Numerical Analysis of Unsteady Vortical Flows Generated by a Rotorcraft Operating on Ground: a First Assessment of Helicopter Brownout

摘要

Helicopter taking-off and landing in dry sandy or powdery snow conditions usually experience brownout or whiteout phenomena where the rotor downwash creates a cloud of dust or snow that can dramatically reduce pilot visibility to zero. In case of brownout, dust can cause a total loss of situational awareness involving possible collisions with unseen obstructions or causing lateral drift during the touchdown which may generate dramatic rollover of the rotorcraft. Helicopter brownout has led to numerous accidents in particular during the last military operations in the Gulf theatre. For said reasons, the accurate prediction of such phenomenon is currently challenging the global helicopter aeromechanics community in several ways. In this paper, the ongoing development in AgustaWestland of an advanced physics-based numerical tool able to analyze rotorcraft operations in brownout conditions is presented together with several examples of its application. Numerical results showed that the compactness and extension of sand clouds are directly related to a change in mode within the flowfield surrounding the aircraft in the different flight regimes occurring in ground effect. In case of a single-rotor configuration, flight operations within 'recirculation regimes' generate the greatest entrainment of dust and sand, since the ground particles tend to follow the air streamlines and the vortex structures recirculating through the forward part of the rotor disk. High levels in velocity unsteadiness were found to occur in the zone immediately forward the fuselage nose before the entry in 'ground-vortex' regime. In case of both tandem and tiltrotor configurations, enormous amounts of sand, dust and ground debris were found to be entrained in the free air. Stronger ground vortex structures and important recirculation zones within the two counter-rotating rotors were found to occur for such dual-rotor systems. Numerical results presented in this paper clearly confirmed the anecdotal evidence of the better capabilities of a single-rotor helicopter to operate in sandy-desert environments.