Study on the aerodynamic interactions of a coaxial rotor hovering in-ground effect

Abstract

Counter-rotating coaxial rotors (CCR) offer notable advantages over conventional single-rotor systems in flight speed, payload capacity, and maneuverability. As a result, CCR configurations are being considered for future vertical flight applications ranging from urban air mobility to space exploration. However, operating two rotors in proximity on a shared axis results in complex aerodynamic interactions between the rotors that significantly influence individual rotor performance. Understanding these interactions is critical to developing safer and more efficient aircraft. This thesis aims to quantify the aerodynamic rotor-rotor interactions in a CCR operating in-ground effect (IGE), where an additional effect, the rotor-ground interaction, competes with the rotor-rotor interactions in driving the performance characteristics. An experimental facility consisting of two mechanically decoupled rotors with a height-adjustable ground plane was developed. Load measurements were collected to assess the rotor performance. They were coupled with a momentum theory-based theoretical framework to develop empirical correction factors that quantify how these interactions influence the induced power required to hover. The results indicated that in IGE conditions, the rotor-ground interaction dominates rotor performance, leading to significant performance improvements in each rotor. However, rotor-rotor interactions were modified by the presence of the ground, varying by up to approximately 8% in certain conditions, suggesting a complex coupling between rotor-rotor and rotor-ground interactions. A secondary study investigating the effects of Reynolds number on these interactions revealed that the lower rotor interactions were altered. In contrast, the upper rotor interactions were invariant to the Reynolds number, suggesting a discrepancy in the wake characteristics of the upper rotor as it impinges on the lower rotor. This thesis introduces a formulation that isolates the rotor-rotor and rotor-ground interactions of a CCR operating IGE. It provides a fundamental understanding of how ground proximity impacts the power requirements for a hovering CCR and establishes a framework for decomposing the complex aerodynamic interactions between the rotors and the ground.