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The pre-critical, critical, and post-critical nonlinear response of an imperfect due to loading eccentricity two-bar frame is thoroughly discussed. In seeking the maximum load-carrying capacity of this non-sway frame, it was qualitatively established that its loss of stability occurs through a limit point and hence, the case of an asymmetric bifurcation can be considered only in an asymptotic sense. After deriving the nonlinear equilibrium equations with unknowns for the two bar axial forces, we can consider such a continuous system as a two-degree-of-freedom model with generalized coordinates the above axial forces. Then, the equilibrium equations and the stability determinant of the frame can be determined in terms of the first and second derivatives of its total potential energy (TPE) with respect to the axial forces. The vanishing of the second variation of the TPE together with the equilibrium equations allows a simple and direct evaluation of the buckling load. Numerical examples demonstrate the efficiency and the reliability of the proposed method.

An experimental investigation of the wake of an Unmanned Aerial Vehicle (UAV) model using flow visualization techniques and a 3D Laser Doppler Anemometry (LDA) system is presented in this work. Emphasis is given on the flow field at the wingtip and the investigation of the tip vortices. A comparison of the velocity field is made with and without winglet devices installed at the wingtips. The experiments are carried out in a closed-circuit subsonic wind tunnel. The flow visualization techniques include smoke-wire and smoke-probe experiments to identify the flow phenomena, whereas for accurately measuring the velocity field point measurements are conducted using the LDA system. Apart from the measured velocities, vorticity and circulation quantities are also calculated and compared for the two cases. The results help to provide a more detailed view of the flow field around the UAV and indicate the winglets' significant contribution to the deconstruction of wing-tip vortex structures.