Optical measurements of flutter mode shapes

The usefulness of an optical motion capture system in aeroelastic wind-tunnel testing is investigated. A system consisting of four infrared charge-coupled device cameras, observing flat passive reflecting markers, is installed in a low-speed tunnel to measure Butter mode shapes. Free vibration and aeroelastic measurements are performed on four wing configurations consisting of thin flat orthotropic composite laminates with varying laminate orientation. The laminate orientations are chosen to result in dissimilar flutter mode shapes. The wings are equipped with up to 20 markers, and the motion is sampled at 240 Wt. Quantitative scalar comparisons between analysis and experiments, with respect to both amplitude and phase are done using the modal assurance criterion (MAC). Measurements of mode shapes on free vibrating wings (ground vibration tests), as well as limit-cycle Butter oscillations, show good agreement with numerical results. MAC ratings consistently exceeding 0.96 are achieved, However, it is clearly seen that the agreement is better for free vibration comparisons than for Butter This is expected considering the higher complexity of the Butter problem. Thus, the cause cannot be attributed exclusively to insufficiencies in the optical system but also to inaccuracies in the modeling. The good quality of the measurements proves the usefulness of such a noncontact positioning system in experimental wind-tunnel testing, not only in the present flutter context, but in a variety of experimental work affected by aeroelastic deformation.