loop + line speed - due to pullback

[Merlin]

Hi everybody

There is a point to remember about line size. Let's take floating lines for example: there is a relationship between loop size and line size defining the limit for a level speed at the beginning of the flight (see the graphic posted by Gordy, for different lines, the speed appears to be constant at the beginning of the cast). A smaller line size will decelerate instead of flying at constant velocity (at the beginning), and this is the reverse situation for a higher line size (accelerate).

In practice, I find that this limit is around a number four line with my rods (on the high speed side and tip action side). You know you cannot expect a number three to behave like a number six in terms of distance with a given loop size.

Merlin

[VGB]

Gordy

You know better than that! The net acceleration due to gravity and drag effects approach zero, but there will definitely be some finite terminal velocity. If all terminal velocities were close to zero it would be safe to jump out of an airplane without a parachute.

Thank you for sight of the Perkins paper, I shall read it properly tomorrow when the rain returns.

It is the velocity going to zero turns you into strawberry jam, nobody has ever died due to heights, only grounds. Similarly the Perkins paper is looking at maximum velocity of the fly line prior to impact with the ground which is a correct use of the term terminal velocity. Bearing in mind that:

"In fluid dynamics an object is moving at its terminal velocity if its speed is constant due to the restraining force exerted by the fluid through which it is moving."

A discussion of terminal velocity does not apply to a fly line when the cast is constrained by the line and not the air All normal casts end with zero velocity because they are constrained, the speed primarily varies dependant upon the mass distribution of the line and the tension on the line.

That shows the terminal velocity of a floating 12 wt line will be higher than for a two weight line because the linear mass density increases as the square of the diameter while the drag only increases in proportion to the diameter.

That is what I have been saying all along, the mass is the dominant factor not drag. Similarly the Lindgard DT lines are not coming to a halt because of drag and the level lines do not stop at all.

Contrary to what you might think, the mass density of the line is a key part of all of this.

The only thing I have been saying different is that where you put the mass is more important than mass density.

One of the things that gets up my hooter is when you quote a single line and miss out the context. You have just put this from one of my posts:

What I have been trying to get across is that mass density is not very important

When I went on to say this:

However, mass profile is what we should be looking at

You are using rho_l liberally in your explanations, where I use mass profile. The reason I do this is because rho l is a property of a material and its construction. It tells you about the material at a given point not how much of the material is there.

If you tell me that you are using a material that has a mass density of 1kg/m, it is useful information. But to put it in the context of a line construction, it does not tell me whether 900g is stuck in 10cm and the remaining 100g is a thin filament. Knowing where that mass is concentrated is vital for understanding how your fly line will perform.

To make life more complicated. For a sinktip you would encounter different values of rho_l in the line and for for parts of the line you may have 2 values of rho_l where materials overlap. I understand it has been kept to a single material to keep the model manageable but I have not seen that constraint stated and why it is misleading to elevate the importance of rho_l.

It is a question that most people would get wrong because we tend to think of wind resistance more so than mass density when thinking about what determines terminal velocity.

I have been saying all along that wind resistance (drag) is not the dominant factor. Perhaps people think that because of statements like this being misunderstood:

For most of the fly propagation distance these two competing forces (changing mass and aerodynamic drag) tend to balance each other out,

Mass density is why the net effect of drag on a sinking line is less than it is for a floating line. Same thing for ping pong balls and golf balls.

Put a ping pong ball and a tennis ball of the same size in a wind tunnel and tell me which one has the largest drag co-efficient and exerts the most drag force.

I agree with Magnus, Lindgard presents a concept not a real life model.

Vince

[VGB]

James

That's a simple one Vince - the floater will have the greater diameter assuming they are the same weight.

You are assuming that I will make them both solid and that I am using only one of my choice of materials. Look at the material densities again; what if I make my floater out of the denser material but put voids in it. Then make my sinker out of the less dense material bt with no voids.

Like I said previously, if you have exactly the same weight of two materials, one of which floats and one of which sinks, then there is no possible way that the sinker can occupy more volume - none. I can't see why you don't get this .

You are moving the goalposts, I was designing a sinking line of the same mass density and volume as a floating line but with higher drag. That's where we started from.

I reckon we could crack this problem over a beer when I unleash my new square sinking line

Vince

[VGB]

Merlin

Let's take floating lines for example: there is a relationship between loop size and line size defining the limit for a level speed at the beginning of the flight (see the graphic posted by Gordy, for different lines, the speed appears to be constant at the beginning of the cast). A smaller line size will decelerate instead of flying at constant velocity (at the beginning), and this is the reverse situation for a higher line size (accelerate).

You have intrigued me, what is the property of the lne that causes this effect?

Vince

[James9118]

Hi Vince,

You are assuming that I will make them both solid and that I am using only one of my choice of materials. Look at the material densities again; what if I make my floater out of the denser material but put voids in it. Then make my sinker out of the less dense material bt with no voids.

No, I'm not - If you make the floater out of lead then I'm fully aware that you're going to have to put voids in it - enough voids to make the overall density < 1g/cc :p .

Do you agree that there's a threshold that determines sink or float i.e. 1g/cc. The density of the material used to make the line has no significance because for it to float it must me 'diluted' to < 1g/cc (by the addition of voids). The sinker must be > 1g/cc.

Perhaps we should chuck some numbers at this?

You are moving the goalposts, I was designing a sinking line of the same mass density and volume as a floating line but with higher drag. That's where we started from.

I don't think I've moved the goal posts at all. If the lines weigh the same per unit length then the sinker will have less drag because it will have a lower diameter - irrespective of what material are used in the construction of the lines.

Probably one for another thread but if I make my floater out of silicone and sinker out of solid PTFE, which one would have the greater diameter given that silicon has twice the material density of PTFE.

What's your answer to your question. As stated I believe the floater will have the greater diameter in this (and all other) case. Perhaps we should run the numbers in another thread :kungfo: .

[VGB]

James

I probalby owe you an apology for yanking your chain with esoteric solutions but this is a common misapprehension:

No, I'm not - If you make the floater out of lead then I'm fully aware that you're going to have to put voids in it - enough voids to make the overall density < 1g/cc

No matter how much air you blow in it the density of lead will always remain at 11340 kg/m^3. If you try to determine its density with voids in, you will get a misleading figure.

One of the reasons I have ducked the maths is that you have to determine the True or Absolute Volume of what you are measuring when you include voids in a material. If you look at Figure 2 in the link below you will see why it is so difficult to hang numbers off:

http://www.particletesting.com/docs/density_determinations.pdf

However, how much air you blow into your lead will change its surface area and determine its drag force or buoyancy without changing its density.

Linear density is determined as mass/length not surface area which is why I suspect Lindgard had to change the line radius to get mass and drag values. They are good for proving his tapering concept but do not apply to a real world line because their construction is more complex.

Vince

PS Was there a beer on this? :p

[James9118]

James

I probalby owe you an apology for yanking your chain with esoteric solutions but this is a common misapprehension:

No, I'm not - If you make the floater out of lead then I'm fully aware that you're going to have to put voids in it - enough voids to make the overall density < 1g/cc

No matter how much air you blow in it the density of lead will always remain at 11340 kg/m^3. If you try to determine its density with voids in, you will get a misleading figure.

One of the reasons I have ducked the maths is that you have to determine the True or Absolute Volume of what you are measuring when you include voids in a material. If you look at Figure 2 in the link below you will see why it is so difficult to hang numbers off:

http://www.particletesting.com/docs/density_determinations.pdf

However, how much air you blow into your lead will change its surface area and determine its drag force or buoyancy without changing its density.

Linear density is determined as mass/length not surface area which is why I suspect Lindgard had to change the line radius to get mass and drag values. They are good for proving his tapering concept but do not apply to a real world line because their construction is more complex.

Vince

PS Was there a beer on this? :p

Vince,

By 'voids' I thought it was clear I was talking about sealed volumes within the denser material (otherwise you would not achieve a density reduction).

We're obviously going to have to do this bit by bit, so here goes:-

I've got 11.43 g of natural lead, it's volume is 1 cm^3. This will sink - do you agree?

I now take the same 11.43 g of lead and cast it into a balloon, the balloons volume is 11.44 cm^3. My lead balloon will now float - do you agree?

I'm going to deserve a beer after this .

[James9118]

I can put a void through the middle of a fly line and the mass will remain the same when the diameter increases. I was accused of being unrealistic but putting more hollow glass beads in a floating line will increase its diameter with a minimal effect on its mass

Vince,

It's clear from these posts that you are also talking about including a sealed 'void'.

You are assuming that I will make them both solid and that I am using only one of my choice of materials. Look at the material densities again; what if I make my floater out of the denser material but put voids in it. Then make my sinker out of the less dense material bt with no voids.

The maths of this is very, very simple - if the density of the composite material is <1 g/cc then it will float. Hence my lead balloon above will float. Do you disagree?

Look at the material densities again; what if I make my floater out of the denser material but put voids in it. Then make my sinker out of the less dense material bt with no voids.

Lets do the maths on this then - supply me the densities of the material that you want to make the respective floater and sinker from.

[James9118]

Incidentally, don't be thinking you can make your sinker with massively 'open' porosity - that really wouldn't just be moving the goalposts, it would be changing the game! Open porosity, when discussing immersion in water, does not count as additional volume.

[Merlin]

You have intrigued me, what is the property of the line that causes this effect?

Vince,

One one side, the energy transmission depends on the linear mass of the line (the line mass per unit of length), and on the other side the energy loss is linked to the diameter of the line. The result (acceleration, constant velocity, deceleration) depends on these two components. For each line size, there is a loop size limit corresponding to constant velocity.

For the same loop size, a heavier line size will do better than a lighter one.

Merlin

[VGB]

James

No tricks, cubs honour:

The maths of this is very, very simple - if the density of the composite material is <1 g/cc then it will float.

I will make my object from 65,000 tons of mild steel at a mass denisty of 7.5g/cc and make it boat shaped a bit like this.

http://www.google.com/imgres?....rl=http

Closed or open voids, your choice. At a density of 7.5g/cc, my mild steel boat shaped thingy will float even though my mild steel has a mass density >1 g/cc.

I will now melt down the boat shaped thingy into a 65,000 ton cube and launch it with tears in my eyes as it sinks. It still has a mass density of 7.5g/cc.

The reason I can do this is because buoyancy is a force acting over an area.

By 'voids' I thought it was clear I was talking about sealed volumes within the denser material (otherwise you would not achieve a density reduction).

The density of your material does not change you are spreading it over a wider area.

If I put my sinking brick and floating boat shaped thingy in a huge wind tunnel I will find that with its higherCd and larger surface area facing the flow that my sinking brick has more drag than the floaty boat.

regards

Vince

Can we postpone the beer until I feel better, I had football stress overload last night.

[VGB]

Merlin

Thank you for your explanation. I am comfortable with the momentum/drag effects but this is the bit I was getting to:

For each line size, there is a loop size limit corresponding to constant velocity.

What drives the loop size limit?

regards

Vince

[Merlin]

Vince

It is the relative importance of gain from momentum versus loss from drag: a smaller loop size will improve the speed. Linear density goes along with the square of line diameter, drag forces with line diameter and loop size. The relationship is not straightforward but if you use a rather simple model, you can identify one.

Merlin

[gordonjudd]

What drives the loop size limit?
and
The relationship is not straightforward but if you use a rather simple model, you can identify one.

Vince,
Dr. Robson used a simple model that used a rectangular loop shape to come up with a loop size limit where the drag on the loop would equal the momentum change for a floating line.

He calculated that the drag for a loop in a .7mm diameter line and a 2 meter loop (with a square front) would equal the positive acceleration that comes from the momentum change, and thus that square loop would:

collapse in front of the caster.

It would need to be larger than that for an elliptically shaped loop that has much less form drag.

Gordy

[VGB]

Thank you Merlin and Gordy (we were posting simaltaneously). I had read the term "loop size limit" to mean something different.