Downwash and Outwash of Counter-rotating Coaxial Rotors Hovering in-ground Effect

Abstract:
This study investigates the evolution of axial and radial velocities in the downwash-outwash region of a counter-rotating coaxial rotor hovering in-ground effect (IGE). The presence of the ground deflects the axial flow of the rotor wake radially outward, with mean radial velocities reaching approximately 2Vh along the ground. Based on the observed velocity profiles, the wake was classified into three distinct regions: the downwash region characterized by maximum wake contraction, the transition region where flow turns from axial to radial, and the outwash region exhibiting wall jet behavior. Results show that increasing inter-rotor spacing d/R and rotor height above ground (z/R)l extends the downwash and transition regions, delaying the onset of radial outwash. Aerodynamic loads on personnel were estimated, showing maximum mean forces and moments of 120N and 120Nm, remaining within safety thresholds for untrained personnel. However, the loads exceeded these limits for heavy-category helicopters with higher disk loading. Instantaneous velocities revealed unsteady behavior driven by tip vortex convection, with peak velocities reaching 2.8Vh and peak loads approximately 50% higher than mean values, as identified using the 98th percentile approach. Frequency analysis confirmed that fluctuations at the rotor frequency dominated the unsteadiness in the rotor wake. These findings provide critical insights for safe rotorcraft operation near ground surfaces and contribute to informed vertiport design and personnel safety guidelines.

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