Sled Kites

From FoilDesign

MMMmmm arcs...

Designing an Arc

This article explains some of the theory behind designing an Arc, or sledfoil. It’s not a detailled guide, but hopefully it helps to understand arcs better.

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What’s an Arc?

An Arc is a ram air inflated foil without a bridle. Because there is no bridle, the kite has a semi circulair shape. Normally, flying lines are directly attached to both wing tips. Arcs can be flown on two or four lines.

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How do Arcs fly?

In case of an Arc, there’s no bridle to hold the foil into a desired shape. Instead of that, you could say that each section of an Arc is bridled by their adjacent sections. This way it’s easy to see that the way each part of an Arc behaves, is greatly determined by the overall shape of the foil.

fig. 2.1 An arc with the centre section collapsing

As with all foils, one of the main properties of kite behaviour is the Angle of Attack (AoA). This is the angle the foil profile makes with the apparent wind. A too high AoA will cause the kite to stall (hang still in the wind window) while a too low AoA causes the kite to luff (the front of the kite collapses towards you). In case of an Arc, a main problem is tip collapse. Instability at the tips cause the tip AoA to become suddenly negative and the tips collapse towards each other.

For a normal foil, the AoA for each section can be adjusted by adjusting the bridle. In case of an Arc aerodynamic forces and kite shape determine the AoA.

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Pivot line and CoP line

For every kite there’s a balance between the force caused by tension on the flying lines and the generated lift. Not only the amount of lift and line tension is balanced, also the resulting moments caused by those forces should compensate each other.

fig. 2.2 Tension and Pivot line

For an arch style foil, line tension is introduced at the tips by two or four lines, and will be spread troughout the whole span of the kite. It is imaginable that most of the spanwise tension will be between the front lines, or in case of tension on four lines, in a pane between those four lines. To simplify this theory it’s possible to define an imaginary line that can be seen as an average of all spanwise tension forces introduced in the kite. This line is also called the Pivot line, cause when changing AoA, the wing can be considered ‘pivoting’ around this line. Generally, it’s position will be between the front lines when there’s tension on the front lines only, and will be shifted further back when tension is applied on the brake lines.

fig. 2.3 Lift forces (Xfoil) and estimated CoP

Secondly, every profile in a foil has a Center of Pressure (CoP). This is the point were the aerodynamic lift forces are considered ‘acting’. This point does not really exist, its position is just the average of all lift forces acting at that profile. The position of the CoP varies with changing AoA. Generally, when the AoA becomes lower, the CoP shifts forward, while at higher AoA the CoP shifts backwards.

fig. 2.4 CoP line

The CoP line now is the imaginary spanwise line connecting the CoP of all profiles.

fig. 2.5 Flat

In case of an Arc (and any other foil) there’s a balance between the downward force component caused by line tension, and the generated upward lift. In this case that would mean that the pivot line and CoP line must fall on top of each other. There can only be a difference in position between those two lines if the foil has sufficient structural rigidity to absorb those differences. In the second Peter Lynn patent on Traction kite design, the position were the Pivot line and CoP line meet is referred to as the Load line.

fig. 2.6 Increased AoA

fig. 2.61 New balance

So, what happens if at one moment the two lines are not at the same position, for example when you pull the brake lines? Since the kite will try to find a new balance, the AoA in this situation is likely to increase. By increasing the AoA the CoP and CoP line will shift backwards and will come in line with the Pivot line again. (See picture.)

Now, how can you use this theory in the design of your sled? Since we now know the kites’ AoA will adapt itself to the position of the Pivot line, it’s to us to find a optimum position for this Pivot line to get the desired AoA of the wing.

There are two ways to change the position of the Pivot line:

Change the position of the line attachment points;
Move sections of the foil forward or backwards.

You can see both options applied in the images:

fig. 2.8 Centre section forward

fig. 2.7 Line attachments downward

The distance between the Pivot line and the Leading Edge (LE) at each profile is called the ‘Pivot line offset’. Normally it is expressed as a percentage of the whole chord of the profile. Example: When the distance between LE and Pivot line is 0.2m and the chord of that profile is 1.5m, the Pivot line offset is: 0.2/1.5 * 100% = 13.3%.

fig. 2.9 Pivot line offset

For an Arc it’s important to keep in mind that the kite acts as a whole, so changes at one section of the wing will also influence other sections. It’s not always easy to know how changes work out on the kite. For instance, increasing the offset of the centre section could cause the shoulder section to collapse, etc...

fig. 2.10 Radial forces

The picture to left shows that the line tension and lift forces act at an angle of approx. 90 degrees. It’s difficult to understand how both forces are transferred through the kite to compensate each other, and how this influences flight behaviour.

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Profile choice

Choosing the right profile for both center and tips of an Arc is very important, it can make the difference between a flying and non-flying kite. For the tips, choose an autostable profile. An example of an autostable profile is the speedfoil profile:

fig. 2.11 Speedfoil profile

Such profiles have a CoP very much at the profile nose, but more important, are insensitive to minor variations in AoA. Between AoA’s of approx. -5% and +5% the profile is unlikely to collapse. This feature is very useful in Arcs. Drawback of such profile is that it creates very little lift, so for the center section there’s an other profile needed. For the center section a profile with positive camber at the nose is preferable, like the MH110 profile.

fig. 2.12 MH110 profile

The more postive camber a profile has, the more lift at the same area is created, but a profile with high positive camber is sensitive to luffing (nose collapses down). So it’s a compromise here. It’s preferable to choose a profile with some reflex at the tail, a profile with negative camber at the tail section. This will resist the AoA of the profile becoming negative, at the cost of a little reduction in lift. The MH110 is a good example of such profile.

In both Surfplan and Foilmaker the user can choose how the profiles are morphed from tip to center profile. Theoretically, too much center profile at the tips could cause the center to collapse because of the outward forces generated at the tips, but the pivot line offset is also an important factor here. There’s not really a rule of thumb on how to set the profile morphing.

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Other factors in sled design.

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Concave LE.

fig. 2.13 Example of a concave LE

A second patent on sledfoils of Peter Lynn describes the use of a concave LE in Sled design. In this situation the ‘shoulders’ of the Arc have a large Pivot line offset compared to the centre profiles. Theory behind this, is that the tip profiles will operate at a larger AoA compared to the center section, giving the kite more stability while not compromising depower. A concave LE is only useful when applying wide tips, tips with a large chord compared to the chord of the center profiles. Too much concavity can cause the tips to stall, or the center to collapse.

fig. 2.14 TE contraction on a S-Arc

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Banana effect.

Sleds are prone to ‘bananaing’ due to TE (Trailing Edge) contraction. The tips bend backwards compared to the center section. Looking at a photo of a sled it’s clearly visible the LE is very smooth while the TE is billowed due to internal pressure. This billowing causes the TE to contract, bending the whole kite backwards. This banana effect changes the Pivot line offset, increasing the offset for the center section. So there will always be a difference between the designed offset and the in-flight offset.

fig 2.15 The Peter Lynn Guerilla is optimised for depower

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Depower.

A desirable characteristic of any four line kite is depower. Depower, also called sheetability, is the possibility to control the amount of pull of the kite. This can be done by changing the speed, AoA, or profile shape of the kite by pulling the brake lines. An Arc is depowerable by varying the AoA of the center section. When flying the kite with slack rear lines, the AoA of the center is preferably close to zero, giving minimal pull while the wind window is close to 180 degrees. When pulling the rear lines the power should increase without stalling the kite. The amount of ‘sheetability’ between depower and full power is an important factor and should be optimised. There are a lot of possibilities to experiment in this area.

fig. 2.16 High AR and L/D for buggying: the Hi-Arc

fig. 2.17 Low AR for kitesurfing: the S-Arc

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Different designs for different purposes.

In general, Arcs can be used for powerkiting, buggying and kitesurfing. In case of buggying and powerkiting. The L/D ratio is preferably as high as possible, giving the Arc speed and a big wind window. Slim profiles, small tips, and high AR (up to 8) are preferable. When using the arc for kitesurfing other factors become important, like maximum stability, water relaunchability and steady speed and pull. In most cases, a kitesurf Arc will have thicker profiles, a lower AoA, and wide tips for more stability.

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Arc construction

For pictures on this matter, check the building descriptions of the Speed and Buggy P-Arc :

wingine.nl - follow Speed link
Buggy P-Arc

An Arc is build as air-tight as possible. Since there is no bridle, an Arc holds it shape entirely by the internal pressure. In effect, an Arc is impossible to launch if not filled enough with air.

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Air-intakes

Use a minimum number of air intakes, mostly 3 to 5 will do. Shape can be elliptic or half a circle for instance. Using mesh at the air intakes help to keep their shape better.

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Valves

Behind the air-intakes valves should be placed to prevent air leaking out. Valves are made by placing a tube made of rip-stop cloth behind the air-intake. A good example can be found here: Sledfoil Pictures

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Taped seams

In order to prevent air leakage, all seams must be taped with double-sided tape, especially at those places were the panels are sewed together.

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Reinforcement and webbing

Since there is no bridle, all forces entering the Arc will eventually be concentrated at the tips, especially at the front line attachment points. These tips most be properly reinforced, for instance by using Dacron and thick Dyneema line. Between the frontline attachment points, most of the line tension is introduced in the kite. There is a strong spanwise force in this area that smoothens the LE but will also rip your kite apart if not properly reinforced. Best way to do this, is applying a spanwise webbing between the front line attachment points. This can be a 750kg Dyneema line, or a backpack strap for instance. This webbing must be placed at he bottom skin (not the upperskin!) and can be sewed completely, or for instance just at the bottom of the first three ribs. The rest of the webbing will then be joined at the bottom skin when sewing in the ribs.

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Tip spars

Spars at the tips give the tips more stability, reduce wrinkles, and spread the load more evenly at the tips. Also the steering becomes much more responsive. Carbon spars between 6 and 8mm is best.

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Rib vents

Since only a few cells are provided with air-intakes, the other cells must be filled by cross vents in the ribs. One way is to make several round holes in each rib. An other way is to make the tail of each profile out of mesh instead of rip-stop cloth. This handles sand and water better, but could give some deformation at the tail.

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Pre-fill opening

To fill the kite for first use, it’s handy to leave the TE of the mid cell open and place a velcro closed opening here. When laying this opening in the wind, the kite will fill much faster than if filled through the valved air-intakes. Also deflation after use becomes much easier. An other option is to use a zipper at the bottom skin of the kite. This is especially useful with big kitesurf kites, since this way the kite can also be filled at the edge of wind window.

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Built and tested sled kites

Kitesurf ram air sleds :

Willi Wing
Fat Wing - then sign in go to the 'files' section )

Buggy ram air sleds :

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Sewing Techniques

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AoA and Sleds

Sled's and Angle of Attack had me thinking in circles for ages.

Imagine (or take) a strip of paper 5cm wide by 20cm long and bend it into an upside down 'U', this is your sled.

It is at 0' AoA.

Removing small triangles of paper from the TE will give it more AoA, the nose will tilt up. It also makes it cone shaped, increasing power and reducing the wind window and depower of the kite.

Adding small triangles of paper to the TE will decrease the AoA. Useful for a kite that stalls too much, or has a small wind window. Or removing triangles of cloth to the LE would have the same effect, but this is hard to do in real life.

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Kite Mod's

So you're sled won't fly, or won't fly as well as you think it should. Here are some tips:

Single LE wedge in the center cell. This involves taking a triangle of cloth out of the LE of the kite (the triangle should reach as far back as 75% (is this right?) of the chord length This will decrease the over all AoA of the kite. Use if the kite window is too small, or it stalls. This can cause the kite to be more prone to luffing (due to the reduced AoA.) Open the TE to get acces to the cell (this may be possible through the deflation bum hole), measure, mark, sew. (photo's and instruction at Arc Wedge [dead])
Two LE wedges at the shoulders. Same as the single wedge (above), but this will change the plan view a little. It pushes the center of the kite back in comparison to the tips, making it a little more stable and may increase the depower. Open the TE seam for access to the cells. Measure, mark, sew.
Top skin tuck. A diamond shaped tuck is performed on the top skin of the center cell, the tip of the diamond starts at the LE. The widest point at the middle of the profile (approx 60mm wide for a 6 meter kite) and the TE tip of the diamond at 75% of chord. This tuck is used on old Arcs that have started to luff. The top skin has stretched a little causing the AoA to drop and the kite over fly and luff. This tuck increases the AoA a little and gives the profile a little more relfex making it less likely to luff. Draw a line from the LE to 75% of chord. Find the center of pressure of the profile (the highest point from a side view) which is approx 30% of chord. This point has two 30mm dot's on either side. Join the LE to these two dots, and on to the TE point. See Arc Modifications
TE tuck. Wedges in the TE, shortening it and increasing the AoA. This reduces overfly and luffing. Increases power, but reduces depower. Open TE, measure wedge (so it reaches forward 60% of chord (is this right?)) mark, sew, close TE. See TE Tuck

These mod's should be applied one at a time and the kite flown to judge the effect.

Try each mod with duck tape or carpet tape before commiting to sewing the wedge in. If the mod failed then the tape can be removed and a different solution tried.

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