Aero-Structure Manufacture : Airframe

Technorati Tags: Airframe,Aero Structure,Material,Manufacturing

Airframe is described as the mechanical structure of an aircraft that are generally used excluding engines. Designing airframe is a challenging field of engineering, as balance of performance, reliability and cost is to be achieved by combining the knowledge of aerodynamics, materials technology and manufacturing methods. [Wikipedia, Et. al.]
Manufacturing Airframe is becoming a tough process. Strict quality control and government regulations have to be followed by Manufacturers. The crash on takeoff of an Airbus A300 in 2001, after its tail assembly broke away from the fuselage, called attention to operation, maintenance and design issues involving composite materials that are used in many recent airframes. The A300 had experienced other structural problems but none of this magnitude. The incident bears comparison with the 1959 Lockheed L-188 crash in showing difficulties that the airframe industry and its airline customers can experience when adopting new technology.
Advanced materials are widely used for high performance aircrafts to manufacture airframe. The reduction of weight was always a challenge by the replacement of aluminium with composites or other superalloys. Aircraft manufacturers are always on a look out for new technologies that can provide better value with reducing the total life cycle costs of a commercial aircraft. This can be achieved with composites compared with the metals since the design and the production costs must be lower when compared. There are many advantages of using lighter; the main advantage is
maintenance cost is lower when compared with the metal structures. The composite materials are affordable, easy to maintain and have some flexibility for manufacturing where as for metal, maintenance is a problem. High fuel costs has made manufacturers look for a alternative method to cut the cost, which forced to adopt and improve the composite materials.
Four major eras in commercial airframe production stand out are all-aluminum structures which started in 1920s, high-strength alloys and high-speed airfoils in the beginning of 1940s, with beginning of 1960s long-range designs and improved efficiencies is achieved, and composite material construction began in 1980s. Boeing has claimed a lead, designing its new 787 series flagship airframes scheduled for first delivery in 2008 with a one-piece carbon-fiber fuselage, said to replace "1,200 sheets of aluminum and 40,000 rivets." These airframes are designed to transport 220-300 passengers, while chief competitor Airbus has designed its A380 flagship airframes to transport 550-850 passengers. The A380 is also built with a large proportion of composite material.[ Peel Et. Al. 1995]
Since 1970s, the use of advanced composite materials in transport aircraft structures has widely increased. The nature of composite materials and fabrication processes used to make these structures has tended to promote an integrated, multidisciplinary, approach to product development. The main benefits of composites over metals technology have been structural weight savings, fatigue resistance, and corrosion suppression. Additional strategic defence benefits have led to more military applications such as wing and fuselage, than commercial. Relatively high fuel costs helped to justify the initial commercial applications based on composite weight savings. In the 1980s and 1990s, fuel costs dropped relative to other airline costs, such as ownership. Composite manufacturing costs are currently the most critical barrier to expanded commercial applications.
Modern aerospace vehicles has performed remarkably to a large degree, as a result of the high performance materials and manufacturing technology used in both the airframes and propulsion systems. A commercial aircraft will fly over 60,000 hours during its 30 years life with over 20000 flights, will taxi over 100,000 miles. For obtaining, continuous performance increase, designers and researcher are constantly searching for the lighter, strong and more durable materials. The most efficient way of reducing airframe weight and improving performance is by reducing material density which is recognised. [Lin Ye Et. Al. 2005]

Source: Boeing 787
Airframe durability is becoming a greater concern since the life of many aircraft, both commercial and military, are being extended far behind their intended design lives. Even for the aircraft with only a 30 year lifetime, it has been estimated that the cost of service and maintenance over the 30 year life of the aircraft exceeds the original purchase by a factor of two.
Since the early 1920, airframe have been built largely out of metal, aluminium in particular has been the material of choice. With high performance composites i.e. first boron and the carbon fibers, started being developed in the mid 1960s and early 1970s, the situation started changing. The earliest developers, and users, of composites were the military. The early application of these in the military, resulted in significant weight savings (approx 20%), they accounted for only small amounts of the airframe structural weight. However, composite usage was expanded from only 2% of the airframe to as much as 27% by the early 1980s.
Similar trends have been followed for commercial aircraft, although at the slower and more cautious pace. Until recently, Airbus has been somewhat more aggressive in using composites than Boeing, primarily for the horizontal stabilisers and vertical fins on their A300 series of aircraft. However, Boeing recently made a major commitment to composites, when it decided to use upwards of 50% on its new 787, which includes both a composite wing and fuselage, as shown in the above figure.[ Barington Et. Al 2002]
Aluminium alloys have been has been the main airframe material since they started replacing wood in the early 1920s. Even though the role of aluminium in future commercial aircraft will probably be somewhat eroded by the increasing use of composite materials, high strength aluminium alloys are and will remain important airframe material. The attractiveness of aluminium is that it is relatively low cost, light weight metal that can be heat treated to fairly high strength levels, and it is one of the most easily fabricated of the high performance materials, which usually correlates with lower costs. Improvements in the compositional control and processing have continually produced improved alloys. Reducing impurities, in particular iron and silicon, has resulted in higher fracture toughness. Along with tightening compositional controls and eliminating unwanted impurities, the development of improved aging heat treatment for 7XXX alloys has resulted in greatly reduced stress corrosion cracking susceptibility and improved fracture toughness, with only a minimal impact on strength. Improvement in aluminium manufacturing technology includes high speed machining and friction stir welding. Aluminium-lithium alloys are attractive for aerospace applications because the addition of lithium increases the modulus of aluminium and reduces the density. Each 1 wt% of lithium increases the modulus by about 6% while decrease the density about 3%.
Titanium is often used to save weight by replacing heavier steel alloys in the airframe and super alloys in the low temperature portions of gas turbines. Titanium is becoming even more important as an airframe material due to its outstanding resistance to fatigue, its high temperature capability and its resistance to corrosion. Titanium alloys are also used extensively in the lower temperature regions of jet turbine engines. Near net shape processes can lead to savings in materials, machining costs and cycle times over conventional forged or machined parts. Investment castings, in combination with hot isostatic pressing (HIP), can produce aerospace quality titanium near net shaped parts that can offer significant cost savings over forgings and built-up structures.
While high strength steels normally account for only about 5-15% of the airframe structural weight, they are often used for highly critical parts such as landing gear components. The main advantages of high strength steels are their extremely high strength and stiffness. This can be extremely important in landing gear applications where it is critical to minimize the volume of the gear component.[ Freeman Et. Al. 1993]
Superalloys are other enabling materials for modern flight where they are used extensively in the jet turbine engines. Some superalloys are capable of being used in load bearing applications in excess of 80% of their incipient melting temperatures while exhibiting high strength, good fatigue and creep resistance, good corrosion resistance and the ability ton operate at elevated temperatures for extended periods of time. The remarkable role superalloy technology had played in allowing higher engine operating temperatures.

Composite materials usage on the 777
The field of composite materials covers polymer matrix composites. The advantage of high performance polymer matrix composites are many, including lighter weight; the ability to tailor lay ups for optimum strength and stiffness; improved fatigue life; corrosion resistance; and with good design practice, reduced assembly costs and due to fever details parts and fasteners. The specific strength (strength/density) and specific modulus (modulus/density) of high strength fibre composites, especially carbon, are higher than comparable aerospace metallic alloys. This translates into greater weight savings resulting in improved performance, greater payloads, longer range and fuel savings. However, to realise the type of structure in the commercial aircraft world, the cost of composite structures still needs to be reduced through innovative design and manufacturing technologies. (Ilcewicz et al., 1997)
For the next generation in aero vehicles, aircraft manufacturers are involved in the development of functionalised materials and structures, with the target of essential safety, cost efficiency, airworthiness, system integrity, environmental compatibility.

Utility of composite structures on A380: (a) monolithic CFRP and thermoplastics; (b) materials distribution (weight breakdown).
Since large quantity of composite materials are used for the production, motivates to innovate improve material. Automated fibre placement (AFP), automated tape laying (ATL), resin film infusion (RFI) and resin transfer moulding (RTM) are few latest manufacturing technologies introduced by Airbus. Jumbo has developed concepts such as GLARE, laser beam welding (LBW). GLARE, is a made by alternative overlapping the layers of aluminium foils with unidirectional glass fibers, it is classified as hybrid material. GLARE can improve the corrosion and fire resistance of the material whereas LBW is used to reduce cracks in aircraft skins by eliminating traditional rivets.
The future of the composite can be judged with the example, 777 empennage structural properties were derived from a size scaling approach, which correlated analysis with a balance of tests at the coupon, element, and subcomponent levels. Some structural behaviour was predicted using reliable analysis methods and basic material properties, reducing or eliminating the number of element and subcomponent tests. Difficult strength predictions used additional element and subcomponent tests. The subcomponent tests helped relieve overly conservative analysis assumptions, which result in cost and performance penalties. This approach also proved to be more cost effective than developing sophisticated analysis methods and testing many coupons at the lower level. (Ilcewicz et al., 1997)
Conclusion

To summarize, composite panels provide a feasible technical solution at a cost level which is competitive with metal structure. Automated fibre placement and hand-lay-up are both manufacturing possibilities which are not limited by size constraints. Advanced composite technologies must earn their way into future commercial aircraft applications by adding value to the product and gaining customer acceptance. To date, the main benefits of composites over metals technology have been structural weight savings, fatigue resistance, and corrosion suppression. Other increases in value, such as reduced manufacturing and maintenance costs.
Recent advances in composite manufacturing and maintenance technology appear pro missing in reducing the total life cycle costs of an aircraft. Manufacturing concepts, which utilize inexpensive tooling and achieve tight tolerances in large components, have the potential to drive costs below that of traditional aluminium built up structure (Ilcewicz et al., 1997). There is also evidence that products with the highest value will incorporate advances in both metal and composite technology to achieve the lowest total costs in a hybrid design. Airlines, manufacturers, and maintenance companies have also been working together to solve existing composite maintenance issues. Since the early 1990s, one such group has made significant progress in this area, providing documentation on favoured repair practices, maintainable design details, and databases.

REFERENCE:

1. Peel, C.J., Gregson,P.J., “Design Requiremen for Aerospace Structural Materials”,in high performance materials in aerospace, Chapman & Hall, 1995, pp.1-48
2. Barington, N., Black, M., “ Aerospace Materials and Manufacturign Processes at the Millenium”, in Aerospace Materials, Institute of physics publishing, 2002, pp. 328-341.
3. Cotton, J.D., Clark, L.P., Phelps, H.R., “ Titanium Alloys on the F-22 Fighter Aircraft”, Advanced Materials & Processes, May 2002, pp. 25-28.
4. Williams, J.C., Starke, E.A.,” Progress in structural Materials of Aerospace systems”, Acta Materialia, Vol. 51, 2003, pp. 5775-5799.
5. Freeman, W.T., “The Use of Composites in Aircraft Primary Structure”, Composites Engineering, vol.3, Nos 7-8, 1993, pp. 767-775.
6. Lin Ye ,Ye Lu , Zhongqing Su , Guang Meng,”Functionalized composite structures for new generation airframes: a review”, Composites Science and Technology 65 (2005) 1436–1446.
7. ILCEWICZ. L. B., HOFFMAN. D. J., FAWCETT ,A. J.,” Composite Applications in Commercial Airframe Structures”, 1997.
8. Airframe, http://en.wikipedia.org/wiki/Airframe