Operational Modal Analysis Of Moving Aircraft Landing Gear Doors In Flight

The vibrations on the aircraft undercarriage triggered by unsteady aerodynamics forces are relevant for the design of these airframe components. Being generated by unsteady flow around itself or excited by vortices shed from other upstream components, e.g. the nose landing gear, these vibrations may occur on movable surfaces in the undercarriage area like the main landing gear doors, even though they are opened only for a short sequence while the landing gear is in transit.The numerical simulation of the vibration level of the undercarriage is associated with uncertainties because of detached flow, vortex shedding and buffet, which needs experimental flight test results. Consequently, research activities comprises instrumented flight tests with a transport aircraft adapted for this purpose and operated by the German Aerospace Centre DLR. Different types of sensors were installed on the movable components of undercarriage, comprising accelerometers, strain gauges, travel and pressure sensors. To get detailed knowledge of the dynamic interaction of pressure loads and structural response on the nose and main landing gear doors, the simultaneous data acquisition of all signals was mandatory during deployment and retraction.The flight test measurements are evaluated with output-only modal analysis based on the data-driven Stochastic Subspace Identification method (SSI). The vibration level results show trends for increasing flight velocity and different flight manoeuvre.