The following was taken from Ultraflight Lazair Newsletter #8:
“In the past five months we have had four reports of loosening of Tedlar covering, apparently due to poor tape adhesion. As the reports are all quite different, there is no indication of any one particular problem, and therefore determining the cause (and remedy) is not easy. However, based on the information we have available, we can make the following suggestions.
(a) Avoid overheating the tape (and the Tedlar) when heat shrinking. As stated in the manual, overheating the tape will cause it to shrink excessively and will lift it at the edges. It is also probable that excessive heat will have an adverse effect on the adhesive. Overheating the Tedlar will cause it to shrink excessively and could tend to pull it away from the tape.
(b) When cleaning the aluminum prior to the application of the tape (whether on a new aircraft or when recovering) use only lacquer thinner as suggested in the assembly manual. There is some indication (but no proof) that the use of acetone for cleaning the aluminum may effect the acrylic adhesive on the tape. Do not use metal cleaners (such as Met-All, Nev-R-Dull, Flitz etc.) as many of these are designed to apply a protective coating as well as clearnthe metal. These coatings (especially the ones which contain silicones) can severely impede tape adhesion.
(c) Make sure there Is sufficient overlap of tape on the aluminum (as described in the assembly manual), especially along the D-cell and along the root rib. If in doubt, additional tape should be applied with at least 3/4 of an inch in contact with the aluminum.
(d) If there is any indication of Inadequate adhesion around the perimeter of the Tedlar, some of the wide single face tape could be removed and replaced, or additional tape could be applied as In Cc) above.
(e) Lack of adhesion of the foam tape on the ribs, while not a common problem, could be a bit more difficult to fix. We have only seen this problem once, and the effected area was so small, it was just left (though watched closely) and the condition has not worsened. If you should ever encounter this situation (and assuming you don’t wish to recover the wing), you could rivet an additional aluminum capstrip to the effected ribs on the outside of the covering (similar to C4 on the wingtip’). However, if you do this, be sure to put at least one layer of 1 1/2″ or 2″ tape over the Tedlar before the capstrip Is put on and use double face tape under the capstrip. Be sure to file or sand the edges of the capstrip so they do not cut Into the covering. In any case, do not (as one customer suggested) attempt to rib stitch the Tedlar. Rib stitching a non rip-stopped material could potentially create many more problems than it could cure. While the additional capstrip suggested above does necessitate drilling rivet holes through the covering, the stress on the covering is distributed by the relatively large area of the capstrip. If rib stitching were used, the stress would be much higher due to the small diameter of the rib cord.
(f) If you paint your Tedlar and/or tape, use a light colour. There is some indication that painted a dark colour. and left in direct sunlight for a prolonged period, the covering creep under the tape due to the extremely high temperature developed.
It is may tend to
(g) To check for overshrinkage on your wings, put a straightedge on the trailing edge and measure the deflection of the T25 trailing edge tub& between each pair of ribs. A deflection of one sixteenth of an inch is about right. An eighth of an inch is excessive but acceptable. A quarter of an inch deflection indicates that the particular panel has been overshrunk. The covering and tape on that particular panel should be inspected and watched very carefully or replaced.
(h) An inspection of the covering and tape should be included in every preflight.
The following two paragraphs have been added to the assembly manual, and should be observed if you recover your Lazair’·:
“As with most acrylic adhesives, the initial tack with this tape is only moderate, but the adhesion improves as it ages. For this reason, it is essential that the tape be firmly pressed down to make sure there is 100 percent initial contact. Then, as the adhesive cures, a proper bond will develop.”
“Unlike Mylar and most other heat shrinkable covering materials, Teldar will continue to shrink significantly after the heat source has been removed. Therefore, to avoid overheating the Tedlar, apply the heat for a few seconds, then remove it and check for signs of shrinkage. If there is no indication, heat it a bit longer, then remove the heat and check again for shrinkage. As the heating period is Increased, you will find the correct exposure so most of the shrinkage will occur after the heat source has been removed. If the heat is maintained on the Tedlar for a significant period of time after it begins to shrink, it is possible to overheat the material and reduce the adhesion of the tape.” ”