Restoring Flight Performance of a Sonic 2 13 Meter

[lightandfrost]

Light and Frost(L&F) obtained a used Flysurf Sonic 2 13 meter kite. The owner stated the kite began folding up in abrupt stall. The kite lines had been tuned but the problem still persisted. This paper will deal with the tuning performed on the line system. L&F has no knowledge on the kites performance, flight characteristics or durability respect other kites of similar design, and we have no issue with Flysurf Products having used them for over 20 years.

Upon receipt of the kite, the outer cloth surface was inspected visually and an internal cursory inspection for obvious damage was done on the internal straps. Nothing was found.

The lines had only very small 1 to 2 cm larks head extensions on only a few of the leading and trailing edge lines (i.e. the "a" and "br" lines on the line plan) . No adjustments were seen on the "b" and "c" hard points or on any branch lines except possibly BR-Main.

This kite as mentioned above was folding up abruptly. Ram-air kites loose their flight performance thru three(3) main factors. The lines shrink, the internal straps stretch, or the kites monocoque structure(ie skin of kite) become porous, reducing the ram air induced internal pressure. Internal strap problems are usually a manufacturing defect in that a thinner skin is used, but the designer does not add stronger internal straps to offset the stretching of the kite length due to thinner skin. This problem is generally rare. Porous skin is not addressed in this paper. There is off course internal damage, but that is case by case and can usually be seen when the kite surface becomes deformed when airborne or inflated. The most likely problem with a Ram-air kite loosing flight performance are the lines.

Restoring the Flight Performance by Adjusting Line Lengths To Factory Settings

Download the Line plan from the manufacturer showing the factory lengths of all lines on the kite. Flysurf has a substantial library available.

Perform a line audit by measuring the existing line length and then compare it to the factory length. Add larks head extensions to each line returning the kite to factory settings.

L&F have done many line audits on ram air kites from several manufacturers, not just Flysurf. L&F has never found a single case of a line stretching. Lines have always been seen to shrink on every kite from Psycho I to Speed III and some other manufacturers that underwent this line audit process. Exceptions here are the two(2) steering lines and two(2) center lines. These lines were never measured against factory lengths. The lines were just balanced by adding larks head kite line extensions to the shorter lines.

Line Shrinkage - Not an Urban Myth

Some say the line shrinkage is caused by fine sand and dried salt absorbed within the lines themselves expanding the line diameter thus shortening the line length. However L&F think there is something else also going on. For example two(2) 7 meter Psycho II kites used maybe three(3) times total and kept in air-conditioned rooms out of the sun in their storage bag were later test flown after a few years. Neither kite would fly, and all the lines had shrunk.

We have seen comments that line shrinkage can be ignored as it is equal across all lines. This is false. Shorter lines have smaller shrinkage while longer lines have greater shrinkage. In general we have found that trailing edge lines have a much higher shrinkage rate then other lines but this is not always the case.

L&F prefer to add larks head line extensions rather then stretch the lines, as stretching is a temporary measure. There seems to be a limit to the shrinkage that can occur. When our Psycho One, Speed I 17 meter and Speed II 15 and 19 meter kites began to become difficult to fly years ago, line audits were conducted. These kites were restored to factory setting with larks head extensions and of course a mixer test conducted when applicable and the kites have flown fine after that with no further problems till present. Note these Psycho One kites are two decades old. Only one(1) line audit was ever performed and the kites were flown within the last weeks just fine.

Adding Larks head Extensions

Tools Required

1. Tape Measure - Suggest a sewing cloth tape measure be used over a metal one.
2. Dacron 100 lb test fishing line for the thin lines
3. Medium line for the thicker lines
4. Kite line for the longer thick lines
5. Knife or Box Cutter
6. Notebook with Factory length and data on the kite
7. Pen or Pencil
8. Lighter
9. Tweezers used to separate larks heads on the kite.

You can use a larks head extension in most cases to extend the length

Making a larks head extension

Measure the difference to adjust then double it and add 7cm for the knot for thin lines and 8-10 cm for thicker lines. Hence on a thin line, if you want to extend the line 5cm then:

5cm X 2 = 10cm + 7cm = 17cm

Fold the line in half and tie the two ends together with an 8 knot or overhand knot at the end so from the knot to the other folded end is 5cm in length. Then burn the ends with a lighter to seal the line ends to prevent fraying

If you have a helper this process can go very quickly. One person measures the line and installs the extensions and the other person keeps track of the larks head lengths required, cuts the lines and makes the larks heads.

Start with a line audit by adding larks head extensions to only the lines attached directly to the hard points of the kite. On the sonic line plan the hard points would be labeled a,b,c and br. The exception is if you are doing a kite where the kite lines are attached directly to the hard point. In that case you have to work up thru the branch lines.

On the Sonic the line shrinkage was small on those lines attached to kite hard points. However the b and c lines were very difficult to remove from the small hard point larks heads installed by the factory. A difficulty in removing the line from the hardpoint larks head attachment almost always indicates that the line is under higher tension. We suspected that the longer branch lines were going to have larger amounts of line shrinkage and this proved to be true.

This line audit of lines attached directly to the hard points can be done in a very small space by rolling up the kite from tip to tip(i.e. do not fold and roll) and then slowly unroll and expose a row of a,b,c,br, then measure and correct the lengths and then continue toward to the next a,b,c, br hard point column working toward the kite center rolling the tip up and unrolling toward the center.

Once the lines attached to the kite have been returned to factory settings, the branch lines must be audited. The kite was taken outdoors, the lines spread out and the branch lines restored. Suggest you only take one line off at a time when possible, especially on short lines. If you take two(2) lines off on the same branch and become confused as to which is which, the shrinkage can make determining the lines by measuring the length impossible on the short lines near the tip. When doing branch lines do one side of the kite completely and then the other so you always have an example of how the lines are arranged on your kite.

Finally a mixer test is performed

Initial Conclusions

Long thin high aspect ratio kites are probably more sensitive to line shrinkage then lower aspect models and will tend to stall more abruptly then a lower aspect kite. Lower aspect kites tend to become sluggish and require more wind to function as lines shrink.

The longer A1-A6,B1-B6,C1-C6,BR1-BR6 and all the I lines i.e. AI thru BRIII showed extensive line shrinkage. Racers looking for that extra edge should carefully monitor the line lengths especially in these lines.

The kite was taken out for its initial test flight. The kite flew up from dead downwind quite easily in 3-4 knots of wind but the tip area on the right side a12 thru a9 was deformed and the kite tended to tip stall and fold during turns. It was expected the lines were either installed incorrectly, or not extended properly as the left side flew perfectly. An inspection was conducted on all right side lines for hard point stations 12,11,10 and 9 lines. Sure enough during installation one set of lines c10,C9, and C5 were installed improperly. The next day the kite was again air tested and the kite flew just fine. 30 minutes of flight time was conducted to set all the larks head knots in place.

As the kite was put down a hydrofoil racer flying a large ram-air kite came in. During conversation it was pointed out to him that the lines on these ram-airs tended to shrink over time and suggested he walk over and look at the amount of kite line shrinkage on the Sonic lying 30 meters away from him. He said "how can that be!?!" indicating that was impossible then abruptly turned and walked away?

The data on the Sonic showing you line shrinkage is provided as a text attachment due to the length of the document.

[direnc]

Why not just perform a long mixer test? Will fly just as well…

[jakemoore]

At some point in kite age the long mixer test (LMT)will not account for different rates of shrink across the span of the kite. Also many kites are not marked for the long mixer test.

For myself I prefer to extend the z bridles at the level of the first riser and there maybe 4 or 6 pigtail extensions rather than ~=100. I will tune a kite to my preference in flight using stock bridles LMT only as a starting point.

Finally it is not a bad idea to put marks on the Z bridles representing the LMT when a new kite has 10 hours on it.

[Herman]

Thanks for all the feedback and info, I found it very interesting.

Regarding shrinkage, in my experience materials based on long carbon chain molecules naturally shrink because the long chain molecules like to curl. Cross linking of the molecules greatly reduce this tendency but does not eradicate it. Simple example is to put an old style crisp packet in an oven and it will come out not much bigger than a postage stamp. I would expect this phenomenon to apply to dyneema. Perhaps a polymer chemist could confirm.

[OzBungy]

In the paragliding world trim measurement and adjustment is very common. People home build rigs and there are numerous businesses who do it professionally.

Most high performance wings have additional wraps in the the main bridle lines that are released at 80-100 hours to lengthen the lines to bring the wing back into trim.

The most common practice is to add a series of different hitches to shorten lines to bring the wing back into trim. The actual line lengths don't matter all that much as long as the trim is correct.

All lines are measured under 5kg load for single thin lines and 10kg for thick lines or groups of lines. Measuring lines without tension is useless. You can eyeball the tension but it's far easier to use weights or water containers. If you pull the line with your fingers you can see it stretch but it reaches a point where is doesn't stretch much more.

The example above talks about about 10-12cm of length difference. That is astounding. My paragliders have had line differences up to around 14mm. Kite flying lines might get up to that much shrinkage over 25m.

If I had to make large adjustments to bridle lengths I would be making whole new lines. It's easy to splice new bridle lines and the finish is far superior to adding bits of line with knots.

[tsuneo]

Adding extensions to the bridles is often needed to get a kite flying well again. Like jakemoore said these usually need to be made on Z at the kite towards the tips to help correct for shrink. The mixer will also need quite a lot of altering. The search function here will turn a a wealth of info on the subject.

[jbrodin]

Light and Frost(L&F) obtained a used Flysurf Sonic 2 13 meter kite.

Question: the larks head extensions are dobled up lines with a knot one one end. Fo you make a larks head around the end of the line you extend around the end with the knot, or do you thread the loops through one another? I assume the end without the knot goes in a larks head around the knot on the knot on the canopy attachment? A picture would be great.

[lightandfrost]

L&F had no intention of implying that ALL Pansh Kites were unflyable. We also have two(2) 15 meter Aurora II kites. One flew fine upon factory receipt and the second is still in the original factory packing and not for sale.

The two 12 meter kites referenced above were modded as indicated and also flew fine afterwards. In fact one of the 12 meters had been damaged internally by the user. One of the intake ram air sleeves was ripped and a large bulge on the bottom leading edge existed on one side. The kite was still tested with a twin tip. When the bulged damage area was on the top in flight on the water the kite flew fine and the rider went up wind. When the bulge was on the bottom the kite stalled but the rider was still able to return to shore.

Reference larkshead knots

Larksheads are just a length of rope etc with a knot at one end

They can be considered a knot with a loop at each end OR a knot with a loop on one end and an attachment point ie knot on the other.

The attached zip file shows several examples. Different sized ropes have been used. A kite string with loops at each end is the same as a larkshead. On a larkshead one end just has a knot closing the loop but it is still a loop.

Examples of larks heads strung together in long strings or looped around the knot ends of larks heads on the hard points put on by the factory exist on most kites. Also two lines looped thru one line to make a Y or branch is shown and can be found on all kites

We use larks head knot like this as they are easy to adjust the length quickly. Bu there are other ways as noted in the user suggestions above.

As a side note if we were to buy a kite and it does not have larkshead attachment points put thru the kite hard points allowing easy removal of the primary strings attached to the kite we would not want the kite even if it was free and we certainly would not buy it.

Kite hard points are the cloth loops sewn into the kite bottom where the primary kite lines first attach to the kite. Flysurf and Pansh loop a small larkshead threw these hard points allowing easy removal.