Nonlinear and Unsteady Multi-rotor Aerodynamics in Ground Effect

Ground effect can significantly enhance the thrust characteristics of multi-rotor aircraft, but the nonlinear and unsteady thrust fluctuations in ground effect flight may lead to altitude instability, manifesting as periodic bouncing phenomena. This research aims to systematically analyze the nonlinear thrust mechanisms in the ground effect, the influence of the overlapping structure on rotor aerodynamic characteristics and the various flow patterns. Using a combined approach of computational fluid dynamics (CFD) and thrust measurement experiments, this study proposes a novel overlap-ping hexa-rotor configuration. The thrust and flow characteristics at various heights and rotor speeds in the ground effect are thoroughly computed and compared with experimental results. It was found that the thrust characteristics in ground effect are significantly nonlinear, with a maximum thrust gain of 37% and an effective range defined as 0–1.75D (where D is the propeller diameter). Furthermore, under the specific conditions of a rotor speed below 9515 RPM and an altitude below 0.75D, the thrust gain was observed to weaken as the altitude decreased. Additionally, a computational analysis for the case with a UAV altitude of 1D and a propeller speed of 8500 RPM showed that the thrust fluctuations of the lower rotor were six times greater than those of the upper rotor. These findings provide crucial insights for altitude control of multi-rotor air-craft operating in ground effect, enhancing understanding of aerodynamic interactions and precise altitude control in ground effect for complex rotorcraft configurations.