In the present numerical investigations the unsteady flow about a helicopter airfoil section under deep dynamic stall conditions has been influenced by dynamic airfoil deformation. First attempts with a dynamic variation of the airfoil thickness have already shown the favorable effects on the dynamic stall phenomenon: the dynamic stall onset could be shifted to considerably higher incidences within the oscillatory loop. In the present paper the extension of this method to more arbitrary types of dynamic deformations is discussed and the influence of a dynamic change of he airfoil leading edge curvature (nose droop) is investigated in detail. Special emphasis is placed on the variation of the Mach number. It is shown that for incompressible flow (M-alpha(sub infinity) = 0.1) the shedding of a dynamic stall vortex can be avoided at all. At a slightly higher Mach number (M-alpha(sub infinity) = 0.3) however the complete suppression of the dynamic stall vortex is a much more difficult task. The present results show the way how to proceed successfully also in the compressible flow cases. Further is is obvious that the concept of deforming airfoils has considerable potential in other areas of helicopter aerodynamics, i.e. reduce or even avoid shock motion on the advancing side and therefore reduce the problem of compressibility noise radiation.


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