Canopy damage that can be easily overlooked

During the last few months the workshop has been very busy servicing wings in anticipation of the new season. A small proportion of those gliders have needed repair work when the pilot was unaware that there was any damage. So what kind of damage can easily go unnoticed?

We can see from the jobs we get in that it’s easy to miss broken or pulled stitching. Most stitching is internal to the wing, but there are three areas where external stitching is often used: the trailing edge itself, mini ribs near the trailing edge, and along the leading edge next to the air intake. It’s the exposed threads near the back of the glider that are the ones most likely to get caught on vegetation when ground handling. A missing stitch here or there may not represent a significant danger, but a stitch in time saves nine, they say.

Abrasion can be difficult to spot unless you look quite closely. Mostly it appears to be caused by contact with something solid like a hard surface, rock or post. Occasionally we see fabric with friction burns from lines that have become tangled around a wing and then come under tension. It only takes a fraction of a second to wear through fabric if there is a combination of significant pressure and rapid movement.

Nylon rods can concentrate pressure onto a small area of fabric, increasing vulnerability to abrasion damage. The first generation of wings to use wires, designed between 2010 and 2012, have a particular problem as their nylon rods often terminate at the surface of the wing: many subsequent designs set the end of the wire a little inside the cell. For all wings, the tight curves of the top surface at the leading edge, and the fabric over any wires towards the rear of the wing are both places that you should examine carefully if you’ve had any exciting moments ground handling recently.

Our suspension lines are very strong, so if one of them gets caught causing the tension on the wing becomes concentrated on just a few points, the line tab may separate from the wing, or the glider fabric may tear at the seam next to the line tab. Again, that’s something to look out for carefully after a tangle.

You may not be familiar with the term “gravel rash”, but it refers to the peppering of a section of fabric with small holes typically 1 – 5mm long caused by treading on a wing. It may not have been apparent that there were stones or gravel just under the surface of a grassy field. Although each hole or groups of holes can safely be patched, that could leave the panel looking badly pock‑marked, and replacement of the affected fabric is the better option.

It’s rare for us to find significant tears in the canopy when the pilot thinks his wing is intact, but it does happen. More often we find damage that is more extensive than was thought. It’s understandable that we hope the damage isn’t so bad, and maybe we can be reluctant to look carefully for every last rip. It’s quite common for the owner to direct our attention towards one part of the wing, only for us to discover after a full examination that another area of the wing has also sustained damage. We had one fairly new wing in recently which a pilot had bought as seen after a mate’s accident, thinking the damage was significant, but limited. It turned out that 12 cells were badly ripped, nearly twice as many as he had thought, turning a bargain into a rather more expensive purchase.

Well executed repairs are every bit as strong as the original. There is usually a range of options to make a repair, but better serviceability is rarely a factor: rather, the difference between cheaper and expensive options is purely a matter of cosmetics. The most expensive option, replacing the entire damaged panel, can be indistinguishable from new if the fabric matches exactly. That may be worth doing for a newish wing, but for older wings, new fabric may well stand out from the existing canopy, undermining the benefit of selecting this approach.

Replacing only the damaged section of the panel will be less expensive, and is the most frequently chosen repair method. There will be at least one additional seam visible, perhaps across the cell or parallel to the existing cell walls, but with care, the visual impact can be minimised.

Patching damage without sewing is very simple and easy to do, but unlike a permanent sewn repair, should be regularly checked to confirm that the patch is still adhering properly and doing its job.

Patching small tears yourself

A small tear, well away from the seams can be simply patched with sticky repair tape. But what are the potential complications?

  • Size – as a rule of thumb, if you can put your thumb through the hole, it’s not a small hole. You may still be able to tape it safely, but tape both sides, and leave at least 25mm around the tear for good adhesion and to spread the tension in flight.
  • Seam – another rule of thumb: if it’s closer to the seam than the width of your thumb, then it’s close to the seam. As a temporary fix, make sure you put tape well across the seam, and regularly check before every flight to ensure that the movement of fabric at the seam is not causing the patch to detach.
  • Square corners are best avoided. Round the corners of your patch to reduce their vulnerability to starting to peel off.
  • Stickyness – the coating on some fabrics makes them more difficult for repair tape to adhere to, so try taping on the other side of the fabric. If neither side works well, an ugly-looking gaffer or duct tape repair could represent a better option if you really must have one immediately. Bear in mind other tapes use different adhesives that may damage the fabric over time and necessitate replacement of the panel when a permanent repair is made.

If you want to chat anything through, we’re always happy to offer help and advice over the phone (01433 627195) or by email (info@aerofix.com).

2018-10-10T12:17:31+00:00Canopy, Repairs|