The Changing Role of Aeroelasticity in Aircraft Design

Kurzfassung

Almost from the beginning of aviation, aeroelasticity was feared for its negative impacts on aircraft stability and performance, which became more and more severe over years. Today however, dramatic changes are expected for the performance and efficiency of future airplanes from new "Active Structures" design concepts, exploiting aeroelastic effects in a beneficial way to reduce aerodynamic drag and design loads, improve manoeuvrability, and enhance aircraft stability. This article is a review of past, on-going, and planned research and development activities where the main objective is an adaptive change of the shape of the airplane by active deformations of the structure. In addition to the intended effects from the structural deformations, these concepts are classified by the types of devices that initiate them: aerodynamic control surfaces, adaptive stiffness systems for the attachment or actuation of an aerodynamic surface, and active structural components. Some of these concepts are mainly concentrating on the creation of large structural deformations - without special considerations about aeroelastic effects, while others are mainly concentrating on the stimulation of deformations by aeroelastic effects. In the first case, the aeroelastic impacts from these concepts and on these concepts will be addressed. The required new analytical design methods will be discussed. Besides the new quality of additional physical properties for the description of structure´s reactions and actions, the field of structural and multi-disciplinary analysis and optimization (MDO) is essential for the effective integration of active elements into an otherwise passive structure. This new design process may also be considered as an extension of "Aeroelastic Tailoring", where the required stiffness distribution of the structure for desired aeroelastic characteristics is achieved by formal structural optimization methods. If properly designed, only a small portion of the total deformation of the structure needs to be carried out actively. This initial step triggers the aeroelastic deformation and aerodynamic load redistribution in the desired way. The major fraction of the required input power can then be taken at no extra costs from the surrounding air. For future designs, an MDO framework is presented to calculate the sensitivities of static aeroelastic and flutter characteristics with respect to the parameters which define the shape and structural layout of a wing. The aerodynamic and structural models are based on one common parametric geometry description which employs the Design Elements Method and is created by a model generator. While the aerodynamic model includes the description for the application of the lifting surface theory the structural model description is based on the Finite Element Method and includes a detailed layout concept with arbitrarily oriented spars and ribs. The characteristics of interest in these studies are the presentation of the model generator with a wide range of geometrical variations defined by small set of parameters and the implementation of the model generator in a design process including the calculation of aerodynamic loads and structural optimization. All of this allows for the study of trim conditions, rolling moment, and induced drag. Shape parameters are the aspect ratio and sweep angle. A wing box layout parameter set includes the position and orientation of spars and ribs defined on the wing planform. A third parameter set consists of structural variables, including element thickness.