Aeroelasticity is the study of the dynamic interaction between unsteady aerodynamics and structural dynamics on flexible streamlined bodies, which may include rigid-body dynamics. Industry standard solutions in aeronautics and wind energy are built on the assumption of small structural displacements, which lead to linear or quasi-linear theories. However, advances in areas such as energy storage and generation, and composite material manufacturing have fostered a new kind of aeroelastic structures that may undergo large displacements under aerodynamic forces.In particular, solar-powered High-Altitude Long-Endurance (HALE) aircraft have recently seen very significant progress. New configurations are now able to stay airborne for longer than three weeks at a time. Extreme efficiency is achieved by reducing the total weight of the aircraft while increasing the lifting surfaces' aspect ratio. In a similar quest for extreme efficiency, the wind energy industry is also trending towards longer and more slender blades, specially for off-shore applications, where the largest blades are now close to 100-m long.These longer and more slender structures can present large deflections and have relatively low frequency structural modes which, in the case of aircraft, can interact with the flight dynamics modes with potentially unstable couplings. In the case of offshore wind turbines, platform movement may generate important rotor excursions that cause complex aeroelastic phenomena which conventional quasi-linear methods may not accurately capture.

Two different nonlinear aeroelastic tool sets, SHARPy and the Modal Rotation Method (MRM), have been employed to predict and design a wind tunnel flutter test campaign of a very flexible wing, the Pazy Wing, as part of the 3rd Aeroelastic Prediction Workshop. The first method, SHARPy, uses geometrically exact beams coupled with an Unsteady Vortex Lattice, which is linearised about a deformed configuration, reduced by means of Krylov subspaces and analysed to compute the stability boundaries of the wing. The MRM is based on structural modal data, from either beam models or finite element models, coupled with a doublet-lattice aerodynamic model from ZAERO of the straight wing configuration. The excellent agreement between numerical and experimental data for structural-only and static aeroelastic analyses paves the way for predicting the stability boundaries of the Pre-Pazy wing with sufficient confidence for the safe design of a flutter wind tunnel test campaign.

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Nonlinear Moving Horizon Estimation (MHE) and Model Predictive Control (MPC) strategies for very flexible aircraft are presented. They are underpinned by a nonlinear reduced-order model built upon the structure's natural modes of vibration. This internal model aims for a minimal realisation of the aircraft which retains sufficient information to enable efficient real-time estimation and control. It is based on a modal intrinsic description of geometrically-nonlinear beams and a linearised unsteady vortex lattice aerodynamic model. Numerical evidence has shown that models of this form are able to capture the main nonlinear geometrical couplings at a very low computational cost. This opens the door to MHE and MPC strategies, which are naturally more computationally demanding than other linear conventional strategies, but are more versatile and able to provide control in the usually neglected nonlinear regime. The proposed control framework is tested on models built in an in-house open-source nonlinear aeroelasticity simulation and analysis package, to emulate the controller performance on a realistic plant model. Very satisfactory results are obtained in a flutter suppression problem involving a very flexible clamped wing, where the nonlinearity of the problem is leveraged by the internal model to achieve stabilisation, and a payload drop control of a very flexible HALE aircraft.

The stability boundaries of a very flexible wing are sought to inform a wind-tunnel flutter test campaign. The objective is twofold: to identify via simulation the relevant physical processes to be explored while ensuring safe and non-destructive experiments, and to provide a benchmark case for which computational models and test data are freely available. Analyses have been independently carried out using two geometrically nonlinear structural models coupled with potential flow aerodynamics. The models are based on a prototype of the wing for which static load and aeroelastic tests are available, and the experimental results have been successfully reproduced numerically. The wing displays strong geometrically nonlinear effects with static deformations as high as 50% of its span. This results in substantial changes to its structural dynamics, which display several mode crossings that cause the flutter mechanisms to change as a function of deformation. Stability characteristics depend on both the free-stream velocity and the angle of attack. A fast drop of the flutter speed is observed as the wing deforms as the angle of attack is increased, while a large stable region is observed for wing displacements over 25%. The corresponding wind tunnel dynamic tests have validated these predictions.