A Lost Art
Fabric covering restoration
By Katie Heuermann
Working on fabric covered airplanes is almost a lost art. Many of us hone our fabric skills in A&P school, only to never put those skills to use after we get our certificate. However, there are still many fabric aircraft flying, and many more stored with clipped wings — waiting to take to the skies again. With the cost of all aircraft rising, restoring an old classic may be the only way for someone to realize the dream of flying. Tackling a restoration project may require a bit of review of what was learned in A&P school.
In the old days, the covering options were Grade A cotton or Irish linen fabrics, with nitrate, and later, butyrate dope. But, increases in cost and the short lifespan of these natural fabrics helped drive the market towards other materials. Today, there are many options. Some processes use a fiberglass fabric, some use polyester, others use vinyl instead of dope.
The original processes had a lot working against them. The natural fabrics tended to break down quickly from sun and mildew, and required covering every few years, especially since most aircraft were not hangared as they are now. The polyester fabrics are used now and though impervious to battery acid, still require protection from UV rays. If the UV blocking coats are not used, the fabric will lose 70 to 85 percent of its strength in one year if left outside. However, with a properly applied coating, the fabric’s service life is practically infinite.
More important than the longevity of the fabric covering is the flammability. The original cellulose dope was nitrate. It was extremely flammable, and butyrate was soon substituted. The flammability factor went down some, but it was still flammable, even after years of being on the aircraft. Poly-Fiber® Aircraft Coatings System and Covering System advertises that it is not flammable after the solvents have evaporated out after application. Poly-Fiber will not support combustion. The fabric itself is combustible, but after the process has been applied per the STC and cured, it will not burn — an important consideration when choosing what process to use. Ray Stits, developer of this system and founding member of EAA’s Chapter 1, demonstrates the flammability characteristics on samples of different processes in his video "Fabric Covering with Ray Stits."
Starting with a good foundation is essential. Ensure all structure is free of defects.
The first step in any restoration project is to start with a good foundation, in this case the framework. The steel tubing should be inspected to see if it has good welds, is corrosion free, solid, and is in reasonable alignment. It should be cleaned, prepped, and primed as necessary. Any bad pieces must be cut out and replaced using the welding method the maintenance manual or AC 43.13.1B recommends. Steel is normally used for overall strength, with wood or aluminum used to give form or shape, as in wing leading edges, longerons, ribs, and bows. Ensure any wood is free of cracks and rot, and is intact. Some glue joints may need help. Aluminum should be checked for proper alignment, cracking, corrosion, or any other damage.
Each process has its own products to be used in a given sequence, and for the most part they are not interchangeable. Poly-Fiber has its own STC, as does Ceconite®, etc. If the aircraft is covered in Poly-Fiber, and only one wing needs to be recovered, Poly-Fiber has to be used to cover it. The same process must be used on the entire aircraft. If recovering an entire aircraft that previously had Ceconite, and you want to go with Poly-Fiber now, first make sure Poly-Fiber has an STC for that aircraft, then file a Form 337 for a major alteration when the job is done. If your aircraft is not listed on the STC’s approved aircraft list, plan on a field approval.
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