Paraglider line lengthsRepair, servicing and re-trim at The Loft

Ruth Kelly, 28 January 2015

While flying at Durdle Door on 5 October 2014 I managed to get a line on my glider - a Gin Sprint Evo - caught round a thorny twig on take off. When I inflated the wing it came up fast but the snagged line stopped the port wing tip rather abruptly. I didn’t realise it at the time but one seam near the port wing tip on the bottom surface of the wing was torn, and the internal cross bracing was ripped away from its rib. I didn’t appreciate the extent of the damage at the time, and went on to enjoy a rather nice flight. I spotted it while packing up, though, and its next outing was to The Loft in east Sussex for repair.

I took it down personally and discussed options with Mike and Eddie. For the repair it was either a matter of re-joining the damaged parts with tape, or stitch in a new half-panel. I chose the latter—and a very neat job they did. The wing had not been serviced from new – it had only 55 hours flying time – so I thought I would also get a normal service, normally priced at £90. This entails inspection, porosity testing and checking the length of a sample of lines. However, for £130 they offer a full line-length check with re-trimming as necessary, and I quickly decided that this would be good to do.

Five working days later and the job was finished, and the wing was delivered back to me about a week later, nicely packed, by courier. No complaints about service. The service report was interesting of course—nice to know that the porosity was excellent and the general condition of the wing was good or very good. But what most interested me was the re-trim report.

This came in the form of two tables. The first shows the designed line lengths and the deviation from these as measured on the glider. Lines that are too short are shown as negative numbers; lines that are too long are positive. The tolerance on line length is plus or minus 10mm, so any difference greater than 10mm is flagged as needing attention (although in fact The Loft does more than this bare minimum when carrying out a full re-trim). The second table shows the line lengths as measured after re-trimming, again as a set of deviations from the designed lengths.

Line lengths - before trimming

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Line lengths - after trimming

I was a bit startled to see the state of my lines. As you can see, many were a bit too short, but a group of A lines, a group of Ds and all of the brake lines seemed to be too long. Now I’m not known for pulling fully developed spirals or other high-g manoeuvres so it’s hard to see how this came about. I phoned Eddie. The only thing he could suggest was that they were like that from new.

Next question: how do you shorten a line that is too long? Easy: they just take an extra loop around the maillon. To lengthen lines they stretch them. But where does the line length data come from? The answer is that The Loft (and other approved servicing and repair businesses) get their data from the manufacturers, or via DHV in Germany. The reason that the “DHV” columns in the table are very different from the measured lengths is that the measurements include the risers—which are shown on the report as 550mm.

Now Gin, I discovered, doesn’t publish line lengths. However, a after a quick scratch around on PG Forum I found that someone had posted a copy of Gin’s line length data for the Sprint Evo, which I happily downloaded. Being me, I had to take this all the way: so I set up a spreadsheet and typed in the Gin line lengths and the DHV line lengths and discovered something curious: most of the lengths in the Gin document were 24mm longer than those used by the Loft. The exceptions were the stablio lines, which were only 9mm longer, and the brake lines, which were 37mm longer.

But investigating that I went in search of information on approved techniques for measuring line lengths. Having tried it in the back garden on Will’s Merlin, and more recently at the Elm Tree on Neil’s U-Sport, I knew how awkward it was with a tape measure—and how difficult it was to reach conclusions. After a bit of a surf on PG Forum I found a rather lengthy video on Vimeo showing some guys checking the trim of World Cup competition gliders. See the video for yourself here. But this is how they did it:

Lines should be tensioned with a load of 50 Newtons when they are measured. Here on the surface of the earth you can generate a force of 50 Newtons with a mass of 50 divided by 9.81 (gravitational acceleration), which is 5.097kg. The chaps in the video used a plastic container full of just over 5 litres of water—and they checked the weight with a spring balance.

What you do is tie the end of the rope to the weight; take it over the pulley and then tie it onto the riser, where the carabiner would normally be. Now you go to the wing. Select the line you want to

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measure, hold it by the tab and pull, lifting the weight off the ground. You now have a 50N load on the line.

Lengths are measured with a laser device. As well as the rope and weight, attach a small metal disc to the maillon for the line group you are measuring; the disc exactly aligned with the end of the line to provide a good reflective target for the laser. The laser unit is then held with its base aligned with the point where the tab joins the fabric of the wing. The red dot of the laser is then pointed at the metal disc; the weight is lifted from the ground, and with a press of the button on the laser device the length is measured. Some of these laser measures have Blue Tooth so measurement can be transmitted to a nearby laptop. The guys in the video simply called out the numbers and typed them in. But I see now that this is a fast, accurate and repeatable way to check line lengths.

Subsequently Grant Oseland shared a link on Facebook to another video, showing a machine that uses the same principles, which is well worth checking out.

Having gone this far I also played for a while, trying to find a good way to represent the line plan in tabular form. The Gin line length data was divided up by level and attachment point—you have to add several sets of numbers to find the actual length from maillon to tab. So I laid out a spreadsheet to show this more visually. It also meant that I could work out the total line length used—another piece of data that Gin doesn’t publish. It’s not the same as adding together all the individual line lengths, because many of them are common. The answer for the Gin Sprint Evo (size S), since you’re asking, is roundly 319m. That’s a lot by modern standards, but then the Evo does combine modern technologies like plastic wire stiffening in the leading edge with a very traditional four-liner line plan.

Pursuing the issue of line-length data a bit further, I wrote to Patrick Holmes at UK Airsports—the main importer of Gin wings to the UK. Regarding publication of line lengths he said “… they are not generally publicised because it's too easy to use them in the wrong way, or make the wrong assumptions when given the data, and pilots might either incorrectly compare individual lengths to the specs, incorrectly add up the individual lengths and get the wrong total length, or not measure in the right conditions or use the correct methodology.” He kindly sent me the spreadsheet that they use, which includes the individual line lengths (matching the document I found on PG Forum), and total lengths from maillon to tab.

He explained the difference: “When lines are made, the actual stitching of the loop shortens them as well as the joining of the lengths. The Stabilo [for example] has thinner material and fewer connections.” Commenting more specifically on the length of my brake lines - which were all over length - he said “… this is not a problem. When measuring the brakes all technicians will have a different method. Some will include the gathering system at the trailing edge, some will include the handle and some may even undo the knot and measure from the mark on the line. As long as the all the brakes are “out” by a similar amount this is fine.” In other words, the majority of the problem could be corrected simply by re-tying the handle. At least I think that’s what he said!

The only other thing we discussed was the behaviour of different line materials. The two most common are Dyneema and Kevlar. These are both brand names of course: the former is a type of high molecular weight polyethylene; the letter is an aramid. I’ve quite often people say that Dyneema shrinks. Now I happen to know that this isn’t true: Dyneema actually creeps slightly under load—that is, it stretches continuously, and doesn’t recover when you take the load off. The reason that Dyneema appears to shrink is because of the braided covering on the core. This is stretched out tight when the line is made, and because Dyneema is a very slippery material, it bunches up when the load is taken off. One of the reasons that paraglider lines must be tensioned before you measure them!

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In conclusion I’d recommend checking your line lengths. My glider has now been trimmed a consistent set of lengths, with good symmetry: if you look at table two you’ll see that the accuracy of all lines is generally much better than +/- 10mm. I should say that I never had any complaints with its performance beforehand, and it’s hard to say whether re-trimming has made a noticeable difference. However, it has been out at Barton since then, and it felt very nice. I’m happy!

Ruth Kelly

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