Andreas Wildschek

Flight Test with an Adaptive Feed-Forward Controller for Alleviation of Turbulence Excited Wing Bending Vibrations

On large, and thus highly flexible airliners, atmospheric turbulence excited wing bending vibrations generally cause high dynamic loads in the wing roots and degrade the ride comfort. In the past, these problems have been tackled by the use of feedback control laws for active wing bending damping. Active wing bending damping reduces fatigue loads on the one hand, and reduces vertical accelerations in the cabin on the other. Additional feed-forward compensation of wing bending vibrations can provide further improvement of ride comfort and, assuming sufficient control authority, has the potential to also reduce peak loads. Best performance is obviously achieved by a combination of both active wing bending damping and feed-forward compensation of atmospheric turbulence excited wing bending vibrations. However, the performance of feed-forward control is very sensitive to plant uncertainties. Regarding wing bending vibration alleviation on large airliners, one major plant uncertainty is the fuel mass. A promising solution to overcome problems related to the sensitivity of performance in regards to plant uncertainties, is the adaptation of the feed-forward controller towards optimum performance using a least mean square algorithm. Such an adaptive feed-forward vibration control system has been successfully flight tested on the DLR ATTAS (Advanced Technologies Testing Aircraft System.) A reference for atmospheric turbulence was obtained by a nose boom mounted flight log sensor. Symmetrically commanded high bandwidth Direct Lift Control (DLC) flaps served as actuators. The obtained flight test data shows robust convergence of the adaptive feed-forward controller towards its optimum, as predicted by previous stability analysis of the adaptation algorithm. The ATTAS is a relatively small and stiff aircraft with a first wing bending frequency of 5 Hz. The converged feed-forward controller thus reduces the power spectral density of modal wing bending accelerations by only 20%, at least showing that the use of a conditioned alpha probe signal for compensation of atmospheric turbulence excited wing bending vibrations is feasible. A 20% reduction in terms of the power spectral density is not much, but the result is well in accordance with performance estimates drawn from rough calculations with the two-dimensional von Karman turbulence model. The same model predicts a 75% reduction of the power spectral density of modal wing bending accelerations (50% reduction of the magnitude of modal wing bending accelerations respectively) for large airliners where the first wing bending frequency lies at about 1 Hz. A 50% reduction of the magnitude of modal wing bending accelerations is comparable to what is achievable with active wing bending damping control systems today, thus showing that for large airliners the proposed feed-forward compensation of atmospheric turbulence excited wing bending vibrations provides a powerful method for further reduction of dynamic loads and improvement of ride comfort.

Integrated adaptive feed-forward control of atmospheric turbulence excited rigid body motions and structural vibrations on a large transport aircraft

An adaptive feed-forward controller for simultaneous compensation of atmospheric turbulence excited rigid body motions and structural vibrations is designed. Proposed feed-forward control is intended as an add-on to current gust load alleviation systems. The objectives thereby are increased passenger comfort and handling qualities, as well as a more efficient reduction of dynamic wing loads. A steepest descent algorithm is applied in order to increase the robustness of the performance of the feed-forward control system against modeling errors and variations of wing tanks configuration and Mach number. The proposed algorithm is tested in numeric simulations with the state space model of a conventional four-engine transport aircraft. The simulation results illustrate the proposed control systems high potential for simultaneous compensation of atmospheric turbulence excited pitch and wing bending accelerations.

Active Wing Load Alleviation with an Adaptive Feed- forward Control Algorithm

The latest subsonic civil transport aircrafts with high aspect ratio at low structural weight/payload ratio have been equipped with active wing-load control systems based on robust feedback of modal accelerations. The aim of these systems is to increase handling qualities and passenger comfort as well as to reduce dynamic wing-loads mainly induced by atmospheric disturbances. Structural control must be robust regarding variations in the plant transfer functions caused by the varying flight and load conditions to which an aircraft structure is exposed to. Such robustness criteria limit performance of robust wing-load control. Since robust structural control law design requires very accurate aero-elastic plant models, it is generally optimized iteratively during the flight test phase, where the accuracy of plant models is successively improved and the flight and load envelope is continuously expanded. This paper shows that introduction of an adaptive structural feed-forward control system could dramatically increase attainable performance of active wing-load control due to the feed-forward character on the one hand, and due to adaptivity on the other. The authors found that the best performance and robust stability of the adaptive wing-load alleviation system can be reached with an adaptive feed-forward controller in combination with error-feedback based on a stochastic-gradient-descent algorithm. The proposed algorithm rapidly adapts to any changes in plant transfer functions and excitation, so that extra robust stability margins no longer have to be taken into account. Additionally, the plant transfer functions have to be known in advance only approximately, since the controller optimizes itself online to the actual plant transfer function. Iterative design with the help of flight test optimized plant models is no longer necessary to perform. Thus it is estimated that the design period and design costs for active wing-load control can be dramatically decreased. After introducing the basic principle and integration of the proposed adaptive controller, the mathematical background of the algorithm is briefly discussed. This is followed by a detailed stability analysis to ensure stable adaptation of the algorithm. Finally, results of numeric simulations are presented to underline the validity of found stability conditions and to highlight improved performance, compared to a robust structural feedback control system.