Theoretical and Experimental Study of Unsteady Rotor/Body Aerodynamic Interactions

Theoretical and experimental results are presented which clarify the nature of the unsteady aerodynamic interactions between a rotor/body combination in forward flight. Experimental measurements of the time-dependent body pressures are compared with predictions made by two models of the body, rotor and its wake system. The first model is a fairly elementary representation using a vortex ring wake and a one-dimensional surface model for the body. The second model consists of a more sophisticated unsteady source panel representation for the body, a lifting line for the rotor, along with a prescribed wake system. Appropriate unsteady terms are included in these analyses. The results show that the unsteady pressure loads on a large part of the body are dominated by the periodically induced pressure field due to the rotor itself. These loads are well predicted using either model. Additional unsteady effects on the body are due to either close passage or direct impingement of the rotor wake vortices. It is shown that if the rotor wake geometry is accurately modelled, then the unsteady pressure signatures can be readily predicted in all locations except where the wake impinges directly on the body. In such cases, viscous interactions appear to play a strong role in the process and are not easily predicted.