Axial Descent of Multirotor Configurations -- Experimental Studies for Terrestrial and Extraterrestrial Applications

Axial descent, specifically the vortex ring state (VRS), poses great challenges for rotorcraft operation as this flight stage is typically accompanied by severe aerodynamic losses and excessive vibrational loads due to the re-ingestion of rotor downwash. Given the hazardous nature of this flight stage, its fluid dynamic properties in regards to single, large-scale rotors have been extensively investigated since the early stages of manned helicopter flight. In light of the rapidly expanding use of small-scale multirotor systems, the field of VRS research has recently received increased interest, with a shifted focus towards small-scale rotors, as the thrust generation and stability of these aerial systems have also been shown to be adversely affected by complex descent aerodynamics. While experimental studies have started examining low Reynolds number rotor aerodynamics in steep or vertical descent, the influence of small-scale rotor geometry and aerodynamic coupling between neighboring rotors have not yet been sufficiently explored. The objective of this work is, therefore, to extend the current understanding of rotorcraft vortex ring state aerodynamics to low Reynolds number multirotor systems. A series of experimental studies employing various wind tunnel setups and flow visualization techniques is presented with the aim of identifying the underlying fluid-structure interactions, and quantifying rotor performance losses during multirotor axial descent. The work is divided into two fundamental experimental approaches, one utilizing statically mounted rotor systems and one utilizing free-flight testing. The first part of this work (Chapters 4 and 5) presents the results of wind-tunnel tested statically-mounted rotors for precise aerodynamic identification of rotor performance under simulated descent conditions. Chapter 4 covers a parametric analysis to comprehensively assess the extent to which relevant geometric parameters of a small-scale rotor inf