by Chappy » Fri Nov 26, 2004 7:52 pm
Hi Steve,
Maybe I can try to help a little more. Both the Mylar and the Tedlar films supplied by Ultraflight were of the stretched variety. When heated, they "shrunk", or more properly described, stress relieved, attempting to return to their original dimensions. They seemed to be stretched more in the long dimension than in the width of the material, but I'm not sure, as I never actually tested it. The later Mylar supposedly had some form of UV protection applied as a coating. I was very suspicious, but the last Mylar I used was much better at tolerating UV than the earliest Mylar I used. It was also much clearer, as the earlier material had a slightly cloudy appearance, although still much better than the Tedlar materials I have used. Both the Mylar and Tedlar I used had some form of lubricant on them straight from the roll, but it didn't seem to keep the adhesives from adhering to the films. Then again, the rubber adhesives used on the Mylar tapes seemed to stick noticeable better than the acrylic adhesives used on the Tedlar tapes, especially in shear. Over heating Mylar tapes caused the adhesive to burn and bubble up. When moisture got into this damaged adhesive, it turned white and powdery. The acrylic adhesive, when overheated, didn't exhibit the same visual signs. but the adhesive was damaged too. It would not "set up", and would get sticky and slip in shear much easier than if not overheated.
Fabric covering is fine, I just really like the look of Lazairs in clear or translucent coverings. I also find real value in being able to see clearly what's going on inside my wings. I think covering. and recovering, is much easier and faster when done in film/tape than when done in fabric/glue/UV block/paints. And I guess I just think our planes are cool because they are so unique looking in see-through films.
The Catto brothers pioneered film covering. They used Mylar on their first Gold Wing Ultralights - that's where Dale Kramer got the idea to use it. Back in the day, it was not unheard of to cover homebuilt hang gliders with plastic shower curtains. Dale covered his homebuilt Super Floater with it. Dale's contribution was to use "glue on a roll" (tapes) rather that the contact cements used by the brothers, and to try Tedlar. They used Plyobond glue, which Dale thought was messy and more difficult to use than tapes.
Twenty-25 years later, it's hard to believe there aren't newer film type materials available that would be even more suitable covering materials for our planes. I'm sure there are people on this list that have the intelligence to research and test these new materials if they decide to commit the mental effort and time to the task.
Many years ago, I played around with Tyvek and it looked like it might have potential, but needed to be glued with something like Plyobond, as tapes didn't have anywhere near enough grip on it. It would also need to be painted, and I never tested whether that was practical.
One winter I was able to rescue some metalized, red Mylar from a commercial Christmas display. I used it to cover the tail of my old Series 1. It was quite stunning in appearance. It was, however, not the best choice in the long run, as it failed in only a couple of years. There were several reasons for that I think. It was only 150 mills thick. Metalizing and/or coloring Mylar is done by post processing, and (at least back then) was done by companies other than DuPont. The coating process utilizes some heat in the process. Some, if not all, of the stretch was relieved out of the film. When attached to our planes, there wasn't a whole lot of shrinkage possible, so you ended up applying more heat than usual. Also, the metalized film reflects heat back at you. It just made it much easier to cook the tapes when attempting to tighten up the covering. Uncoated Mylar is very good at absorbing UV radiation. You can easily witness this in a Mylar covered Lazair that's been left out in the sun for extended periods of time. The top wing covering can become very weak, and actually fail and blow off the wing in flight. This happened to a friend's Lazair I was following one time. I thought he was throwing out foil pieces as he flew along. When we landed, I mentioned it to him. We looked at his plane, and he had lost the top panels on both wings behind the engines. The lower surface was like new, and since he couldn't even tell the panels were gone in the flight out, he flew his plane home with the missing panels, and had no problem at all (I'm NOT advocating doing that!). Anyway, the point being that the metalized coating, because it doesn't allow any UV light to penetrate through it, reflects the UV light back up through the material where it can do even more damage. The thinner material I used just rotted too quickly. Ultraflight also covered a factory demonstrator in silver metalized Mylar, and they told me they had the same problems that I experienced with my red covering. It was, however, AWESOME looking! It looked like the whole plane had been dipped in chrome.
I wish I could come up with the specs on the Mylar and Tedlar supplied by Ultraflight way back when, but I just don't know them ;-(
Chappy