## 1 Introduction
A smaller number is usually required for the design of control surfaces. Moreover, many manoeuvre loads depend on trim conditions, which imply the availability of a flight simulator of the aircraft, something that is not possible when the structural design begins, especially in a fly-by-wire control system [10]. Modern design approaches are based on decoupling the load analysis from the flight control laws design. These methods represent an agreement between the loads and flight mechanics, where the former analyses the boundaries of a predefined envelope for the structural design and the latter ensures that the flight control laws do not work outside this area [11; 12]. A similar approach is presented in this article for the design of adaptive structures, where multiple loads depend on the aerodynamic shapes that the morphing device can achieve, which are unknown when the design begins. The idea is to define aerodynamic shapes that can maximize the aircraft performances before, and then to design an adaptive structure that can match, once actuated, all of the aerodynamic shapes previously defined, while accounting for the conflicting structural requirements mentioned above. Most of the work in the literature on morphing deals with performance optimization [13; 14], modeling of morphing wings [15], and design of specific morphing devices [16; 17; 18]. On the one hand, the issue related to multiple load conditions and conflicting structural requirements needs a dedicated design methodology, and, on the other hand, other open issues are related to the Technology Readiness Level (TRL). The technology maturity of the adaptive structures concerns the estimation of overall benefits that should overcome the weight and complexity penalty [1]. More specific issues concern the integration between the actuation, morphing devices, and existing wings [13], and the design of flexible skins that cannot be considered too innovative with respect to the materials, to be installed on the modern aircraft. Adaptive structures represent conformable and gapless control surfaces that can be attached to the traditional wing-box. This approach is a clear advantage for strength-to-weight ratio considerations and because of the need for fuel storage. This morphing solution can be accepted by industries and certification boards and organizations, but the aeroelastic effects and the mutual interactions, in terms of the functionality and accuracy of the target deformed shape, must be evaluated. Concerning the deformable skin, the most critical aspect is that the external shape must be kept smooth in order to not compromise the aerodynamic efficiency, without exceeding the material strain limits. The structural solution proposed in this article for the design of adaptive structures is based on the distributed compliance concept. This concept was originally proposed by Kota [19], as an alternative to the distributed actuation concept, and then it was successfully adopted by Politecnico di Milano (PoliMi) in many European projects, as the basis for the design of morphing wings [20] and wind tunnel models equipped with adaptive compliant leading and trailing edges [21]. Compliant structures are optimized to efficiently distribute the flexibility in the structure and to transform the input actuation energy into strain energy to achieve the desired motion or the required shape change. They have been shown to be suitable for realizing seamless and gapless wing control surfaces and do not require the use of unconventional actuators or excessively innovative skin solutions [22]. Due to the peculiar characteristics of compliant structures, dedicated design procedures must be developed, and specific tools are required, to achieve a feasible design. An intensive research activity that was conducted over the past ten years at PoliMi, has led to the continuous development of a design procedure based on the two-level approach described in [23]. However, the design of adaptive structures is a naturally multiobjective optimization problem (MOOP).