Two heads cast at the same time

[Paul Arden]

Sure I know you disagree, that’s why I was asking for your clear explanation, Graeme. I learn things better by reading them than by hearing them, so if you could give me a little bit of time here it would be very much appreciated.

Thanks dude,
Paul

[Will]

Paul

I am now wondering if both points of view have merit. Merlin’s post points to differing effects depending on how thin the line is, so up to a point a taper will accelerate the line (which I think Lee’s Scandi video shows), but the effect depends on the rate of taper (Lee’s Powerspey video), and the actual thickness (and density) of the line being cast.

So, maybe air drag is of little consequence on heavier/thicker lines, and is of some consequence for finer tapers?

Interested to hear Merlin’s view on this.

Will

[Paul Arden]

The answer is in air drag Paul. You can have better acceleration conditions due to the fact that more mass leaves the loop by comparison to the mass entering the loop at the same time, but air drag can, under some circumstances, spoil that trend to the point that there is no acceleration any more and a deceleration takes place. It depends of several parameters (e.g. loop size, fly drag, etc.) and on line diameter.

This raises a few questions for me, Merlin. Surely more mass entering the loop also sets up acceleration conditions as we see in Lee’s videos? It appears that under most conditions acceleration offsetting drag forces is very much in balance for a level line. That’s either a miracle or a universal law at play. I don’t know about you but I expected to see tapers causing far greater effects to the unrolling loops that we are seeing.

Thanks, Paul

[John Waters]

Thanks Lee, interesting video.
Just a simple question from me, if you cast each profile with the same rod, one at a time, which one casts further?

Thanks in advance,

John

[Paul Arden]

Crossposted Will. It’s also possible that they’re both wrong. Noting would surprise me

[Will]

Hah!

[Graeme H]

Sure I know you disagree, that’s why I was asking for your clear explanation, Graeme. I learn things better by reading them than by hearing them, so if you could give me a little bit of time here it would be very much appreciated.

Thanks dude,
Paul

Sorry Paul, there are a whole bunch of things wrong with the underlying premise of Gordy's explanation. I and others have already explained why over the past year (there's a 75 page thread about it.)

It will take me about 10 minutes to show you with a piece of fly line in my hand. I might have to make a video.

This post in the Angular momentum thread also explains why Gordy's premise is wrong. He wants to use the whole loop shape to explain angular momentum. In reality, the curved part of the line path is small. The line path NEVER mimics the loop shape in a normal tethered cast.

Gordy's premise of form drag on the line is false, so his argument is not valid. Even tilted line is following the piece of line in front of it. Watch the videos I've posted .... He has continually used "form drag" as both a retarding factor and a lifting factor in his model. His penchant for tracing static images of line without tracing the motion of the parts of the line ignores the actual path of the elements of the line. It has led him astray time after time and has forced him to find spurious supporting articles to convince himself and others he is right.

Cheers,
Graeme

[Paul Arden]

I’m sorry I must have missed your explanation Graeme!!

[Merlin]

There is not one single behavior of lines, there are too many parameters influencing it. Lee's videos are just that: hard to see with big long lines, more visible with lighter lines, but the basic mechanisms are still there. I do not see any contradictions with a simplified theory; momentum changes acts on change in speed always in the same direction (acceleration) but with a variable intensity (TT or reversed taper). Air drag always slows down the rollover and can create a significant deceleration, especially at the end of the rollover of a light line. Big lines are not really disturbed to that point by air drag. What you can rely on to know what is happening is the sag in the rod leg. More sag means more deceleration; a tight rod leg means acceleration and the basic level of sag is linked to the level of speed of the fly leg.

Graeme disagrees with Gordy I suspect (an likely with me as well ) but I do not see on what at this time, or on what it could be because all this is basic physics: change of momentum and air drag on the loop and fly mainly. What else would say George Clooney?

Where does the story of limits for line diameter come from? It is just the expression of the differential equation governing the motion of the line when using linear momentum in the direction of the cast (I do not speak of vertical momentum here). This equation can be written in different forms, and the one I use says that the relative variation of fly leg speed (V) is linked to the relative variation of fly leg length (l). There is a function in between, which depends on line and fly parameters (linear density, drag coefficients), including line section, loop height, etc., called F here. It is written like that:

dV/V = - F * dl/l

dl is negative since the fly leg shortens, so - dl is positive and the line can accelerate if F is positive and conversely, the line decelerates if F is negative. For sake of simplicity, I consider a level line. Now if I set F=0, I find a first limit diameter corresponding to a constant speed situation. Below that diameter the line will decelerate. Interestingly, if I set F=-1, then the deceleration contains an exponential term. So I can calculate another limit diameter which correspond to a stronger deceleration. No need to say that if I set air drag coefficients to zero, then the line accelerates indefinitely (COE).

Such diameters can indicate in which situation the line speed is going to change. Is it extremely precise? No since it depends on different variables and because it is an estimate through a rather simple calculation (if you want details Will, send me your mail address on a board message, I shall provide you with a spreadsheet).

Can I be wrong here? I do not think so as long as we speak about trends, exact numbers are unfortunately not available, they depend on too many things and we still wait the one who will completely describe the physics of the line flight. Lines seem to behave differently, but they do since they are different, which has consequences on momentum changes and air drag. There is no universal behavior apart the impression that most of the time, the line seems to travel at a more or less constant speed, with some changes which are caused by line taper or by thin leaders. Keep an eye on the sag in the rod leg, this is a good indicator of what happens in the fly leg.

Merlin

[Graeme H]

I’m sorry I must have missed your explanation Graeme!!

This post for choices: viewtopic.php?f=8&t=2986&start=10#p48550

This post for why it slows: viewtopic.php?f=8&t=2986&start=30#p48589

This post for why it speeds up: viewtopic.php?f=8&t=2984&start=30#p48510

Innumerable posts on the physics of fly lines.

Cheers,
Graeme