THE interactions of lightweight flexible airframe structures, steady and unsteady aerodynamics, and wide-bandwidth active controls on modern airplanes lead to considerable multidisciplinary design challenges. More than 25 years of mathematical and numerical methods' development, numerous basic research studies, simulations and wind-tunnel tests of simple models, wind-tunnel tests of complex models of real airplanes, as well as flight tests of actively controlled airplanes, have all contributed to the accumulation of a substantial body of knowledge in the area of aeroservoelasticity. 1-5 A number of analysis codes, with the capabilities to model real airplane systems under the assumptions of linearity, have been developed. 6" 12 Many tests have been conducted, and results were correlated with analytical predictions. A selective sample of references covering aeroservoelastic testing programs from the 1960s to the early 1980s, 13 as well as more recent wind-tunnel test programs14'19 of real or real-istic configurations, are included in the References section of this paper. An examination of Refs. 20-29 will reveal that in the course of development (or later modification), of almost every modern airplane with a high authority active control system, there arose a need to face aeroservoelastic problems and aeroservoelastic design challenges. It has become evident that aeroservoelastic analysis and synthesis are not just endeavors or tasks of choice, which the designer is free to adopt or reject, depending on whether he wants to pursue technologies such as active flutter suppression or gust load alleviation. Even if the designer chooses, as part of his design approach, not to create desirable interactions and not to try to harness these multidisciplinary interactions to his benefit, still, as long as there are onboard high-power active control systems for flight mechanics, he has to face some multidisciplinary problems in the form of undesirable interactions. Aeroservoelasticity, thus, must be addressed in the course of modern airplane design. And whether benefits of aeroservoelastic interactions are to be pursued, or problems avoided in a cost-effective way, aeroservoelastic analysis and synthesis should be included in as early a stage of the airplane design process as possible, and not be postponed until the design is almost complete.

This paper aims at examining the integrated aeroservoelastic design challenge in the context of the evolving field of multidisciplinary design optimization (MDO), and at reviewing the status of the effort to make aeroservoelasticity an integral part of airplane conceptual and preliminary design. 30 Emphasis is not on aeroservoelastic analysis, where aeroservoelastic behavior is evaluated for given configurations and control sys-tems. The drive discussed here is to develop integrated aeroservoelastic synthesis, the capability to simultaneously, and in an integrated way, synthesize aeroservoelastic systems across their contributing disciplines, quickly, efficiently, and reliably. The work discussed here is limited to airframes and control systems of fixed-wing airplanes operating without significant aerodynamic heating. Selected references, which can serve as starting points for the review of developments in aeroservo-elasticity and multidisciplinary interactions on panels, hyper-sonic vehicles, rotary wing aircraft, and also addressing the problem of whirl flutter, can be found in Refs. 31-38. In the following sections, the integrated aeroservoelastic synthesis problem is defined, and different levels of complexity and difficulty identified. Mathematical modeling for design, behavior response analysis, behavior sensitivity analysis, and approximation …