July 2008 Issue
Build Your Skills: Fabric
We begin our new fabric series with a bit of a history lesson.
Aircraft covered with fabric have been in existence since the very beginning of flight. During the late 1800s, kites and gliders were constructed and flown. The Wright brothers' curiosity resulted in their study of several glider and kite designs, each using fabric material as a covering. They discovered that without fabric on a wing it was useless as a lifting surface. In 1899, the Wrights constructed and flew their first kite from which further knowledge of flight was gained, and it was then used in the design of their first powered aircraft.
All of these designs were covered with a fabric material to hold the structure together in addition to providing the foundation for lift. Fabric also provided for flexibility of the wing structure, allowing control of the aircraft using the wing-warping technique.
The first fabric the Wright brothers employed was "Pride of the West" muslin. It was a tightly woven cotton material used largely for women's undergarments. Strips of fabric were sewn together to provide covering for the wings and control surfaces. For later model aircraft they used cotton infused with rubber, making it airtight and waterproof.
Along Comes Cotton and Linen
Aviation pioneers largely used cotton and linen material for covering aircraft through WW-I. The fabric used on aircraft was later termed Grade A cotton, a name that is still recognized today, and it became the standard for builders of early airplanes. They covered their aircraft with fabric because it was light and easy to apply.
But just covering the wings with fabric wasn't enough. Just as the Wright brothers learned, early aviators found that unless they tightened and sealed the fabric, the wings would not create lift, so they used varnish to seal the fabric. However, they discovered that the varnish would soon turn yellow and crack. They tried furniture lacquers next, with limited success. Finally, they developed furniture lacquer into nitrate dope, and the dope we use today is still made from the same raw material used in lacquer
The term dope as often used in our society today has an obvious and negative connotation. But to people who cover their airplanes with fabric, it has an entirely different meaning. Used for decades, the term describes a liquid that seals and tightens an aircraft's fabric. Nitrate dope is flammable, and problems associated with this hazard were an issue during the early days of flying. A crash with any type of spark would almost certainly cause a destructive fire. Mechanics doing welding repairs or a careless cigarette smoker could cause major destruction of aircraft covered in cotton and nitrate dope.
Polyester material was developed in England during the 1940s. DuPont obtained the rights to manufacture this product in the United States under the trade name Dacron. This fabric, first manufactured in the U.S. in 1953, was widely used in many industries, including the garment trade. During this time, DuPont published information on shrinking this material through the application of heat. Colonel Daniel Cooper, a military officer, began testing polyester fabric for aircraft applications. In 1958, Cooper termed his new material Ceconite and began selling it to cover aircraft.
Butyrate dope had also been developed and was being used as a coating over aircraft fabric. It has the advantage of being less flammable than nitrate, but the disadvantage of not adhering to polyester fabrics. The old nitrate dope was found to stick on polyester fabric very well. Colonel Cooper later began applying nitrate dope to polyester fabric for the initial coats followed by finishing with butyrate dope. This process lessened the fire hazards associated with an all nitrate system, and it's still in use today.
Today's Covering Processes
The aircraft builder or restorer has three different types of covering systems available to cover an airplane. The first is polyester or Grade A cotton fabric using nitrate and butyrate dope as a chemical process. The most common polyester cloth is Ceconite, using Randolph dopes. Legal Grade A cotton is difficult to obtain. There are a couple of sources of cotton or linen material, but you need to ensure that these fabrics meet the Supplemental Type Certificate (STC) requirements of the dope manufacturer. Also, certain Technical Service Orders (TSOs) apply to fabric. For example, medium weight Ceconite meets the requirements of TSO C-15d. If you are covering a production aircraft, you must stay within the STC guidelines. The Ceconite STC allows the use of aircraft grade nitrate and butyrate dope.
The next type of system is polyurethane, and there are two systems to choose from: Air Tech and Superflite. The Air Tech covering system uses polyurethane products throughout, including primers, coatings and topcoats. Air Tech uses any C-15d polyester fabric including both Ceconite and Poly-Fiber fabrics. Superflite also has its own STC using Superflite polyester fabric.
The most widely used covering system today is the Poly-Fiber system. This process was formerly known as the Stits covering process named after the inventor of the system, Ray Stits, who developed it in the mid-1960s. His system uses vinyl-based chemicals that do not shrink the fabric after they are applied, nor do they support combustion. This system is entirely different from the others. It is easy to use and is supported by a detailed manual and video/DVD.
First-time coverers should investigate each process and choose the one that they want to work with. The cost of each system is within the same range. Issues to consider are flammability, ease of use, longevity, final appearance and quality. Another important issue to consider is availability of instruction. The Experimental Aircraft Association (EAA) presents fabric covering workshops throughout the country on a regular basis. They are called EAA/SportAir workshops. More information and a schedule may be found atwww.sportair.com. Workshops and forums are also conducted at major fly-ins including Sun 'n Fun and AirVenture.
Regardless of the process you choose, you must adhere to the materials and instructions specific to that system. In other words, do not mix and match covering systems. Pick one and stay with it. Do not use one type of fabric, another type of coating and a different type of topcoat. This not only voids the STC, but several of these products are not chemically compatible. You will have problems mixing systems—guaranteed.
Legality of Covering Your Own Airplane
Can you legally do the covering work on your airplane project without a mechanic's license? If you are building an Experimental airplane, doing the fabric work is legal. There are no restrictions. Should you be restoring an antique or classic production airplane that requires fabric work, you are allowed under FAR Part 43 to cover the airplane under the supervision of a licensed mechanic. Now, if you are covering a production aircraft, you have a further 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 manufacturer. To do this you must use a covering system that has been issued an STC by the FAA. In addition, the materials included in the 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 maintains a listing of all aircraft that are approved to use their respective systems.
Each approved fabric process must have an instruction 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 tape that is not part of the manufacturer's STC, your airplane will not be considered airworthy. If you mix and match different chemicals, in the same way the final product will not be legal. Again, the best advice is to choose a covering system and follow the manual explicitly. Each company has tested its products for years and knows what works and what will cause problems.
If you are covering an Experimental aircraft, you are free to choose any type of fabric and chemicals you desire to complete this process. However, common sense should prevail. If you do not follow the manufacturer's covering manual, you will have the same disastrous results that occur from mixing systems. Your Experimental aircraft flies through the same airspace as a production airplane, and from a maintenance standpoint it should be treated with the same care. Just because a custom aircraft builder can legally mix systems and experiment with products, the temptation to do so should be avoided. Otherwise, you may be re-covering your airplane within a short period of time.
Often people are unsure about whether they are capable of covering their airplanes. Let me assure you that you can do it. Manuals, videos and/or DVDs help explain the covering process, and workshops provide you with a weekend of hands-on covering experience. You will save a considerable amount of money if you do this work yourself. Typically, a shop will charge $10,000 or more to cover an airplane depending upon the type. This along with approximately $3500 for materials adds up. If you follow the manual you can install fabric and coatings on your airplane that will not only result in a quality appearance but will also provide a service life of over 15 years. So let's get to work and learn the steps of fabric covering.
Tools and Workshop Space
At this point you will need to decide where to do the actual covering and spraying. I recommend having your workshop as close to home as possible. If you have a garage, you have a workshop. Having your workshop in or close to your home will provide you with the opportunity to work more often and also to involve your family.
Fumes will be a problem. If you are working in your garage, you may want to cover the surfaces there and then take them to another location for spraying. You can, however, minimize the fumes and overspray by building an inexpensive paint booth with a wood or PVC frame that is large enough to house a wing or a fuselage and has room for you to walk around while you are spraying. You can hang the frame from your shop ceiling and then lower it with a set of pulleys. Cover the roof and sides with cheap plastic sheeting stapled to the frame. We'll have more on constructing a paint booth in a later article.
The Basic Steps
We'll be getting into much more detail as this series unfolds, but here are the basics.
Removal of old fabric (restorations only) is followed by the preparation and inspection of surfaces. Then comes the selection of fabric type, attachment of that fabric, followed by shrinking the fabric. Next up are the chemical coats, taping, making inspection holes and drain grommets, spraying UV protection, color coats and trim. Yes, that's a shorthand outline, but we'll be getting into great detail as the process unfolds.
But let's first talk about preparing the surfaces, which may vary somewhat depending upon the material involved. Usually, you will encounter aluminum, steel, wood or fiberglass as the underlying structure.
Always make sure you use epoxy primer on metal and fiberglass parts and epoxy varnish on wood parts. The majority of other primers and varnishes will be lifted from the aircraft surface by the chemicals found in covering processes. MEK and reducers found in covering process chemicals will not affect most epoxy primers and varnishes. But they will lift some paints and varnishes like a paint stripper, allowing moisture to collect in the metal or wood with obvious consequences.
If you have already primed or varnished with some other product, test it before applying the fabric. Soak a rag with MEK and leave it on the surface for about 30 minutes. If it lifts the paint or varnish, you need to redo the surface. Often you can simply spray epoxies over the existing finish without having to strip them. This, too, can be tested. Spray the epoxy over the existing surface on a small area to be sure it does not act as a paint stripper prior to applying it to the entire piece.
Before priming or varnishing a surface, be sure it is completely clean and free of all oil and other contaminants. Do not prime over rusted pieces. Remove the rust and immediately prime. A piece of bare steel will rust within hours if a primer is not in place. Make sure you fill all dents in leading edges, etc. I would recommend a product called SuperFil rather than Bondo, which is a polyester filler that will shrink with age. I do not recommend using Bondo on aircraft surfaces unless you are prepared to redo the filled portion of the surface after it shrinks and cracks the topcoat. SuperFil is an epoxy filler. That means it will not shrink over time. It may be used on wood, fiberglass or metal with equally good results. You may want to use cloth padding on a really dented leading edge. Polyester padding is often used between the leading edge aluminum or plywood and the fabric itself.
All of the sharp edges that could potentially cut the fabric should be covered with anti-chafe tape. This usually includes rivet heads and metal seams. Let your sense of touch be your guide. If you feel something sharp, cover it with the anti-chafe tape. Do not use masking tape for this purpose. It will retain moisture and cause problems later on. Also, paper masking tape will turn brown with age and possibly show through a light colored paint.
That's it for the background and some of the prep work. In the next issue, we'll continue with a detailed presentation of the covering steps and processes you'll be using on your own project.
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