Acceleration

As we progress with our detailed discussions of the physics of the fly cast we go deeper into what we know AND we approach what we do not know !

For those of you who may feel that all this is a bit much, I'll say, "hang on and try to stay with these experts as far as you can !"

I don't pretend to be one of these "experts" ....... just your monitor. One of the monitor's jobs is to ask questions which bring out answers and information from the experts. As we do so, we'll explore the depths of their knowledge. We'll all learn.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Answer to my comments about acceleration in our last message from Stephan Siikaavara :

That's very interesting, it's exactly the opposite for me, I used to think constant acceleration was the thing to strive for and I used to teach it.

We are using a Richards/Perkins casting analyser here in Piteå. I am referring to the smoothness values calculated by the CA.

Constant acceleration is close to what happens when you drop something. Exponental acceleration is close to what happens when you throw something. Are we throwing the line?

I heard about that statement, that you need a constant acceleration to perform a good cast. It does not seem to apply to a distance cast or a high speed cast with a sharp pointy loop. Not for us and not for heaps of friends that throw distance either, and we can cast the occational good loop. (((-:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Your loops may well be better than mine ! Bruce has studied the incredible tight loops of some of our champian distance casters. I havn't

In a way we "throw" the line which carries the lure (fly) as opposed to "throwing" the lure which carries the line when spin casting.

However, when we cast a fly line, I like to look at it the way Lefty described it .... as unrolling a loop to a target. Not exactly the same thing.

What I need the mathematicians to teach me is this : Can we have acceleration as either a constant numerical progression as well as a constant exponential progression ?

Suppose I drop a bullet from the top of a building. It will fall at an acceleration of approximately 32' /sec./sec less the effect of air drag.

Now, I'll shoot that same bullet down from the same position. Obviously, that bullet will hit the ground a great deal sooner than the first one. Is it not possible that both were examples of constant acceleration ? The one shot from the gun perhaps even at an exponentially greater rate of change of velocity ?

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Before deciding on how velocity changes with respect to time we must decide where and how the measurement is being made.

For instance a constant angular acceleration delivered via a rod tip produces very different rod tip velocities wrt time compared to a constant linear acceleration of the rod tip. In addition to that, if we rightly consider only the rod tip (ignoring hauls) then it has a latency due to rod loading/unloading.

Well that is one of the fascinating things about the fly cast. We apply angular force to the rod which then rotates but this results (theoretically) in a straight line path of the rod tip because of the rod's flexibility as the rod loads provided the rod bend matches the casting arc. G.

If constant angular acceleration is considered together with rod loading effects the line segment adjacent to the rod does not normally accelerate linearly and in all probability it is much more like a cubic or exponential velocity increase.

I hope that Walter and some of the other engineering, maths and physics types will respond to this and also consider how the position of line inertia when the cast starts affects where the cast ends and how that is affected by the velocities of the rod tip and the adjacent line segment as they separate at the launch point.

If we could get someone to attach a strain gauge to measure the variable force applied to the line during a cast the answer to this question would be easy because the line mass is sensibly constant. If it was captured on high speed video and synchronised with the visual it would be even better.

From Bruce Richards. His comments in Stephan's text prefaced by **** in italics. :

When I cast something, a rock, a ball of whatever, I do not feel that the most efficient way to do it is by accelerating the object in a constant manner? I think I peak the acceleration pretty clearly exponential once the cast starts to involve my elbow and wrist joint. Do spear casters accellerate in a constant manner and how about baseball pitchers?

******The reason that constant acceleration is important in fly casting, and my not be in other types of "throwing", is that we are using a flexible lever in fly casting. To make tight, efficient loops the rod tip must track in as straight a line as possible. How the rod flexes is a major determiner in rod tip path. CA graphs, coupled with high speed video, clearly show that acceleration that is more constant results in rod bend that yields a much straighter tip path than acceleration that is more exponential. The flexible lever is the key, the other sports mentioned don't rely on a flexible lever so how the object is accelerated is much less critical, although I don't have specifics on that. BR

*****Extreme casts can yield unusual numbers, of course. Large amounts of drag, drift, roatational readjustment can skew the results, obviously. We have not studied all the extremes, our interest to date has been mostly more moderate casts, the kinds that instructors deal with every day. I do think that if you eliminate rotational rod "adjustments" in extreme casts you will find that the casting stroke itself will have very constant acceleration. But, as I said, we still have lots to learn, but learn best from actual data....

Hi Gordy, thanks, very interesting. I see nothing to disagree with, but am not sure I understand the comment below, by Aitor. I have known both Aitor and Stefan for many years, nice guys, and very sharp.

"A constant acceler. applied to a mass that remains constant equals a constant force (F = M x A). During a casting stroke the mass doesn't change, if we apply a constant acceler. to the rod butt the force exerted against the rod tip is constant also so the bend in the rod doesn't increase."

Obviously there is a problem with the statement above. If the rod starts to accelerate from a straight position, as is usually the case, the initial acceleration will cause the rod to bend. If the rod is accelerated without a line attached I would think that a certain amount of bend would be reached and then maintained if acceleration at the same rate was to continue. But, when casting we are pulling on the line with the rod and it takes a while to get it all up to speed. We know that rod bend continues to increase as long as the rod is accelerating, we've measured that with a strain guage. With humans casting I don't think we have the range of motion needed to achieve a state of static rod bend. If the rod/line could be accelerated over a much longer distance I would assume that eventually rod bend would stabilize and stay the same as long as the rate of acceleration remained constant. But within human limits, we don't reach that stage. I haven't thought about rod bend in quite this way before as I'm more interested in real world casting, but will discuss with Noel to make sure I'm on track....

Bruce

Acceleration - more