Nonmetallic Aircraft Materials

The use of magnesium, plastic, fabric, and wood in aircraft construction has nearly disappeared since the mid-1950s. Aluminum has also greatly diminished in use, from 80 percent of airframes in 1950 to about 15 percent aluminum and aluminum alloys today for airframe construction. Replacing those materials are nonmetallic aircraft materials, such as reinforced plastics and advanced composites.

Wood 

The earliest aircraft were constructed of wood and cloth. Today, except for restorations and some home-built aircraft, very little wood is used in aircraft construction.  

Plastics 

Plastics are used in many applications throughout modern aircraft. These applications range from structural components of thermosetting plastics reinforced with fiberglass to decorative trim of thermoplastic materials to windows. 

Transparent Plastics 

Transparent plastic materials used in aircraft canopies, such as windshields, windows and other similar transparent enclosures, may be divided into two major classes or groups: thermoplastic and thermosetting. These plastics are classified according to their reaction to heat. Thermoplastic materials soften when heated and harden when cooled. These materials can be heated until soft and then formed into the desired shape. When cooled, they retain this shape. The same piece of plastic can be reheated and reshaped any number of times without changing the chemical composition of the materials. 

Thermosetting plastics harden upon heating, and reheating has no softening effect. These plastics cannot be reshaped once being fully cured by the application of heat.

In addition to the above classes, transparent plastics are manufactured in two forms: monolithic (solid) and laminated. Laminated transparent plastics are made from transparent plastic face sheets bonded by an inner layer material, usually polyvinyl butyryl. Because of its shatter resistant qualities, laminated plastic is superior to solid plastics and is used in many pressurized aircraft.

Most of the transparent sheet used in aviation is manufactured in accordance with various military specifications. A new development in transparent plastics is stretched acrylic. Stretched acrylic is a type of plastic, which before being shaped, is pulled in both directions to rearrange its molecular structure. Stretched acrylic panels have a greater resistance to impact and are less subject to shatter; its chemical resistance is greater, edging is simpler, and crazing and scratches are less detrimental. 

Individual sheets of plastic are covered with a heavy masking paper to which a pressure sensitive adhesive has been added. This paper helps to prevent accidental scratching during storage and handling. Be careful to avoid scratches and gouges which may be caused by sliding sheets against one another or across rough or dirty tables. 

If possible, store sheets in bins that are tilted at approximately 10° from vertical. If they must be stored horizontally, piles should not be over 18 inches high, and small sheets should be stacked on the larger ones to avoid unsupported overhang. Store in a cool, dry place away from solvent fumes, heating coils, radiators, and steam pipes. The temperature in the storage room should not exceed 120 °F.

While direct sunlight does not harm acrylic plastic, it causes drying and hardening of the masking adhesive, making removal of the paper difficult. If the paper does not roll off easily, place the sheet in an oven at 250 °F for 1 minute, maximum. The heat softens the masking adhesive for easy removal of the paper.

If an oven is not available, remove hardened masking paper by softening the adhesive with aliphatic naphtha. Rub the masking paper with a cloth saturated with naphtha. This softens the adhesive and frees the paper from the plastic. Sheets so treated must be washed immediately with clean water, taking care not to scratch the surfaces.

Note: Aliphatic naphtha is not to be confused with aromatic naphtha and other dry cleaning solvents, which have harmful effects on plastic. However, aliphatic naphtha is flammable and all precautions regarding the use of flammable liquids must be observed.

Composite Materials 

In the 1940s, the aircraft industry began to develop synthetic fibers to enhance aircraft design. Since that time, composite materials have been used more and more. When composites are mentioned, most people think of only fiberglass, or maybe graphite or aramids (Kevlar). Composites began in aviation, but now are being embraced by many other industries, including auto racing, sporting goods, and boating, as well as defense industry uses. 

A “composite” material is defined as a mixture of different materials or things. This definition is so general that it could refer to metal alloys made from several different metals to enhance the strength, ductility, conductivity, or whatever characteristics are desired. Likewise, the composition of composite materials is a combination of reinforcement, such as a fiber, whisker, or particle, surrounded and held in place by a resin forming a structure. Separately, the reinforcement and the resin are very different from their combined state. Even in their combined state, they can still be individually identified and mechanically separated. One composite, concrete, is composed of cement (resin) and gravel or reinforcement rods for the reinforcement to create the concrete. 

Advantages/Disadvantages of Composites 

Some of the many advantages for using composite materials are:   

• High strength-to-weight ratio 

• Fiber-to-fiber transfer of stress allowed by chemical bonding 

• Modulus (stiffness-to-density ratio) 3.5 to 5 times that of steel or aluminum 

• Longer life than metals 

• Higher corrosion resistance 

• Tensile strength 4 to 6 times that of steel or aluminum 

• Greater design flexibility 

• Bonded construction eliminates joints and fasteners 

• Easily repairable 

The disadvantages of composites include: 

• Inspection methods difficult to conduct, especially delamination detection (Advancements in technology will eventually correct this problem.) 

• Lack of long-term design database, relatively new technology methods 

• Cost 

• Very expensive processing equipment 

• Lack of standardized system of methodology 

• Great variety of materials, processes, and techniques 

• General lack of repair knowledge and expertise 

• Products often toxic and hazardous 

• Lack of standardized methodology for construction and repairs 

The increased strength and the ability to design for the performance needs of the product makes composites much superior to the traditional materials used in today’s aircraft. As more and more composites are used, the costs, design, inspection ease, and information about strength-to-weight advantages help composites become the material of choice for aircraft construction.

Composite Safety 

Composite products can be very harmful to the skin, eyes, and lungs. In the long or short term, people can become sensitized to the materials with serious irritation and health issues. Personal protection is often uncomfortable, hot, and difficult to wear; however, a little discomfort while working with the composite materials can prevent serious health issues or even death. 

Respirator particle protection is very important to protecting the lungs from permanent damage from tiny glass bubbles and fiber pieces. At a minimum, a dust mask approved for fiberglass is a necessity. The best protection is a respirator with dust filters. The proper fit of a respirator or dust mask is very important, because if the air around the seal is breathed, the mask cannot protect the wearer’s lungs. When working with resins, it is important to use vapor protection. Charcoal filters in a respirator remove the vapors for a period of time. When removing the respirator for breaks, and upon placing the mask back on, if you can smell the resin vapors, replace the filters immediately. Sometimes, charcoal filters last less than 4 hours. Store the respirator in a sealed bag when not in use. If working with toxic materials for an extended period, a supplied air mask and hood are recommended.  

Avoid skin contact with the fibers and other particles by wearing long pants and long sleeves along with gloves or barrier creams. The eyes must be protected using leak-proof goggles (no vent holes) when working with resins or solvents, because chemical damage to the eyes is usually irreversible.  

Fiber Reinforced Materials 

The purpose of reinforcement in reinforced plastics is to provide most of the strength. The three main forms of fiber reinforcements are particles, whiskers, and fibers. 

A particle is a square piece of material. Glass bubbles (Q-cell) are hollow glass spheres, and since their dimensions are equal on all axes, they are called a particle.

A whisker is a piece of material that is longer than it is wide. Whiskers are usually single crystals. They are very strong and used to reinforce ceramics and metals.

Fibers are single filaments that are much longer than they are wide. Fibers can be made of almost any material and are not crystalline like whiskers. Fibers are the base for most composites. Fibers are smaller than the finest human hair and are normally woven into cloth-like materials.

Laminated Structures 

Composites can be made with or without an inner core of material. Laminated structure with a core center is called a sandwich structure. Laminate construction is strong and stiff, but heavy. The sandwich laminate is equal in strength, and its weight is much less; less weight is very important to aerospace products.

The core of a laminate can be made from nearly anything. The decision is normally based on use, strength, and fabricating methods to be used.

Various types of cores for laminated structures include rigid foam, wood, metal, or the aerospace preference of honeycomb made from paper, Nomex, carbon, fiberglass, or metal. Figure shows a typical sandwich structure. It is very important to follow proper techniques to construct or repair laminated structures to ensure the strength is not compromised. Taking a high-density laminate or solid face and back plate and sandwiching a core in the middle make a sandwich assembly. The design engineer, depending on the intended application of the part, decides the selection of materials for the face and the back plate. It is important to follow manufacturers’ maintenance manual specific instructions regarding testing and repair procedures as they apply to a particular aircraft.  

Reinforced Plastic 

Reinforced plastic is a thermosetting material used in the manufacture of radomes, antenna covers, and wingtips, and as insulation for various pieces of electrical equipment and fuel cells. It has excellent dielectric characteristics that make it ideal for radomes; however, its high strength-to-weight ratio, resistance to mildew, rust, and rot, and ease of fabrication make it equally suited for other parts of the aircraft.  

Reinforced plastic components of aircraft are formed of either solid laminates or sandwich-type laminates. Resins used to impregnate glass cloths are of the contact pressure type (requiring little or no pressure during cure). These resins are supplied as a liquid, which can vary in viscosity from water like consistency to thick syrup. Cure or polymerization is affected by the use of a catalyst, usually benzoyl peroxide.

Solid laminates are constructed of three or more layers of resin impregnated cloths “wet laminated” together to form a solid sheet facing or molded shape.

Sandwich-type laminates are constructed of two or more solid sheet facings or a molded shape enclosing a fiberglass honeycomb or foam-type core. Honeycomb cores are made of glass cloths impregnated with polyester or a combination of nylon and phenolic resins. The specific density and cell size of honeycomb cores varies over considerable latitude. Honeycomb cores are normally fabricated in blocks that are later cut to the desired thickness on a band saw. 

Foam-type cores are formulated from combinations of alkyd resins and metatoluene di-isocyanate. Sandwichtype fiberglass components filled with foam-type cores are manufactured to exceedingly close tolerances on overall thickness of the molded facing and core material. To achieve this accuracy, the resin is poured into a close tolerance, molded shape. The resin formulation immediately foams up to fill the void in the molded shape and forms a bond between the facing and the core. 

Rubber 

Rubber is used to prevent the entrance of dirt, water, or air, and to prevent the loss of fluids, gases, or air. It is also used to absorb vibration, reduce noise, and cushion impact loads. The term “rubber” is as all-inclusive as the term “metal.” It is used to include not only natural rubber, but also all synthetic and silicone rubbers.   

Natural Rubber 

Natural rubber has better processing and physical properties than synthetic or silicone rubber. These properties include flexibility, elasticity, tensile strength, tear strength, and low heat buildup due to flexing (hysteresis). Natural rubber is a general-purpose product; however, its suitability for aircraft use is somewhat limited because of its inferior resistance to most influences that cause deterioration. Although it provides an excellent seal for many applications, it swells and often softens in all aircraft fuels and in many solvents (naphthas and so forth). Natural rubber deteriorates more rapidly than synthetic rubber. It is used as a sealing material for water/methanol systems.

Synthetic Rubber 

Synthetic rubber is available in several types, each of which is compounded of different materials to give the desired properties. The most widely used are the butyls, Bunas, and neoprene.

Butyl is a hydrocarbon rubber with superior resistance to gas permeation. It is also resistant to deterioration; however, its comparative physical properties are significantly less than those of natural rubber. Butyl resists oxygen, vegetable oils, animal fats, alkalies, ozone, and weathering.

Like natural rubber, butyl swells in petroleum or coal tar solvents. It has a low water absorption rate and good resistance to heat and low temperature. Depending on the grade, it is suitable for use in temperatures ranging from −65 °F to 300 °F. Butyl is used with phosphate ester hydraulic fluids (Skydrol™), silicone fluids, gases, ketones, and acetones. 

Buna-S rubber resembles natural rubber both in processing and performance characteristics. Buna-S is as water resistant as natural rubber, but has somewhat better aging characteristics. It has good resistance to heat, but only in the absence of severe flexing. Generally, Buna-S has poor resistance to gasoline, oil, concentrated acids, and solvents. Buna-S is normally used for tires and tubes as a substitute for natural rubber.

Buna-N is outstanding in its resistance to hydrocarbons and other solvents; however, it has poor resilience in solvents at low temperature. Buna-N compounds have good resistance to temperatures up to 300 °F and may be procured for low temperature applications down to −75 °F. Buna-N has fair tear, sunlight, and ozone resistance. It has good abrasion resistance and good breakaway properties when used in contact with metal. When used as a seal on a hydraulic piston, it does not stick to the cylinder wall. Buna-N is used for oil and gasoline hoses, tank linings, gaskets, and seals. 

Neoprene can take more punishment than natural rubber and has better low-temperature characteristics. It possesses exceptional resistance to ozone, sunlight, heat, and aging. Neoprene looks and feels like rubber. Neoprene, however, is less like rubber in some of its characteristics than butyl or Buna. The physical characteristics of neoprene, such as tensile strength and elongation, are not equal to natural rubber but do have a definite similarity. Its tear resistance, as well as its abrasion resistance, is slightly less than that of natural rubber. Although its distortion recovery is complete, it is not as rapid as natural rubber. 

Neoprene has superior resistance to oil. Although it is good material for use in nonaromatic gasoline systems, it has poor resistance to aromatic gasoline. Neoprene is used primarily for weather seals, window channels, bumper pads, oil resistant hose, and carburetor diaphragms. It is also recommended for use with Freon™ and silicate ester lubricants. 

Thiokol, known also as polysulfide rubber, has the highest resistance to deterioration but ranks the lowest in physical properties. Petroleum, hydrocarbons, esters, alcohols, gasoline, or water, in general, does not seriously affect Thiokols. Thiokols are ranked low in such physical properties as compression set, tensile strength, elasticity, and tear abrasion resistance. Thiokol is used for oil hoses, tank linings for aromatic aviation gasoline, gaskets, and seals. 

Silicone rubbers are a group of plastic rubber materials made from silicon, oxygen, hydrogen, and carbon. The silicones have excellent heat stability and very low temperature flexibility. They are suitable for gaskets, seals, or other applications where elevated temperatures up to 600 °F are prevalent. Silicone rubbers are also resistant to temperatures down to −150 °F. Throughout this temperature range, silicone rubber remains extremely flexible and useful with no hardness or gumminess. Although this material has good resistance to oils, it reacts unfavorably to both aromatic and nonaromatic gasoline. 

Silastic, one of the best-known silicones, is used to insulate electrical and electronic equipment. Because of its dielectric properties over a wide range of temperatures, it remains flexible and free from crazing and cracking. Silastic is also used for gaskets and seals in certain oil systems.

Shock Absorber Cord 

Shock absorber cord is made from natural rubber strands encased in a braided cover of woven cotton cords treated to resist oxidation and wear. Great tension and elongation are obtained by weaving the jacket upon the bundle of rubber strands while they are stretched about three times their original length.

There are two types of elastic shock absorbing cord. Type I is a straight cord, and type II is a continuous ring known as a “bungee.” The advantages of the type II cord are that it is easily and quickly replaced and does not need to be secured by stretching and whipping. Shock cord is available in standard diameters from 1/4 inch to 13⁄16 inch. 

Three colored threads are braided into the outer cover for the entire length of the cord. Two of these threads are of the same color and represent the year of manufacture; the third thread, a different color, represents the quarter of the year in which the cord was made. The code covers a 5-year period and then repeats itself.