Contribution of the ground vibration tests for the preparation of flutter suppression flight test campaigns: Case of the flipased P-Flex UAV

Kurzfassung

With the everlasting reach for lighter and more fuel-efficient aircraft structures, aircraft designs become more and more flexible. This leads to both increased deformation of the airframe under aerodynamic load, as well as more coupling between the structural static and dynamic behaviour and the surrounding airflow, potentially leading to the dreaded phenomenon of flutter. This dangerous behaviour has limited potential aircraft design optimization, hence impaired carbon footprint reduction. In order to circumvent this constraint, active flutter suppression through fast actuators and control surfaces was investigated during the FLEXOP and subsequent FLIPASED projects, on a research high aspect ratio fixed wing UAV. Such control strategies, though of great interest thanks to their capability to control a wide variety of behaviour, also come with great risks if badly designed. They must therefore be conceived with very good knowledge of the controlled structure, as well as properly verified through an extensive test phase. This paper focuses on the identification of the structural dynamic behaviour of the FLIPASED P-FLEX UAV during a Ground Vibration Test (GVT) campaign. The paper presents the GVT organization, from instrumentation to analysis, including test configurations accounting for high-bandwidth actuators as well as devices, designed to help increase or decrease the flutter coupling, so-called flutter stoppers. The paper describes the structural dynamic behaviour of the P-FLEX UAV and the influence of the different mechanical components on this behaviour. Numeric and experimental modal models are compared and discussed to highlight the main differences. Due to multiple specific features of this aircraft including high aspect ratio and custom designed high-bandwidth actuators, this GVT proved difficult to achieve for both organizational and technical reasons. The paper presents the main problems associated with these specific features and proposes improved test strategies for future GVTs. Although the P-FLEX UAV is of a peculiar design with respect to commercial aircraft, the observations made on this aircraft will be of great interest for future civilian aircraft designs that tend towards very high aspect ratio wings equipped with numerous active flutter suppression control surfaces.