Category: Fly Casting Physics

Rod Loading by Daniel le Breton

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As promised, here is Daniel le Breton’s latest paper on rod loading. I’ve added an addendum which contains results of a simulation to act as an example of how rod loading and rod selection can affect the cast. (Note that there are two documents).

[embeddoc url=”http://wildoutfitting.com/testwp4920/wp-content/uploads/2016/01/Road_loading_Jan5.docx” download=”all” viewer=”microsoft”]

[embeddoc url=”http://wildoutfitting.com/testwp4920/wp-content/uploads/2016/01/Addendum-by-Walter-1.docx” download=”all” viewer=”microsoft”]

Physics of the Fly Rod by Daniel le Breton

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How does a fly rod work? It is amazing machinery which seems to escape common understanding, and there are good reasons why. Fly rods have been developed over more than 100 years, and space age materials and computing science have contributed to improve their design. Impressive modeling of a cast has been performed but the complexity of this approach does not help us understand the underlying basics quickly.

Viewed from the bridge, a fly rod is both a lever and a spring, sometimes we use it as a pure spring (bow and arrow casting), sometimes as a nearly pure lever (leader casting only), and most of the time we take benefit from both functions. The lever allows us to generate a high speed using a simple rotation, but what about the spring?

Casting instructors are familiar with the casting essential of SLP, or straight line path of the rod tip, but the path of the rod tip is only part of casting efficiency. Mechanically speaking, it is most efficient to move an object along a trajectory by pulling along that trajectory. This is why to cast a line; it is desirable to generate speed on the flattest possible trajectory and to align the line with the path of the rod tip, aiming at the smallest possible loop. The flexibility of the spring allows us to achieve that goal with more or less success, and we can say that this property of the spring is complementary to the advantage of the lever.

But there is more than that coming from the spring function. There are hidden properties that I shall try to show you, and you will realize, I hope, that there is some kind of magic in a fly rod.

Let’s start by the beginning: the spring is also a speed amplifier (the lever also has this property, but it works differently). Casting a line with a fly rod is similar to launching a marble by pushing it with a spring along a flat table. Depending on the way we push on the spring (along a straight line), the speed of the marble can be increased up to 40% by comparison to the maximum speed of the push. It can also have no amplifying effect if you do not push properly on the spring: it is a question of timing. On top of that, it is a question of spring and marble characteristics, which goes along with the push. So let’s concentrate on that “spring & marble” system, which is our “rod & line”.

The single characteristic which is important for the speed amplification is the frequency of oscillation of our system. It depends on the natural frequency of the spring and on the mass of the marble. The heavier that mass, the slower our system becomes. So we have to adapt our timing, and this is why DH rods are different from SH rods in terms of frequency (which we usually call speed). DH rods are slower to adapt the tackle to our physical capabilities.

The frequency of the tackle drives its response to our input, and there is nothing straightforward here since we are under what we call “transient conditions”: the spring oscillates just one time up to the launch of the marble. It is rather tolerant. If your timing is not perfect (acceleration followed by deceleration to a stop), then the speed amplification is still significant. You must cast a noodle (extremely slow rod) or a broomstick (extremely stiff or fast rod) to lose speed.

The speed of the marble is also determined by the distance covered during the push (the casting arc): the greater that distance, the greater the marble speed.

The spring function is able to explain the importance of adapting our casting arc to our objective. Most of the time we use a more or less constant timing (longer for a DH rod), and thanks to rod designers the frequency of the tackle we are using is matched to our physical capabilities. This simple approach of spring function allows defining a line scale if you need one. Have we found the basics here (tip trajectory, casting arc, timing)? I think we can say yes, but there is more than that.

Fly rods are hard springs (non linear), which means that they resist more and more to bending as we increase the load or bend: you begin at the tip but you face the butt after, and it is really stiff. This increase in stiffness occurs as you lengthen the line, and this allows us to cast a longer line, because it maintains the frequency of the rod & line system on the high side. Consequently the line speed is also maintained on the high side, which is needed if we want to cast for distance. This is not a small effect; you may get something like 40% again by comparison to a rod having little hardening of its spring, and this comes on top of the speed amplification due to the basic spring effect that we described before. So if you look for distance you will have to choose a “hard” rod, but this may not be useful if you fish at short distance a small stream. It is worth knowing some basic rules: short rods are more “non linear” that long rods. A steeper taper has a non linear effect but it is difficult (it depends on the material) to have a long non linear rod and a short linear one.

If you fish at short distance you may wish to benefit from another strange aspect of the lever. This aspect is due to the spring somewhere, since it is linked to the fact that the tip and the butt have very different speeds during the cast (at the same time). The swing weight of a fly rod (its inertia for a mechanic) is relevant to the difficulty (resistance) to rotate that rod. During the cast the rod deflects and its swing weight diminishes during the acceleration of the butt, which helps reduce the effort for the caster, and it increases again as the rod is decelerated and the rod returns to straight position, which contributes to that deceleration. Swing weight variations are limited most of the time, mostly less than 10% (if the distance between tip top and butt end is reduced by 10%, the swing weight is reduced by 5%; this is just a rule of the thumb). This is why “slow butt action” rods are more comfortable to cast than “stiff fast action” ones, but this is a general statement, not a universal law.

Swing weight seems to be a single number but in fact we can consider that it is made of three components that all have a role to play during the cast. One is linked to the tip and is a major parameter of its natural frequency. The second one is linked to the butt and is important for casting comfort, and the third one is an in between parameter linked to both tip and butt, and it is responsible for the (magic) effect I am going to describe now.

This intermediate component of the swing weight manages the kinetic energy between tip and butt; it tends to counteract the influence of these parts of the rod to our advantage. Let’s describe some phenomenon coming from its influence. As we start casting forward, the tip begins moving backwards for a short while. This is not to be confused with the potential effect of a higher vibration mode (the second one, with a node in the tip), we are accelerating progressively here, and we do not use sharp wrist acceleration. Later in the cast, as we decelerate the butt, the resulting effect is a boost of tip speed, something like 30% for a short line. You get that on top of the 40% given by the spring function, but you get little from the non linearity of the spring under such conditions. So if you are casting a short line, you will appreciate the benefit of this phenomenon. It is the designer’s job to provide this faculty into his rod.

Casting at intermediate distance benefits from both the non-linearity effect and swing weight transfer effect. All in all you may get an additional 30% there too, and that’s good news. If one could make a rod with maximum transfer effect and high non linearity that would be a good one (in fact there are already some on the market).

And this is not yet the end of the story. The intermediate component of the swing weight, by providing more kinetic energy in the tip, takes it from the butt, so it helps to decelerate the butt of the rod just when we need it. At some stage (line length), this deceleration phenomenon is amplified by the effect of the mass of the line being cast. This deceleration effect could take place even if we could stop driving the rod (letting it go at the end of acceleration), this is why I call it the self deceleration mechanism. The mass of the line limits the tip speed boost during deceleration, because there is more mass to move for a limited quantity of kinetic energy available, but then there is a transfer in the deceleration of the butt to our advantage. I am convinced that this is key for tuning a line to a rod, but as usual, the various possibilities in casting style can affect that tuning. It can also explain why we think to cast “stop less” a long line. The weight of the line is helping us to decelerate the rod, up to the point that we do not need to make any effort to stop it if that line mass is large enough.

To conclude, a short summary of the lever and spring functions:

  • Lever: amplifies speed by rotation, allows (because it is flexible) a kinetic transfer between tip and butt to the caster’s advantage
  • Spring: basically a speed amplifier which can be reinforced by a hardening stiffness as load increases.

To me it is not possible to separate these functions completely, they are linked together. At the end of the day, the fly rod has visible and (nearly) invisible properties than can be used by a designer to maximize a desired effect. It has been fascinating me for more than 35 years now, and I think it will still be a subject of investigation for understanding its complexity. I did not describe other properties here, a fly rod has several natural frequencies, the three first being the most influential, and it has some consequences on rod behavior too.

Enough for today!

Daniel, 4 Dec 2015

 

 

 

170 cast vs not 170 cast

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[WS] With respect to rod loading we did have some discussion in this group and the consensus was that “loading” is not a great term and that bending and straightening were better terms to describe what happens with the rod during the cast. I think that saying that a loaded rod is a bent rod is still acceptable.

[DlB]A rod bends because it is loaded by forces: e.g. torque from caster, inertial forces (line, rod weight) and drag (air).Both are linked so it is not a question of “acceptability” but of what you are looking at. Bend speaks to all, load speaks to specialists.

[WS] I like that. In the past I’ve talked about load being a mechanical engineering term that actually makes good sense in this context but most people are not mechanical engineers and are not familiar with this term or its use in this context. Bend is a term everyone understands.

The issue is that saying that a rod is loaded creates the impression that the purpose of the casting stroke is to load potential energy into the rod which can then be released at will to throw the line similar to loading a gun or a catapult. This has a few issues. To begin with the energy for the cast comes from the caster. During the casting stroke some energy is stored in the rod and it is recovered when the rod straightens at the end of the casting stroke but this straightening does not increase the amount of energy that the caster puts into the stroke.

[DlB] But to get that stored energy, the caster had to produce it anyway, of course there is no external generation of energy apart from the caster, then it takes various forms: kinetic (rod, line), elastic (bended/loaded rod), losses (air drag).That stored energy goes back mainly into the line at the end (RSP, some is lost through air drag.

[WS] With a gun the energy generated by the bullet is much greater than the amount of force the shooter exerts on the trigger. If we look at European style distance casting there is little or no recovery of the energy that is stored in the rod by bending it.

[DlB] I cannot see a technical reason for that lack of recovery. How would you explain it? Because of the angle between the rod and the line at the end? For me it goes into the line, or should I say into the fly leg and also into the loop for a 170 style.

[WS] I know we’ve had this discussion before and I don’t think we will come to complete agreement on it. Given that force is a vector quantity we need to apply force in the direction that we want an object to move in order to make it accelerate and move in that direction. If we apply force in any other direction than we intend the object to move in then the object will not move in the direction we want it to move but it will move in the direction we apply force. That should be obvious when we are talking about point masses but the line is a long flexible object so we have to consider that even though we may be pulling the line at right angles to the direction (let’s call this the X direction) we want it to go the line will not move as a point mass in the direction we are applying force, the end we are pulling on will move in that direction but the line following this will form a curve resulting in forces in the Y direction along the curve as well. If we look at rod straightening in a non-170 cast, i.e. we stop the rod so that the rod tip straightens in the direction of the cast, it’s easy to see that whatever potential energy there is in the rod is going to contribute to line speed in the direction of the cast. In the 170 cast the rod does not unload or straighten in the direction of the cast. It unloads at nearly a right angle to the direction of the cast. Only a portion of the kinetic energy that is generated as the rod straightens goes into the fly leg, much of it actually goes into the rod leg and results in the large loops we typically see in the 170 cast. I will attach a picture to make this clearer (I hope).

Another issue is that we can’t do much to prevent the rod from straightening once it has reached its maximum bend. We can perhaps delay the straightening by a few hundredths of a second but short of hitting a solid object with the rod it is going to straighten no matter what we do. One of the main benefits of rod bending and straightening is that it contributes to our ability to maintain a straight line path of the rod tip during the casting stroke. Using the term loading doesn’t really describe this.

[DlB] Above the contribution to a longer and straighter tip path, rod bending allows the caster to put a greater part of the energy he produces into the line. This is why the “flexible lever” is better than the “broomstick” for casting.

[WS] Agreed, but this is really the advanced stuff we are talking about now. Briefly, you are talking about how the spring effect allows us to apply more force to the line over the casting stroke than we would be able to with a “broomstick” or a rigid lever.

I’m comfortable with bending or loading, but maybe bending is more practical for instruction since anyone can see it.

Thanks Daniel! This is a very interesting discussion.

[embeddoc url=”http://wildoutfitting.com/testwp4920/wp-content/uploads/2015/09/170vsnot1701.pptx” viewer=”microsoft”]

 

Superman vs The Flash, Force vs Power

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Force vs power or torque vs power is a difficult concept for many people to understand. Most people who have an interest in cars will have an idea but have a hard time explaining it.

In our Superman vs the Flash analogy, Flash is the only person who is faster than Superman but Superman is capable of generating far more force than the Flash. We know that F=ma which means a=F/m so we should expect Superman to always win in a race. More force equals more acceleration equals greater speed. Right?

So why does Flash win in a race if Superman is capable of generating far more force than Flash?  Is this just comic land ignoring real world physics or is thus scenario possible in real life?

The answer is in the relationship between force, work, energy and power.

If I apply force to an object and get it moving I am doing work on the object. The amount of work I do on the object  is equal to the force I apply times the distance I apply the force (W=fd). When I do work on an object I change its kinetic energy by the amount of work I do on the object. Let’s say I apply one Newton of force on an initially stationary 1 kilogram mass for a distance of 1 meter.  Ignoring gravity and losses due to friction I will do one Joule of work on the mass and, hence, raise its kinetic energy to 1 Joule. Since we know the mass of the object is 1 kg we can determine its final velocity with the formula KE=1/2 mv^2. The velocity of the mass is now roughly 1.4 meters/sec.

The final velocity of the mass is directly related to the amount of work I do on the object. If I do that work in 1 second or 2 seconds it doesn’t matter, the final velocity will remain the same.

This is where power comes into the discussion. Power is the rate at which I do work on an object, i.e. P=W/t. If I do 1 Joule of work on an object in 1 second I will be applying twice as much power than if I do 1 Joule of work on the object in 2 seconds. In the first case my power output is 1 watt, or 1 joule/s, because I increased my energy level to 1 joule in second. In the second case my power output is only 0.5 watts because I increased my energy level by 0.5 joules for two seconds for a final energy level of 1 joule.

Think of power as the ramp up rate of energy and the amount of energy I give to an object determines how fast the object will end up moving. Power is how fast I increase the energy of the object.

Okay this is still confusing because if I apply more force to an object I give it more acceleration and it gets up to speed that much faster so more force still means a greater increase in energy in a shorter amount of time. How is force different from power? And why do I care?

This is where the car buffs say, “But the amount of force (torque) my vehicle can apply to the road varies with how fast my vehicle is moving. The faster my car is moving the less torque it can exert.”

Bingo!

When we measure a person’s strength we measure how much force they are able to exert against an unmoving or slow moving object. When we measure a person’s power we look at how much force they are capable of exerting against an object that is in motion. We measure that value at various speeds and generate a power curve, i.e. how much force can I exert on an object that is moving at various velocities.

A typical power curve for a person can be found here:

Muscle Force Velocity Relationship

This is similar to the automobile analogy where a truck built for towing can exert far more force than a race car on a stationary or slow moving object but the race car is capable of far greater speeds. This is because the amount of force the truck is capable of exerting with its tires on the road (i.e. torque) decreases rapidly as its speed increases. The amount of torque the race car exerts also decreases as its speed increases but not as rapidly as the truck. At speeds of a little over a hundred miles per hour most trucks are only capable of exerting enough force to overcome drag and friction in order to keep the truck moving at a constant speed. The race car on the other hand continues to exert sufficient force to overcome drag and friction and to continue accelerating up to speeds in excess of 200 mph. When the race car reaches its maximum speed it too will top out and will only be able to continue at that speed without further acceleration.

Since power is equal to force times velocity (P=Fv) , at lower speeds the truck has more power but at higher speeds the race car has more power.

When it comes to casting we have another factor to consider. The time it takes to reach peak force. I may be physically much stronger than someone else but if someone else is faster than I am they will reach their peak force more quickly than I will. In casting we want to reach peak force quickly and then continue to apply a constant force to the rod for the duration of the casting stroke until we stop the rod butt. This was described by Bruce Richards in his article about Casting Analyzer Traces. If we take too long to reach our peak force the resulting casting analyzer trace will show a lack of “smoothness” resulting in tailing loop.

For most casting that we do we are capable of reaching the desired force for our casts quickly enough to be smooth and we have sufficient power to generate the line speed we need. These items are more of interest to distance casters or anyone looking to increase their distance cast.

How does one increase power and improve time to peak power for distance casting? I think that is a subject for a separate post which I plan to work on later.

 

Superman vs Flash and, as a casting instructor, why do I care?

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Based on the discussion of F=ma it would sound like the stronger I am the farther I can cast. Twice the force, twice the acceleration, blah, blah, blah, etc., etc.

But if it was that simple then the biggest, brawniest men would cast the farthest and we all know that isn’t the case. Paul Arden may have put on a few pounds in the last couple of years but he is still not the largest person in distance casting and he can still outcast many people larger than he is. At the same time there are many people of slighter builds who can outcast him. Joan Wulff and Lefty Kreh can probably build a two story apartment out of one of my parkas but if it comes to a casting competition between the 3 of us the smart money will be on me to place third.

Those of you who remember Tom White will remember two things. He was very tall and he could throw more line than they sold him (I stole that last one from Lefty Kreh by the way). Most people watched Tom cast and said, “Of course he can throw a lot of line, he has really, really long arms!”. Tom would respond by slipping his arm into his shirt so that only his hand was sticking out of the neck of the shirt and then cast. Even with this encumbrance Tom could outcast most people.

So if it isn’t size, strength or long limbs that make you able to cast far then what is the key?

I discussed this with a friend who has coached a number of Olympic weightlifters. Dan’s response was that it was strength vs power. In physics we talk about force vs power.

How does this relate to Superman vs the Flash? I hope everyone knows the characters I’m talking about. Superman has what seems like an unlimited number of super powers. I recall an episode where he pushed the earth out of its current orbit (super strength), he can fly, he can use xray vision to see what color of underwear Lois Lane is wearing today, and he has super speed. He can travel faster than light even. The Flash, on the other hand, has only one super power. He’s the only person can move even faster than Superman. In a race he leaves Superman sucking oxygen from the vacuum he leaves behind him. He can punch Superman so many times with his left hand that Superman is left begging for a right. Sorry for all the bad jokes but I think you get my point.

Now here’s the question(s) – both Superman and Flash have similar body types – same height, same weight, same glove size, etc. Ignoring the fact that a punch from either one will immediately dissolve you in to atoms, which one would you rather be punched by and why? Superman has the strength but Flash has the speed.

As a casting instructor do you care and why?

Casting Analyzer Traces from Bruce

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Gordy Hill had asked to see what a casting analyzer chart corresponding with various tip paths would look like. The embedded diagram is the response from Bruce Richards. For those of you who are unfamiliar with casting analyzer charts the X axis displays time and the Y axis displays the angular velocity of the rod butt. You can determine the angular acceleration of the rod butt by the steepness of the curve. Additional information about the fly casting analyzer is available here.


BruceCATraces

How much physics do we need to know? by Daniel le Breton

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How much physics do we need to know to understand and improve casting?

I can imagine five levels of understanding, the scale being not linear. This is arguable but this may be useful as a starting point.

First level: efficiency of the cast

Let’s start with Bill Gammel’s five essentials. They all turn around keeping a straight line path: no slack, straight line before casting, proper power application (avoiding tailing loops) and finally adapting arc size to line carry.

Why do we need a straight line path? For line control for sure but mechanically speaking, this allows an efficient transmission of energy from the caster to the line. The energy is given to the line by moving a force at the tip of the rod over a suitable distance. To be efficient the force must be in the same direction than the line. This force is following the tip path and if this path is curved significantly, a part of the work will be spoiled and not transmitted to the line to give it speed. Think about pushing a wagon on rails: you will never try to push perpendicularly to the rails.

If you increase the carry there is more energy to transmit to the longer line which is more difficult to roll out in the distance, so you have two options like increasing the distance over which you move the force, or increasing the intensity of that force. The fact is when you increase the arc; both the path and the force are increased if you keep the same overall timing for the cast, meaning you have to make your cast faster on a larger arc.

The most difficult point to understand is “proper power application”. There is no exact explanation of how to get it (not easy for sure). If you put power too soon you will spoil the tip path (tip moving down), so it is recommended to increase speed progressively, and to delay the wrist rotation at the very end of the cast (use with moderation for fishing casts).

The acceleration phase is in fact responsible for something like three quarters of the final line speed, so it is very important to apply acceleration properly. The deceleration phase is also important since it controls loop size (short deceleration, small loop), but also the degree of ease to stop the rod (long deceleration, easier stop). Mastering the whole process needs training, especially if you change your #4 outfit for a #12 one.

If the students can understand these points, they have the basics.

Second level: how does the line works?

I have explained that in the paper I wrote for answering to Paul’s challenge. Let’s try to make it even simpler. Since a fly cannot cast itself, our ancestors discovered that it was smarter to attach the fly to a “massive” line that would be the carrier and will be able to place the fly in the distance. You cast the line and the fly follows. The trick is to roll over the line which is possible through a rather simple mechanism. As you give speed to the line and stop it to shape a loop, the part of the line which is holding the fly can overtake the part which is stopped. The energy given to the line when launched is transferred to the moving part, and is used to fight air resistance until the fly can land in front of the line. Since the mass of the moving line diminishes it may accelerate if air resistance is not too large. Air resistance being related to the area of line facing the air under speed (you can compare with your own experience if you walk with a front head wind), you have to minimize the size of the loop to use as little energy as possible. Air resistance uses to be the winner and the fly can land softly if it is launched with sufficient speed. If this can be understood, that’ fine for a beginner. Do not use a mechanical principle that the beginner cannot understand or he is not familiar with, avoid any technical word at this stage, e.g. “this is due to conservation of momentum”: nice to impress the girls at the bar but pretty wrong.

Third level

Up to this point, we have not mentioned rod characteristics or the way a fly rod works, and said nothing about what are the real important parameters of the fly line. Does one need to know about these things and how much?

To make a long story short, a fly rod is a “harmonic oscillator”, in other words, it is as if you were pushing a marble on a table in a straight line (e.g. in a V block), but there is a spring (the rod) between the marble (the line) and you, which is attached to your finger. As you start pushing, the spring is contracting because the marble is resisting the motion (imagine a bowling ball instead), but after some time, the spring decides to unload itself and launches the marble. You can imagine that things will vary depending on the weight of the marble and the stiffness of the spring. It will take more time for a soft spring to react and unload. Things will even change if you change the weight of the marble. The characteristics managing the launch speed of the marble are the stiffness of the spring, the weight of the marble, and your input. It is ever simpler than that (for a specialist), the actual mechanical characteristic involved is the frequency of the loaded spring (attach the spring vertically with the marble and make the marble go up and down, then measure the frequency of this motion). It combines both the spring stiffness and the weight of the marble. We are speaking of the role of the “speed of the tackle” here. The “harmonic oscillator” can tolerate a reasonable variation of this speed characteristic and give a suitable speed to the line, this is important to know. Only few casters are able to adapt their cast to the tackle, for most this is not necessary. With experience, rod makers have adapted their tackle to our physical capability to cast. Now it would be fine to find another wording than “harmonic oscillator”.

The key parameters for a fly line are its density per unit of length and its diameter. The first one is easy to capture, a denser liner is thinner and you can test yourself the impact with a sinking line. For a given type of line, the mechanical rule may not be straightforward: the larger the diameter, the easier it is to roll out. You can compare a #2 with #8, which should be convincing

Fourth Level

If you understand quite well what I explain in my large paper (Flycasting 2014), you have this level of expertise.

Fifth Level

If you can understand what modeling is and the principles governing mechanical equations, you have achieved this level of expertise and you can argue against your peers. A good subject is (there are many other subjects of course): what is the use of the elastic energy placed in the rod, for example. Of course, be ready to justify your argument by numbers if you want to minimize the duration of the dispute, but there is no guarantee it will not last.