An experimental investigation of the interaction between a model rotor and airframe in forward flight

The flowfield between a lifting rotor and airframe is dominated by the presence of a vortical rotor wake structure which strongly influences various components of the airframe. The presence of a nearby rotating blade also has a major effect on the pressure distribution experienced by the airframe. In forward flight the wake flow combines with the freestream flow creating a non-steady flowfleld which surrounds the airframe. The result is that in the presence of the rotor, the airframe loads are completely different from those experienced by an identical isolated airframe in a uniform flow.;Rotor and airframe components of modern rotorcraft have exhibited problems resulting from aerodynamic interactions which have compromised the overall capability of the vehicles. The need to understand the causes and effects of such aerodynamic interactions has led to the present investigation.;A series of experiments were conducted in the John J. Harper 7 x 9-foot wind tunnel to measure the interaction effects for an idealized rotor-airframe configuration. Detailed pressure measurements on the airframe surface were correlated with quantitative flow visualization data. Correlations allowed for specific structures in the wake flow to be identified with the corresponding effect measured on the airframe.;Impingement of the rotor tip vortex on the airframe surface was found to create a low (instantaneous) pressure region on the surface. The interaction of the vortex sheet structure with the airframe also created a low instantaneous pressure on the surface. The effects of the rotor blade on the airframe were significant. A large positive pressure pulsation occurred on the airframe each time a blade passed over the airframe. Suitable theoretical models for explaining these interaction effects were developed, and a comprehensive data base was generated to allow future analytical efforts for predicting these interactions to be tested.;Unsteady aerodynamic rotor-airframe interactions in forward flight. This method is based on the extension and coupling of a lifting line/free-wake rotor analysis (Scully free-wake code) and a source/doublet panel fuselage analysis (VSAERO). Coupling is achieved by iterating on the disturbance velocities induced by the air-frame at the rotor inflow plane, and on the effect of the rotor and wake on the airframe. Physical flow features such as the energy addition in the wake and the blade passage effect have been modeled and included in the analysis. Provisions have been made to allow the rotor wake to distort freely in the presence of the airframe. In addition, the code has been extended to account for rotor wake/lifting surface interactions. Preliminary results (both time averaged and unsteady) from this study are presented along with a discussion on possible ways to improve correlations with experimental results.