The flow field generated by a helicopter in flight is extremely complex and it has been recognized that interactions between different components can significantly affect helicopter performance. Recent experimental results indicate that significant impulsive loads may be exerted on a helicopter airframe due to the influence of the tip-vortex and a simplified model for this interaction has been developed. The Biot-Savart law is employed to describe the flow induced by the vortex and the flow is assumed to be inviscid and irrotational outside the core of the vortex. On the left figure is a sketch of a single bladed rotor showing how the tip-vortex will impinge on the airframe. On the right is a sketch of the local behavior as the vortex collides with the airframe.
Present work suggests that the process is essentially inviscid.
This is illustrated on the right figure just above. Characteristic of collisions is that the dominant flow features occur on local length and time scales different from those before the collision. We define a two-dimensional collision as a collision in which the axis of the vortex is aligned with the axis of the body and there is no velocity component in the vortex in the direction of its axis. Helicopter aerodynamics problems always involve three-dimensional collisions and the axial flow in the core of the vortex is a critical feature and generally the primary cause of the collision. Experiments suggest that a strong suction peak persists on the side of the airframe where the axial flow in the vortex is directed away from the surface and is removed where the axial flow in the vortex is directed toward the surface.