Rod Loading - Spring v Lever

[Merlin]

Walter,

There is always air drag, on the rod and on the line: you start by accelerating slowly against inertia and then the rod stays in a nearly constant deflection as the pace becomes nearly constant, you can change the pace slowly and describe circles, figure eight or whatever; the rod will remain loaded without too much variation in bending and will not unload as long as you control that bend. If you start accelerating significantly at a constant rate, then you are introducing a casting motion and the rod with load deeper and unload, even if you keep accelerating for a long distance, if practically possible (pretty theoretical)


Mark,

I spoke of translation, and this introduced confusion since English is not my native language. I meant something like a relatively constant pace, something very different from a distance cast to caricature the situation. Maybe my comment to Walter can help understanding my point. I’m sure that you are mastering a fly rod much better than Joe (and me), you certainly have a large control of your input and you can anticipate rod reaction. I should not have said “loss of control” because I opened a Pandora box, but maybe this will be another opportunity to clarify things.

I agree that the descriptions of phenomenon we talk about from the output of a model are not always visible; I would even say that they are hardly visible but they are there. I am not talking about oscillations due to higher harmonics, which do not correspond to a loading / unloading sequence but which are part of the rod response to the caster input.

I never said, or it is a misunderstanding again, that the rod does what it wants to do as the caster drive its motion. A rod responds with its own mechanical properties to a driving motion controlled by a caster, and we make the assumption that the caster has sufficient training with that rod. The caster controls the acceleration and the deceleration. He has something like half a second to do that with a SH rod, a little bit more with a DH rod. If he does not master the input, he will lose control of the cast (a common situation for non expert casters) and consequently the control of the rod and of the line.

If this perception of “the rod does what it wants” remains, then we have to take this out of caster’s mind. Viewed differently, knowing what the rod does because of the input driven by the caster (linear or not linear acceleration and deceleration) helps to understand how they should be designed (more to come on that point with the characteristics of the Hot Torpedo).

May I suggest we swap the discussion on the new Board, opening a new thread different from the existing one (Spanish experiment)?

Merlin

[TrevH]

I think one of the mistakes made by myself here is showing the results of the model for situations that are a million miles away from a cast.

I don't think so. I found it useful to look at it in that way. I don't think anyone was thinking that was a reflection of what was happening in a real cast, but to see what was happening over a relatively long period I found helped to understand what would happen over a shorter period such as, as you say, something around half a cycle.

And of course, even that is only a fraction of the story, but to look at one factor at a time and then layer the other factors on top of that and seeing how they all interreact is something I find, well interesting at least and, I think maybe even useful in some way.

Cheers
Trev

[Hal Jordan]

Walter,

There is always air drag, on the rod and on the line

Merlin - that is what we call assumptions. We assume the model includes air drag or we assume the model does not include air drag. We state these in the description of our models as I did. Why would we leave air drag out of a casting model in this case? For the same reason we would ignore centripital forces. So that we can understand how acceleration vs velocity affects a rod. What is really cool about assumptions of this nature? I can make a prediction and then run experiments to verity the prediction. If the predictions match the experiments then I know my model is telling me something useful. In this case I could predict how the rod would behave differently if I cast in space vs casting in the atmosphere. I say that at constant velocity a loaded rod will unload in space. Maybe if someone is through trying to decipher the complex free body diagram that I provided at their request they could take the time to try this experiment and provide something useful to this board. That is why it is important to state those assumptions.

[Merlin]

Walter

We are switching from model world to real world and vice versa. Mark was talking about DH rod real world and likely some form of Spey casting. We cannot model that for the time being. We can describe the general conditions affecting real world motion of complex cast, like movîg around the rod before firing at the target for example.

At the same time we were commenting the usefullness of knowing or ignoring the intricacies that a model can help to identify.

The question was not to perfectly align the assumptions to reality. As Vince said, checking the physics of a simple model does not need to go up to that point. Incidentally we are neglecting the friction on the table in the case of Aitor's experiment.

Merlin

[Stoatstail50]

Trevor

If I didnt have a raft of even more deeply humdrum and pointless tasks to do already in my life I would trawl through threads for the last three or four years which repeatedly claim that these models are exactly a reflection of what is happening in a real cast...or a real rod. I've been chewed out more than once for asking exactly how.

For example, we have been told again and again that the rod will "unload" of its own accord, a thing I have never been able to reconcile with experience. Now, it turns out, in this particular case anyway, you have to be the size of Bob Blob the Angling Amoeba to have any scale concept of the phenomena actually occurring, and it doesn't refer to the rod "unloading", as anyone currently uses the term anyway, but to imperceptible oscillations in the rod that wouldn't wobble a half pissed weeble.

It is an unpleasant personal conceit perhaps but I haven't been able to understand why I don't understand WTF these important models are telling all these bright people about real casts and real rods. How can they see things that I can't ? What are these graphs and maths and grand concepts saying and why can't I make head nor tail of it. So for the last four odd years I've been asking questions that have, on and off, made me look, and feel, like a proper knacker..then you say this...

I don't think anyone was thinking that was a reflection of what was happening in a real cast,

Well...I was.... :oh:

[Stoatstail50]

The tallest turbine in the world is tapered to 160m high with 90m rotors.....this will oscillate in a mightily macro sort of way much as the models on here predict.

From my perspective however, it won't be these oscillations that make the thing a real bugger to cast.... :glare:

[VGB]

The question was not to perfectly align the assumptions to reality. As Vince said, checking the physics of a simple model does not need to go up to that point.

Merlin

I think this is the point being made, Where simplifications are made they should be declared and the impacts explained.

I have not been on the board for 3-4 years but if decimal place accuracies and claims of high fidelity to the real world have been made then the intent of the models, to explain that they are indicative not a replica should be clarified. Moreover, where claims have been made and later found to be inaccurate, then it should be declared. If the technical analysis does not have demonstrable integrity then it is a waste of bandwidth.

I think that most people on the board understand and appreciate the work that goes into producing a model but do not appreciate being talked to as "laymen" who know nothing at all about fly casting when the model and real world experience appears different.

regards

Vince

[Merlin]

Vince

Models are clearly a replica rather than an extremely accurate desciption of what is happening in an overhead cast. However, they have been tested along recorded casts and give a fairly good estimate of reality sometimes, and not such a good one sometimes too which I mentionned a few times. For sure there are casting situations we are not able to model, e.g. the Spey cast, but we do not pretend to.

One can use models to derive hidden information. Now maybe giving such information to the audience from time to time is likely a source of trouble on both sides: to understand and to be understood. If this is not visible and difficult to understand, why mentioning them (e.g. unstability is not the cup of tea of many on the board).

The intention is not to show how good we are at modeling, per se there is little interest if any, it is to give some clues about how the whole thing runs. I realize that it is difficult to use an universal language that will fit anyone from the skilled technician to the one for which the physics course was just a pain to follow. I think that casters have a different interest than rodbuilders in this field, and I do not know where to put the limit.

If we had a blackboard and a few bears available, that would help a lot, I'm pretty sure we could have interesting discussions, not limited to a post to post like this one.

Merlin

[Stoatstail50]

If this perception of “the rod does what it wants” remains, then we have to take this out of caster’s mind.

Yes, we do and fair play for seeing the problem now but whilst I don't see any particular benefit in rubbing noses in anything here I think we should be aware how this came about....

I never said, or it is a misunderstanding again, that the rod does what it wants to do as the caster drive its motion

Merlin 2013

Well, I think you did or certainly implied something like it. Heres an example....

I said the rod will unload over a period of time even we keep on accelerating.

Merlin 2011

Walter has obviously been under the same impression that this was exactly what you were saying....he said to Gordy...

On the topic of harmonic oscillation you and Merlin have already identified that the rod is going to unload whether we try to make it unload or not as soon as we stop accelerating the rod.

Walter 2011

Same thread...different issue, levers...Gordy this time..sounds like modelling a cast to me...

The rod is acting like a flexible lever in casting. The forced harmonic oscillator model is just a simple means to predict how it is going to respond to different acceleration functions applied at the butt and different tip loads that are related to length of the line being cast.

Gordy 2011

Now...

I think that we can agree that the broomstick functions as a third class lever, but because of the more complicated dynamic response of the spring in the rod to external forcing accelerations the rod (even to first order) does not.

Gordy 2013

Really ??!! ....nothing confusing or contradictory there.... ... and thats just comparing the comments in one random thread from 2011 with this one.

All this seems to me to be talking about modelling real casts and real rods with real casters and real lines on each end of them. There are bloody hundreds of comments all implying similar things going back to 2009/10.

If you are modelling oscillations that are only observable in something the size of the Empire State then dimwits like me would, I'm sure, prefer to be made aware just so that I can shut down the synapses and get back to watching braindeath TV with a nice cuppatea because I have at least enough wisdom to recognise that this bears as much scalar relevance to a cast, a rod, or a caster as my foot to a fkn fur coat....

May I suggest we swap the discussion on the new Board, opening a new thread different from the existing one (Spanish experiment)?

Yes I think there should definitely be a new thread on the new board and its objective should be to quite explicitly explain the limitations of the models that are being employed in these discussions.

[VGB]

Merlin

In the past, my day job required me to deal with models of serious complexity such as predictions of the behaviour of stores releases from Eurofighter. I then had to agree test programmes to verify those models and provide data to tune them. I do understand how you can and cannot use them.

I also have considerable sympathy with how difficult it is to explain, the forum makes life very difficult in that regard. However, like Mark I can point to multiple places where the data has been used inappropriately and how it appears that being right is more important than being correct, at times it has been laughable. I wrote to you off-line to explain why I would not join the technical discussions on the new board and I have yet to see anything that would make me change my mind.

regards

Vince

I'd be grateful if you could swap the bears for beers, then I might join you

[Merlin]

OK, so my last comments in this thread: a rod can start unloading before the end of acceleration, it is a matter of a few 1/100 seconds and nobody cares but me.

I could not imagine that all I said is scrutinized from the beginning. I promise not to do the same with your posts.
Now I will have to use very few words to explain something, and I shall limit myself to what is "visible".

Good basis, no?

Merlin, looking forward seeing you on the new Board.

"Tirez les premiers, Messieurs les anglais" (battle of Fontenoy)

[VGB]

"Tirez les premiers, Messieurs les anglais" (battle of Fontenoy)

Je suis chaud - to my belle mere at breakfast 2010

[Hal Jordan]

OK, so my last comments in this thread: a rod can start unloading before the end of acceleration, it is a matter of a few 1/100 seconds and nobody cares but me.

Merlin -

F=ma

In this case the F can be measured by the bend in the rod (remember all the times I've said that the rod, although not linear, obeys Hooke's law in that the greater the force applied the greater the bend in the rod?). So the greater the acceleration the greater the bend in the rod. If I decrease acceleration, even though I am still accelerating, the bend in the rod will decrease. This is called unloading. There is nothing earth shattering about this. And, yes, there are many people who care and understand about Newton's laws.

[grunde]

OK, so my last comments in this thread: a rod can start unloading before the end of acceleration, it is a matter of a few 1/100 seconds and nobody cares but me.

Merlin -

F=ma

In this case the F can be measured by the bend in the rod (remember all the times I've said that the rod, although not linear, obeys Hooke's law in that the greater the force applied the greater the bend in the rod?). So the greater the acceleration the greater the bend in the rod. If I decrease acceleration, even though I am still accelerating, the bend in the rod will decrease. This is called unloading. There is nothing earth shattering about this. And, yes, there are many people who care and understand about Newton's laws.

Hal,

this is not correct! There is not a one-to-one relation between the acceleration of the rod handle (both rotational and translational) and the overall bend in the rod.

Therefore; Merlins comment on the timing of the unload is correct. Maximum overall rod bend and maximum rod handle acceleration will (generally) not happen at the same time (and even for constant acceleration of the handle; one will observe a whole range of different rod bends/loads). I also still believe that at the end of the casting stroke "the rod will happily unload by itself" even if we keep on applying force in the direction of the cast (you know the Server Sadik thing).

And I also still believe that all of Newton's three laws (including the 3rd :p which someone falsely believe is not valid in the dynamic case) and Hookes law is all valid and applicable in flycasting

Cheers,
Grunde

[VGB]

And I also still believe that all of Newton's three laws (including the 3rd :p which someone falsely believe is not valid in the dynamic case) and Hookes law is all valid and applicable in flycasting

Hi Grunde

Does that mean we can say things like Dynamic Amplification Factor instead of mechanical advantage to talk about the beam response to input conditions?

...and if the Dynamic Amplification Factor is real then something close to a step input might cause a rapid bend in the rod and a consequent tendency to tail?

regards

Vince

PS Please could you point me at the Server Sadik thing, I have heard of it but never seen it.