Static and Dynamic Morphing Characteristics of a Chiral Core Airfoil

Aeroelastic tailoring requires structural compliance and thus often conflicts with stiffness requirements to carry prescribed aerodynamic loads. Recently however, the application of cellular structural concepts has suggested the potential to achieve compliance while conserving required load-carrying capacity. Among the proposed concepts, a chiral geometry in particular is a novel configuration which features an in-plane negative Poisson’s ratio which leads to a very high shear modulus, while maintaining some degree of compliance. In particular, the chiral geometry allows large continuous deformations of the airfoil assembly, with the constitutive material remaining in the linear region of its stress-strain curve. The ability to sustain large deformations without exceeding yield conditions is required to recover the original shape and to provide smooth deformations as required by aerodynamic considerations. In previous work, a coupled-physics model, comprising of simultaneous CFD and elastic analyses, is developed to investigate the influence of the chiral core geometry on the behavior of a given airfoil. The modification of geometric parameters defining the considered layout leads to significant variations in mechanical properties, which can be exploited to achieve various levels of compliance. The morphing capabilities of the proposed airfoil, quantified as camber changes, are evaluated for various design configurations of the internal core structure. Specifically, three such airfoils have been constructed to study the influence of core geometric parameters on the elastic behavior observed in numerical simulations. ∗Address all correspondence to this author. Experiments on the aforementioned airfoil samples are characterized by imposing large camber-wise deflections, via static loading, and measuring the resulting strain, both in the honeycomb core and in the airfoil profile. The experimental results confirm the ability of the airfoils to sustain large deflections while not exceeding yield strain limits, in addition to producing continuous deformations, which are critical for the implementation of aeroelastic tailoring. INTRODUCTION Since the first attempts of powered flight, biologically inspired researchers have tried to devise techniques to implement aeroelastic tailoring as a form of flight control. Albeit few exceptions such as the Wright Flyer and more recent experimental aircraft, aeroelastic tailoring has proven elusive [1]. Aeroelastic tailoring requires structural compliance and thus often conflicts with stiffness requirements to carry prescribed aerodynamic loads. Recently however, the introduction of smart materials and structures, such as composite materials, has encouraged engineers and researches to revisit aeroelastic tailoring, as it promises higher authority and efficiency over current flow control mechanisms. To this day various configurations have been proposed. Among others, Among other forms of innovative smart structures, cellular solids have been suggested for the design of structural components with superior mechanical properties and multifunctional characteristics. The chiral geometry [2] in particular is a novel configuration which features an in-plane negative Poisson’s ra1 Copyright c © 2006 by ASME tio which leads to a very high shear modulus, while maintaining some degree of compliance. This unique mechanical behavior can be exploited for the design of sandwich structures with a core composed of a macroscopic chiral truss, laid out across the thickness. This concept, also denoted as ”truss-core,” lends itself for the design of airfoils with morphing capabilities. In particular, the chiral honeycomb allows for large continuous deformations of the airfoil assembly, while remaining in the linear region of the stress-strain relationship. The ability to sustain large deformations without exceeding yield conditions is required to achieve repeatability, while smooth deformations are imperative for aerodynamic applications such as aeroelastic tailoring. In previous work [3], a coupled-physics model, comprising of simultaneous CFD and elastic analyses, was developed to investigate the influence of the chiral core on the static aeroelastic behavior of a given airfoil. The alteration of geometric parameters defining such layout leads to significant variations in mechanical properties, which can be exploited to achieve different functionalities. The morphing capabilities of the proposed airfoil, here quantified as camber changes, are evaluated for various design configurations of the core. Specifically, three such airfoils have been constructed to study the influence of honeycomb core geometric parameters and relative density on the elastic behavior observed in numerical simulations. In particular, the chiral honeycomb is defined by circular elements acting as nodes, connected by ribs or ligaments tangent to the nodes. Experiments on the aformentioned airfoil samples are characterized by imposing large camber-wise deflections, via static loading, and measuring the resulting strain, both the in the honeycomb core and the airfoil profile. Finally, experimental results confirm the ability of chiralcore airfoils to sustain large deflections while not exceeding yield strain limits, in addition to producing continuous deformations, which are critical for the implementation of aeroelastic tailoring. CONSIDERED CONFIGURATIONS The design of deformable systems may be driven by kinematic or mechanic considerations, according to the manner in which the system’s deformations take place. Deformations may be desired to alleviate structural forces, they may be passive in nature and arise from low structural stiffness, or they may be actively induced, as in the case of structural mechanisms. Often, the ability of a structural system to deform is coupled with strength requirements, as in the case of multifunctional structural components. Coupled deformability-stiffness requirements arise often in applications for which weight considerations drive the design. Aircraft are a prime example of such requirements. Given the current state of the art in aerospace design, it is common practice to select an aircraft configuration based on the most frequent conditions encountered during a given mission. for a passenger aircraft, for example, cruise conditions dictate the design. The lifting surfaces of such aircraft, as an example, are optimized to produce the highest lift-to-drag ratio (L/D) at cruise conditions; however, they need to operate properly even for off-design conditions. Such requirements are satisfied by wing reconfiguration, which is often justified in terms of efficiency, while it is in fact required to sustain flight. The deformations to which lifting surfaces are subjected can be divided into passive, due to aeroelastic phenomena, and active, due to the actuation of mechanisms for reconfiguration, such as flaps and slats. Within the realm of passive deformations, elastic deformations may be further differentiated into span-wise and chord-wise directions. Span-wise deformations are in turn characterized by torsional, bending and shearing phenomena. Span-wise bending is usually sought to relieve wing-root stresses, while span-wise torsion is to be avoided as it is one of the major causes of aeroelastic divergence, aileron reversal and flutter [1]. Chord-wise elastic deformations, on the other hand, are currently avoided as they alter wing-section aerodynamic characteristics, and, more importantly, alter the span-wise characteristics of a wing [1, 4], while highly coupling the design of reconfiguration mechanisms with elastic phenomena. Currently employed wings, hence, are highly anisotropic components. The multifunctionality of wings may be improved if elastic deformations could be exploited as a means to achieve aeroelastic tailoring [1], yielding complexity and weight reduction. To this end, Anisotropy may be exploited to satisfy often conflicting requirements. The aim of the current work, then, is to present a novel wing-section assembly that exploits the exotic mechanic characteristics of the chiral honeycomb [2] to achieve chord-wise deformations, useful for wing-reconfiguration purposes, while possessing high torsional rigidity, paramount to avoiding divergence and flutter. Airfoil core layout The novel geometry defining the airfoil core is that of the chiral honeycomb. The geometry of such layout is shown in Fig. 1, where the geometric parameters L, R, r, t, β and θ are also presented. The relation between the aformentioned geometric parameters is suggested by [2], and is defined as follows: