Lidar-Based Gust Load Alleviation - Increasing the Load Reduction Potential through a Two-Degree-of-Freedom Controller Architecture

Kurzfassung

This paper proposes a two-degree-of-freedom (2-DoF) gust load alleviation (GLA) control architecture. Each "degree of freedom" was developed independently, either solely for load reduction purposes (in the case of the feedforward GLA part) or as a combination of a basic control law (load factor/C* normal law) and a feedback GLA control law. The basic control law influences the structural gust loads, even if not directly intended. The feedforward GLA part consists of a lidar-based controller, using wind field measurements ahead of the aircraft. To prevent unnecessarily aggressive controller commands for flight points at which lower load alleviation levels are required, the gains of the feedforward GLA control law and the gains of the feedback GLA control law are both scheduled via the true airspeed. Furthermore, a precompensation term is included to prevent the feedback laws (basic control law and GLA) from fighting the feedforward GLA commands. This precompensation works through the use of a reduced-order prediction model and additional connections from the feedforward GLA controller to the inputs of the feedback controllers. In a sense, the prediction model and the connections allow the feedforward GLA controller to "inform" the feedback controllers of its intentions. The load alleviation performance is evaluated on a representative hybrid (continuous and multi-rate discrete time) environment for a wide range of mass cases, flight points, and gust lengths, leading to results based on 864 different load cases. The proposed control architecture yields a reduction in peak bending moment of about 23 % in the area between the important wing root and the engine pylon, and still over 10 % near the wing tip.