Henrik Hesse

Reduced-Order Aeroelastic Models for Dynamics of Maneuvering Flexible Aircraft

This paper investigates the model reduction, using balanced realizations, of the unsteady aerodynamics of maneuvering flexible aircraft. The aeroelastic response of the vehicle, which may be subject to large wing deformations at trimmed flight, is captured by coupling a displacement-based flexible-body dynamics formulation with an aerodynamic model based on the unsteady vortex lattice method. Consistent linearization of the aeroelastic problem allows the projection of the structural degrees of freedom on a few vibration modes of the unconstrained vehicle but preserves all couplings between the rigid and elastic motions and permits the vehicle flight dynamics to have arbitrarily large angular velocities. The high-order aerodynamic system, which defines the mapping between the small number of generalized coordinates and unsteady aerodynamic loads, is then reduced using the balanced truncation method. Numerical studies on a representative high-altitude, long-endurance aircraft show a very substantial reducti...

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

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.

Model-based identification and control of the velocity vector orientation for autonomous kites

In this paper we address a control problem for an autonomous tethered kite system for the purpose of airborne wind energy generation. In particular, we design a tracking controller for the velocity vector orientation of the kite. Motivated by empirical data we model the kite steering behaviour as a delayed dynamical system and explicitly utilise the derived model information for the controller design. We identify the involved parameters from experimental data. To adapt to changes in operating conditions we update the parameter estimation on-line. We present results of the derived approach successfully tested in real-world flight experiments.

Numerical Investigation of Edge Flame Propagation Behavior in an Igniting Turbulent Planar Jet

The effects of strain rate and curvature on the edge flame propagation characteristics in an igniting turbulent coflowing planar jet are studied based on 3-dimensional compressible Direct Numerical Simulation (DNS) with a modified single-step Arrhenius chemistry. In the present configuration the high-speed jet fluid is considered to be fuel-rich, whereas the slow-moving coflowing fluid is taken to be fuel-lean. Consistent with previous work with DNS without mean flow and shear, the resulting flame from localized forced ignition at the jet exhibits predominantly premixed edge flame structure where premixed flames are formed on both the fuel-rich and fuel-lean sides and the intersection between these two branches propagate on the stoichiometric mixture fraction isosurface. The edge flame propagation behavior has been studied in terms of the statistics of the edge flame displacement speed S d , which refers to the speed at which the fuel mass fraction Y F isosurface moves normal to itself, relative to an initially coincident material surface at the intersection between the fuel-rich and fuel-lean premixed flames on the stoichiometric mixture fraction isosurface. The probability density function of the density-weighted edge flame displacement speed shows nonzero probability of finding negative values of at later stages of self-sustained flame propagation. The mean value of decreases after the energy deposition is switched off but eventually settles to a value that does not change appreciably with time. The is found to be predominantly negatively correlated with curvature but the correlation between and tangential strain rate shows both positive and negative correlating trends with the positive correlating trend dominant at later stages of flame propagation. It has been found that strain rate and curvature dependences of |∇Y F | have significant influences on the statistical behavior of in response to strain rate and curvature. The observed strain rate and curvature dependences of have been explained in detail in terms of statistical behaviors of the reaction, normal diffusion, and the tangential diffusion components of (i.e., , , and ).

Dynamic load alleviation in wake vortex encounters

This paper introduces an integrated approach for flexible-aircraft time-domain aeroelastic simulation and controller design suitable for wake encounter situations. The dynamic response of the vehicle, which may be subject to large wing deformations in trimmed flight, is described by a geometrically nonlinear finite-element model. The aerodynamics are modeled using the unsteady vortex lattice method and include the arbitrary time-domain downwash distributions of a wake encounter. A consistent linearization in the structural degrees of freedom enables the use of balancing methods to reduce the problem size while retaining the nonlinear terms in the rigid-body equations. Numerical studies on a high-altitude, long-endurance aircraft demonstrate the reduced-order modeling approach for load calculations in wake vortex encounters over a large parameter space. Closed-loop results finally explore the potential of combining feedforward/feedback H∞ control and distributed control surfaces to obtain significant load ...

Model-based flight path planning and tracking for tethered wings

In this paper we propose a guidance strategy for the flight control of a tethered wing. We control the wing trajectory in a cascaded approach via the velocity vector orientation. In particular, we consider a control-oriented model with an input delay to follow a reference path. To account for the delay we design a predictor and use the predictions to compute a reference for a lower level tracking controller. In a path-planning step we design reference figure-eight paths for the wing to follow. The path design explicitly considers the model parameters used in the tracking controller such that limitations induced by the delay are respected. By estimating the model parameters on-line we enable the adaptation of the tracking controller and also the path-planner to time varying conditions, including the input delay and crucially the line length. We present the derivation of the guidance strategy and demonstrate its performance via simulation results.

Assessing the impact of aerodynamic modelling on manoeuvring aircraft

This paper investigates the impact of aerodynamic models on the dynamic response of a free-flying aircraft wing. Several options for the aerodynamics are evaluated, from two-dimensional thin aerofoil aerodynamics and unsteady vortex-lattice method up to computational fluid dynamics. A nonlinear formulation of the rigid body dynamics is used in all cases. Results are generated using a numerical framework that will allow in the near future multi-disciplinary fluid/structure/flight analysis. In this paper, flexibility effects are neglected. A validation for fluid/flight models is presented. The well-established approach based on stability derivatives is also used, and is found in good agreement with solutions obtained from linear aerodynamic models. The uncertainties in predicted trajectories of the free-flying wing are, in general, large and attributed to the aerodynamics only. This suggests that a careful control law synthesis should be done to account for uncertainties from modelling techniques.

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

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.

Predictive Control of Autonomous Kites in Tow Test Experiments

In this letter, we present a model-based control approach for autonomous flight of kites for wind power generation. Predictive models are considered to compensate for delay in the kite dynamics. We apply model predictive control (MPC), with the objective of guiding the kite to follow a figure-of-eight trajectory, in the outer loop of a two level control cascade. The tracking capabilities of the inner-loop controller depend on the operating conditions and are assessed via a frequency domain robustness analysis. We take the limitations of the inner tracking controller into account by encoding them as optimization constraints in the outer MPC. The method is validated on a kite system in tow test experiments.

Range-inertial estimation for airborne wind energy

An estimation approach is presented for an autonomous tethered kite system for the purpose of airborne wind energy generation. Accurate estimation of the kite state is critical to the performance of automatic flight controllers. We propose an estimation scheme which fuses measurements from range sensing, based on ultra-wideband radios, and inertial readings from an inertial measurement unit. Ranges are measured between a transceiver fixed to the moving kite body and a number of static range beacons scattered on the ground. Estimates are computed using the multiplicative extended Kalman filtering scheme with a sensor-driven kinematic process model using a quaternion representation of the kite attitude. Furthermore, we assume only approximate prior knowledge of the range beacon locations and consider the problem of estimating the kite state and localizing the range beacons simultaneously. We present results of the estimator tested within a simulation environment of an airborne wind energy system and compare performance to an existing estimation scheme based on tether-angles and tether-length measurements.