Aviation and Aerospace technology is a funny thing. Frequently, the newest and most innovative technologies seem much like fashion. For instance, a brand or style of jean becomes fashionable for a duration of time and then simply disappears. Look at jeans over the years, rolled up leg’s, bell bottoms, button-fly, stone washed, and the eponymous, skinny jean. Many aviation and aerospace technologies also pinnacle and then fade away, or are improved upon. Avionics, the gauges in the cockpit that translate performance and navigation data to the crew, have morphed greatly over time. Older aircraft utilized simple direct reading needle gauges, then LED (Light Emitting Diode) backlit gauges, and flight management computers with Apple Macintosh style interfaces. Now most avionics are cathode ray tubes (CRT’s, almost HD televisions), driven by display electronic units that gather complex data and flash them upon a multi-function display screen.
However, the same metaphor cannot be said of aircraft structures. Aluminum structured aircraft have stood the test of time and remained relevant for over 90 years. Sure, the sophistication in the metallurgy has improved, but aluminum designed aircraft seem an amazingly resilient technology. Once a niche technology and a favorite of maverick designer Burt Rutan (Scaled Composites), Composite structures are taking over aviation and aerospace in a profound manner. This leaves aluminum a dying technology. Why? What is it about composites that make them the giant killer, the newest structural material to take down aluminum’s 90 year reign?
If your looking for a simple definition of composite structures, here is a simple explanation, “Composites are formed by combining materials together to form an overall structure that is better than the sum of the individual components.” Simple right? However, this definition hardly explains it’s pervasive use on the newest Boeing, the 787. Boeing has traditionally resisted the temptation to commit to composite technology for years, but have done a 180, and embraced composite’s. In contrast Airbus has utilized composites in the past, primarily mating larger flight surfaces such as the vertical stabilizer, wings, or horizontal stabilizer’s to aluminum fuselages. In 2013, they released an all composite fuselage and wing design in the A-350.
But for a more in depth understanding of composites, and their structural applications, I spoke with Dr. Peng Hao Wang, a Graduate of Purdue University in Aviation Composite Technologies. Dr. Wang has written a text on the subject called, Structural Composites: Advanced Composites in Aviation, published by AVOTEK.
Dr. Wang is also an Adjunct Faculty member at Lewis University teaching lab and lecture courses on the subject. He was gracious enough to take some time out to answer questions that will hopefully help clarify my questions. My first question was obvious, “what makes composite technology so much better than aluminum when it comes to aircraft construction? Dr. Wang replied that, “composite materials, specifically carbon fiber composites, have a better weight to strength ratio than aluminum.” This means a significantly lighter airplane is flying through the air, one that saves commercial airlines more money in overall fuel burn. But if these airplanes are so much lighter, don’t they sacrifice structural integrity which aluminum has so succesfully provided? Composites are lighter, yet the strength of carbon fiber is significant. In fact, “composites provide twice the strength and half the weight of aluminum.”
The three main ingredients in aviation composite materials are resin, fibers, and heat pressure. Different fibers allow for different characteristics such as heat resistance, durability, and flexibility. Primarily three types of fibers are used, glass (i.e.fiberglass ), carbon, and Kevlar (a Dupont brand name) or aramid. The crucial ingredient according to Dr. Wang is, “the resin.” Resin, the amount of heat, and the type of fibers are crucial in determining what type of application the composites will be best suited. Aerospace fibers are carbon fibers which tend to be expensive, but provide maximum strength. Dr. Wang believes aramid is the toughest fiber, “but handling and fabrication of the material is difficult. i.e. difficult to machine and aramid composite absorbs moisture.” Dr. Wang also see’s future potential in, “organic fibers,” rather than synthetic.
However, composites are not a perfect replacement for aluminum aircraft construction. Aluminum has always been susceptible to corrosion, yet if properly maintained will ultimately last decades. Composite materials have similar problems, but rather than corrode, composites erode over time. That is, the constant wind resistance over the composite structure will erode like sand from a rock. Furthermore, ultraviolet light will degrade the material when not painted or maintained properly. Composite’s also lose much more structural integrity than aluminum if they are hit with blunt force. For instance, say a baggage tug accidentally bumps into the side of a composite fuselage. It is more difficult to detect any damage on the aircraft. The trauma can also take more “overall strength” away from the structural integrity than aluminum. Aluminum is still used in conjuction with composites to prevent erosion, the leading edge of the wing on the 787 is mated with aluminum. With a similar number of disadvantages doesn’t it seem logical that aluminum should continue to be utilized over composites. Dr. Wang disagrees, “the real advantage of composites over any existing technological aircraft building material is that it can make complex shapes with minimal parts.” A traditional aluminum wing has hundreds of thousands of parts to construct it, attach it to the fuselage, etc. But the 787 wing is literally, “one single manufactured composite piece, albeit a large one.” This makes it virtually unbreakable. During static wing loading tests, the Boeing 787 composite wing was repeatedly stretched and pulled to evaluate for fatigue and load resistance. Every aircraft certification requires this rigorous testing procedure. During the test, “the wings on the 787 were flexed upward approximately 25 feet which equates to 150 percent of the most extreme forces the airplane is ever expected to encounter during normal operation.” It passed with incredible results, it could not be broken, in fact the wing could practically clap over the fuselage. In 2008, Boeing had to isolate one specific section of the wing and apply incredible load factors just to get the wing to break for FAA approval. The durability and single piece construction make composites incredibly effective for aviation construction. Dr. Wang feels, “that to prove the durability of composites, aircraft manufacturers are actually over-engineering them, and that slowly composites will supplant aluminum designs.”
So it seems that composites will continue to be utilized in aircraft design and construction. As the manufacturing process’s improve, damage detection, and repair methods safer, the proliferation of composites will continue. This hardly seems a death sentence for aluminum structures. In fact, it seems both technologies are enjoying a happy marriage on current and new aviation designs. However, it is just a matter of time until safety, economics, efficiency, and public perception eventually move aluminum completely out of aircraft construction. History tells us that at some point in the future, composites will also be rendered obsolete, moved out of the way by some new organic structural system perhaps. I just hope skinny jeans are long gone before composites are…..