Development of an Indicial Model of Helicopter Tail Rotor Orthogonal Blade Vortex Interaction

This paper describes the development of an indicial model of orthogonal blade vortex interaction. This type of interaction can be experienced by the tail rotor of a helicopter in certain flight states and is a known noise generator. The work uses a substantial database of experimental measurements, made at the University of Glasgow during a five-year research program, to validate the indicial model predictions. The blade response during orthogonal blade vortex interaction is strongly dependent on the axial flow within the vortex core. Depending on the vortex core size, however, it is likely that only a small proportion of the chord is subject to this cross-flow at any instant in time. In addition, after the initial vortex cutting, the axial flow stagnates quite rapidly. These features present particular modeling challenges and this is demonstrated by showing that a simple indicial model driven by a classical vortex core model, in which the blade is represented as a point, significantly overpredicts the blade response. It is also shown that a more sophisticated representation, in which the chordal distribution of cross-flow is accounted for in the indicial model, produces considerably improved predictions. At this stage it becomes apparent that comparison of the model with the experiment is compromised by the three-dimensionality of the measured data. This is explored by numerically simulating the experiment with a vortex wake code. Indicial predictions based on this simulation are found to display many of the essential features of the measured data.