Physical evidence on stall inception from visual studies, mean-velocity-profile correlations, shear measurements, and fluctuations in separating boundary layers in the neighborhood of stall are discussed. Direct visual studies suggest that stall inception in the laminar boundary layer follows the classical model, but does not necessarily do so in the turbulent shear layer. It is useful to describe stall as a certain type of transition region, which can be long or short. Adoption of these ideas is shown to lead to better correlation of stall data and more complete understanding of available physical evidence. However, physical data on the relation between the various types of evidence in the turbulent case and their respective connections with the events in the transition region leading to stall are not presently complete. This suggests experiments of a certain type which should lead to further clarification of the process of stall inception in the turbulent boundary layer.

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