Walter & Group.....
Bruce Richards' answers and comments in Eric's Cook's text. I highlighted Bruce's comments in bold red italics. Also, each of Bruce's comments are preceded by ****.
G.
Gordy,
I agree with what Paul, Mac and others are saying about when and magnitude
of power applied has a great influence in determining the shape of the
loop.
Along these lines, isn’t the thrust nothing more than additional
acceleration at the end of the stroke?
****The way most people do thrust, it is simply an arm extension that
coincides with final rotation, it is not a separate motion that happens
after RSP, at least that isn't what I've seen. In most casts there is a
combination of translation and rotation and they usually happen at the same
time. If rotation is delayed as long as possible, and that rotation is also
matched with some fast hand translation at the same time, that is "thrust",
as I see it... (Bruce)
This past summer, Mac got me
experimenting with later and later rotation. Prior to that, I was
consistently having climbing loops and even had side arm casts (loops in
the
horizontal plane) that would be climbing perpendicularly to the loop
(climbing in the vertical plane). As my rotation was delayed ever closer to
the end of the stroke, the more the loops started to take on a wedge shape.
*****Typically, when rotation starts early it is relatively slow which
doesn't load the rod as it should which causes the rod tip to rise,
throwing the top leg of the loop higher than we like. Delaying rotation
forces faster acceleration initially which bends the rod more keeping it
from rising. Tip path is much straighter which is what causes the wedge
loops. Rotation that starts too slowly, too early is the single biggest
thing we change when improving intermediate and advanced casters. Often it
is the only change needed... (Bruce)
Also, in order to get that rotation later, I had to forget about any kind
of
stop and to essentially stop when I ran out of arm (in the case of distance
casting). I was thinking that this " casting stroke til I ran out of arm"
was doing both; a) allowing me to accelerate longer through the stroke and
*****In reality, delaying rotation typically results in shorter
acceleration through the stroke, not longer. Since there is translation
happening at the same time, the longer the rotation is delayed the more
extended the arm will be, hence, "running out of arm". (Bruce)
b) creating high jerk for transference of
energy (more background on jerk later). I had concluded that a wedge shaped
loop is created when the greatest amount of casting energy is concentrated,
in the direction of the cast, at the moment of RSP (loop formation).
*****True, and this comes from constant angular acceleration which leads to
the straightest tip path. (Bruce)
In the
case of a morphing loop, I assume that the point on the fly line that
coincides with the rod tip at RSP is still accelerating well beyond loop
formation. Is this possible?
*****Yes it is possible. Noel has calculated loop speeds and has found that
it is possible, and likely, that tight, top pointed wedge loops do
accelerate for a time after leaving the rod tip. With increased speed comes
an exponential increase in wind resistance so there is a limit how long
this can last, but it does happen.
Does this violate what we accept in loop speed
is ½ the fly leg line speed or ½ the sum of the fly leg and rod leg speeds?
****No, loop speed remains half the sum of the speeds of top and bottom
legs. The top leg is moving very fast and is subjected primarily to skin
drag, not form drag. As the top leg gets shorter, and lighter it is
continually trying to accelerate. If the top leg is very straight it is
subjected to mainly to skin friction which is minimal so acceleration can
occur. If the top leg is not very straight, it is subject to both skin and
form friction which very effectively prevents acceleration. This is the
main reason that only tight, top pointed loops accelerate, because the top
leg must be very straight. (Bruce)
Can this one point on the fly line still be accelerating even when the rest
of the fly line may be at constant velocity? For this last question, I
think
this would have to be true for the loop to change shape.
*****Assuming you mean the point of the loop, it cannot accelerate unless
at least one of the two legs is also accelerating.... (Bruce)
When I wrote to you about jerk, I failed to give an example for better
understanding. I also forgot to mention that I believe the English refer to
it as jolt. Jerk applies to both acceleration and deceleration. Currently,
I deal with jerk in industrial robotics and CNC controls. We have one
particular CNC routing machine that we cut plastics with at a rate of 1200
inches per minute. The CNC can accelerate from a stop to this 1200 ipm in
about 1 second (without jerk inhibited) as well as decelerate to a stop in
about 1 second. The jerk parameter in the control is dealt with in
percentages, so if I were to adjust the jerk parameter to 50%, the
acceleration on this machine to 1200 ipm would double to 2 seconds.
Remember, jerk is not a percentage (it is the time rate of change of
acceleration), that is just how this particular control inhibits it.
Related
to fly casting; as you can see from the CNC machine example, if a caster
where to possess a high tolerance for jerk (strength to d
ecelerate the rod rapidly, or run out of arm) they would be able to
accelerate longer into the casting stroke.
Eric
*****And we all know that a "hard" stop results in tighter loops than a
"soft" stop. The faster the rod decelerates the less it rotates during
deceleration which keeps the rod tip higher, which impacts where the bottom
leg will be. (Bruce)
Bruce
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Comment: I think Bruce has placed this in perspective. I, for one, will archive this messge string.
Gordy
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