Given that they have virtually superseded aviation-grade aluminum alloys of the previous generation, are composite materials really represents a genuine advance in aviation technology?
By: Ringo Bones
From the vantage point of most people – including me in most circumstances – who only have an insider-like glimpse of the inner workings of the engineering side of the aviation and aerospace industry through the Discovery Channel and National Geographic. One can easily be excused of harboring a superficial perception that composite materials – like Kevlar and carbon fiber – has advanced the aviation and aerospace industry by leaps and bounds. Especially when only knowing that the advantages of composite materials are their high strength-to-weight ratio and their only disadvantage are the high initial cost when compared to Duralumin and other aviation-grade aluminum alloys. But do we “civilians” really seeing the big picture when it comes to the aviation and aerospace industries’ current fascination with composite materials from an engineering standpoint?
In our present energy consumption and global warming conscious globalized society, the high strength-to-weight ratio of composite materials can be seen as a godsend when it comes to their use by the airline industry. The lighter weight of aviation-grade certified composite materials means more fuel savings that easily translates not only in reduced airline running cost but also in substantially reduced carbon dioxide emissions without sacrificing the safety previously provided by significantly heavier aviation-grade aluminum alloys of the previous generation. Given these advantages, are composite materials really represented a significant advance in aviation from an engineering standpoint?
During the heyday of aviation-grade aluminum-based alloys, special attention was particularly paid to the wings. Almost every conceivable sort of test was conducted. Small sections of the wings are purposely cut with a saw, and then the section is artificially “aged” on testing machines which apply and release pressure in just the way it would occur in flight. Whole wings were taken into the strength testing laboratory and repeatedly bent up and down as would occur in an airplane’s typical lifespan. Back in those days, a Boeing 707 were undergoing such test were a design prototype of its wing was bent upward nine feet without breaking. And a lift of 425,000 pounds was required to actually buckle the wing.
Even back then, testing techniques in the aviation have in fact reached the stage where it is at least theoretically possible to guarantee that an airplane rolling off the end of a production line, will take off and fly for its entire lifetime without any significant failure. This, however, would assume a Utopian State of affairs of faultless materials in the airplane’s construction along with impeccable maintenance and use.
Testing does not end when a typical jetliner is delivered to an airline company. The Federal Aviation Administration, from the results of earlier tests, certifies that it will not require a major airframe overhaul until the plane has amassed 6,000 flying hours. Although during that period the airplane’s structure is frequently given a visual “twice-over” by airline maintenance personnel. When it reaches the 6,000-hour mark, however, the plane is sent to the company’s overhaul base for a complete examination.
The aircraft is first inspected visually and by x-ray machines similar to those used by a physician. In another test, ultrasonic generators send high-frequency sound waves flowing through a section under inspection and the wave pattern is displayed on an oscilloscope, like a test pattern on a television screen. If a flaw is present, it will show up as a deviation in the pattern.
Still another inspection technique is the dye check. Structural metal is first treated with a penetrating red dye, then covered with a white liquid, which dies into a powder. If there are substantial cracks, the red dye will bleed through the powder along the length of the crack. If the detected flaw is minor, it is repaired; if major, the whole structural section – a wing panel, for instance – is replaced. These extensive tests to maintain airline safety were already de rigueur during the heyday of aluminum alloy based planes.
But when it comes to the inspection of the airworthiness of our newfangled composite material-based aircraft, signs of damage and of structural fatigue are unfortunately invisible to the naked eye and our unaided senses. Detecting flaws in composites-based aircraft structures requires specialized equipment like ultrasound generator-based testing machines to detect minor cracks and micro-fractures. This will be an increasing necessity, especially when current and upcoming civilian air transports will be almost all totally made up of composite materials like Kevlar and carbon fiber. Something the airline industry will be very reluctant to invest during our still fiscally austere global economic climate. And economics, if you recall, is an integral part of good and sensible engineering.
With the new generation of composites-based jetliners slated to replace the older aluminum alloy-based jetliners like the Airbus A380 and the Boeing 787 Dreamliner – which are made mostly of composite materials, especially in the wing and fuselage section – be seen by most airline companies as not economically viable? The climate-friendly prestige surrounding new generation composites-based jetliners might prove too tempting to resist by most airline companies, given the potential fuel savings that could result. But if the investment in newer maintenance and diagnostic test equipment to maintain the well-being of these newfangled composites-based jetliners prove more costly than the resulting fuel savings, airline companies won’t be buying these jetliners as their manufacturers have been hoping to sell like hotcakes. Not even if these newfangled aerospace-grade composite materials enabled those new generation of fighter jets to fly up to Mach 1.5 without resorting to afterburners – i.e. supercruise capability.