Constant Acceleration

[Walter]

Is the Casting Analyser still somewhere in use?

Greetings,
Torsten

I came across mine while going through some old stuff recently. Not sure if the palm pilot still works. I’ll see if I can find it again and see if it works…

[Merlin]

There are several casting analyzer records showing nearly constant rotation acceleration, but more frequently the acceleration level increases during the cast. Tip acceleration records may exhibit a constant acceleration level at the end of the casting phase, but there is no large technical advantage to get such a configuration (protecting a fragile fly?). There is no such constant force at rod tip corresponding to a given deflection since all parameters change during a cast, deflection included, at any location of the rod shaft.

In the case of rotation acceleration the best results (high line speed, SLP) should occur when the caster uses an acceleration level at the limit of the kickback all along the cast. The level of acceleration should then increase as the rod deflects more and more (harder spring, less keen to kick back). However it is not easy to simulate, since this should correspond to a time step by time step calculation looking for kick back limit at each time point. Using so called constant but moderate rotation acceleration is clearly on the safe side once you have moved from the initial kickback. That is mostly applicable for fishing conditions and cool casting style.

I cannot see an advantage by using a mostly constant acceleration for rod butt rotation in distance competition. One should use an increasing level of rotation acceleration of rod butt, which is observed in casting records.

Merlin

[Mangrove Cuckoo]

Merlin,

So... is the dreaded "dip" in the rod tip, during the stroke, that is suspect of causing tailing loops, essentially the same as a "kick back", just later than at the very beginning of rod rotation?

Or is kick back only something that happens to a stationary mass when something happens in a beam elsewhere?

Thanks!

[Merlin]

Yes Gary

A dip in the tip path (expected to create a tail) can be obtained by a sudden increase in acceleration of rod butt rotation which makes the rod bend backwards (that is a kick back in fact). Since the rod is rotating forward with speed, the rod tip deviates from the original path. It corresponds to the classic mistake of beginners who accelerate too much too soon and get a tail. It justifies to place the highest rotation speed late (wrist/fingers action) in the cast, using the larger stiffness of the rod as it is bent to limit kick back during the cast (in that case the potential dip can be cancelled as the extra bend is such that the tip deviation takes place in the direction of the path).

At the very beginnng of the cast, the kick back can be hidden by the forward rotation of the rod, and this is why one should accelerate gently at start before using a large acceleration of rod butt rotation.

Merlin

[Bernd Ziesche]

What happens if the acceleration is not constant, but increases through the stroke?

John Waters wins GOLD!

[Few]

To instantly start at a target constant* acceleration requires a huge jerk, a jack rabbit start. There's no such thing as constant acceleration in initiating any human movement. Further, in fly casting smoother is better

*Constant as in remaining at a fixed m/s², if I correctly understood James'

Correct. If you differentiate a perfectly linear increase in velocity you obviously get a step function as the acceleration. If you then differentiate the acceleration (the step function) you get something that is sometimes referred to as 'jerk', as you state above. The differential of the step function has a huge initial 'jerk'.

This is fairly academic though - the human body cannot just jump to an acceleration rate.

James.

Sorry to resurrect an old thread; I'm a new subscriber. If there's any continuing interest in this thread, could someone help me clarify my thinking? If I drop an initially stationary ball (in a vacuum), it accelerates downward at 9.8 m\(\cdot \)s\(^{-2}\) because of gravity. A graph of its velocity would show zero velocity until the ball is released, followed by a linear increase in velocity magnitude. Where is the expected "jerk" in this constant acceleration? If I step off a roof I don't feel a sudden jerk downward (unless I'm Wile E Coyote). I understand the result of differentiating the velocity graph, but in what sense does the ball experience a jerk?

Thanks in advance!
Few

[Merlin]

Hi Few

In your example there no acceleration jerk since the ball is always subject to gravity, released or not.

The situation is different when you consider accelerating the rotation of a fly rod, starting from zero.

Merlin

[Paul Arden]

Ah… thanks! That certainly makes it clearer.

[Few]

casting machine drawing.jpg (23.64 KiB) Viewed 335 times

Here's a thought experiment I devised as I try to wrap my head around how one might physically realize constant acceleration (while acknowledging that it's not widely adopted as an appropriate goal). If a medieval casting machine were constructed out of a mass \(m\), supported by a trap door, and hung off a rope wrapped around a pulley, as I’ve tried to sketch above, and a fly rod were rigidly mounted to the pulley, what would happen if the trap door were suddenly released? I’m assuming the fly line is loosely held in a straight back cast configuration before the experiment, and the little circle to the right of the fly rod acts as a stop that defines the end of the casting arc.

If the trap door opens “instantaneously,” then it no longer counteracts the force due to gravity, and the mass \(m\) feels a net downward force that abruptly jumps from zero to \(m\cdot G\) (where \(G= 9.8 m\cdot s^{-2}\)). That force imparts a torque around the pulley’s center of rotation which, for simplicity, I arranged to align with the rod’s reel seat. Would the torque jump from zero to some fixed value that depends on the pulley geometry? If so, what happens to the angular velocity of the rod butt?

If the rod were perfectly rigid, would its angular velocity start at zero and linearly increase until the rod strikes the stop? And if so, would that constitute a jump from zero to constant angular acceleration (as I interpret Mr. Richards as prescribing)?

If a more realistic flexible rod were substituted, would the applied torque remain constant but yield a time-varying angular acceleration? It seems to me that the effective length of the rod would decrease as it bends, and that would temporarily reduce its moment of inertia. In addition, the rod's compliance would complicate the mechanical impedance the rod presents as a load. The casting analyzer measures what the rod does, not what the applied torque was.

How “jerky” would the cast be? Calculus says there would be a big jerk when the trap door is released, but in my mind’s eye it’s not obvious the result would be jerky casting motion. My intuition may just be faulty. In any case, I'm not trying to be a jerk about it…

Few

[Paul Arden]

The trebuchet handy casting device. Can also be used to throw fish back in the river. One of my friends built something that could be modified…

IMG_4605.jpeg (74.83 KiB) Viewed 318 times

He did this to take out the human element, so that we could discover how far a cast could really go! It turns out about 73’ with a tailing loop.

To answer your question: pass! But I think what would happen is initially the tip would travel backwards. What happens after this I don’t know. But if it’s anything like Jon’s machine it’s going to be ugly

Cheers, Paul