Easy Trim Check

The EASY TRIM

In 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

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.

Picture 1; The Easy-Trim-Check can be carried out with a helper or alone. If there are two of you, the helper on the canopy holds the innermost loops of the A-line and the brake line firmly together at the same height. Picture 2; If no helper is available, the two loops are simply looped with a short auxiliary line and attached to a fixed point - for example on the wall of the room, a door handle or a tree. Measurements are always taken to the end of the lines. Loops and gathering systems are not taken into account! Picture 3; Ideally, the measuring point is the A-line shackle on the riser and the knot of the brake line above the brake handle. Both lines are now tensioned with the same force. A tension of five kilograms per line is completely sufficient. Picture 4; With the ruler/tape measure, the difference between the brake knot and the line shackle of the A-line is now measured and compared with the target value. Alternatively, the target value can be marked on the brake line beforehand with a marker pen. After the measurement has been taken, it is repeated on the other side of the wing.

5Picture 5; Deviations of two to three centimetres are completely uncritical. In case of larger differences, however, it is absolutely necessary to adjust the length of the brake lines. For this purpose the brake knot is loosened and the brake line is adjusted to the specified value with the knot specified in the manual.

Detailed instructions can also be found on the skywalk website at: skywalk.info/2019/09/10/info-skywalk-easy-trim-check/

Frequent flyers in particular and all pilots who practice often with their glider on the training slope should regularly check the brake line lengths. This is especially important for pilots who shorten the empty brake travel by a few centimetres because they expect a more direct or faster response to brake line impulses. With the “Easy-Trim-Check”, every pilot can do the checks themselves.

If you need professional support, just knock on the door of your dealer!