Characterization of the near wake of a helicopter rotor

Abstract

Vortex characteristics in the near wake of a 2-bladed teetering rotor in steady forward flight are measured using a laser velocimeter. The vortex passage at a measurement point is seen to be repeatable to within 1° of rotor revolution. Velocity was measured in planes intersected by the rotor-blade tip at rotor azimuths of V> = 0°, 90°, 180°, 270°. Variations in tipvortex characteristics such as core radius, circulation and axial velocity were obtained as functions of rotor azimuth and vortex age. The core circulation and core-radius distributions conform to a simple harmonic function of the rotor azimuth. However, the core axial velocity has a fundamental and a 2-per rev variation with rotor azimuth. Empirical curve fits are described for these variations. The core axial velocity is of the order of the peak tangential velocity at all locations. It is higher than the tangential velocity over part of the advancing side of the rotor. The axial velocity is in all cases wake-like, being directed towards the blade of origin. Wake geometries obtained from the LDV data are also are presented here.