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Gravity and the loop

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Paul Arden
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Gravity and the loop

Post by Paul Arden »

Hi guys, I've been having an offline discussion about the effects of gravity on the unrolling loop - I thought I'd bring it here.

There is a cliff I throw off in NZ overlooking the Waiau river. I park the truck at the top, and throw for distance. I can carry around 105 by standing on the roof. It's very interesting and great fun. However because of obstructions on the backcast, I have to tilt my forward cast considerably below the horizontal.

Why does the loop not plummet to the ground?

Thanks, Paul
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Post by Paul Arden »

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Marc LaMouche
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Post by Marc LaMouche »

hey Paul !

could very well be because of hotter air rising (and lifting the line) from the face of the cliff.

i used to do 3-D archery and a somewhat problem that happened regularly was shooting to a target over a ravine.
if you shot the same target from the same distance in cooler weather you would have to aim a bit higher than necessary for the given distance and conversely a bit lower when the temps where higher.
this is without wind but i'm sure barometric factors would come in to play as well.

the example above was with arrows flying at 300+ fps and at 40 m.
i can only imagine how much more a line would be influenced by this given the slower speed and increased overall surface.

sometimes the term "aim for the stars" just isn't right :D

cheers,
marc

ps- something i've always wondered about with Carlos's pic.
why in the world is he wearing waders at the top of the mountain ? :p
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Post by Paul Arden »

Yes but when the loop has unrolled the line falls and you have to be bloody quick to make your backcast!
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Post by Marc LaMouche »

that's called gravity :p

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Post by Magnus »

I think you need another question Paul :D
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Post by Paul Arden »

OK I'll rephrase it. I'm being told that this: http://www.sexyloops.com/flycasting/loop%20dynamics.pdf
is irrelevant, or a "myth" (by Walter). When I throw a loop off a mountain even with a trajectory below the horizontal, it appears to hang in the air for much longer while it's unrolling as opposed to once it's unrolled.

So something is holding that line in the Y-direction while it is travelling in the X-direction. Once the loop has stopped propagating it falls.

If Noel's discovery is a myth then what's happening?

Thanks, Paul
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Post by gordonjudd »

If Noel's discovery is a myth then what's happening?


Paul,
I too have seen that the y velocity of the propagating loop appears to be slower than that predicted by Noel's terminal velocity values based on the acceleration force due to gravity and the opposing force in the +y direction due to form drag.

When Iget home I will try to find the relevant measurements to get some real values. But from that experiment I remember it did not take much of an upward angle going from the bottom of the loop to the rod tip for the tension in the bottom leg of the loop to apply another upward directed force on a line segment where the loop is joined to the bottom leg.

That would explain the additional upward force at least for that segment, but I do not know how to extend it to cover rest of the line.

If you had a pointed loop then that would also provide some lift as described in the Perkins-Bono paper. My loop was rounded so that was not a factor in my measurements, but it certainly could be in yours as I would assume iyou were casting a floating line with a narrow intial loop and high line speed.

I think I was using a Teeny TS-450 head when I did my measurements. As a result it would not morph into a rising loop shape, but it would have higher line tension to provide the added upward force. That force would be equal to T*sin(phi) where T was the tension in the bottom leg and phi was some positive phase angle.

When the lne straightens out the tension would go to near zero, and then the line should fall with the terminal velocity calculated using the equations in Noel's paper assuming you took the up draft velocity into account.

Gordy
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Post by grhen »

So are you saying that when casting over a cliff it takes longer to unroll than when casting over flat ground?

Or are you talking about the time it takes to drop to the ground after it unrolls? If so this could be because, even though you are casting at a downward angle the line may be rising relative to the ground level.
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Post by Magnus »

Guy

I think Paul is talking about how long it takes for a loop to travel, straighten and fall, compared to dropping the line from the same height.
Shoot a bullet horizontal and drop a bullet at the same time from the same height and they reach the ground at the same time. Is that what happens when we cast?

If a loop takes longer than it would in 'free fall' to fall the same distance one way to explain the difference might be that the caster angled the trajectory of the loop upwards.

The explanation Noel offered says that friction, both 'skin friction' and 'form drag' contribute to the loop staying aloft - he uses the term "lift" meaning the rate of fall, the downward acceleration due to gravity, is reduced - the line doesn't actually rise.

1. Skin friction reduces the downwards acceleration of the loop.
2. This reduction grows with the square of the speed of the upper leg. Thus, a loop travelling twice as fast generates four times the “lift” due to skin friction.
3. This reduction is inversely proportional to the fly line diameter and the fly line density. Thus, “thin” lines of lesser density will generate more “lift” due to skin friction.
4. This reduction is proportional to the drag coefficient for skin friction. Thus, increasing this drag coefficient will generate more “lift” due to skin friction.
5. This reduction is independent of loop radius. Note however that larger loops will decelerate more rapidly leading to smaller Vo and therefore the dependence on loop diameter is really implicit through the speed Vo . The point is that smaller loops are generally launched with higher speeds and therefore they achieve greater initial “lift” due to skin friction.


Imho Paul may be misinterpreting what Walter has said? I think this http://castanalysis.com/links/jam2004.pdf is the paper Walter was referring to and in particular to what they call a 'Climbing Loop'.
They conclude:
)the lift generated by a climbing loop is approximately four times greater than that of a semicircular loop with the same characteristic dimensions. The source of this additional lift is the contribution of form drag on the ‘‘belly’’ of the fly line that has a positive angle of attack. The
negative angle of attack for the falling loop shape results in a net negative ‘‘lift,’’ again due to the form drag on the belly. The symmetrical loops ~circular and pointed! generate approximately the same lift. These results may be readily generalized to other loops shapes.


Noel's note is much less specific and really says that faster loops fall more slowly and narrower loops are more aerodynamic. The Gatti-Bono & Perkins paper contends that loop shape contributes lift. Personally, I'm not clear what they mean by "belly".
By that I mean the line between the loop front and the rod tip, which is usually in a curve with the loop front at one end and the rod tip at the other. The curve is shallower or larger depending several factors but it's always a curve.
How that contributes 'lift' I'm not at all clear - that would be like the line I'm shooting pushing the line in the fly leg up. Personally if I want to prolong the flight of a loop I might try a thinner lighter running line, aim the loop slightly upwards and hit it as fast as I can.
That is not what the Gatti-Bono & Perkins paper seems to say - that says I should be trying to make that chisel shaped climbing loop.

Am I right that within tournament casting distance is either casting from a raised box or standing on the ground? I would assume casting from higher up leads to greater distance - is that true?
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Post by Paul Arden »

Just thinking aloud here but wouldn't one of the things that allows a loop to seemingly defy gravity be a domed tip path? Upside-down loops don't seem to travel very well...

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Post by Magnus »

Is the tip path of an upside down loop not curved?
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Post by Paul Arden »

Yes but towards the ground.
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Post by Bernd »

Hi Paul,

if I got it right - you are comparing aerializing a long line horizontally to aerializing it while aiming the forward loop below the horizontal line.

And you find out that the loop while being in the unrolling process won't drop down as fast as it would when casting horizontally.

Do I get it right ?

I think the line needs a specific minimum speed to "stay resistant against the effect of gravity".
Mostly when the caster gets to his limit in aerializing a long line the line speed in the very end of the unrolling process will not be high enough anymore. The line will start to significally drop while still unrolling.

When you are casting slightly downwards the gravity will be in a better angle to help not loosing too much speed in the very last part of the unrolling process.

So I think the answer is in the loss of line speed for the last part of the unrolling process.

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Post by Paul Arden »

No, what I'm saying is that I believe that an unrolling loop will stay in the air for longer than a line in free-fall. I think that's common knowledge. Many years ago there was a big argument here involving Frank, Paul Burgess, Mike Conner, Tim Rajeff, Carl H, Bruce and a few others. The argument was that a loop had to be cast above the horizontal because it was a projectile - that's what Paul Burgess was arguing anyway. Many people disagreed.

This was the catalyst for Noel and Bruce writing the free-fall line paper you can find archived in the flycasting contents section as well as the loop dynamics PDF. Now I have to say that when I see all those equations I start to think "OK just tell me the results". Which I'm fine with because anyone can cast a line horizontally and drop another and see which lands first, and its pretty clear that a cast one remains in the air for longer.

However a number of people now dispute Noel's paper, it's not that they disagree with it, it's just that they don't see how the loop front can prevent the line behind from free-falling in the y-axis. Walter and Grunde suggest that it's a low back cast that sets up a climbing loop, however I've spent a lot of time throwing off cliffs and mountains - mostly to the sound of Pink Floyd - with a loop often thrown below the horizontal and I don't see that loop plummeting to the Earth either.

So it crossed my mind that maybe it's the curved top leg of the loop that sets up the force that prevents the loop from falling and giving a force in the y-axis - because there must be one. Look at the Sexyloops logo at the top of the page for example, with a curved top leg, imagine line travelling through this curve would be travelling up as well as forward. Now my proposal is if the curved top leg is responsible for lift then wouldn't a curved tip path be responsible for setting up the curved top leg? If that's true then our first essential should in fact be [1] we need a curved tip path, not an SLP.

OK it's just a theory. Shoot it down :p

Cheers, Paul
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