The Whip Effect

[Merlin]

Hi Bernd

The inertial effect is directly linked to the acceleration or deceleration of the rod butt. Bending a rod with a fixed butt will show nothing in terms of inertial effect, rod unloading under such condition only depends on rod speed (frequency, first mode).

Merlin

[Mangrove Cuckoo]

Merlin,

If there were a hypothetical rod with a soft butt section, but a much stiffer mid section, could you get the inertial effect first, but then only the frequency response from the upper section?

Would it feel different when you cast it? Or would all that take place so quickly as to be undetectable?

[Merlin]

Gary

The inertial effect comes along with spring behavior. When accelerating the butt forward, rod bends under line inertia (its mass) and rod MOI. And as soon as the rod butt is decelerated, the spring energy comes back into the line just as the inertial effect adds forward speed to the line. It is not possible to separate both phenomenon. One indication of the action of inertia may be seen in the speed pattern, if the effect is rather strong then tip acceleration changes (more pronounced with the inertial effect), and you can detect two slopes in speed history. I shall elaborate on that later, with the case of "no line cast" illustrated by Gordy's video and mentioned by Bernd.

Merlin

[Bernd Ziesche]

The inertial effect comes along with spring behavior. When accelerating the butt forward, rod bends under line inertia (its mass) and rod MOI. And as soon as the rod butt is decelerated, the spring energy comes back into the line just as the inertial effect adds forward speed to the line. It is not possible to separate both phenomenon.

Hi Merlin,
I thought Newton ended this to be a phenomenon when publishing his first law?!

Are there any percentages available for the key factors resulting in rod bend?

A bit off topic now, but talking about typical tip path in a fly cast I was wondering how much percentage of the rod bend comes by the air resistance especially against the fly line. Often for the initial tip path videos showed me an initial rising motion (further away from the caster's head) for the tip. I thought that the air resistance against the line is too small (zero at the start...) to start with adequat increase of rod bend in order to move the tip less on a convex path. Then the air resistance increases in square which should help more. Most casters can't add high force application at the beginning while still controlling the perfect increase in rod bend without having the tip dipping at some point. Casting with less force application helps for more control, but the initial tip path easily gets more convex according to my observations.
Having said this I remember how significant the difference in feel is when changing density of a fly line. More density = less surface = less friction by air resistance = the line feels significant lighter.

I would love to cast in space! "Small cast for me, huge cast for the Flycasting Physics forum on SL." (maybe)
Cheers
Bernd

[Merlin]

I thought Newton ended this to be a phenomenon when publishing his first law?!

Second law, Bernd, but if you try to apply it to the fly rod in the classic way you will end up nowhere. This is not the simple "F=ma" but a more sophisticated version which is named the Lagragian equations. And hopefully you get a relationship between inertia (part of it) and butt acceleration or deceleration, and this is far from being straightforward, hence the difficulty there is to give a simple explanation of the inertial effect. Have a look to what was said by 1937: "When spring power expands itself through a fly rod, it creates power by momentum. Momentum goes into casting power up to the point where spring power ceases". And this last point is exactly what I found with modeling.

The figures available are in terms of energy. It should be possible to gather corresponding bending data and show line and rod inertia influences.

The air resistance problem is pretty difficult, at least because it is not so easy to describe the part due to the line and the part due to the rod. Up until now, studies I am aware of carefully leaved that aside (Steve Fry gave up). Maybe I shall have a try one of these days, since apparently, there is something to identify behind (line "feel").

Merlin

[gordonjudd]

I was wondering how much percentage of the rod bend comes by the air resistance especially against the fly line.

Bernd,
I don't know how you would compute the added bend in the rod that is due to air resistance.

Calculating the varying resistance force due to form drag on different parts of the rod would be doable, but how much extra bending it would cause would be much more involved. The added bending as a percentage of the total deflection of the rod may not be that much when compared the deflection produced by the tip force from the line especially for larger deflections.

Calculating the drag force on the line and comparing that value to the F=ma force from accelerating the line should be straightforward.

The problem would be in taking the the tilt of the line into account since the form drag coeffiecient (1.0) is so much larger than the skin drag coefficient (.015). The form drag velocity component would be equal to sin(tilt angle) * horizontal velocity, so the two drag Cd*v.^2 values would be equal for a tilt angle around 7 degrees. The skin drag area would always be larger than the form drag area by a factor of pi so that would required the tilt angle to increase to around 12 degrees to produce the same (drag force)/(m) value.

Gordy

[Paul Arden]

Flycasting Astronauts

[Paul Arden]

By wave I did not mean from rod to line, I meant from butt to tip. To my eyes it often appears to look like this.

Thanks, Paul

[James9118]

Without wanting to get drawn back into this section - has anyone looked at the effect of damping (i.e. resulting from a relax of the grip) on whip?

[Merlin]

James

Although I did not studied that point in details nor looked after publications (if they can exist), I am pretty sure that the "not so tight" grip at the very beginning of a cast and the "relaxed grip" at the end of it moderate inertial effect. It is really effective when one accelerates and decelerates the rod butt rotation with a firm grip. After RSP, when the rod rebounds, butt rotation speed is low and damping associated to it just put inertial effect at a very low level, IMHO. Anyway the main benefit takes place just before RSP.

Merlin