Finely woven organic fabrics, such as Irish linen and cotton,  were the original fabrics used for covering airframes, but  their tendency to sag left the aircraft structure exposed to the  elements. To counter this problem, builders began coating the  fabrics with oils and varnishes. In 1916, a mixture of cellulose  dissolved in nitric acid, called nitrate dope, came into use as  an aircraft fabric coating. Nitrate dope protected the fabric,  adhered to it well, and tautened it over the airframe. It also  gave the fabric a smooth, durable finish when dried. The  major drawback to nitrate dope was its extreme flammability. produced with fabric coverings.  [Figure 3-1]  The nitrate/ butyrate dope process works well, but does not mitigate  the short lifespan of organic fabrics. It was not until the  introduction of polyester fabric as an aircraft covering in  the 1950s that the problem of the limited lifespan of fabric  covering was solved. The transition to polyester fabric had  some problems because the nitrate and butyrate dope coating  process is not as suitable for polyester as it is for organic  fabrics. Upon initial application of the dopes to polyester,  good adhesion and protection occurred; as the dopes dried,  they would eventually separate from the fabric. In other  words, the fabric outlasted the coating.  Eventually, dope additives were developed that minimized  the separation problem. For example, plasticizers keep the  dried dope flexible and nontautening dope formulas eliminate  separation of the coatings from the fabric. Properly protected  and coated, polyester lasts indefinitely and is stronger  than cotton or linen. Today, polyester fabric coverings are  the standard and use of cotton and linen on United States  certificated aircraft has ceased. In fact, the long staple cotton  from which grade-A cotton aircraft fabric is made is no longer  produced in this country.  To address the flammability issue, aircraft designers tried  a preparation of cellulose dissolved in butyric acid called  butyrate dope. This mixture protected the fabric from dirt  and moisture, but it did not adhere as well to the fabric as  nitrate dope. Eventually, a system combining the two dope  coatings was developed. First, the fabric was coated with  nitrate dope for its adhesion and protective qualities. Then,  subsequent coats of butyrate dope were added. Since the  butyrate dope coatings reduced the overall flammability of  the fabric covering, this system became the standard fabric  treatment system. The second problem, lack of durability, stems from the  eventual deterioration of fabric from exposure to the elements  that results in a limited service life. Although the mixture of  nitrate dope and butyrate dope kept out dirt and water, solving  some of the degradation issue, it did not address deterioration  caused by ultraviolet (UV) radiation from the sun. Ultraviolet  radiation passed through the dope and degraded not only the  fabric, but also the aircraft structure underneath. Attempts to  paint the coated fabric proved unsuccessful, because paint  does not adhere well to nitrate dope. Eventually, aluminum  solids were added to the butyrate coatings. This mixture  reflected the sun’s rays, prevented harmful UV rays from  penetrating the dope, and protected the fabric, as well as the  aircraft structure.  Re-covering existing fabric aircraft is an accepted maintenance  procedure. Not all aircraft covering systems include the use  of dope coating processes. Modern aircraft covering systems  that include the use of nondope fabric treatments show no  signs of deterioration even after decades of service. In this  Regardless of treatments, organic fabrics have a limited  lifespan; cotton or linen covering on an actively flown aircraft  lasts only about 5–10 years. Furthermore, aircraft cotton has  not been available for over 25 years. As the aviation industry  developed more powerful engines and more aerodynamic  aircraft structures, aluminum became the material of  choice. Its use in engines, aircraft frames, and coverings  revolutionized aviation. As a covering, aluminum protected  the aircraft structure from the elements, was durable, and  was not flammable.  Although aluminum and composite aircraft dominate modern  aviation, advances in fabric coverings continue to be made  because gliders, home-built, and light sport aircraft, as well  as some standard and utility certificated aircraft, are still  Figure 3-1.  Examples of aircraft produced using fabric skin.  3-2