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Double covering fabric covered wings
I just found out about a method being used by some experimenters to
cover their wings that is intriguing. They are literally covering the wings twice with fabric. The technique involves covering the wing initially with a heavy grade fabric, taughten it with the iron as specified, apply the rib tape and then stitch the ribs as normal. Up to this point, everything is absolutely by the book. But then, they apply a lightweight second layer of fabric on top of the original covering using polytack. The second layer becomes a defacto enormous tape. What I mean is normally, the next step after stitching is to cover the stitches and leading edges with pinked tapes. In effect this method is doing that, it's just that the tape is the width of the entire wing. This second layer is then ironed with the iron at it's lowest calibrated setting. I got to inspect two wings side by side, one with the finished double covering and the other one with just the single fabric (prior to it getting the second layer). The difference between the two wings was incredible. The double layered wing seemed like it was covered with sheet aluminum compared to the single layered wing. Snap your finger on the double layered wing and you heard a reasonant "PING". Do the same to the single layered wing and you hear a dull "thud". The guy doing the covering said that he understood that this method for wing covering was a certified process for the Beech Staggerwing. He's seen several airplanes with the two layers at airshows and they really impressed him. Why would anyone want to do this? Well it makes a very stiff fabric, no bulging up between the ribs. It also eliminates the pinked tapes applied on top of the stitches and elswhere because the second layer constitutes those tapes. There is much less sanding required because you don't have to spray and sand around each of the pinked tapes. Basically you just spray the proper silver and primer, scuff slightly and you're ready for the color coat. Has anyone else heard of this method? Corky Scott |
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On Thu, 06 May 2004 15:04:54 GMT, Richard Lamb
wrote: The Poly Fiber manual specifically recommends NOT to do this because Poly Tack, when dry, it too hard to stand up to the flexing of the fabric. That would mean using Poly Brush to attach the second layer - and how now do we shrink the second layer??? Richard Lamb I dunno. The second layer of fabric might be applied with polybrush and not polytac. I'm not familiar enough with the process to state accurately. I'll find out. As to shrinking the second layer, that's done with the iron calibrated for 250 degrees. I've seen the results and even without any anything applied to the fabric to seal it yet, it looks very nice. Corky Scott |
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On Thu, 06 May 2004 19:57:24 GMT, Richard Lamb
wrote: 250 degrees is only the _starting_ point. Last pass is at 350 degrees! If the first layer has been coated with Poly Brush, the heat from the iron will quickly soften the coating. PB starts melting at 200 degrees. By the time the iron is up to full temp, it would be hard not to melt into the previous PB layer - possibly causing fabric pulls and gouges. Maybe the outter thin fabric doesn't have to be tightened that much? As long as the first layer is strong enough, the second might only need to be snug enough to pull out the wrinkles. Call it non-structural? Second pass at 250 on light fabric _might_ do it? Bottom line, Corky, I don't really know much, I've never tried it. It seems like there may be some potential pitfalls in the doing part, but it sure did look pretty on that Pacer. Right, 350 is the final pass for the first layer. But the way this was explained to me, you definately do not want to go to 350 on this outer layer. I don't know whether you iron the pinked tapes or not, but if you do, do you go to the full 350 degrees? I thought not but I haven't actually covered a wing myself, I've just helped out here and there. Corky Scott |
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Corky ad Richard,
I have experience with this double covering method. I have done it a few times. It does not match the polyfiber manual but it worked very well on high power and acro airplanes. Feel free to email me directly if you would like more info on how it is done. If you look at the cover of the new June Issue of Kitplanes, 2 of the 4 model 12s on the cover are finished in this method. Kevin Kimball |
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KJKimball wrote:
Corky ad Richard, I have experience with this double covering method. I have done it a few times. It does not match the polyfiber manual but it worked very well on high power and acro airplanes. Feel free to email me directly if you would like more info on how it is done. If you look at the cover of the new June Issue of Kitplanes, 2 of the 4 model 12s on the cover are finished in this method. Kevin Kimball Thanks Kevin. Were we even close? Richard |
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Any idea of what the weight penalty would be in dope and fabric vs. reduced
drag? And is this method more or less expensive than pinked tape? Just wondering, Harry Shellac dope would have added 76 lbs. to the weight of a Wright 1902 glider, and almost 155lbs. to the 1903 "Flyer." ...Based on tests made with my 1899 kite, August 2001 |
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Corky, Richard and Harry,
Corky was pretty close to having the method down that we used. I have heard of those who have used polybrush for the bonding of the 2 layers. We don't. The example of the tri pacer mentioned is interesting in that if the airplane is still in standard catagory, the recoverer must have gotten a special field approval for using the method. We have not used it on certified airplanes. Only experimental. Jim Younkin did use the double cover method on 3 staggerwings he converted from D models to G models a few years ago. He got the covering included in the long list of field approvals for those 3 airplanes only. No STC for it. Double cover is not standard on staggerwings from the factory. The double covering we have done is based on the techniques used on these 3 airplanes. We made some changes in how the coatings are applied and have had great luck thus far. I would be more than happy to discuss the way we have done it with you directly. It is a bit too invloved to fully list here. If you are wanting a bare minumum finish and gloss or slickness is not important, don't double cover. If you have a low powered non acro airplane, don't use this method. This is a method that is not only intended for a slick finish but is more valuable as a tougher, stiffer, higher durability system for big engines, big props, and hard acro. The Staggerwing and pitts model 12 are good examples of airplanes that can benefit from such a covering system. It is not approved of by Polyfiber nor is covering wings and not using some form of mechanical fastener like stitches or screws. There are airplanes out there that simply glue the fabric to the ribs and have success because of the type of airplane it is ad the design of the structure. Double covering is not the norm but rather is a special use fix for a problem of losing tapes in the prop wash on big engine acro airplanes. The weight difference is always a question. I can only give example from the airplanes we have done which are pitts model 12s, 4 of them and the McCullocoupe. We have a standard finish we like to see which is good enough to place well at flyins. We use urethane top coat colors on all the airplanes we build. So, if we want the same finish on a model 12, we have to fill the 3.4oz heavy duty fabric with a few additional coats of polyspray, sand more, and possibly sand the color to get a satisfactory gloss out of the course fabric. This is by no menas a bury the tapes finish. The tapes will still be fully proud above the base fabric. Still, with all that effort, the weave is still strong through the paint and we have to use 4" tapes on the ribs to keep them from blowing off the wings. When we double cover, we end up with no tapes to blow off, 5.1 oz per square yard of fabric weight, a fine weave fabric to fill that takes far less coatings to do so and a finished fabric/coatings weight that is within 1 or 2 pounds of single cover on a model 12. A model 12 and a staggerwing have ribs that are very close together. When you use 4" wide tapes over the ribs, you have double covered the wing 70-80% with just very small strips of base fabric being exposed. The process is not for the first time cover job. You need to know your way around the fabric system before trying it. We advise our builders to go the standard way unless they have experience and are willing to go it alone without tech support from the covering supplier. Labor time is about equal to the standard method. Base fabric install is the same labor either way. Second layer can tape more time than taping would. But, sanding is faster. In terms of material cost, you can price 3.4 oz and 1.7 oz fabric for the size airplane you are covering. Then price tapes. Coatings cost will be basically the same. Kevin |
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