Aircraft wings are a compromise that allows the aircraft to fly at a range of flight conditions, but the performance at each condition is sub-optimal. The ability of a wing surface to change its geometry during flight has interested researchers and designers over the years as this reduces the design compromises required. Morphing is the short form for metamorphose; however, there is neither an exact definition nor an agreement between the researchers about the type or the extent of the geometrical changes necessary to qualify an aircraft for the title ‘shape morphing.’ Geometrical parameters that can be affected by morphing solutions can be categorized into: planform alteration (span, sweep, and chord), out-of-plane transformation (twist, dihedral/gull, and span-wise bending), and airfoil adjustment (camber and thickness). Changing the wing shape or geometry is not new. Historically, morphing solutions always led to penalties in terms of cost, complexity, or weight, although in certain circumstances, these were overcome by system-level benefits. The current trend for highly efficient and ‘green’ aircraft makes such compromises less acceptable, calling for innovative morphing designs able to provide more benefits and fewer drawbacks. Recent developments in ‘smart’ materials may overcome the limitations and enhance the benefits from existing design solutions. The challenge is to design a structure that is capable of withstanding the prescribed loads, but is also able to change its shape: ideally, there should be no distinction between the structure and the actuation system. The blending of morphing and smart structures in an integrated approach requires multi-disciplinary thinking from the early development, which significantly increases the overall complexity, even at the preliminary design stage. Morphing is a promising enabling technology for the future, next-generation aircraft. However, manufacturers and end users are still too skeptical of the benefits to adopt morphing in the near future. Many developed concepts have a technology readiness level that is still very low. The recent explosive growth of satellite services means that UAVs are the technology of choice for many investigations on wing morphing. This article presents a review of the state-of-the-art on morphing aircraft and focuses on structural, shape-changing morphing concepts for both fixed and rotary wings, with particular reference to active systems. Inflatable solutions have been not considered, and skin issues and challenges are not discussed in detail. Although many interesting concepts have been synthesized, few have progressed to wing tunnel testing, and even fewer have flown. Furthermore, any successful wing morphing system must overcome the weight penalty due to the additional actuation systems.

Shape memory alloys (SMAs) are a unique class of metallic materials with the ability to recover their original shape at certain characteristic temperatures (shape memory effect), even under high applied loads and large inelastic deformations, or to undergo large strains without plastic deformation or failure (super-elasticity). In this review, we describe the main features of SMAs, their constitutive models and their properties. We also review the fatigue behavior of SMAs and some methods adopted to remove or reduce its undesirable effects. SMAs have been used in a wide variety of applications in different fields. In this review, we focus on the use of shape memory alloys in the context of morphing aircraft, with particular emphasis on variable twist and camber, and also on actuation bandwidth and reduction of power consumption. These applications prove particularly challenging because novel configurations are adopted to maximize integration and effectiveness of SMAs, which play the role of an actuator (using the shape memory effect), often combined with structural, load-carrying capabilities. Iterative and multi-disciplinary modeling is therefore necessary due to the fluid-structure interaction combined with the nonlinear behavior of SMAs.

Multiple flight regimes during typical aircraft missions mean that a single unique optimized configuration, that maximizes aerodynamic efficiency and maneuverability,\ud cannot be defined. Discrete components such as ailerons and flaps provide some adaptability,\ud although they are far from optimal. Wing morphing can significantly improve the performance\ud of future aircraft, by adapting the wing shape to the specific flight regime requirements,\ud but also represents a challenging problem: the structure has to be stiff to maintain its shape\ud under loads, and yet be flexible to deform without collapse. One solution is to adopt structural\ud elements made of smart materials; Shape Memory Alloys (SMAs) have demonstrated their\ud suitability for many static applications due to their high structural integration potential and\ud remarkable actuation capabilities.\ud In this work, the airfoil camber at the wing trailing edge on a full scale wing of a civil\ud regional transportation aircraft is controlled by substituting a traditional split flap with a\ud hingeless, smooth morphed flap. Firstly, the development and testing of an actuator device\ud based on a SMA ribbon, capable of a net rotation of 5 deg, is presented. Then, a flap bay is\ud designed and experimentally tested in presence of static loads, based on a compliant rib built\ud as a series repetition of the proposed actuator. An aero-thermo-mechanical simulation within\ud a FE approach was adopted to estimate the behavior and performance of the compliant rib,\ud integrating both aerodynamic loads, by means of a Vortex Lattice Method (VLM) code, and\ud SMA phenomenology, implementing Liang and Rogers’ constitutive model. The prototype\ud showed good actuation performance even in presence of external loads. Very good numerical\ud experimental correlation is found for the unloaded case, while some fatigue issues emerged\ud in presence of static load

Chord extension morphing of helicopter rotors has recently been shown to be highly beneficial for stall alleviation, with the ability to reduce power near the envelope boundaries and increase maximum gross weight, altitude, and speed capability of the aircraft. This article presents a morphing mechanism to extend the chord of a section of the helicopter rotor blade. The region aft of the leading-edge spar contains a morphing cellular structure. In the compact state, the edge of the cellular structure aligns with the trailing edge of the rest of the blade. When the morphing cellular structure is in the extended state, the chord of that section of the blade is increased by 30%. In transitioning from compact to extended states, the cellular structure slides along the ribs which define the boundaries of the morphing section in the spanwise direction. The cellular section has mini-spars running along the spanwise direction to attach the flexible skin and provide stiffness against camber-like deformations due to aerodynamic loads. This article presents a finite element analysis and design of the morphing cellular structure, ensuring that the local strains in the elastic ligaments of the cellular structure do not exceed the maximum allowable, even as the section undergoes a large global strain. The morphing cellular structure itself is designed to be stiff enough to support the pre-stretched skin attached to its surfaces. Various methods of flexible skin attachment to the morphing substructure, and their ramifications, are considered. A model of a blade section is fabricated and shown to undergo chord morphing, as designed.

The adaptive structures concept is of great interest in the aerospace field because of the several benefits which can be accomplished in the fields including noise reduction, load alleviation, weight reduction, etc., at a level in which they can be considered as compulsory in the design of future aircraft. Improvements in terms of the aerodynamic efficiency, aeroelastic behavior, stability, and manoeuvrability performance have already been proved through many international studies in the past. In the family of the Smart Materials, Shape Memory Alloys (SMA) seem to be a suitable solution for many static applications. Their high structural integrability in conjunction with actuation capabilities and a favorable performance per weight ratio, allows the development of original architectures. In this study, a morphing wing trailing edge concept is presented; morphing ability was introduced with the aim of replacing a conventional flap device. A compliant rib structure was designed, based on SMA actuators exhibiting structural potential (bearing external aerodynamic loads). Numerical results, achieved through a FE approach, are presented in terms of trailing edge induced displacement and morphed shape.