 Advances in Technology Innovation , vol. 1, no. 1, 2016, pp. 13 - 15 13 Copyright © TAETI Composite Elements for Biomimetic Aerospace Structures with Progressive Shape Variation Capabilities Alessandro Airoldi * , Paolo Bettini, Matteo Boiocchi, Giuseppe Sala Department of Aerospace Science and Technologies , Politecnico di Milano, Italy Received 01 February 2016; received in revised form 11 March 2016; accept ed 02 April 2016 Abstract The paper presents some engineering solutions for the development of innovative aerodynamic surfaces with the capability of progressive shape variation. A brief introduction of the most significant issues related to the design of such morphing structures is provided. Thereafter, two types of structural solutions are presented for the design of interna l co mp liant structures and fle xible e xterna l skins. The proposed solutions explo it the properties and the manufacturing techniques of long fibre reinforced plastic in order to fulfil the severe and contradictory require ments related to the trade-off between mo rphing performance and load carrying capabilities . Keywor ds: morphing structures, composite structures, chiral topologies, corrugated laminates 1. Introduction Morphing structures have been intensively studied in the last decades in the aerospace field, with the objective of developing innovative, more fle xib le and effic ient method s to change the shape of aerodynamic surfaces. Imitation of nature plays an important role in conceiving such type of structures, since organisms have solved the problems related to flight control and adaptation to different flight phases without the use of rigid moveable surfaces , which a re currently used in aircra ft. For instance, a fle xib le wing with the capability of shape variation can increase the curvature, when higher lift is required at lo w veloc ity, whereas, at high speed, curvature can be reduced to decrease drag (1). Another concept, called “chira l sail” is proposed in (2) and is based on wing with a centra l morph ing part that increases its camber when angle of attitude is changed (Fig. 1). Th is can lead to noticeable advantages for the surfaces that generate the forces for the stabilization of a vehicle, like the tail e mpenn ages of aircra ft. Indeed, these surfaces could be reduced with overall we ight saving and drag reduction. However, a lthough morphing is an appealing concept, there are critica l engineering issues to be solved for the development of such type of structures, which are hereby summarized in the following point: a) Co mpliance of s tructures must be finely tuned to accomplish shape variations induced by aerodynamic loads (passive morph ing) or of actuators (active mo rphing). b) Shape can vary but must retain the aerodynamic effic iency, without angular points , surface waviness, and anomalous modification of profiles . c) Aerodynamic loads acting in morphing directions must be transmitted, so that morph ing structures must exhib it fle xib ility and strength at the same time (passive morph ing), or reacted by load bearing actuators (active morphing). d) Stiffness and strength in non -morphing directions must be ma ximized to avoid the need of additional structural parts that would increase structural weight, thus reducing or eliminating the advantages of morphing concepts. Fig. 1 Variable ca mber wing with centra l morphing part * Corresponding aut hor, Email: alessandro.airoldi@polimi.it Advances in Technology Innovation , vol. 1, no. 1, 2016, pp. 13 - 15 14 Copyright © TAETI In the following sections , two concepts will be presented to fulfill the afore mentioned severe and contradictory requirements. 2. Composite Chiral Structures Chiral topologies are special non centre-symmetric geometries that consist of circula r e le ments, called nodes, connected by straight liga ments (Fig. 2-A). The ir defo rmation mechanis m leads to a transverse expansion, when a tensile load is applied, and a contraction, under the action of a co mpressive load (Fig. 2-B). Hence, if the chiral tessellation is considered as a meta-materia l, it turns out to be characterized by a negative Poisson’s ratio (au xet ic behavior). Such response avoids the development of localized displacements and weak points, and allo ws the achieve ment of controlled shape variations, as it is shown in Fig. 2 -C, refe rred to deformation modes of the chira l sail depicted in Fig. 1. For such reasons, chiral topologies were proposed to develop the internal structure of morph ing airfoils [ 1], but manufacturing of chiral honeycombs represent a critical proble m for application to rea l world structures. The process devised at Politecnico di Milano (4), allo ws the production of composite chira l ele ments by means of a procedure based on the bonding of composite units , which are produced in a prev ious step and then uniformly pressed together during bonding by means of elastomeric inserts (4,5). The resulting composite chira l e le ments can be produced with very thin liga ments and different types of composite materia ls, thus enhancing the design fle xib ility o f the concept. So me e xa mp les of manufactured chiral ele ments are provided in Fig. 3. Fig. 2 Chira l topologies (A) defo rmation mechanis m (B) and deformation mode of a chiral airfoil (C) Fig. 3 Composite chiral honeycomb (A)and composite chiral rib for the chiral sail concept (B) 3. Composite Corrugated Laminates The development of morphing surfaces always requires the development of a fle xib le skin to collect ae rodynamic forces . The require ments presented in the introduction are valid also for the skin, which has to undergo large recoverable strains, carrying and transmitting aerodynamic pressures to the internal structure. Moreover, in traditional aeronautical constructions , skin also provides a valuable contribution to structural stiffness, so that optima l solution s should present adequate structural response in non-morphing directions. Co mposite corrugated la minates have been proposed (6) to develop skins thanks to their inherent anisotropy that allo w high compliance and strains at failure in corrugation directions , and noticeable stiffness and strength in non-morphing directions (Fig. 4-A). Fig. 4 Corrugated laminate (A) composite chiral rib for the chiral sail (B) Unfortunately, the usage of co mposite corrugated la minates as morphing skin has a ma jor dra wback represented by the surface irregularities that increase ae rodynamic drag and reduce the generated lift (7). For such a reason, a solution was developed at Politecnico d i M ilano (8) to integrate in the corrugated laminate an elastomeric layer, supported by honeycomb inserts (Figs. 4-B and 4-C). The developed skin system was tested up to elongation of 20% (Fig. A B Advances in Technology Innovation , vol. 1, no. 1, 2016, pp. 13 - 15 15 Copyright © TAETI 4-D), proving that the elastomeric layer provides a smooth efficient aerodynamic surface without interfering with the mechanica l response of corrugated laminates. The composite corrugated la minates can be designed with appropriate lay-up and guarantee very high bending stiffness in non-morphing directions and even a not negligible shear stiffness, which can exceed 50% of the shear stiffness obtained by a conventional panel of the same we ight and lay-up (8). 4. Concluding Remarks The development of innovative solutions for internal structure and skin of morphing aerodynamic surface has been accomplished by jointly e xplo iting special structural geo metries and the properties of composite materials. The presented solutions fulfil the peculia r require ments of mo rphing structures and have been applied to develop a demonstrator of the chiral s ail concept, which is shown in Fig. 5, in order to assess technological feasib ility, functional aspects and structural response. Fig. 5 Chiral sail demonstrator References [1] D. Bo rnengo, F. Scarpa, and C. Re millat, “Evaluation o f he xagonal chiral structu re for moprh ing airfoil concept,” Journal of Aerospace Engineering, vol. 219, pp. 185-192, 2005. [2] A. Airo ldi, M. Crespi, G. Quaranta, and G. Sala, “Design of a morphing a irfo il with compos ite chira l structure,” Journal of Aircra ft, vol. 49, no. 4, pp. 1008-1019, 2012. [3] P. Pan ichelli, A. Gilarde lli, A. Airo ldi, G. Quaranta, and G. Sala, “Morphing composite structures for adaptative high lift devices ,” Proc. 6th Int. Conference Mechanics and Materials in Design, Ponta Delgada, Azores, pp. 26-30 July 2015 [4] P. Bettin i, A. Airoldi, G. Sala , L. Di Landro, M. Ruzzene, and A . Spadoni, “ Co mposite chiral structures for morphing a irfoils: numerical ana lyses and development of a manufacturing process ,” Co mposites Part B - Engineering, vol. 41, no. 2, pp. 133-147, 2010. [5] A. Airoldi, P. Bettini, P. Panichelli, and G. Sala, “ Chira l topologies for co mposite moprh ing structures – Part II: novel configurations and technological process”, Physica Statu Solidi (b), vol. 252, pp. 1446-1454, July 2015 [6] T. Yo koze ki, S. Takeda, T. Ogasawara, and T. Ishikawa , “ Properties of corrugated composites for candidate fle xible wing structures ,” Composites Part A: Applied Science Manufacturing, vol. 37, no. 4, pp. 1578-1586, 2006. [7] Y. Xia, O. Bilgren, and M. I. Friswe ll, “The effects of corrugated skins on aerodynamic performances ,” Journal of Intelligent Material Systems and Structures, vol. 25, no. 7, pp. 786-794, 2014. [8] S. Fournie r, A. A irold i, E. Borlandelli, and G. Sa la, “Fle xib le co mposite supports for morph ing skins ,” Proc. XXII AIDAA Conference, Naples, Italy, pp. 9-12, 2013.