Auto-zenith

Kitesurfing/Kiteboarding on Liquid H2O

Auto-zenith

Postby Snake » March 16th, 2013, 12:15 am

What makes a kite auto-zenith? For those that don't know, auto-zenith is a quality that makes a kite automaticly return to zenith(12:00) without user input. Peter lynn Twinskins are the only kites that do this. There are 2 main reasons why they do this.

The most prominent cause is the spars in the wing tips. When the kite drifts to one side, gravity pulls the spar downward. This increases the angle of attack in the upper wing tip and decreases the angle or attack in the lower wingtip. The angle of attack change causes the upper wing tip to have more lift than the lower wing tip, lifting the kite back to zenith. It also causes the upper wing tip to have more drag than the lower which turns the kite upwards toward zenith.

The second prominent effect is the distribution of canopy. When the kite veers off to one side, there is always more canopy pulling upwards than downwards. This returns the kite to zenith from to balance the forces. This also causes stability in arcs and other power kites. The downward curve of the wing tips causes more of the lift to pull the kite upwards, counteracting the force of gravity on the kite, adding stability.

There are other effects but they don't add much to autozeinth. I hope you found this interesting little write up interesting. I am planning on makeing more articles about other kite related things in the futre.
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Re: Auto-zenith

Postby tungsten » March 16th, 2013, 7:52 am

Hey Snake, great to see this kind of subject discussed on here.

The ARC auto zenith is basically a special case of the general question: "how do single line kites know which way is up?"

Here's some more detailed information about the subject from Peter Lynn himself - Newsletter 01/2009. Enjoy!


Single Line Kite Stability. by Peter Lynn

For a kite to fly on a single line, it must, as the most basic condition, have some way to detect which way is up.

All single line kites that aren't under some sort of remote control do this by having their centre of lift position (CL, where lift forces act) above and forward of their centre of gravity (CG, where weight forces act). The pendulum effect that this creates causes such kites to point upwards, and upwards they will fly, until they get to a line angle at which wind generated lift exactly matches the kite's weight (when the kite is said to be at its apex)- disregarding dynamic effects of course.

But unfortunately, we can't disregard dynamic effects- because they very often prevent kites from flying stably at their apex.

And, at some upper wind speed, they will always prevent kites from doing so. This is because, while the lift (and drag) forces that drive dynamic instabilities increase with the square of wind speed, the weight force (from which the kite derives its upward seeking tendency) is constant. At some wind speed therefore, the pendulum effect will be overwhelmed by aerodynamic forces and the kite will crash- if it doesn't break first.

Dynamic instabilities derive from apparent wind effects; changes to the air speed experienced by a kite that are caused by its own movements. Of particular significance for dynamic instability is the relationship by which, when a kite is turning, the lift on the faster wing will increase by more than the lift on the slower wing decreases.

It's useful to consider two main failure modes for single line kites. One, overcorrection, is when a kite reacts too aggressively while re-aligning itself with the wind and triggers dynamic effects. The other, undercorrection, is when it reacts too slowly.

An example of overcorrection is when recovery from some directional displacement (a change in wind direction for example) initiates a series of increasing amplitude lateral oscillations that build until the kite starts to loop uncontrollably.

An example of undercorrection is when a kite takes so long to recover from a directional displacement that while doing so it traverses completely to one side or other of its wind window and collapses.

In addition to the relative magnitude of a kite's pendulum effect, the four main elements that influence overcorrection/undercorrection are tail drag (tails, trailing drogues etc), laterally disposed drag (drag sources to each side), lateral area (keels, flares, dihedral, anhedral), and longitudinal dihedral (often called 'reflex').

Tails are clever because they don't begin to apply any corrective force to a kite until there is substantial angular displacement (tail drag increases with the sine of the angle of displacement, so by 10degrees, say, are providing 17% of the maximum corrective effect they are capable of). The beneficial effect of this is that tail drag allows a kite to adapt quickly to minor wind direction changes (quickly enough so that the kite will not shift too much laterally while doing so) but comes in with rapidly increasing corrective force if for some reason the kite gets seriously tipped. Tails will therefore rarely if ever make a kite's response so slow as to cause undercorrection - unless their end catches in a tree or they are REALLY long. The bad bit about tails is that they cost lift to drag ratio (L/D). (L/D is a general measure of aerodynamic efficiency. For gliders it defines how many metres they fly forward for every metre of sink. For traction kiting it measures how well you can go upwind. For single line kites, it determines line angle- in fact the tangent of the angle, relative to the horizontal, of the flying line at the kite, is exactly the kite's L/D).

Laterally disposed drag- that is, having sources of drag out to each side of the kite, also has a clever effect: Because drag rises with the square of wind speed, when a single line kite with substantial outboard drag gets into a destructive turn, the drag on the faster side will increase by more than the drag on the slower side decreases- providing active damping. Such drag elements will also decrease L/D of course, except if they are an intrinsic and essential part of the kite anyway. The insight being offered here, and it's a major one, is that aspect ratio (AR, effectively width to length ratio) is the most powerful 'costless' (by L/D) dynamic instability cure available to kite designers. A way to make this understandable is to consider a square kite, 1m on each side, lifting area 1sq.m (aspect ratio 1.0). If such a kite is built and is found to be inclined to overcorrect and go into destructive looping, then if it's rebuilt to 1.25m span x O.8m long (still 1 sq.m but now AR 1.56), it will have much less tendency to overcorrect- may even be inclined to undercorrect. This is because the drag associated with the wingtips, while still having similar cost with respect to L/D, is further out from the kite's centre of lift, so will be more effective in resisting any rotations (in the plane of its lifting surfaces) that the kite becomes subject to (that is, it slows turns). Adjusting a kite's aspect ratio is therefore a way to get correction that's neither too fast (loops out of control) nor too slow (flies off to one side or the other and crashes or stalls). Wingtip drag isn't referenced in any way to up/down, all it can do is slow down turns- and of course this can be a bad thing when it slows a desirable recovery- but on balance it is hugely beneficial because it slows down all the movements which energise dynamic instability, unplugs their power source so as to speak.

The third main useful stabilising element, lateral area (flares, keels, dihedral, anhedral etc), is also relatively costless by L/D, and can be very effective at damping out any incipient overcorrection but has to be of appropriate magnitude and carefully positioned. If a kite with substantial lateral area (as a proportion of its lifting area) is subject to an angular disturbance (that is the longitudinal axis of the kite gets out of alignment with the wind direction), the aero forces acting on this lateral area can cause the kite to move a long way sideways across the wind window before the pendulum effect gets it back in line- that is, excessive lateral area can promote undercorrection. Clearly, the longitudinal placement of lateral area will have an effect also. If disposed mainly behind the kite's CG, it can promote rapid re-alignment but may also exacerbate dynamic effects (overcorrection). If in front of the kite's CG, it will tend to cause undercorrection and make it very difficult for the kite to fly centrally (that is, directly downwind of the line tether point). Although dihedral (upward angled wings) and anhedral (downward angled wings) have some different effects on how single line kites react, they are primarily both just ways to get lateral area. There is a mistaken belief that dihedral is 'stable' while anhedral is 'unstable' but this comes from aeroplane experience and doesn't generally apply to kites. When an aeroplane rotates around its longitudinal axis, if the downside wing loses projected area at a faster rate than the upside wing gains projected area then the rotation will become self promoting. Aeroplanes are made with dihedral so that they are auto-stable in rotation about their longitudinal axis. For kites, bridles generally prevent this sort of rotation anyway. Kites with centre line bridling (most diamond kites for example), require dihedral for the same reason that aeroplanes do, but kites with laterally disposed bridles (like sleds for example) don't.

Longitudinal dihedral, or reflex, the fourth and last major single line kite stabilising element has the obviously beneficial effect of reducing or eliminating luffing tendencies, but its underlying influence is more profound: Because aerodynamic lift forces drive instability (of both the overcorrection and undercorrection types), anything that decreases lift without changing other things too much, will generally improve a kite's stability. "More longitudinal dihedral" is just another way of saying "less camber"- and having less camber will cause less lift to be generated, (a generally applicable aerodynamic effect). Introducing longitudinal dihedral therefore deals directly to overcorrection, but it's a rather ugly solution, a last resort (usually taken when graphics considerations don't permit other more efficient form changes), because it also directly reduces L/D, and by a lot if it's to be effective. It's influence on undercorrection is equivocal: Reducing lift does reduce the driving force that makes a kite traverse off to the side before it's pendulum effect can straighten it up- less lift means that it won't get as far before correction occurs. But, longitudinal dihedral also shifts the kite's CL rearward (nearer to its CG), which reduces the effective pendulum length and therefore its corrective effect (while adding to its usefulness against overcorrection of course).

This is a brief description of a complex and indeterminate field. Like all things that are subject to turbulent flow (the weather for example), single line kites will never be fully predictable.
But, there are some things that are both true and useful that can be established- which is what I've tried to do.
I've tested the above against the kites I see flying, and don't think I've seen anything that falsifies any of it. However, there are so many overlapping effects and other influences that it's sometimes difficult to see through all this fog to the fundamental relationships. No doubt I've made errors in at least some respects.
I'll modify and correct when these come to light, and plan to add descriptions of various special conditions as time and opportunity permits (von Karman affects for example).

Peter Lynn. Ashburton, New Zealand, January 1 '09. (Back in 1973 I reckoned I'd have this done by my 30th birthday (1976), but it's taken a bit longer).
PS, Just had a terrible thought; what if it's all wrong- wasted life, AAGH!
Last edited by tungsten on March 17th, 2013, 10:12 am, edited 1 time in total.
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Re: Auto-zenith

Postby Hardwater Kiter » March 17th, 2013, 1:37 am

Makes my head hurt.
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Re: Auto-zenith

Postby tungsten » March 17th, 2013, 10:07 am

Hehe, it took Mr. Lynn a good 36 years to get that far and he still isn't sure his findings are all true, so I guess you're excused. A couple hours kiting take care of this kind of head aches though. 8-)
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Re: Auto-zenith

Postby Andreas » March 28th, 2013, 8:48 pm

Interesting.

I never thought of the weight of the spars affects. However I pretty sure the auto-zenith working when I flew my old 1120 without spars, but I dont remember if it was less prominent?

The second effect I cant see how it differs from a C-shaped LEIF?

The P.L. text is a bit over my level :mrgreen:
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Re: Auto-zenith

Postby tungsten » March 29th, 2013, 10:28 am

The spars add to stability as they are denser than the kite material, effectively lowering the CG. By how much you have to guess. The kite construction is such that they auto zenith without the spars.

The C-shape is what PL calls anhedral. It's the same on a LEI, so this part of the effect is same on both types.

Reason for auto zenith is, as Peter Lynn says, that CL is above and forward of CG, which causes a pendulum effect leading the kite to zenith. This is the case with ARCs.

What is different on a LEI is weight distribution. LEIs have a heavy LE tube, thus the CG (center of gravity) is way forward on most LEIs, and thus there's no (not much) stabilizing pendulum effect - to the contrary, it usually is destabilizing, so the kite will drop out of the sky if it's not steered actively. There are some LEI's though which have a rather evenly distributed weight and stay up more or less.

CL (center of lift) is the point where all the lift forces act. It's like the kite would be pulled upwards by a string attached in this point.

CG (center of gravity) is the point where all the weight forces act. Imagine a string attached to this point, pulling downwards.

CG is forward of the CL for LEIs, and rearward of CL for ARCs, as you see in the pic. In frontal view, CG and CL are in the middle of the kite (on a line from your harness hook to the middle of the wing).

auto zenith.jpg
auto zenith.jpg (45.98 KiB) Viewed 9640 times



auto zenith2.jpg
auto zenith2.jpg (37.07 KiB) Viewed 9640 times
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Re: Auto-zenith

Postby ikke » April 6th, 2013, 12:30 pm

There is another aspect, unique to the arc concept. The tips are unconstrained with respect to torque. With the centre of gravity behind the centre of pressure the lower tip anlge of attack decreases, while the higher one increases if the kite is not directly overhead. The resulting net force will steer the kite back to 12
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Re: Auto-zenith

Postby Andreas » October 3rd, 2013, 11:09 am

Aha, that is why the auto-zenith almost disapears with long lines (50m on an old 1120 or 30m on 460). The ARC flares out more and the CG point moves upwards.

The tips falling down at the rear edge when the ARC is low i didn't think of before either.
C-QUAD 3.2, C-QUAD 4.2, ARC 1120, ARC 460, Waterfoil 3.6 (Now rep :-) ), Flysurfer MastAir 16, Vade Retro Satana 7 m2 (under rep), VenomII 16 (waiting for rep), Synergy 15.

Top speed on skies: 71,0 km/h or 44.1 mph (feb 24 -2011, Syn 15)
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Re: Auto-zenith

Postby Pitu » October 20th, 2013, 11:24 am

Well.. I would like to reduce autozenith on Ch1.. how can I do this?
I was thinking to add some weight to the front tips, to balance the spars.
There is also the couple moment of the reverse airfoil at tips, who is useful to keep arc open, but pulls down the upper back tip.
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Re: Auto-zenith

Postby tungsten » October 20th, 2013, 3:40 pm

Pitu, there's two kinds of auto zenith:
- auto zenith when you let go of the bar, and
- auto zenith on a tack, you move in one direction with your board, and the kite keeps constantly pulling upwards, which you have to correct all the time.

Which one is your issue?
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Re: Auto-zenith

Postby Pitu » October 21st, 2013, 9:23 am

Hi Tungsten

I've used Arc & Chargers for years, but now I'm using the Escape. Returning to chargers make me feel happy again, but now I feel the limits of this project and the solution at that limits!
One of those are the too strog autozenith. Both in speed and most during landing operation or surfing a wave.
I'm intrested on the 10mt, the only ch I want to keep.
By now i modify the Vpc2 to vpc1in order to save weight removing the 2 pulley back.
On my 10mt I've never had clapping or similar problem (only few tacos in very powerful kiteloops)
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Re: Auto-zenith

Postby tungsten » October 21st, 2013, 12:04 pm

Pitu, it's possible to balance ARCs in such a way that they don't pull upwards in normal flight. I tuned my SYN10 and SYN12 this way back in 2008, for waves, and I fly them this way ever since and love them for this feature. They still have some auto zenith when you let go of the bar, but they don't pull upwards when you sheet in.

It takes some tinkering and fine tuning though. Basically it consists of changing the kite balance rearwards altogether by adding a 4th braid - a leech line right at the TE, from zipper cell to zipper cell. You shorten this line in steps of 5cm until the kite all of a sudden changes balance. When this happens, the kite is sitting way forward in the window and has not enough built in AoA anymore, and at the edge of the window, the middle of the kite collapses downwards.

To take care of this collapsing, you shorten the first and / or second strap in the middle cell(s).

This is a rather advanced mod. Not difficult to do, but you have to have an understanding of the aerodynamics acting in an ARC, otherwise you get no where.
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Re: Auto-zenith

Postby Pitu » October 21st, 2013, 2:22 pm

Can you explain me that with a drowning?
I'm not sure to uderstand (it's my poor english :oops: )

What do you think about adding some weight to front tips and/or use light carbon spars?
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Re: Auto-zenith

Postby tungsten » October 22nd, 2013, 9:13 am

Pitu wrote:Can you explain me that with a drawing?


I will see what pictures I have. Give me a couple days.


Pitu wrote:What do you think about adding some weight to front tips and/or use light carbon spars?


Adding weight in the front will definitely not work. The weight would be in line with the force vector through the front line.

Lighter carbon spars will only have a very very very small effect. It's not worth the effort.
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Re: Auto-zenith

Postby Pitu » October 23rd, 2013, 7:43 am

The 4th braid act like fllapping the TE? It's a different result than shortening the 3th one?

I was thinking to insert a line in the TE to flap the kite between the zipper cells. This line driven by backs lines like fly cronix bridled TE.
What you think about that?
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Re: Auto-zenith

Postby tungsten » October 23rd, 2013, 8:33 am

Yes, on a straight wing you could say "flapping"... but a kite is not a straight wing... you have to consider the amount of reflex in the profile, what happens to built in AoA if you pull in the TE, and so on.

If you look at the position of the 3rd braid, you see it is far forward. Pulling the 3rd braid down (with the adjuster) as a different effect than pulling a leech line down. I'm exaggerating in the pics, but this is to give you a rough idea:

3rd braid:

3rd braid.jpg
3rd braid.jpg (37.16 KiB) Viewed 8305 times




4th braid (leech line):

4th braid.jpg
4th braid.jpg (37.64 KiB) Viewed 8305 times
Last edited by tungsten on October 23rd, 2013, 8:42 am, edited 2 times in total.
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Re: Auto-zenith

Postby Pitu » October 23rd, 2013, 8:39 am

I can't see your pics.

This is a horrible example of my idea:
1382517327084.jpg
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Re: Auto-zenith

Postby tungsten » October 23rd, 2013, 8:44 am

I understand what you mean. A little TE bridle connected to rear attachment point and leech line. If I remember right, funalex did this mod on Venoms and found it works good, enhancing turning speed.
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Re: Auto-zenith

Postby Pitu » October 23rd, 2013, 9:04 am

So this move back the charger's CL, next to the CG, right?
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Re: Auto-zenith

Postby tungsten » October 23rd, 2013, 10:24 am

No. This is what I was trying to explain. The weight force - CG below and behind of CL - is the STATIC auto zenith, when the kite is flying off the front lines.

The STATIC auto zenith is not what you wanted to change. You wanted to change the DYNAMIC auto zenith, when you fly the kite with the bar in your hands and it pulls upwards. This is a different condition. The DYNAMIC forces in flight with back lines sheeted are much higher than the STATIC weight force bringing the kite up to zenith.

In DYNAMIC flight situation, the kite pulls upwards because the profile and AoA overall cause a righting force upwards. When you pull in a TE leech line, you change the AoA and profile. This is rather complex, and I don't think you could easily calculate it. And not only will you change the auto zenith: you change the overall behavior of you kite.

I understand some of the principles, or at least I think I understand them, and have to find the balance point with trial & error.

That's why I never wrote it down as a mod. It would sound like "try this and the other, or the opposite if that does not work, and see how the kite behaves."

It does work on my CH1-12 though 8-)
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