and Columbia line of production aircraft, leading to their high  performance despite their fixed landing gear. Composites also  help mask the radar signature of “stealth” aircraft designs,  such as the B-2 and the F-22. Today, composites can be  found in aircraft as varied as gliders to most new helicopters.  Lack of corrosion is a third advantage of composites. Boeing  is designing the 787, with its all-composite fuselage, to have  both a higher pressure differential and higher humidity in  the cabin than previous airliners. Engineers are no longer as  concerned about corrosion from moisture condensation on the  hidden areas of the fuselage skins, such as behind insulation  blankets. This should lead to lower long-term maintenance  costs for the airlines.  Another advantage of composites is their good performance  in a flexing environment, such as in helicopter rotor blades.  Composites do not suffer from metal fatigue and crack growth  as do metals. While it takes careful engineering, composite  rotor blades can have considerably higher design lives than  metal blades, and most new large helicopter designs have all  composite blades, and in many cases, composite rotor hubs.  Composite construction comes with its own set of  disadvantages, the most important of which is the lack of  visual proof of damage. Composites respond differently from  other structural materials to impact, and there is often no  obvious sign of damage. For example, if a car backs into an  aluminum fuselage, it might dent the fuselage. If the fuselage  is not dented, there is no damage. If the fuselage is dented,  the damage is visible and repairs are made.  In a composite structure, a low energy impact, such as a  bump or a tool drop, may not leave any visible sign of the  impact on the surface. Underneath the impact site there may  be extensive delaminations, spreading in a cone-shaped area  from the impact location. The damage on the backside of  the structure can be significant and extensive, but it may be  hidden from view. Anytime one has reason to think there  may have been an impact, even a minor one, it is best to  get an inspector familiar with composites to examine the  structure to determine underlying damage. The appearance  of “whitish” areas in a fiberglass structure is a good tip-off  that delaminations of fiber fracture has occurred.  A medium energy impact (perhaps the car backing into the  structure) results in local crushing of the surface, which  should be visible to the eye. The damaged area is larger than  the visible crushed area and will need to be repaired. A high  energy impact, such as a bird strike or hail while in flight,  results in a puncture and a severely damaged structure. In  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, the use of composites reduces drag for the Cirrus  Composite construction offers several advantages over  metal, wood, or fabric, with its lighter weight being the most  frequently cited. Lighter weight is not always automatic. It  must be remembered that building an aircraft structure out of  composites does not guarantee it will be lighter; it depends  on the structure, as well as the type of composite being used.  Carbon fiber is generally stronger in tensile and compressive  strength than fiberglass and has much higher bending  stiffness. It is also considerably lighter than fiberglass.  However, it is relatively poor in impact resistance; the fibers  are brittle and tend to shatter under sharp impact. This can  be greatly improved with a “toughened” epoxy resin system,  as used in the Boeing 787 horizontal and vertical stabilizers.  Carbon fiber is more expensive than fiberglass, but the price  has dropped due to innovations driven by the B-2 program  in the 1980s and Boeing 777 work in the 1990s. Very well- designed carbon fiber structures can be significantly lighter  than an equivalent aluminum structure, sometimes by 30  percent or so.  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 disadvantage is that it is  somewhat heavy, and it is difficult to make a fiberglass load- carrying structure lighter than a well designed equivalent  aluminum structure.  Figure 3-15.  Composite aircraft.  Disadvantages of Composites  Advantages of Composites  3-10 