This article is closed access. It was published in the AIAA Journal [© American Institute of Aeronautics and Astronautics] and is available from: http://www.aiaa.org/content.cfm?pageid=406&gTable=jaPaper&gid=39571Stall flutter is a nonlinear aeroelastic phenomenon that can affect several types of aeroelastic systems such as\ud helicopter rotor blades, wind turbine blades, and highly flexible wings. Although the related aerodynamic\ud phenomenon of dynamic stall has been the subject of many experimental studies, stall flutter itself has rarely been\ud investigated. This paper presents a set of experiments conducted on a NACA0012 airfoil undergoing stall flutter\ud oscillations in a low-speed wind tunnel. The aeroelastic responses are analyzed with the objective of characterizing\ud the local bifurcation behavior of the system. It is shown that symmetric stall flutter oscillations are encountered as a\ud result of a subcritical Hopf bifurcation, followed by a fold bifurcation. The cause of these bifurcations is the\ud occurrence of dynamic stall, which allows the transfer of energy from the freestream to the wing. A second\ud bifurcation occurs at the system’s static divergence airspeed. As a consequence, the wing starts to undergo\ud asymmetric stall flutter bifurcations at only positive (or only negative) pitch angles. The dynamic stall mechanism\ud itself does not change but the flow only separates on one side of the wing

The experimental determination of the vibration characteristics of rotating engine blades is very important for fatigue failure considerations. One of the most promising techniques for measuring the frequency of blade vibrations is blade tip timing. In this paper, three vibration analysis methods were specifically formulated and applied to the tip timing problem for the first time, using data obtained from a simple mathematical blade tip timing simulation. The results from the methods were compared statistically in order to determine which of the techniques is more suitable. One of the methods, the global autoregressive instrumental variables approach, produced satisfactory results at realistic noise levels. However, all of the techniques produced biased results under certain circumstances.