The Use of Lightweight Composites in Satisfying the Unique Structural Requirements of Aircraft Design by Brad Peirson School of Engineering Grand Valley State University Term Paper EGR 250 – Materials Science and Engineering Section 1 Instructor: Dr. P.N. Anyalebechi July 14, 20051 Abstract The design and manufacture aircraft wings require attention to several unique structural demands. Among other traits all of the materials used must have be lightweight and have a high strength. Since the first airplane flight over 100 years ago the materials and processes used in the manufacture of wings have evolved greatly. As the technology employed in providing the thrust for flight has advanced so too has the physical requirements of the material. As aircraft become larger and faster the stresses applied to their wings increase. Wood frames and doped cloth construction eventually gave way to all metal airframes. The current trend in the aerospace industry is the use of lightweight, high strength composites. Introduction More than a century ago two men forever changed the face of history. In the winter of 1903 Orville and Wilbur Wright piloted the first powered heavier than air vehicle. This maiden flight lasted only moments but it ushered in an era of constant technological advancement in the field of aeronautics. While the techniques have been perfected over the year, the basic concepts developed by the Wrights are still in practice. The wings of the Wright flyer consisted of a hardwood truss covered in fabric [1]. The truss allowed for maximum strength and minimum weight in the aircraft. This basic combination remained the staple of aircraft design until the twenties. In this decade Jack Northrop pioneered the shift to stressed skin designs. Fabric is not strong enough for a stressed skin design so Northrop perfected a method of forming plywood sheets to the required shape of the airframe [1]. This advancement increased the overall rigidity of the plane. It allowed for faster planes and increased cargo capacities. This stressed skin design is the same essential method used in modern aircraft. The ultimate difference is in the advancement of the materials used. In the decades after Northrop’s innovation, technology had advanced to the point of metal framed and skinned aircraft.2 Figure 1: Examples of Jack Northrop’s stressed skin designs [1] The evolution of metals to the point of being useable in aircraft construction marked the beginning of the jet age. In the middle part of the twentieth century metals allowed planes to be built larger and faster. A metal airframe allows the plane an excellent rigidity. This added rigidity in the wings of an aircraft allow for greater lift forces and thus greater payloads. Taking aircraft technology into the 21st century are lightweight, high strength composite materials. Composite materials are made by combining two different materials in order to gain properties greater than either of the components. Composites typically consist of particulate or fibrous material suspended in a matrix of the different material [2]. This composition can give excellent properties to a composite material such as high stiffness, fatigue resistance and thermal shock resistance. The properties exhibited by a composite3 material are directly determined by the properties of the constituent materials [2]. Such materials are continuously being applied to aerospace technologies. Figure 2: Applications of composites in the Boeing 777 aircraft [2] Functional Requirements of Airplane Wing Material There are two primary functional requirements that must be considered when considering materials for use in an airplane wing. The first is high strength. As aircraft become larger they naturally become heavier. The heavier aircraft requires a more lift force to obtain flight. Greater lift directly translates to greater stress on the wings. This effect on the wing can be illustrated through the use of finite element analysis software. If a given material was not strong enough it would fail under the high stresses generated by large passenger aircraft such as the Boeing 777 or the Airbus A380, which has the largest wings ever produced on an aircraft.4 The second required property of a wing material would be light weight. Again, as aircraft become larger they become heavier. If the materials used were not of sufficient light weight the payload of the aircraft would be decreased. When the structure of the aircraft is made as light as possible the weight savings can be used to carry extra cargo or passengers. Lightweight is especially important in the Airbus, which will carry 35% more passengers (555 people) than the current largest airliner. Because of unique weight saving materials and processes the Airbus will use 20% less fuel per passenger than current airliners [3]. Figure 3: FEA of aircraft wing under maximum loading (units in psi) [4] ` Additionally, the material must be able to resist extreme temperature changes. Within the troposphere the atmospheric temperature can have an extreme amount of variation. Aircraft such as the Airbus A380 have an operational ceiling of around thirteen kilometers [5]. At this altitude the temperature is considerably lower than that at sea level. There are two possible modes of failure for a wing material at such altitudes. The first is that the temperature drop could make the material brittle. This would lead to cracking and general failure as the wing would still be subjected to the lift forces.5 Figure 4: Temperature vs. Altitude in the Earth’s atmosphere [5] The other possibility is that the material would experience creep, or shrinkage, due to the extreme temperature change. This could cause a void in the skin of the wing. The resulting airflow disruption could cause a resonance in the wing’s structure that could tear it from the plane. A similar failure was catalogued by the Transportation Safety Board of Canada in May of 1998. A small Skyhopper aircraft scraped its wing on the runway prior to takeoff. The impact left no visible evidence on the runway. The initial damage to the wing is shown in Figure 5. Figure 5: Damage to the wing tip of a Shyhopper aircraft on May 1, 1998 [6]6 The photograph in Figure 5 was taken of the recovered wing. The photograph of the entire wing is shown in Figure 6, after a resonance