Aeroelastic tailoring of high- aspect ratio wings via tow-steering
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Date
2024-07-08
Authors
Ζιάκος, Νικόλαος
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Abstract
Over the past several decades, composite materials have been used progressively more in the aerospace industry due to their superior performance, lightweight design and inherent ‘tailorability’. Traditionally, unidirectional composites stacked to form a laminated structure are used for high performance load carrying applications. However, with the introduction of automatic fiber placing machines, it is now possible to manufacture composites with fibers that follow curved paths inside the laminae. This process is known as tow-steering and the resulting composites as variable stiffness composites.
Tow-steered composites allow for even greater flexibility in the design of structures by taking advantage of the anisotropic behavior of fiber composites to enhance performance with no additional weight penalty compared to conventional composites. However, given the highly coupled nature of aeroelastic problems, it is not obvious nor intuitive to find the optimal fiber patterns, and sophisticated computational methods and optimization algorithms are required.
In this thesis, a structural optimization framework is developed for the aeroelastic benchmark uCRM-13.5 aircraft wing. More specifically, the wing structure is represented by a Finite Element model utilizing beam and shell elements, while a medium fidelity DLM model is incorporated to model the aerodynamics. A series of optimizations are performed to evaluate the benefits of using tow-steered composites over their conventional counterparts. Static strength, buckling and flutter constraints are imposed in all optimization runs.
Initially, a mass optimization is performed on the wing, using conventional composites for the skins, in order to determine the optimum unsteered configuration. Then, tow-steering variables are introduced into the problem, resulting in a 12.5% reduction in structural mass. Subsequently, two single-objective optimizations are performed on the optimum unsteered wing configuration using only the tow-steering design variables. First, the wing is optimized for stength, by maximizing the aggregated value (KS function) of the skin first-ply failure indices. A 47% reduction in the KS function was achieved in the tow-steered wing compared to the optimum unsteered configuration. Next, the wing is optimized for flutter velocity, achieving an increase of 29% in the optimized tow-steered design. Finally, a multiobjective optimization run is conducted to explore the Pareto front between skin strength (KS
function), buckling and flutter velocity in terms of the fiber steering patterns.
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Keywords
Aeroelasticity, Optimization, Aeroelastic tailoring, Variable stiffness composites, u-CRM wing, Flutter, Tow-steering, NASA common research model