Challenges of Morphing Wings for Airborne Wind Energy Systems

Dominic Keidel PhD Researcher ETH Zurich

Laboratory of Composite Materials and Adaptive Structures Tannenstrasse 3 8092 Zurich Switzerland keideld@ethz.ch www.structures.ethz.ch

Challenges of Morphing Wings for Airborne Wind Energy Systems

Laboratory of Composite Materials and Adaptive Structures, ETH Zurich Aircraft wings commonly utilize hinged control surfaces

to alter their aerodynamic properties, with the aim of achieving controllability of the aircraft and adaptabil-ity to different flight conditions. However, the discrete shape changes resulting from the deflections of the con-trol surfaces result in sub-optimal airfoil geometries and in a decrease in aerodynamic efficiency. On the other hand, morphing wings enable spatially smooth and con-tinuous geometrical changes of the wing shape by using smart materials and novel distributed compliance struc-tural concepts. Thereby, more aerodynamically efficient deformed shapes can be achieved, and the wing’s per-formance can be improved for different flight conditions, compared to conventional wings.

Airborne Wind Energy (AWE) presents a very promising application field for morphing, since AWE aircraft expe-rience a wide range of flight conditions, including take-off, traction phase, retraction phase, and landing. Fur-thermore, the wide range of wind speeds - and the result-ing flight speeds - encountered by the aircraft, result in a constantly changing environment for the aircraft. By con-trolling the camber deformation, and thereby optimally adapting the airfoil shape, the extracted power for every flight situation can be maximised.

This study presents the design, manufacturing and test-ing of a rigid AWE aircraft with a selectively compliant wing with an area of 1.5m2_{and a span of 5m. The}

morph-ing concept developed by the authors consists of a

con-tinuous skin and a selectively compliant internal struc-ture, enabling smooth camber changes, constant or vary-ing along the span. In order to solve the conflictvary-ing re-quirements of stiffness for load carrying and compliance for morphing, CFRP are utilized. The wing is comprised of a rigid wingbox, carrying the majority of the structural loads, and a compliant trailing section, made of a com-posite skin. Actuators are used to introduce mechanical energy in the system and the deformation is guided by the distributed compliant ribs, thus achieving favourable camber morphing.

The ideal wing characteristics are determined by an op-timization, which accounts for aeroelastic interactions in the assessment of the wing behavior. The optimiza-tion variables describe the aerodynamic shape, the inner compliant structure, the wing skin composite layup, and the actuation strategy. The objective of the optimization is to maximize the produced power of the aircraft. The result of the optimization is a morphing wing capable of achieving an optimized shape and a favourable lift distri-bution along the span for a wide range of wind speeds. A full scale experimental demonstrator is manufactured according to the optimized shape and structure. Fol-lowing the manufacturing, a test campaign is performed to evaluate the performance benefits of the morphing wing. Preliminary results show that the performance is improved and the weight can be decreased, while achiev-ing sufficient rollachiev-ing moment.