Innovative Structural Concepts for Deep-Winged Large Transports

Aircraft wing structures experience comparable levels of pushdown and pullup forces, which subject them to near-reversible (tension-compression) stress systems. Fiber reinforced polymer composites provide distinctly high tensile strength and modulus, which cannot be matched by their compressive performance. Reversible stress cycles are also detrimental to the fatigue life of composites. Hence, compressive stresses govern the design of aircraft wing composite structures, leaving their superior tensile attributes largely under-utilized. This undermines the structural efficiency of aircraft wings, and carries important weight penalties which grow with aircraft size. Our approach subjects the wing structure to a pre-tension which controls the compressive stresses in service. This approach balances the peak stress-to-strength ratios of wing structures in tension and compression, thereby improving their structural efficiency. The theoretical analyses of Phase I research verified the potential for major weight savings and significant gains in fatigue life through prestressing of composite wing structures. The experimental work conducted in Phase I research confirmed that prestressing renders statistically significant benefits to the compressive and flexural performance of composite panels, and also to the flexural performance of composite box sections. The experimental trends agreed with theoretical predictions. The advantages of prestressing were preserved after impact damage. The proposed Phase II project will: (i) develop refined analysis and design techniques for prestressed wing structures; (ii) optimize the design of prestressed wing structures; (iii) experimentally verify the prestressing effects on representative structural components, and evaluate the benefits of prestressing to aircraft wing structures; and (iv) evaluate broader applications of prestressing to aircraft structures. BENEFIT: The weight savings and the gains in fatigue life resulting from prestressing of composite wing structures promise important benefits in terms of fuel efficiency, pollution control, service life, maintainability, and initial and life-cycle economy. These benefits can be realized with relatively small changes in design and fabrication of aircraft structures. Major airframers constitute the primary customers for use of prestressing in aircraft structures. Aircraft production is a $220 billion market, about $35 billion of which may be attributed to aircraft structures. Prestressing is a simple and low-cost technique which promises close to $3.5 billion annual saving in aircraft production. The eventual benefits of the technology in terms of fuel saving and pollution control are even more significant. Prestressing is a complement to (and not a competition for) alternative efforts towards development of advanced materials and novel structures for reducing the aircraft production cost and weight. We are preparing a patent application which promises to provide us with a comprehensive proprietary position with respect to the technology. The estimated production cost savings have been used to develop financial projections for market transition of the technology. The Emerging Technologies Fund of Michigan has committed $125,000 towards transition of the technology to marketplace (pending success of Phase II proposal). Other arrangements have also been made for financing and management of the commercialization process. Boeing is a partner in development of the technology, and plans to take part in follow-up efforts to transfer the prestressing technique to aircraft structures.