# Design of a Variable-Stiffness Compliant Skin for a Morphing Leading Edge
## 1 Introduction  
China is becoming one of the world's largest aviation markets, whose fleet, according to Boeing's Commercial Market Outlook 2019-2038 [1], will account for nearly 20% of the share of the world by 2037. To meet the strict requirements of green aviation, China has been actively encouraging aircraft designers to develop innovative technologies [2]. Seamless and smooth morphing leading edge has been one of the most potential aviation technologies in the short-term future from the perspectives of noise abatement and drag reduction [3, 4, 5]. Firstly, the gap and slot introduced by conventional leading edge produce a large part of noise during takeoff and landing, and eliminating them is a good cause [2, 6]. Secondly, the morphing leading edge is detrimental to laminar flow, which requires the skin surface quality to an extremely high extent [7, 8]. Therefore, developing such a new system has a remarkable advantage in noise abatement and drag reduction.  
Much work has been carried out for this purpose [3, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24], and such work can be roughly divided into two types, depending on whether it adopted a uniform-stiffness compliant skin or a variable-stiffness one. Due to good deformation accuracy and scalability, the latter is the concept with the most potential. The core issue, however, for this kind of concept is the optimization of the stiffness distribution of the complaint skin. To address this issue, [PERSON] firstly employed an empirical design method to determine the stiffness distribution of a composite laminate compliant skin [19] and, in 2011, adopted a Simplex method to further optimize its thickness distribution [21]. With a similar design method, [PERSON] et al. designed a morphing leading edge with a variable compliant skin and aninternal compliant structure in 2014 [23]. Based on homogeneous material and a shape optimization method, [PERSON] et al. designed a variable-thickness compliant skin [9], [PERSON] et al. chose equivalent axial and bending stiffness as a design variable to optimize the distribution of a composite compliant skin, without any consideration of the actual layup sequence [12]. In short, to realize a variable stiffness concept, the previous research studies either adopted a variable-thickness homogeneous-material complaint skin or a variable-stiffness composite one. For the homogeneous-material complaint skin concept, it is difficult to obtain high deformation accuracy, as the search space of the variable is lower than the composite one. However, for the variable-stiffness composite one, the proposed design methods in the previous works cannot take manufacturing constraints or layup sequence of the composite laminate into account, as they either empirically determined it or did not care about them at all, which could cause high deviation of the final deformed profile from the aerodynamic target profile. In addition, the employed optimizer of these works was mainly a gradient-based method or the Simplex method, which is prone to be trapped in a locally optimal solution, decreasing the deformation accuracy further.  
This paper, therefore, proposes an innovative two-step design method, including layup optimization and layup adjustment, for a variable-stiffness compliant skin based on composite material. The combination of these two steps can not only improve the deformation accuracy of the final profile of the compliant skin but also easily and effectively determine the sequence of the composite layups at the same time.