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Moment of Inertia and the casting stroke

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Graeme H
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Re: Moment of Inertia and the casting stroke

#61

Post by Graeme H »

gordonjudd wrote: Wed Oct 30, 2024 11:59 pm Not simple.

Gordy
Especially if one is determined to make it as complex as possible.

"Don't let perfect be the enemy of good." Perhaps we only really need "good enough" to explain the concept of swing weight, eh?

Cheers,
Graeme
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VGB
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Re: Moment of Inertia and the casting stroke

#62

Post by VGB »

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Merlin
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Re: Moment of Inertia and the casting stroke

#63

Post by Merlin »

Here is the exact (simple) formula of the MOI relative to the base of the cone:

MOI = rho (kg/m^3) * L^3/30 * pi/4 * (D^2+3Dd+6d^2)

Set D=d and you get 1/3*m*L^2

For a fly rod you typically find MOI = 0.17 m*L^2

Merlin
Fly rods are like women, they won't play if they're maltreated
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Walter
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Re: Moment of Inertia and the casting stroke

#64

Post by Walter »

Questions for analytical types out there.

When looking at the moi of the rod alone is it acceptable/sufficient to assume a smooth taper from butt to tip?

Is is okay to assume the rod is solid or do we need to consider it as a thin walled conic frustum?

For the axis of rotation is the butt tip acceptable or would it be best to assume rotation about elbow (wrist and shoulder not rotating) or shoulder (wrist and elbow not rotating). If not the butt then what offset angle from point of rotation to butt tip?

Is a semicircular bend form of the rod acceptable for small tip displacement? What about for larger displacements?

Do we need to include moi of the reel? Especially when rotating about elbow or shoulder.

That’s it for now. If I have other questions I will post them as they come up.
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Torsten
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Re: Moment of Inertia and the casting stroke

#65

Post by Torsten »

Walter wrote: Wed Oct 30, 2024 5:16 pm Torsten,

If I keep the same simplifying assumptions:

1) 9 foot rod with a swing weight of 70
2) the rod does not bend during the casting stroke
3) the line is straight and remains horizontal throughout the stroke
4) rotation about the butt

Then yes, the results would be the same.
I've computed charts with the parallel axis theorem and above assumptions:
MOIs.png
Line is like the rod a long, slender, rigid cylinder, line is always horizontal above ground.
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Walter
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Re: Moment of Inertia and the casting stroke

#66

Post by Walter »

Torsten,

Thank you so much for that.

Two things come to mind:
1. Obviously the moi of the line changes during the stroke so I hope we can put that to rest.
2. This is a different result than what I show in post number 2 so my simplifications were too extreme. I don't think eliminating the y component of force on the line accounts for that difference. I will try a different approach and let you know the result.

I think we all know how intuition works in physics, but I'll throw this out there anyway. I'm attaching a copy of your slide from post number 56 for reference. Looking at this slide and your result above this would indicate that the moi in the top part of the diagram has a significantly greater moi than the lower part. Viewing the line as a stiff rod it seems to me (totally intuitively so I could be out to lunch) that the bottom figure would have greater moi given the mass distribution of the line relative to the point of rotation.
MOIs.png
MOIs.png (2.97 KiB) Viewed 275 times
"There can be only one." - The Highlander. :pirate:

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Walter
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Re: Moment of Inertia and the casting stroke

#67

Post by Walter »

Torsten,

Actually, I’ve been thinking of the cast proceeding from left to right. If we reverse it so that 180 degrees is the start of the cast and 0 degrees the end of the cast then that would make sense intuitively as well as calculated.

Sorry for the confusion.

Walter
"There can be only one." - The Highlander. :pirate:

Physics for physics sake. Faith for casting sake.
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Merlin
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Re: Moment of Inertia and the casting stroke

#68

Post by Merlin »

Walter
When looking at the moi of the rod alone is it acceptable/sufficient to assume a smooth taper from butt to tip?
Yes it is
Is is okay to assume the rod is solid or do we need to consider it as a thin walled conic frustum?
You can consider it solid if you adapt the density and Young modulus of the material to compare with an actual rod (mass, stiffness).
For the axis of rotation is the butt tip acceptable or would it be best to assume rotation about elbow (wrist and shoulder not rotating) or shoulder (wrist and elbow not rotating). If not the butt then what offset angle from point of rotation to butt tip?
I would chose the butt otherwise you have to consider the MOI of forearm, arm, etc.
Is a semicircular bend form of the rod acceptable for small tip displacement? What about for larger displacements?
Even if it is wrong I think you can take this assumtion to get an estimate of MOI variation for the rod.
Do we need to include moi of the reel? Especially when rotating about elbow or shoulder.
I would forget it for the time being.

When simulating the casting arc, you should consider the angle at line launch as the limit for calculation, take 45 degrees from vertical as a maximum.

Merlin
Fly rods are like women, they won't play if they're maltreated
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Walter
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Re: Moment of Inertia and the casting stroke

#69

Post by Walter »

Hi Torsten,

I've done the exercise using integration.

Conditions:
1. 10 m of line as a stiff rod
2. line remains horizontal during the cast
3. rod length = 2.75 m, rod swing weight = 70
4. no rod bend

Results are:
moi of rod and line by integration.jpg
moi of rod and line by integration.jpg (45.98 KiB) Viewed 150 times
Thanks for your help. Next step for me is to add slope to the line.

Walter
"There can be only one." - The Highlander. :pirate:

Physics for physics sake. Faith for casting sake.
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Walter
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Re: Moment of Inertia and the casting stroke

#70

Post by Walter »

Moving in baby steps here. I've done the calculation of the MOI of rod and line adding tilt to the line. In this case the line is considered to be a stiff rod that forms a straight line from the rod tip to the "fly". I've assumed that the fly end of the line remains at the same y component throughout the cast. Pictorially this looks like the following:
line as rod with tilt.jpg
line as rod with tilt.jpg (14.51 KiB) Viewed 107 times
I know this isn't what the line does during the cast but... baby steps...

This time the moi of the rod and line was calculated using the parallel axis theorem for those who may be wondering. Same assumptions as previously but with line tilt assuming one end of line follows the rod tip while the other remains at the same y component throughout the casting stroke.

Results as follows:
moi of rod and line with tilt.jpg
moi of rod and line with tilt.jpg (47.66 KiB) Viewed 107 times
Next step will be to add a more realistic line shape during the cast.
"There can be only one." - The Highlander. :pirate:

Physics for physics sake. Faith for casting sake.
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