of a sink tip line enters the loop as shown below for a Tenny 450 line.
Or maybe it would narrow in this case anyway because the fly leg is simply following a trajectory governed by gravity ...
Do you have any examples showing a cast with a less abrupt change in linear mass and/or a cast with fewer waves in the rod leg? Those waves really complicate the image because in this static image, they show the loop narrowing, widening and finally narrowing again. Why did the loop get narrow one third of the way in from the right before widening again if mass change is the reason for the loop to narrow? (They look like waves from tip counter flex, but I could be wrong.)
Torsten, so by the same reasoning you'd dismiss hypotheses based on conservation of linear momentum then, i.e. most of the published papers on fly casting?
James,
Could you give references for a couple of the published papers you found that mentioned the conservation of linear momentum in regards to loop propagation?
I have seen that contention in many sites on the web, but never in published technical paper. As Torsten stated since the line has external forces acting on it you would not expect the conservation of momentum to apply to loop propagation.
Conservation of linear momentum expresses the fact that a body or system of bodies in motion retains its total momentum, the product of mass and vector velocity, unless an external force is applied to it.
Due to drag forces the KE in the moving mass of the line is not conserved either,but it is a good place to start.
I know that you guys really want to discuss how fly casting works again, however what we also have here is the line spinning around 1/4 of a circle. In the case of a V-loop or top pointed wedge, this circle is very tight.
So what happens when different tapers go through this rotation? For example the MED?
Even with that in mind I think the conservation of angular momentum helps to explain why the loop diameter reduces when the heavier linear mass momentum of a sink tip line enters the loop as shown below for a Tenny 450 line.
The problem there is that the sink tip line is also a thinner diameter. If it was the tip of a heavier floating line would it still narrow or open?
Actually I have a room full of lines. Tonight I will see what I have that can be chopped, spliced and looped together.
In the old board there is a thread in the tech forum named "Conservation of angular momentum - How does it impact loop size". Most is said in the first pages, after that there is a discussion in the tradition of the board: arguing about disagreements.
If angular momentum is a weird concept for a flexible object (of magical nature maybe), one can use vertical and horizontal momentum to get an idea of what is going on. Some time ago, a scientist (Hendry) studied the vertical momentum (not conservative) to conclude that the size of the loop can be related to the speed of the fly leg: the faster the speed is, the smaller the loop should be (and conversely).
In the case of a 170 competition cast the line is shot and this brings the line flight closer to conservative conditions. If we accept this assumption, solving the equations in a favorable context (little aerodynamic forces) indicates the following: at the start of the shoot, the fly leg speed reduces and the rod leg speed increases. Then comes the time when the rod leg is sufficiently massive to allow the fly leg to accelerate again. If we consider Hendry's conclusion, then we have an explanation for the observation mentioned by Paul. The loop begins by widening (the fly leg speed reduces), and at some time its size is reduced (the fly leg speed increases), and this is linked to the shoot. Sometimes applying conservation of momentum helps to get the trends.
For sure, line taper can blur or emphasize some trends, and loop shape as well.
Merlin
Fly rods are like women, they won't play if they're maltreated
Charles Ritz, A Flyfisher's Life
Thanks Merlin, that’s a good post. I’m cutting up flylines into 4m sections. I can’t find my balance and will try to buy one in Ipoh where I’ll be on Wednesday. I *think* I’m cutting 4, 6 and possibly an 8 which might weigh in at a 9.
The plan is to cast them in different configurations alongside a “control” line and film the loop morph.
