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Are Pulley bridles more tolerant to stretch/shrink?

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Sandras
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Are Pulley bridles more tolerant to stretch/shrink?

Postby Sandras » Sat May 05, 2018 7:58 pm

reo.JPG
reo.JPG (19.13 KiB) Viewed 3617 times
My understanding may be wrong and I welcome you to correct it.
I paste the bridle of Reo as an example of a non pulley kite.

1) For pulley less kites only one of the A1,A2,A3,A4 carries the load of the front lines (the rest have a very small load that just keeps the shape of the kite)
A1 (and the back lines) is carry the load at the maximum sheet in and A4 carries the load when fully de-powered.

2) A bridle with pulleys, because it can move it divides the load in the bridle lines.

My though is that a bridle with pulleys is less sensitive to stretch/shrinking exactly because the pulley can move and redistribute the load. That is that even if +/- a bit it would still work better than a pulley less bridle.

Is this thinking correct? If not, what's wrong?

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Re: Are Pulley bridles more tolerant to stretch/shrink?

Postby Bushflyr » Sun May 06, 2018 7:05 am

No, all the front lines will share the load regardless of pulleys or no.

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Re: Are Pulley bridles more tolerant to stretch/shrink?

Postby grigorib » Sun May 06, 2018 7:28 am

Bushflyr wrote:
Sun May 06, 2018 7:05 am
No, all the front lines will share the load regardless of pulleys or no.
Of course they won't equally distribute the load without pulleys if angles between parent branch and child branches are not exactly the same. in above picture if you pull all the way on one of the front lines the maximum load will be on A1

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Re: Are Pulley bridles more tolerant to stretch/shrink?

Postby grigorib » Sun May 06, 2018 7:35 am

Sandras wrote:
Sat May 05, 2018 7:58 pm
reo.JPG

My understanding may be wrong and I welcome you to correct it.
I paste the bridle of Reo as an example of a non pulley kite.

1) For pulley less kites only one of the A1,A2,A3,A4 carries the load of the front lines (the rest have a very small load that just keeps the shape of the kite)
A1 (and the back lines) is carry the load at the maximum sheet in and A4 carries the load when fully de-powered.

2) A bridle with pulleys, because it can move it divides the load in the bridle lines.

My though is that a bridle with pulleys is less sensitive to stretch/shrinking exactly because the pulley can move and redistribute the load. That is that even if +/- a bit it would still work better than a pulley less bridle.

Is this thinking correct? If not, what's wrong?
You're absolutely right on #1 but I'm not sure why we'd assume for #2 that equally distributed load on leading edge will be "better"

With pulleys you introduce complexity and wear which are risk factors (remember Liquid Force 2012-2013 using wrong type of Ronstan pulley which would get stuck with sand, the bridle would cut through pulley plastic to the axle which consequently will cut the bridle)

Pulleyless wing less might not feel as smooth to steer but it's what they marketed as "direct feeling". Works well for many just as stick shift vs. automatic transmission

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Re: Are Pulley bridles more tolerant to stretch/shrink?

Postby Sandras » Sun May 06, 2018 10:19 am

Bushflyr wrote:
Sun May 06, 2018 7:05 am
No, all the front lines will share the load regardless of pulleys or no.
I can't agree on that.
Just look at the drawing below.
no pulley.JPG
no pulley.JPG (43.21 KiB) Viewed 2002 times
Let's assume that at this AoA (angle of attack) it is as you describe it. All bridle lines have load.
Now imagine the AoA increasing (that is the trailing edge going down - back lines pulling more). What will happen to 6 in that case? I believe it will go slack, no?
Now imagine closing even more, now 4 goes slack.
(When I say slack, you wont see it flapping because the leading edge will move/flatten and compensate but this is a very small load, just tensioning the line)

The more the angle of attack increases, the higher the pull in the lines.
My assumption is that in fixed bridles, line 5 will stretch more than the others disturbing the initial tuning. (e.g. more chances to front stall depowered.)

Reading again my first statement and looking again at the drawing above, it now seems to me that it is not a single line that carries the load but there must be a few slack lines in fixed bridles depending on the AoA.

@grigorib: You added the word "equally" which makes a big difference. Yes with the word "equally" everything is wrong. But dividing the load does not have to be equal!

Let's do the same with a pulley system
pulley.JPG
pulley.JPG (41.66 KiB) Viewed 2002 times
Assume now all lines have a load (not the same but have a few Newtons - not just the deforming of the front tube)
Now you increase the angle of attack. (that is the trailing edge going down - back lines pulling more)
Instead of line 4 going slack, the pulley will move closer to 4 and change the whole geometry giving some load to line 4.
Therefore, this changing geometry makes the system more tolerant in small stretches/shrinkages. (Pulley will move and compensate - as long as it does not hit a stopper)

@grigorib: I'm not checking if one system is better than the other, I'm not even checking which one is more durable or wear resistant.
All I'm comparing is that if a fixed bridle system gets a bit out of tune because of stretches/shrinkages it will have a bigger effect compared to a pulley system with a bit of stretch/shrink. I see the pulley system as more autoadjusting.

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Re: Are Pulley bridles more tolerant to stretch/shrink?

Postby foilholio » Sun May 06, 2018 2:08 pm

Lines always shrink not stretch. Not much has been done to design around shrink. Pulleys make lines shrink lots, because of bending and friction. I would say the second picture is going to have more issues. The fixed LE bridles on foils kites tend to shrink little and evenly, mainly I think because of the higher and more consistent load at the LE. I would say because of the rigid LE on inflatables this is less of a problem too. The main problem inflatables have for shrink is the bridles are too thick.

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Re: Are Pulley bridles more tolerant to stretch/shrink?

Postby grigorib » Sun May 06, 2018 3:36 pm

Sandras wrote:
Sun May 06, 2018 10:19 am

@grigorib: You added the word "equally" which makes a big difference. Yes with the word "equally" everything is wrong. But dividing the load does not have to be equal!

@grigorib: I'm not checking if one system is better than the other, I'm not even checking which one is more durable or wear resistant.
All I'm comparing is that if a fixed bridle system gets a bit out of tune because of stretches/shrinkages it will have a bigger effect compared to a pulley system with a bit of stretch/shrink. I see the pulley system as more autoadjusting.
Agreed on load distribution.
As of stretching - I think for thick LEI bridles the stretching is minimal. Both uneven load or wear/pressure from the pulley would stretch/wear the bridle, but again - on LEIs they’re thick enough. I think the canopy stretches and gets damaged before bridles do.

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Re: Are Pulley bridles more tolerant to stretch/shrink?

Postby Bushflyr » Sun May 06, 2018 6:30 pm

Sandras wrote:
Sun May 06, 2018 10:19 am

I can't agree on that.
Just look at the drawing below.
no pulley.JPG
Let's assume that at this AoA (angle of attack) it is as you describe it. All bridle lines have load.
Now imagine the AoA increasing (that is the trailing edge going down - back lines pulling more). What will happen to 6 in that case? I believe it will go slack, no?
Now imagine closing even more, now 4 goes slack.
(When I say slack, you wont see it flapping because the leading edge will move/flatten and compensate but this is a very small load, just tensioning the line)

The more the angle of attack increases, the higher the pull in the lines.
My assumption is that in fixed bridles, line 5 will stretch more than the others disturbing the initial tuning. (e.g. more chances to front stall depowered.)

Reading again my first statement and looking again at the drawing above, it now seems to me that it is not a single line that carries the load but there must be a few slack lines in fixed bridles depending on the AoA.
Yeah, just no. :-?

Don't "ASS-u-me" just do the vector analysis. It's simple for a string or group of strings. As we say in engineering, "you can't push a rope."

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Re: Are Pulley bridles more tolerant to stretch/shrink?

Postby grigorib » Sun May 06, 2018 6:49 pm

Bushflyr wrote:
Sun May 06, 2018 6:30 pm
just do the vector analysis. It's simple for a string or group of strings. As we say in engineering, "you can't push a rope."
It gets confusing. There’s no pushing anywhere...
With pulleys a split will distribute the load equally.
Without pulleys load split will depend on the angles (vector analysis).

You ca achieve equal load distribution without pulleys placing attachment points right

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Re: Are Pulley bridles more tolerant to stretch/shrink?

Postby Sandras » Sun May 06, 2018 7:25 pm

Bushflyr wrote:
Sun May 06, 2018 6:30 pm
Don't "ASS-u-me" just do the vector analysis. It's simple for a string or group of strings. As we say in engineering, "you can't push a rope."
Oh, I'm glad that we can talk with vectors!

Just take the first drawing (Ozone Reo from the pdf manual)

You see that A1 is parallel to AL1 that simply means that A2 is 0

which contradicts what you wrote above
Bushflyr wrote:
Sun May 06, 2018 7:05 am
No, all the front lines will share the load regardless of pulleys or no.


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