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A large majority of restoration projects - both antique and classi - use fabric as their outer cover. For this reason, fabric covering plays a large role in the restoration process of most airplanes. Fabric has been used to cover the structure on aircraft since the very beginning of aviation. The fabric used along with the process has evolved throughout the years to a very durable and appealing finish.
Often individuals are unsure as to whether or not they can accomplish the fabric-covering portion of their project. Let me assure you that you can do it. The Poly-Fiber manual, as an example, is very clearly written for the person who has never worked with fabric. There is also an EAA video available on the covering process that may be purchased from Aircraft Spruce. In addition, there are EAA SportAir workshops that provide you with a weekend of "hands-on" covering experience. You should also realize that you will save a considerable amount of money if you do this portion of the project yourself. If you follow the manual you can install fabric and coatings on your airplane that will not only have a quality appearance but that will provide a service life of over 15 years.
Covering a production airplane that you are restoring imposes a restriction requiring you to apply a fabric covering process that meets or exceeds the specifications of the original fabric process that was installed by the airplane manufacturer. To do this you must use a covering system that has been issued a Supplemental Type Certificate (STC) by the FAA. In addition, the materials that comprise this STC must be manufactured according to a Parts Manufacturing Authorization (PMA) also issued by the FAA. Your particular type of airplane must be listed under the STC of the respective fabric process for you to use it as a replacement for the original fabric. Each company manufacturing covering materials maintains a listing of all aircraft that are approved to use their respective system.
Each approved fabric process must have a manual outlining how to apply the fabric and coatings. This manual is an integral part of the STC and must be adhered to during the covering procedure. To use different fabrics or chemicals that are not part of the process voids the STC. What does this mean? If you are going to mix one type of fabric with another type of fabric tape that is not part of the manufacturer's STC your airplane is not considered airworthy and therefore is not legal to fly. If you mix and match the chemicals from different processes the same restriction applies. The best advice is to choose a covering system and follow their manual explicitly. They have tested their products for years and know what works and what will cause problems.
Most of the antique airplanes we will encounter were originally covered using grade A cotton fabric along with nitrate and butyrate dopes. Covering early airplanes with fabric required more work and skill than the processes that are available today. Grade A cotton was sewn in place on the airframe and then shrunk with water, which gave the cotton a moderate amount of tautness. Then several coats of clear nitrate were brushed on to fill the fabric's weave. As each coat of nitrate dope dried it shrank, pulling the fabric tighter. In other words, the shrinking dope continued to tighten the fabric.
Flammability was a major problem for airplanes covered with nitrate dope. Any spark generated from an accident would tend to ignite the nitrate covered fabric. And when exposed to the elements, nitrate dope didn't last very long. The search for an alternative coating resulted in the development of butyrate dope.
Butyrate dope essentially replaced the more flammable nitrate dope during WW II due to the fact that it weathered better and was less flammable. When you hear "old-timers" discuss a dope and fabric job, they are usually referring to the systematic process of using cotton fabric along with nitrate and butyrate dope. Because certificated cotton fabric isn't available today, this process is now outdated. Polyester fabric is the choice used on airplanes today, and it has changed the way we cover an airplane.
During the 1950's polyester fabric became available under the trade name of Ceconite. Rather than using dope to shrink polyester fabric (Ceconite), you can use the heat from a household iron. You then place non-tautening nitrate and butyrate dope over the fabric resulting in less shrinkage than occurred with the cotton fabric and tautening dopes originally used. Using nitrate and butyrate dope is still popular with many restorers today. It will provide a very high quality finish that is easy to repair. It is also easily rejuvenated after several years of exposure to the elements. The biggest disadvantage in using dopes still has to do with the flammability issue.
Many people restoring antique and classic aircraft want the deep finish that can be obtained by using dopes, and this type of finish adds to the aircraft's originality. However, it is more difficult to apply for the first time user than many of the other fabric processes available today.
During the early 1960's, Ray Stits developed a fabric covering system using polyester fabric. This system was developed using chemicals other than nitrate and butyrate dope. Stits wanted a system that would not support combustion and also one that would not continue to shrink fabric over time. (Nitrate and butyrate dopes will continue to shrink both cotton and polyester fabrics through the years. This shrinkage is not critical if the fabric is properly applied.) The Stits Poly-Fiber covering system was introduced in 1965 and rapidly found widespread use. It is still the most popular covering system today and it is marketed as simply the Poly-Fiber System.
Your first step in the covering process is to choose the type of fabric covering process you will apply to your project. This decision involves choosing the type of fabric you will use. In choosing a fabric for your airplane project you have limited choices. Polyester is the main type available today. Grade A cotton has all but faded away. To my knowledge, there is no Grade A fabric available today that meets FAA requirements. Fabrics are certified by the FAA for use on production aircraft through Technical Service Orders (TSO). TSO C-15d applies to all aircraft fabrics including Grade A Cotton. Fabrics are certified for use on production airplanes and should have a Parts Manufacturing Authorization (PMA) stamp on the fabric itself. All types of polyester fabric are basically the same regardless of their name. Poly-Fiber and Ceconite fabric both are loomed to the same specifications.
After you have selected the type of process you will use you should then consult the associated covering manual for the proper methods of application. I cannot emphasize enough that you follow the manufacturer's directions. There are also a few pitfalls that you need to be aware of before beginning the covering process. Prior to listing these specific problem areas, it is important to note that there are a few general dos and don'ts to remember. The first is not to rush the covering process. Many builders push to complete an aircraft for an airshow or convention (I am sure you have never done this). Rushing through the fabric covering stage is not conducive to completing a trophy-winning airplane. Fabric covering involves the spraying of several coats of chemicals. Each coat must thoroughly dry before the next one can be applied. Allow plenty of time for each coat to dry. Another mistake made by many first time coverers is to tackle a large surface initially. Do not start covering a wing or fuselage without practicing. Start with a practice panel and then proceed to a small control surface. Then, if you do make mistakes you are not going to have to spend a fortune on materials to correct the problem.
There are seven major problems encountered in covering aircraft. This list incorporates the most common reasons aircraft owners have to recover their airplanes prematurely. These problems include:
1. Inadequate preparation of surfaces to be covered
2. Selecting the wrong weight of fabric for the airplane
3. Improper tautness of the fabric after it has been applied
4. The 1st chemical coat improperly applied
5. Inadequate protection from the UV rays of the sun
6. Topcoat problems
7. Not following the procedure manual
STEPS OF FABRIC COVERING
Most methods of fabric covering will enlist essentially the same steps. The following are found in a typical fabric covering process:
• Removal of old fabric (restorations)
• Preparation of surfaces
• Inspection of surfaces
• Selection of fabric type
• Attachment of fabric to structure
• Shrinking fabric
• First chemical coat
• Securing fabric to wings and control surfaces
• Taping, inspection holes, and drain grommets
• Spraying initial coats of chemicals
• Protecting fabric from UV rays of the sun
• Application of color coats & trim
I want to conclude by repeating this admonition: regardless of the type of covering process you select it is absolutely imperative that you follow their manual to the letter. Do not experiment with fabric covering. By following the manual you will not only stay legal but the results will be a covering job that will last well over 15 years. |
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