" Flight Test for PIOs on Advanced Technologies Testing Aircraft System (ATTAS),”
Oliver Brieger, DLR and Matt Turner, University of Leicester
This presentation documents the progressive evolution in the development of a rate saturation compensation scheme to enhance degraded handling qualities and to suppress Category II Pilot In-the-Loop Oscillations (PIO). In two flight test campaigns in 2006 and 2007 termed SAIFE I and II (Saturation Alleviation In-Flight Experiment I and II) the effectiveness of these rate saturation compensators have been tested using the German Aerospace Center’s (DLR) flying test bed ATTAS (Advanced Technologies Testing Aircraft). Details of the employed compensator design philosophy and flight test techniques such as HQDT as well as qualitative and quantitative data to support this promising concept are given. The rate saturation compensators have been designed based on the Anti Windup (AW) control philosophy, with the aim to reduce the adverse effects of incipient and fully developed rate saturation on the piloted aircraft dynamics, and consequently provide an increased flight envelope (operating envelope) for acceptable aircraft handling qualities and reduced PIO susceptibility. The achievement of this goal has been determined by correlating flight test data with Handling Qualities Ratings (HQRs) and PIO Ratings (PIORs) as supportive evidence. Primary objective of the SAIFE I campaign was to scrutinize the saturation suppression effectiveness of high and low order compensators at predefined flight conditions. These results served as basis to enhance AW compensator performance by including additional design criteria such as the Open Loop Onset Point (OLOP)-criterion to improve robustness over large parts of the flight envelope. The new designs were subsequently tested during the SAIFE II trials. The results demonstrate that the employed compensation schemes are effective in reducing the level of rate saturation in all investigated cases, making the aircraft less PIO prone while exhibiting either unchanged or improved handling qualities. Furthermore, the maturation of the design process was reflected in more robust low order compensator designs, but also highlighted tradeoffs between inherent stability and system performance. In essence, the results provide a basic understanding of the relationship between design parameters and the Pilot Vehicle System (PVS) closed loop response during periods of rate saturation. The conducted flight tests demonstrated the potential of well designed AW compensators to improve the handling qualities of aircraft encountering rate saturation.