Lift force acting on a fly line

[Paul Arden]

Ah YouTube must have cleverly displayed it as the next video to watch and when I scrolled back up there it was!



Look closely at the ones at the end; the dangling line doesn't move away from the rod leg until it starts picking up line from the ground.

Cheers, Paul

[Walter]

Thanks Graeme,

I can think of other complications to add to your Ockhams Razor approach:
- The lift supposedly comes from the inclined section of line between the front of the loop and the rod leg. Not all loops have this "sexy" shape but I don't see them falling any faster than the sexy ones. Are they getting lift from other sources that will be unique to each shape? Ockhams Razor would say that's unlikely.
- Paul and others say they can cast farther at higher altitudes. They attribute this to less drag due to the thinner air which seems reasonable to me. But less drag would mean less lift so you shouldn't be able to cast as far.

[Graeme H]

Thanks Walter.

Here are some other things that indicate Ockham's Razor can be safely applied are:

• Side-casts don't veer to the fly leg side.
• The rod legs of side-cast loops don't fall like stones.
• The fly leg in a vertical loop is often angled down towards the loop. Why doesn't that get forced down due to "anti-lift"?
• The back side of the bump in the sexy loop is inclinded in the opposite direction to the front side of the bump.
• Every cast into the wind should soar like an eagle.
• Every cast with the wind should fall like a stone (or at least not climb as high as the corresponding back/front cast does.) Floating lines should soar more than sinking and silk lines.
• The Mulson Wind Cast works.
• Fly lines are not rigid beams (in the engineering sense). Holding some across the palm of the hand does not lift the line even 3 cm beyond the side of the hand.

lift.jpg (71.98 KiB) Viewed 2108 times

Here's a picture of my hand applying lift to a whole fly line, minus the last 6". This line can't even support more than 1 centimetre beyond the spool's edge with only 6" of line mass to lift. How would ~12" of line at the lower leading edge of a loop fare any better, especially facing a stiff wind travelling at v_loop and a fly leg with its own momentum? Even if it's rapidly rotating and being "flung out" (but only vertically) by centripetal force, there's 90' of line to lift from that log Paul is casting from.

Cheers,
Graeme

[Walter]

Maybe lift is caused by vortices around logs?

[gordonjudd]

What he left out was the bit about the line ALSO being supported at the loop's end through tension induced by the fly leg. ...
Tension from the rod tip is sufficient to prevent free fall of the rod leg near the loop, but any drag acting purely in a horizontal direction will build on that existing tension.
and
If Gordy had remembered to include that information, he could have avoided going on about upwards lift in an inclined section of the loop. You were so close Gordy!

Graeme,
I am always interested in getting a simple answer to a complicated dynamics problem, but then there is also Feyneman's admonition that:

The first principle is that you must not fool yourself — and you are the easiest person to fool.
and
It doesn't make any difference how beautiful [or simple ] your guess is, it doesn't make any difference how smart you are, who made the guess, or what his name is. If it disagrees with experiment, it's wrong.

So applying your idea how much upward force is coming from the rod tip and/or drag forces on the rod leg in this frame of Chris Rowen's cast?


Gordy

[Paul Arden]

Hi Gordy,

Chris’ cast is a dynamic roll cast. The backcast line angle was 45 degrees. Don’t you agree that the momentum direction of the fly leg plays a part in loop propagation?

Cheers, Paul

[Paul Arden]

For me it looks a bit like this.

[gordonjudd]

Don’t you agree that the momentum direction of the fly leg plays a part in loop propagation?

Paul,
I think the vertical momentum change in the fly leg will have an effect, but the assumption is the fly leg will soon reach an equilibrium angle such that the weight of the fly leg is balanced by the upward force of form drag on that inclined length of fly leg. Thus I don't think it will have much effect on the loop propagation later in the cast.

You can see the equilibrium tilt angle of the fly leg was about 4 degrees (probably more like 2 degrees if the camera was pointed perpendicular to the roll out path) so I don't think the change in its vertical momentum due to the changing vertical mass momentum flux=rho_l*v_loop.^2 sind(2) would be very big since sind(2)=.035.

Gordy

[Paul Arden]

Hi Gordy,

Don't you think that it's very strange that Chris' roll-cast loop 'lifts', whereas my overhead cast falls? If lift is linked to line speed then surely mine should lift higher than Chris'? And shouldn't the wedge angle on my backcast also produce more lift?



Cheers, Paul

[Walter]

the assumption is the fly leg will soon reach an equilibrium angle such that the weight of the fly leg is balanced by the upward force of form drag on that inclined length of fly leg.

Whose assumption is that?