Could aircraft capable of changing their shape during flight hold the key to sustainable aviation?
The most pressing challenges facing the future of sustainable aviation centre largely around weight reduction and increased efficiency. One innovative project from Politecnico di Milano is aiming to meet these challenges head on by harnessing the phenomenon of structural instability to develop radically new adaptive composite structures for next-generation aircraft.
Led by Professor Chiara Bisagni of the university’s Department of Aerospace Science and Technology, the NABUCCO research project will develop new design, analysis and optimisation methods to create composite structures such as adaptive wings that could revolutionise the aerospace industry as we know it.
“The current quest for highly efficient and green aircraft demands new concepts, including morphing applications,” says Bisagni. “Practical solutions require a compromise between two counteracting requirements typical of aircraft structures: the design of a structure that can withstand the requested loads without excessive deformation (stiffness requirement), together with the possibility to change its shape without excessive effort (compliance requirement). This is precisely the opportunity offered by modifying and adapting the aircraft wing shape during the flight mission by the direct use of the buckling phenomena, induced by the typical large non-linear displacements and stiffness redistribution.”
In aeronautics, structural instability is generally avoided as it can generate large deformations and, in some cases, can cause catastrophic collapse of structures. The NABUCCO project, however, is flipping the concept of structural instability on its head: rather than seeing structural instability as a phenomenon to be avoided, it is seen as a design opportunity to be explored for its revolutionary potential.
“The solution we propose tries to modify the aircraft wing shape, mainly the twist span-wise, for different flight conditions thanks to the design of selected elements that can switch into different post-buckling configurations, when requested,” Bisagni explains. “In this way, it will be possible to tune the structures toward the desired aerodynamic shape with minimum energy requirements. The potential benefits will be in terms of both aerodynamic and structural efficiency, and are mainly due to the possibility to reshape the spanwise load distribution.”
According to Bisagni, this type of adaptive wing will help to not only reduce the weight of future aircraft, but also improve their aerodynamic and structural efficiency. Meanwhile, the added possibility to adapt the wing stiffness distribution span-wise will enable greater control of the wing load distribution, impacting on the induced drag and on the root bending moment.
ADAPTIVE COMPOSITE STRUCTURES
Central to the project’s success is the design and development of novel adaptive composite structures.
“In NABUCCO we will use composite materials,” she says. “Their potential benefits for aerospace industries in terms of mechanical properties and design simplification for manufacturing processes are many. Nowadays, there are new manufacturing processes, in particular we will benefit from fibre steering technology as it will allow the realisation of efficient load paths, improving at the same time the structural performance of the composites and reducing their weight.
“We will investigate all possibilities of tailored composite materials: optimised laminated composites, thermoset and thermoplastic composites, and fibre steering layers using automated fibre placement manufacturing – yielding a large design space that will allow to explore many and unconventional optimal configurations.”
NEW MATERIALS REQUIRE NEW METHODS
To achieve the revolutionary composite structures described above, Bisagni and her team will create new design, analysis and optimisation techniques based on analytical formulations, neural network algorithms and an integrated multidisciplinary approach.
“A strongly coupled computational-experimental framework will be developed based on novel analytical formulations, approximations and model reduction using artificial neural networks, large multi-objective optimisations, high-fidelity simulation methodologies, and advanced test techniques,” Bisagni explains. “We aim to develop and validate a computational model consisting of a fast tool to calculate possible multi-stable configurations through analytical formulation and deep learning, and high-fidelity finite element models to verify that the structures remain reliable in the post-buckling regime.
“As the buckling phenomenon is very sensitive to several parameters, a robust and reliability-based design optimisation will be conducted, taking into account the parameters that mainly influence the buckling phenomena, already from the first steps of the design, and considering structures and materials no longer as two separate entities.”
To support her research, Bisagni has been awarded an ERC Advanced Grant from the European Research Council for the NABUCCO project. Over the next five years, she and her team will design and formulate the novel adaptive composite structures that could lead to a step-change in sustainability for the aerospace and aviation industries.
