The EASY TRIM CHECK I n many sports, tuning is part of the everyday life of athletes. A thicker front tire here - and the bike has more traction in the bike park, a smaller edge angle there - and the freeride ski bites like a slalom carver on hard slopes.
Text: Christoph Kirsch Photos: Skywalk
Almost every piece of sports equipment can be adjusted to your personal preferences with just a few hand movements. In paragliding, such interventions in the setup are taboo. And with good reason. Even small changes in the trim can lead to massive changes in flight behaviour. Paraglider manufacturers adjust their wings so that they function optimally under different aerodynamic conditions. Pre-calculated tolerances ensure that any changes in line length that might occur between check and trim intervals have no effect on flight characteristics or even safety.
The Special Case of Brake Lines Although only small forces act on the brake lines, length changes still happen frequently. This applies especially to lines made of Dyneema, which are used on almost all models because of their low sensitivity to kinking on the brake lines. Investigations by the German Hang Gliding Association (DHV) have shown that several paragliding accidents have occurred in recent years that were caused by shortening of the Dyneema control lines due to shrinkage. The consequence of this shrinkage is always a shortening of the brake line travel. Shrinkage of up to eleven centimetres has been found on the equipment used in the accidents. In some cases the complete empty travel of the brake lines was used up by the line shrinkage. These paragliders were thus pre-braked while still “hands up” despite the brake lines being fully released. Test flights have shown that gliders pre-braked in this way can develop an unusually aggressive tendency to surge after collapses. The tendency to enter a spin can also increase during turns and delay the recovery after a parachutal stall or full stall. A particular problem is that pilots do not normally notice such line changes, partly because the shrinkage process is gradual and takes place over a long period of time.
A Look at Materials Science To understand line length changes, it is worth taking a look at materials science. Due to their physical properties, all types of lines shrink to a certain extent. Factors such as buckling, bending, abrasion, dirt and moisture have a reinforcing effect. Lines become slightly thicker at kinks, and the lost volume is recovered from the original length by shrinking. This is especially true of Dyneema lines, which are widely used in paraglider construction today due to their excellent properties of resistance to buckling, breaking loads and durability, coupled with their
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thin diameters and minimal elastic recovery values. If force is applied to lines, they recover most of the lost length. However, brake lines are only slightly stressed during flight. Although it sometimes feels different in hard thermals, the maximum forces acting on the main brake lines are only around 10 daN - i.e. ten kilos. In fact, the higher line levels are only loaded by a fraction of that. Thus the ca. 10 kilos of tension on the brake handle of a Mescal amounts to only 300g on each individual line of the uppermost brake gallery. The extent of the line shrinkage is strongly dependent on how the glider is handled during take-offs and landings on demanding surfaces, on the practice slope, during ground handling and packing, and whether the lines get wet and dirty in the process.
Do-it-yourself Easy-Trim-Check In cooperation with the DHV, skywalk has developed a method that allows every pilot to check the length of the brake lines easily and quickly. As a reference for the “Easy-Trim-Check” the A-lines are used, which are only slightly affected by length changes due to the high forces acting on them and are largely constant in length. Every pilot can easily check the length of a brake line himself by comparing it with an A-line at a defined and easily found point over the entire length from canopy to the brake line knot. Because the difference between the length of the complete A-line and the length of the entire brake line is less than one meter for almost all glider models, nothing more than a normal measuring ruler or tape measure is needed. For glider models where the same line material is used on the brake lines over the whole span and where the fork lines do not have extremely different lengths, it is sufficient to check the length of the brake line only at one point to derive conclusions about the shortening.
