2.6.1 History #

The use of composites in aircraft construction dates to World War II, when soft fiberglass insulation was used in B-29 fuselages. By the late 1950s, European high-performance sailplane manufacturers were using fiberglass as primary structures. In 1965, the FAA type-certified the first all-fiberglass aircraft in the normal category, a Swiss sailplane called the Diamant HBV. Four years later, the FAA certified a four-seat single-engine Windecker Eagle in the normal category. By 2005, over 35 percent of new aircraft were constructed of composite materials.

Composite is a broad term and can mean materials such as fiberglass, carbon fiber cloth, Kevlar® cloth, and mixtures of all of the above. Composite construction offers two advantages: extremely smooth skins and the ability to easily form complex curved or streamlined structures. [Figure 2-15]

2.6.2 Advantages of Composites #

Composite construction offers several advantages over metal, wood, or fabric, with lighter weight the most frequently cited. Lighter weight is not always automatic, however: building a structure out of composites does not guarantee it will be lighter; it depends on the structure and the type of composite used.

A more important advantage is that a very smooth, compound-curved, aerodynamic structure made from composites reduces drag. This is the main reason sailplane designers switched from metal and wood to composites in the 1960s. In aircraft such as the Cirrus and Columbia lines, composites reduce drag and yield high performance despite fixed landing gear. Composites also help mask the radar signature of “stealth” designs such as the B-2 and F-22. Today, composites are found in aircraft as varied as gliders and most new helicopters.

Lack of corrosion is a third advantage. Boeing designed the 787, with its all-composite fuselage, to have both a higher pressure differential and higher cabin humidity than previous airliners, since engineers are no longer as concerned about corrosion from moisture on hidden fuselage skin areas. This should lead to lower long-term maintenance costs.

Another advantage is good performance in a flexing environment, such as helicopter rotor blades. Composites do not suffer from metal fatigue and crack growth as metals do. With careful engineering, composite rotor blades can have considerably higher design lives than metal blades; most new large helicopter designs have all-composite blades and, in many cases, composite rotor hubs.

2.6.3 Disadvantages of Composites #

Composite construction has its own disadvantages, the most important being the lack of visual proof of damage. Composites respond to impact differently from other structural materials, and often there is no obvious sign of damage. For example, if a car backs into an aluminum fuselage, it may dent it; if not dented, there is no damage; if dented, the damage is visible and repairs are made.

In a composite structure, a low-energy impact, such as a bump or a dropped tool, may leave no visible sign on the surface. Underneath, there may be extensive delaminations spreading in a cone-shaped area from the impact location. The damage on the backside can be significant and extensive, yet hidden from view.

Anytime there is reason to think an impact occurred, even a minor one, it is best to have an inspector familiar with composites examine the structure for underlying damage. “Whitish” areas in a fiberglass structure are a good tip-off that delaminations or fiber fracture have occurred. A medium-energy impact (such as the car backing into the structure) causes local crushing of the surface, which should be visible; the damaged area is larger than the visible crushed area and will need repair. A high-energy impact, such as a bird strike or hail in flight, results in a puncture and severely damaged structure. In medium- and high-energy impacts the damage is visible, but low-energy impact is difficult to detect. [Figure 2-16]

If an impact results in delaminations, crushing of the surface, or a puncture, a repair is mandatory. While waiting for the repair, the damaged area should be covered and protected from rain. Many composite parts are thin skins over a honeycomb core, forming a “sandwich” structure—excellent for stiffness, but an easy target for water ingress, leading to further problems. A piece of “speed tape” over a puncture protects it from water, but is not a structural repair; neither is paste filler used to cover damage, although it is acceptable cosmetically.

The potential for heat damage to the resin is another disadvantage. While “too hot” depends on the resin system, many epoxies begin to weaken above 150 °F. White paint is often used on composites to minimize this. For example, the bottom of a wing painted black, facing a black asphalt ramp on a hot sunny day, can reach 220 °F; the same structure painted white rarely exceeds 140 °F. Composite airplanes therefore often have specific recommendations on allowable paint colors, which must be followed if the airplane is repainted. Heat damage can also result from fire—even a quickly extinguished small brake fire can damage bottom wing skins, composite landing gear legs, or wheel pants.

Also, chemical paint strippers are very harmful to composites and must not be used on them. If paint must be removed from composites, only mechanical methods are allowed, such as gentle grit blasting or sanding. Many expensive composite parts have been ruined by paint stripper, and such damage is generally not repairable.

2.6.4 Composite Materials in Aircraft #

Composite materials are fiber-reinforced matrix systems. The matrix is the “glue” that holds the fibers together and, when cured, gives the part its shape, but the fibers carry most of the load. There are many types of fibers and matrix systems.

In aircraft, the most common matrix is epoxy resin, a thermosetting plastic. Compared to choices such as polyester resin, epoxy is stronger and has good high-temperature properties. Many epoxies exist, with a wide range of structural properties, cure times and temperatures, and costs.

The most common reinforcing fibers used in aircraft construction are fiberglass and carbon fiber. Fiberglass has good tensile and compressive strength, good impact resistance, is easy to work with, and is relatively inexpensive and readily available; its main drawback is being relatively heavy, making it hard to build a fiberglass load-carrying structure lighter than a well-designed equivalent aluminum one.

Carbon fiber is generally stronger in tension and compression than fiberglass and has much higher bending stiffness; it is also considerably lighter, but relatively poor in impact resistance—its fibers are brittle and tend to shatter under sharp impact (greatly improved with a “toughened” epoxy system, as on the Boeing 787 stabilizers). Carbon fiber is more expensive than fiberglass, but prices have dropped thanks to the B-2 (1980s) and Boeing 777 (1990s) programs. Very well-designed carbon-fiber structures can be significantly lighter than equivalent aluminum—sometimes by around 30 percent.

2.6.5 Fluid Spills on Composites #

Some owners worry about fuel, oil, or hydraulic fluid spills on composite surfaces. These are generally not a problem with modern composites using epoxy resin: usually, if the spill doesn’t attack the paint, it won’t hurt the underlying composite. Some aircraft use fiberglass fuel tanks, with the fuel riding directly against the composite surface with no sealant. If the fiberglass is made with some of the cheaper polyester resins, there can be a problem when using auto gas blended with ethanol. The more expensive polyester resins, as well as epoxy resin, can be used with auto gas, 100-octane aviation gas (avgas), and jet fuel.

2.6.6 Lightning Strike Protection #

Lightning strike protection is an important design consideration. When an aircraft is hit by lightning, a very large amount of energy is delivered to the structure. Whether flying a light general aviation (GA) airplane or a large airliner, the basic principle is the same: the energy must be spread over a large surface area to lower the “amps per square inch” to a harmless level.

If lightning strikes an aluminum airplane, the energy conducts easily through the aluminum structure; the challenge is to keep it out of avionics, fuel systems, etc., until it can be safely conducted overboard, the outer skin being the path of least resistance.

In a composite aircraft, fiberglass is an excellent electrical insulator, while carbon fiber conducts electricity, but not as easily as aluminum. Therefore, additional conductivity is added to the outer layer of composite skin, typically with fine metal meshes bonded to the surface—aluminum mesh on fiberglass, copper mesh on carbon fiber. Any structural repair on lightning-protected areas must also include the mesh as well as the underlying structure.

For composite aircraft with internal radio antennas, there must be “windows” in the lightning-strike mesh in the area of the antenna. Internal radio antennas may be found in fiberglass composites because fiberglass is transparent to radio frequencies, whereas carbon fiber is not.

2.6.7 The Future of Composites #

In the decades since World War II, composites have earned an important role in aircraft structural design. Their design flexibility and corrosion resistance, as well as the high strength-to-weight ratios possible, will undoubtedly continue to lead to more innovative designs. From the Cirrus SR-20 to the Boeing 787, it is clear that composites have found a home in aircraft construction and are here to stay. [Figure 2-17]