Lightweighting design is an extensively explored and utilized concept in many industries, especially in aerospace applications, and is associated with the green aviation concept. The contribution of aviation to global warming phenomena and environmental pollution has led to ongoing efforts for the reduction of aviation emissions. Approaches to achieve this target include increasing energy efficiency. An effective way to increase energy efficiency and reduce fuel consumption is reducing the mass of aircraft, as a lower mass requires less lift force and thrust during flight. For example, for the Boeing 787, a 20% weight savings resulted in 10 to 12% improvement in fuel efficiency. In addition to reduction of carbon footprint, flight performance improvements such as better acceleration, higher structural strength and stiffness, and better safety performance could also be achieved by lightweight design.
Lightweighting optimization of a solar-powered unmanned aerial vehicle (UAV) is an example of using both clean energy and lightweight structures to achieve green aviation operation. Current solar-powered UAV designs face challenges such as insufficient energy density and wing stiffness. Lightweight design is essential for ultralight aviation, enabling longer flight duration.
The principle of lightweight design is to use less material with lower density while ensuring the same or enhanced technical performance. A typical approach to achieve lightweight design for aerospace components is to apply advanced lightweight materials on numerically optimized structures, which can be fabricated with appropriate manufacturing methods. As such, the application of lightweight materials can effectively achieve both weight reduction and performance improvement. Although metal materials — especially aluminum alloys — are still the dominant materials in aerospace application, composite materials have received increasing interest and compete with aluminum alloys in many new aircraft applications.
Structural optimization is another effective way to achieve lightweighting, by distributing materials to reduce materials use, and enhance the structural performance such as higher strength and stiffness and better vibration performance. Conventional structural optimization methods are size, shape, and topology. Manufacturability is a crucial constraint in both material selection and structural optimization. The development of advanced manufacturing technologies such as additive manufacturing, foam metal, and advanced metal forming not only enable the application of advanced materials, but relax constraints, enhancing the flexibility of multiscale structural optimization.
Many examples of lightweight design have been successfully applied in the design of lightweight aircraft. Figure 1(a) illustrates the SAW Revo concept aircraft (produced by Orange Aircraft), which is an ultralight aerobatic airplane with carbon fiber-reinforced composite wings and a topologically optimized truss-like fuselage. The empty weight of this 6-meter-wingspan aircraft is 177 kg. Figure 1(b) shows a high-altitude, pseudo-satellite, solar-powered UAV from Airbus. The Zephyr 7 currently holds the world record for the longest absolute flight duration (336 hours, 22 minutes, 8 seconds) and highest flight altitude (21,562 m) for UAVs, partly from increased energy efficiency by lightweighting. Figure 1(c) shows a model of a future concept lightweight airplane for 2050 from Airbus, inspired by a bird skeleton. Figure 1(d) demonstrates a concept of a box wing aircraft where shape optimization is employed in the wing design. Structural efficiency could be increased by using a box wing structure; higher stiffness and lower induced drag force result from the box wing compared with conventional wing structures.
Selecting Lightweight Materials
The selection of aerospace materials is crucial in aerospace component design since it affects many aspects of aircraft performance, from the design phase to disposal, including structural efficiency, flight performance, payload, energy consumption, safety and reliability, lifecycle cost, recyclability, and disposability. Critical requirements for aerospace structural materials include mechanical, physical, and chemical properties such as high strength, stiffness, fatigue durability, damage tolerance, low density, high thermal stability, high corrosion and oxide resistance, and commercial criteria such as cost, servicing, and manufacturability. Studies have indicated that the most effective way to improve structural efficiency is reducing density (around 3 to 5 times more effective compared with increasing stiffness or strength), i.e. using lightweight materials.
The most commonly used commercial aerospace structural materials are aluminum alloys, titanium alloys, high-strength steels, and composites, generally accounting for more than 90% of the weight of airframes. From the 1920s until the end of the century, metal — because of its high strength and stiffness, especially aluminum alloy — has been the dominant material in airframe fabrication, with safety and other flight performance measures driving aircraft design decisions. Lightweight aluminum alloys were the leading aviation structural materials — accounting for 70%–80% of the weight of most civil aircraft airframes before 2000 — and still play an important role. Since the mid-1960s and 1970s, the proportion of composites used in aerospace structures has increased due to the development of high-performance composites. Figure 2 illustrates the materials distributions for some Boeing products.
Aluminum Alloys. Although high-performance composites such as carbon fiber are receiving increasing interest, aluminum alloys still make up a significant proportion of aerospace structural weight. The relatively high specific strength and stiffness, good ductility and corrosion resistance, low price, and excellent manufacturability and reliability make advanced aluminum alloys a popular choice of lightweight materials in many aerospace structural applications, e.g. fuselage skin, upper and lower wing skins, and wing stringers. The development of heat-treatment technology provides high-strength aluminum alloys that remain competitive with advanced composites in many aerospace applications. Aluminum alloys can offer a wide range of material properties meeting diverse application requirements, by adjusting compositions and heat treatment methods.
Titanium Alloys. Titanium alloys have many advantages over other metals, such as high specific strength, heat resistance, cryogenic embrittlement resistance, and low thermal expansion. These advantages make titanium alloys an excellent alternative to steels and aluminum alloys in airframe and engine applications; however, the poor manufacturability and high cost (usually about 8 times higher than commercial aluminum alloys) result in the restriction of titanium alloys being used extensively. Hence, titanium alloys are used where high strength is required but limited space is available, as well as where high corrosion resistance is required. The current applications of titanium alloys in aerospace are mainly in airframe and engine components, overall comprising 7% and 36% of the weight, respectively.
High-Strength Steel. Steel is the most commonly used structural material in many industry applications due to good manufacturability and availability, extremely high strength and stiffness in the form of high-strength steels, good dimensional properties at high temperatures as well as the lowest cost among commercial aerospace materials. But high density and other disadvantages, such as relatively high susceptibility to corrosion and embrittlement, restrict the application of high-strength steels in aerospace components and systems. Steel normally accounts for approximately 5% to 15% of structural weight of commercial airplanes, with the percentage steadily decreasing. Despite the limitations, high-strength steels are still the choice for safety-critical components where extremely high strength and stiffness are required. The major applications for high-strength steels in aerospace are gearing, bearings, and undercarriage applications.
Aerospace Composites. High-performance composites such as fiber reinforced polymer and fiber metal laminates (FML) have received increased attention in aerospace applications, competing with the major lightweight aerospace materials such as aluminum alloys. In general, aerospace composites have higher specific strength and specific stiffness than most metals at moderate temperatures. Other advantages of composites include improved fatigue resistance, corrosion resistance, and moisture resistance as well as the ability to tailor layups for optimal strength and stiffness in required directions; however, the higher cost of composites in comparison to metals is one of the major obstacles for the application of composites.
Carbon fiber reinforced polymer (CFRP) represents the most extensively used aerospace structural material apart from aluminum alloys, with the major applications being structural components of the wing box, empennage, and fuselage as well as control surfaces (e.g. rudder, elevator, and ailerons). Glass fiber reinforced polymer (GFRP) is used in radomes and semi-structural components such as fairings. Aramid fiber polymers are used where high impact resistance is required. Fiber metal laminates, especially glass fiber reinforced aluminum (GLARE), are other types of composites that have applications in aerospace (especially in the Airbus A380) due to enhanced mechanical properties such as reduced density, high strength, stiffness, and fatigue resistance compared with monolithic metals. The main applications of GLARE are the fuselage skin and empennage.
Shape memory polymer composites (SMPC) are smart materials that can change their form as a result of a certain stimulus such as change of temperature, an electric or magnetic field, particular light wavelengths, etc. by releasing the internal stress stored in the material. The applications of SMPCs in aerospace components and systems include the wing skin of morphing-wing aircraft, and the solar array and reflector antenna of satellites. The advantages of SMPCs over shape memory alloys (SMAs) includes lower density, higher shape deformability and recoverability, better processing, and lower relative cost.