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What is the angle of the canopy "wing/sail" to the lines?

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sergei Scotland
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What is the angle of the canopy "wing/sail" to the lines?

Postby sergei Scotland » Sun Sep 26, 2021 12:40 am

Big simplification of course. And another simple question :wink:

Say I found out a neutral 0 angle of attack for a kites model in a wind tunnel.
Say I wanted to design a bridle for this kite.
What would I be aiming for - with full bar in?

Full bar in is obviously when front and back bar lines are equal.
This does not mean kites canopy or AOA is 90 degrees to the lines of course. There is bridle and shape of the kite etc.

What would a kite designer consider a good starting point for an optimum angle between lines and kite's 0 AOA line?

To clarify - i sheet in a kite at 12 fully - what is angle between bar lines and 0 AOA line going from kite away from me?

Rough estimate even? 80, 90,100,110, 120, 135?

For example if we consider a sailboat going across the wind at 90 to the wind - this angle between imaginary bar lines going straight upwind can be set by sailor to be say around 135 degrees. Or less with stronger wind and more boat speed etc.
On a boat a sailor can usually set this angle almost all the way to 90,may be 95 degrees when fully sheeted in. Obviously one can grab the beam and set it to 45 too - if you want to go backwards for example :lol:. We do not have such luxury. My hands are not long enough to allow that much range :thumb:

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Re: What is the angle of the canopy "wing/sail" to the lines?

Postby Herman » Sun Sep 26, 2021 11:06 am

Take a look at Wikipedia for Angle of Attack, this will help you define the term and see how the Coefficient of Lift varies with AoA.

Then learn how to set up a tether and take a look at the angles yourself. With care and knowledgeable assistant you could vary the kite trim on the tether and gently walk the kite back into the power zone to demonstrate the difference it makes, but you need somebody who knows exactly what they are doing!
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sergei Scotland
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Re: What is the angle of the canopy "wing/sail" to the lines?

Postby sergei Scotland » Tue Sep 28, 2021 12:29 am

Herman wrote:
Sun Sep 26, 2021 11:06 am
Take a look at Wikipedia for Angle of Attack, this will help you define the term and see how the Coefficient of Lift varies with AoA.

Then learn how to set up a tether and take a look at the angles yourself. With care and knowledgeable assistant you could vary the kite trim on the tether and gently walk the kite back into the power zone to demonstrate the difference it makes, but you need somebody who knows exactly what they are doing!
Thanks, I might do just that, but I do not have a wind tunnel so do not really know the 0 aoa angle of my kite :D
So what looks like 0 aoa to me may not be even close.
But I am asking about a different angle. Obviously aoa changes dramatically for a kite in power zone vs kite at the edge of the window - which us a change in the region of may be 80 degrees.
Obviously the throw of the bar can only give 20 to 30 degrees change or less.
Looking at how relatively slow (if at all) an average kite climbs up from the bottom of a sine I guess at equal line length kite (sail) is not that far from 90 degrees to the lines. This is particularily obvious when relaunching the kite from straight down wind posittion as even at full sheet out kite hardly moves toward the edge of the window even if you manage to lift it on the wingtip.Thats how a sail boat sail behaves when fully sheeted in with boat across the wind - not going forward basically.
My guess is that basically bridle sets a trimmed kite to be somewhere around 110 degrees fully sheeted out and around 90 fully sheeted in making kite to climb pretty much overhead at 12. Anything less than 90 would make kite to over fly behind the kiter when at 12 but potentially make kite which tends to sit close to the edge/with better upwind?
Gross oversimpification of cause. :D

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Re: What is the angle of the canopy "wing/sail" to the lines?

Postby nixmatters » Tue Sep 28, 2021 2:04 pm

sergei Scotland wrote:
Sun Sep 26, 2021 12:40 am
...

What would a kite designer consider a good starting point for an optimum angle between lines and kite's 0 AOA line?

To clarify - i sheet in a kite at 12 fully - what is angle between bar lines and 0 AOA line going from kite away from me?
A simplified answer to your slightly overcomplicated questions ;)

2nd question:
Depends on if the kite was designed to sit forward in the wind window or deeper (e.g. wave kites).

1st question: I don't think that's a starting point for a kite designer.

The lowest AoA (bar sheeted out and front lines trim pulled in fully) is somewhere between 2° and 4°.
The max AoA (backstall point) can be easily calculated (6th grade geometry) if you know or can find the tow point of the bridle.
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Re: What is the angle of the canopy "wing/sail" to the lines?

Postby Herman » Wed Sep 29, 2021 12:11 am

Conventionally 0° AoA is just the chord of the aerofoil aligned with the flow, unless I have missed something.

sergei Scotland
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Re: What is the angle of the canopy "wing/sail" to the lines?

Postby sergei Scotland » Wed Sep 29, 2021 12:27 am

Herman wrote:
Wed Sep 29, 2021 12:11 am
Conventionally 0° AoA is just the chord of the aerofoil aligned with the flow, unless I have missed something.
Yeah, I thought it was angle with 0 lift and some airfoils I came across before seem to follow that. But Wikipedia agree with you obviously. I guess for some airfoils 0 Aoa can give plenty of lift and for some even give negative lift.

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Re: What is the angle of the canopy "wing/sail" to the lines?

Postby sergei Scotland » Wed Sep 29, 2021 12:40 am

nixmatters wrote:
Tue Sep 28, 2021 2:04 pm
sergei Scotland wrote:
Sun Sep 26, 2021 12:40 am
...

What would a kite designer consider a good starting point for an optimum angle between lines and kite's 0 AOA line?

To clarify - i sheet in a kite at 12 fully - what is angle between bar lines and 0 AOA line going from kite away from me?
A simplified answer to your slightly overcomplicated questions ;)

2nd question:
Depends on if the kite was designed to sit forward in the wind window or deeper (e.g. wave kites).

1st question: I don't think that's a starting point for a kite designer.

The lowest AoA (bar sheeted out and front lines trim pulled in fully) is somewhere between 2° and 4°.
The max AoA (backstall point) can be easily calculated (6th grade geometry) if you know or can find the tow point of the bridle.
Interesting. So for fully powered kite it can be somewhere in 22 - 27 degrees I guess. And if kite is say 20 degrees into power zone it is 42-47 degrees. No wonder kites lose so much efficiency when sined and sheeted in fully then. They are practically at stall point at the edge of the window already when fully sheeted in!

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Re: What is the angle of the canopy "wing/sail" to the lines?

Postby nixmatters » Wed Sep 29, 2021 8:37 am

I suggest you do the math again, using a kite you can measure and test (stall point flown at 12 o'clock). Sorry for the primitive sketch, but you get the idea.

And don't forget that thing called apparent wind when you sine, loop or just ride the kite. The 42-47° you mention above could be the angle to the wind direction, but definately not to the apparent wind direction when the kite is not static ;)
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Re: What is the angle of the canopy "wing/sail" to the lines?

Postby Herman » Wed Sep 29, 2021 9:00 am

As you know aerofoils produce lift and suffer drag. It is only convention to show the aerodynamic forces that way as it makes the analysis easier, but the forces could be resolved in other ways such as a single force at an angle to the chord. Arguably better for kite analysis as this would be helpful to analyse the effect of the kites central chord angle to the fly lines. Obviously at steady state this single force would be aligned with the fly lines and the kite would be stationary in the window.
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Re: What is the angle of the canopy "wing/sail" to the lines?

Postby GregK » Thu Sep 30, 2021 2:27 am

Herman wrote:
Wed Sep 29, 2021 9:00 am
... Obviously at steady state this single force would be aligned with the fly lines and the kite would be stationary in the window.
It would be pretty close, somewhere between front and rear line's direction.

With the kite stationary with respect to the rider, and he or she not accelerating, the vector sum of the flying lines tension, plus the lift and drag forces, and the kite's weight will sum to zero.


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