Last month I looked at glider fabrics and factors affecting porosity. We saw how the fabric used nowadays is much lighter than it used to be. So how strong does it need to be to be safe, and how should you go about repairing any damage to your wing?
A decade ago, most gliders were made of fabric weighing 45 grams per square metre. But gliders today typically use fabric that is less than 40 grams, with lightweight designs down to as little as 27 grams. During that time, technological advances have resulted in gains in fabric strength, and the glider strength shock test used in the certification process has remained the same over this period. So we can be confident that a glider when new will be as strong as ever, despite the use of lighter fabrics.
But what about once we’ve been using our wing for a few years? Well, the factors I mentioned last month, like UV, heat and damp, and even simply the passage of time, all take their toll on the strength of fabric. Telltale signs that would give us cause for concern about a glider’s fabric strength include bleached-out colours (indicating a lot of exposure to UV), the fabric feeling soft and a poor porosity result.
In the workshop we use a Bettsometer to test the fabric strength. It’s a simple shaped needle connected to a spring balance. Keeping the fabric slightly taught, the needle is pushed through the fabric and then pulled with a force of 600 grams. [Picture of Bettsometer] The needle has to cause a tear going through the ripstop for the glider to fail. It’s rare for us to use the Bettsometer, and even more rare for a glider to fail: in fact, none have failed so far in 2016.
In practice, your glider is more at risk of previous damage becoming worse than it is of weakened fabric suddenly starting to rip simply under tension. In particular, an existing tear could worsen much more readily on a wing with weaker fabric. Stress on the surface of a wing tends to become concentrated around the end of a tear, with the risk of it ripping further. In general, the closer to the leading edge, or the centre of the glider or to a seam, the greater the tension on the fabric in flight is likely to be. And tension is greater across the span of the wing than it is along the chord from leading to trailing edge.
Small repairs can easily be made good by yourself (see panel). A professional workshop can usually make a repair in one of four ways, which are set out in the table below. All of them will be safe, but the choice probably depends on how you want the glider to look after the repair, and also the price you want to pay.
|Patch||Small holes or tears||Patch will match canopy but appear slightly darker when lit from behind|
|Sewn patch||Larger tears or damage nears cell wall seams, leading or trailing edge||Sewing will make a large patch more visible than an unsewn patch|
|Panel section||Significant damage to be repaired cost effectively||Limited – additional seam across the panel at each end of the new section|
|Replacement panel||Extensive damage, or to preserve resale value of a nearly new glider||As new if the fabric matches exactly. If canopy is less than pristine, the new panel may stand out|
We often recommend a panel section repair [Picture of panel section repair] as the best value, quickest and most appropriate approach. When making a repair [Picture of Sebastian], matching the appearance of the existing fabric exactly can’t be guaranteed. Often a workshop will have suitable cloth in stock, and if not, the glider manufacturer should be able to supply the original cloth. However, there can be minor variances in appearance between batches of the same fabric.
Patching small repairs yourself
If you have a small rip on your wing, sticky repair tape should be fine to get you back in the air again.
- If the rip is more than a few centimetres long, you may want to think about putting patches on both inside and outside.
- Don’t cut the patch too small – make sure you leave enough of the repair tape around the rip to achieve a good adhesion and spread the tension.
- Round the corners of your patch as sharp corners are slightly more vulnerable to starting to peel off.
- If the damage is close to a seam, you can make a patch extending over the seam, but unless the damage is very limited, it may be worth getting it professionally repaired. The normal movement of the fabric around a seam may start the patch detaching, so keep an eye on it.
If you want to chat anything through, we’re always happy to offer help and advice over the phone (01433 627195) or by email (firstname.lastname@example.org).