Applications of the unsteady vortex-lattice method in aircraft aeroelasticity and flight dynamics

A review on non-linear aeroelasticity of high aspect-ratio wings

We investigate the control of constrained stochastic linear systems when faced with limited information regarding the disturbance process, i.e., when only the first two moments of the disturbance distribution are known. We consider two types of distributionally robust constraints. In the first case, we require that the constraints hold with a given probability for all disturbance distributions sharing the known moments. These constraints are commonly referred to as distributionally robust chance constraints. In the second case, we impose conditional value-at-risk (CVaR) constraints to bound the expected constraint violation for all disturbance distributions consistent with the given moment information. Such constraints are referred to as distributionally robust CVaR constraints with second-order moment specifications. We propose a method for designing linear controllers for systems with such constraints that is both computationally tractable and practically meaningful for both finite and infinite horizon problems. We prove in the infinite horizon case that our design procedure produces the globally optimal linear output feedback controller for distributionally robust CVaR and chance constrained problems. The proposed methods are illustrated for a wind blade control design case study for which distributionally robust constraints constitute sensible design objectives.

Distributionally Robust Control of Constrained Stochastic Systems

This paper reviews the applicability of some conventional structural design practices to the analysis and design of very flexible aircraft. The effect of large structural deformations and the coupling between aeroelasticity and flight dynamics is investigated in different aspects of the aircraft structural design process, including aeroelastic stability, loads, and flight dynamics and control. This is illustrated with a numerical example of the static and dynamic responses of a representative high-altitude long-endurance vehicle. Suggestions are presented for the development of appropriate frameworks to design and analyze very flexible aircraft.

/pdf/reexamined-structural-design-procedures-for-very-flexible-25jtokebwu.pdf

Reexamined Structural Design Procedures for Very Flexible Aircraft

Aircraft with variable wing geometry (morphable wings) are of considerable interest, not only formission-specific optimization but for enhanced maneuverability as well. In the nascent field of mini or micro unmanned aerial vehicles, large and rapid changes inwing geometry are achievable, resulting in significant changes of the dynamics of the vehicle. In this paper, a simulationmethodology suitable for such aircraft is presented, accounting for the changes in both the aerodynamic and inertial properties. Because of the complexity of the possible wing configurations, the aerodynamics are simulated using an unsteady vortex-lattice approach, solved concurrently with six-degree-offreedom ( ) nonlinear equations of motion. The time dependence of the inertia tensor andmotion ofmass within the body frame are explicitly taken into account, resulting in additional body-frame forces andmoments. The simulation methodology is applied to various gull-wing configurations, and the flight dynamics are analyzed through nonlinear simulation.

Modeling of Flight Dynamics of Morphing Wing Aircraft

The paper investigates the coupled nonlinear aeroelasticity and flight mechanics of very flexible lightweight aircraft. A geometrically-exact composite beam formulation is used to model the nonlinear flexible-body dynamics, including rigid-body motions. The aerodynamics are modeled by a general 3-D unsteady vortex-lattice method, which can capture the instantaneous shape of the lifting surfaces and the free wake, including large displacements and interference effects. The coupled governing equations are solved in a variety of ways, allowing linear and nonlinear time-domain simulations of the full vehicle and frequency-domain linear stability analysis around trimmed configurations. The resulting framework for the Simulation of High-Aspect Ratio Planes (SHARP) provides a medium-fidelity representation of flexible aircraft dynamics, based on an intuitive and easily linearizable structural representation using displacements and the Cartesian rotation vector, time-domain aerodynamics, and at relatively low computational costs. Previous verification studies on the structural dynamics and aerodynamics modules are complemented here with studies on the flexible-body implementation and on the integrated simulation methodology. A numerical investigation is finally presented on a representative high-altitude long-endurance model aircraft, investigating its stability properties and its open-loop dynamic response.

/pdf/stability-and-open-loop-dynamics-of-very-flexible-aircraft-2fhc1v1bqu.pdf

Stability and Open-Loop Dynamics of Very Flexible Aircraft Including Free-Wake Effects

This paper numerically investigates the dynamics of a flexible, lightweight, unmanned aircraft, evaluating its stability boundaries and focusing on the response of the aircraft under atmospheric disturbances. This is achieved by integrating a time-domain 3-D unsteady vortex-lattice aerodynamics method with a geometrically-exact composite beam model encompassing elastic and rigid-body degrees of freedom. The resulting framework is a medium-fidelity tool for the analysis of vehicles that exhibit substantial couplings between their aeroelastic and flight dynamics responses. In its general nonlinear form, the unified model captures the instantaneous shape of the lifting surfaces and the free wake, including large geometrically-nonlinear displacements, in-plane motions, and aerodynamic interference effects. The linearization of the equations leads to a monolithic state-space assembly, ideally suited for stability analysis and control synthesis. The numerical studies illustrate these capabilities, designing linear PID controllers in order to alleviate gust-induced loads and trajectory deviations.

https://spiral.imperial.ac.uk:8443/bitstream/10044/1/9571/1/SDM2012-MuruaPalaciosGraham.pdf

Open-Loop Stability and Closed-Loop Gust Alleviation on Flexible Aircraft Including Wake Modeling

This paper introduces a methodology for the design of gust load control systems directly from large aeroelastic models with relaxation of spatial discretisation. A convenient state-space representation of the vortex-panel unsteady aerodynamics suitable for control synthesis is presented. This allows a full understanding of the dynamics of the linearized vortex aeroelastic model and is suitable for control system design. Through the use of robust controllers, large reductions in loading could be achieved. Comparisons are also made between robust and classical control methods. It further demonstrates that controllers synthesized from models of coarse spatial discretizations and of an order of magnitude smaller in size were capable of rejecting disturbances on fully converged models, with performances comparable to expensive higher order controllers developed from full models.

http://spiral.imperial.ac.uk/bitstream/10044/1/12860/4/Ng%202012.%20Robust%20control%20synthesis%20for%20gust%20load%20alleviation%20from%20large%20aeroelastic%20models%20with%20relaxation%20of%20spatial%20discretisation.pdf

Robust Control Synthesis for Gust Load Alleviation from Large Aeroelastic Models with Relaxation of Spatial Discretisation

This paper presents an aeroservoelastic modeling approach for dynamic load alleviation
in large wind turbines with trailing-edge aerodynamic surfaces. The tower, potentially on a
moving base, and the rotating blades are modeled using geometrically non-linear composite
beams, which are linearized around reference conditions with arbitrarily-large structural
displacements. Time-domain aerodynamics are given by a linearized 3-D unsteady vortexlattice
method and the resulting dynamic aeroelastic model is written in a state-space
formulation suitable for model reductions and control synthesis. A linear model of a single
blade is used to design a Linear-Quadratic-Gaussian regulator on its root-bending moments,
which is finally shown to provide load reductions of about 20% in closed-loop on the full
wind turbine non-linear aeroelastic model.

Jmr Graham

Papers

Applications of the unsteady vortex-lattice method in aircraft aeroelasticity and flight dynamics

Stability and Open-Loop Dynamics of Very Flexible Aircraft Including Free-Wake Effects

Open-Loop Stability and Closed-Loop Gust Alleviation on Flexible Aircraft Including Wake Modeling

Robust Control Synthesis for Gust Load Alleviation from Large Aeroelastic Models with Relaxation of Spatial Discretisation

Model-based Aeroservoelastic Design and Load Alleviation of Large Wind Turbines