MODEL FOR ROTOR TIP VORTEX-AIRFRAME INTERACTION .1. THEORY

The flowfield generated by a helicopter in flight is extremely complex , and it has been recognized that interactions among components can si gnificantly affect helicopter performance. In the present work a simpl ified model for the interaction of a rotor tip vortex with the helicop ter fuselage is developed. The tip vortex is idealized as a single thr ee-dimensional vortex tube, and the fuselage is modeled as an infinite circular cylinder. The Biot-Savart law is employed to describe the fl ow induced by the vortex, and the flow is assumed to be inviscid and i rrotational outside the core of the vortex. The numerical calculations indicate that a large adverse pressure gradient develops under the vo rtex on the fuselage causing a rapid drop in the pressure there; large variations in the curvature of the vortex are not observed. Numerical solutions for the vortex position and for the pressure on the airfram e are calculated for the case where the vortex is embedded in a three- dimensional steady mean flow; the effect of vortex core size is also i nvestigated. The nature of the initial stages of the breakdown of loca l axisymmetry of the core of the vortex filament is suggested based on both the numerical and experimental results.